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The canonical model for the formation of the Solar System involves the gravitational collapse of a nebula into (perhaps) several stars across several light years. Is it possible that two or more of the stars (or protostars) combined to form our Sun? Or would any stellar collision release so much energy that our Sun could not have formed the way we see it today? Have astronomers or astrophysicists created n-body orbital models that allow for a "soft" collision of two stars combined with the ejection of the other(s)?
The dynamics of the Solar System and the chemistry of the Solar System bodies don't support a hypothesis of a stellar merger later than formation of the protoplanetary disk which would have mixed-up things considerably and heavily disrupt any circumstellar disk. Thus this basically excludes any collision after the time one can start talking about a protostar, way before it even entered main sequence (as that's already the end of planetesimal formation).
Stellar mergers are certainly possible, but also relatively rare. Maybe protostars merging is a bit more common since they have less relative velocity.
However, unless the merger is straight it will typically deposit a lot of angular momentum. The sun seems to be a slow rotator for its spectral class. Hence it is not likely it was formed through a stellar merger.
Astrotheology of the Ancients
The further one regresses in time, the more obvious it becomes that the principal and singular religious worship found around the globe has revolved around nature. This nature worship has included reverence not only for the earth, its creatures and their fecundity, but also for the sun, moon, planets and stars. For many thousands of years, man has looked to the skies and become awestruck by what he has observed. This awe has led to the reverence and worship both of the night and day skies, an adoration called “astrotheology.” While fertility worship has constituted an important and prevalent part of the human religion, little has astonished humankind more than the sky, with its enormous, blazing, white day orb in the azure expanse, and with its infinite, twinkling, black night dome. So fascinated by the sky, or heavens, has been man that he has created entire religions, with organized priesthoods, complex rituals and massive edifices, in order to tell its story.
The story begins, as far back as the current evidence reveals, with the night sky as the primary focus of pre-agricultural, nomadic peoples. The night sky held particular importance in the lives of desert nomads, because the fiery sun was a hindrance to them, while the cool night allowed them to travel. In traveling by night, these desert nomads became keenly aware of the night sky’s various landmarks, including the stars, planets and moon. The nomads noticed regularity and began to chart the skies, hoping to divine omens, portents and signs. Others who developed this astronomical science included ancient mariners who journeyed thousands of miles through the open seas, such as the Polynesians, whose long, Pacific voyages have been estimated to have begun at least 30,000 years ago. The astronomical science allowed the ancients to predict weather patterns, the turn of seasons and attendant climate changes, as well as comets, asteroids and meteors menacing the earth. This archaeoastronomy was an accurate prognosticator for daily, weekly, monthly and yearly events. Indeed, it was an augur for the changes of entire ages, some of which, as in the chronologies of the Maya, Babylonians and Hindus, extend back hundreds of thousands or millions of years.
Determining the archaeoastronomy requires the use of astronomy, archaeology, ethnography and other sciences to study legends, texts, artifacts and architectural remains. Such fascinating relics include rock paintings, megalithic structures, calendars and medicine wheels. Cultural remains and ruins globally demonstrate the ancients’ interest in and knowledge of “the complex regularity of the motions of the sun, moon, and stars and…unusual occurrences such as the appearance of a nova or comet in the sky.”… (26-27)
That ancient peoples, including those thought to be “primitive,” possessed this impressive knowledge, which required precise geometrical capacity as well as astronomical expertise, is a fact. That they went to extraordinary lengths to encapsulate and memorialize it is also a fact. Another fact is that the depth of inspiration and passion reflected by these remains is indicative of the ancients’ astrotheological religious tendencies.
The astronomical science of the ancients is the same used today to determine full moons, eclipses, conjunctions and other cosmic events both past and future. It is because of the ancient study that we have this capability today, although our abilities are just beginning to catch up to the archaeoastronomy of such peoples as the Maya and their forebears. This regression and loss of knowledge is due to cataclysm and destruction of human culture. Yet, the basics of this important knowledge were preserved because the ancients used myths as mnemonic devices passed along from generation to generation. This tradition was especially important during the thousands of years when writing was either non-existent or limited. Unfortunately, the key to this knowledge was nevertheless often lost, as the myths became believed as “historical fact.”… (28)
Astronomical or astrotheological knowledge reaches back to the dawn of humanity, appearing widespread and becoming highly developed over a period of millennia. In its entry on “Astrology,” the Catholic Encyclopedia describes the development of this archaic science in the ancient world:
The history of astrology is an important part of the history of the development of civilization, it goes back to the early days of the human race…. Astrology was…the foster-sister of astronomy, the science of the investigation of the heavens…. According to the belief of the early civilized races of the East, the stars were the source and at the same time the heralds of everything that happened, and the right to study the “godlike science” of astrology was a privilege of the priesthood. This was the case in Mesopotamia and Egypt, the oldest centres of civilization known to us in the East. The most ancient dwellers on the Euphrates, the Akkado-Sumerians, were believers in judicial astrology, which was closely interwoven with their worship of the stars. The same is true of their successors, the Babylonians and Assyrians, who were the chief exponents of astrology in antiquity…. The Assyro-Babylonian priests (Chaldeans) were the professional astrologers of classical antiquity. In its origin Chaldaic astrology also goes back to the worship of stars this is proved by the religious symbolism of the most ancient cuneiform texts of the zodiac. The oldest astrological document extant is the work called “Namar-Beli” (Illumination of Bel) composed for King Sargon I (end of the third millennium B.C.) and contained in the cuneiform library of King Asurbanipal (668-626 B.C.)…. Even in the time of Chaldean, which should be called Assyrian, astrology, the five planets, together with the sun and moon, were divided according to their character and their position in the zodiac as well as according to their position in the twelve houses. As star of the sun, Saturn was the great planet and ruler of the heavens…. The Egyptians and Hindus were as zealous astrologers as the nations on the Euphrates and Tigris. The dependence of the early Egyptian star (sun) worship (the basis of the worship of Osiris) upon early Chaldaic influences belongs to the still unsettled question of the origin of early Egyptian civilization.
Thus, astrology – a “godlike science”—dates back thousands of years and has been an important part of human civilization. According to mainstream archaeology, the oldest extant text specifically addressing “astrology” dates from the 3 rd millennium BCE yet, the astrological religion or astrotheology is recorded abundantly in Indian, Egyptian and Sumerian sacred literature as well, some of which represents traditions much older than the third millennium. Also, as noted, megalithic ruins push astronomical knowledge back at least 6,000 to 6,500 years ago, while ancient mariners reveal such knowledge dating to 30,000 or more years ago…. (29-30)
In The Roots of Civilization, archaeologist Alexander Marshack discusses “calendar sticks,” or ancient bones with markings that Marshack determined represented lunar calendars, dating to at least 25,000 or 35,000 years ago. One of these artifacts is the “Ishango bone” discovered at Lake Edward in Zaire, and possibly dating to 18,000-23,000 BCE. Marshack found other such bones, from the Upper Paleolithic (30,000-10,000 BCE) or Aurignacian culture. Marshack’s contention that they are lunar calendars is not “set in stone,” but there is more than good reason to assume it to be accurate. In his book In Search of Ancient Astronomies, astronomer and past-director of Los Angeles’s Griffith Observatory, Dr. Edwin Krupp, relates:
“The Blanchard bone, a small piece of bone found in the Dordogne region of France inscribed by some Cro-Magnon individual about twenty thousand years ago, has a complicated pattern of marks. The shapes of the marks vary, and the sequence curves around in a serpentine pattern. In Marshack’s view the turns in the sequence represent, on one side, the times of dark, new moon, and on the other, bright full moon. Statistical analyses may not support Marshack’ s interpretations, but similar batons and sticks are carved for the same purpose by the Nicobar Islanders in the Bay of Bengal.”
At the very least, these bones demonstrate that the ancients knew how to count, to a certain point. The thesis that these bone markings also reflect the “moons” or menstrual periods of women is likewise sound hence, it has been suggested, women were the “first mathematicians.” One of these women is represented on an 18-inch bas-relief called the “Venus of Laussel,” an image dating to the Aurignacian era, some 21,000 years ago. Originally painted in red ochre, suggesting menstrual blood, the Venus holds a curved bison horn with 13 notches, which represent the crescent moon and, apparently, the “Universal Vulva,” along with the annual lunar months and women’s menses. Significantly, the average menstrual cycle is 29.5 days, the same as the lunar month hence, the two are intimately connected. In all probability, it was women’s observations of their menses that led to timekeeping. Another factor in the development of astronomy was the need for hunters to know the lunar cycle, so they could plan their hunt, based on the waxing or waning of the moon.
In the famous caves of Lascaux in France have been discovered star maps that date to 16,500 years ago and, according to Dr. Michael Rappenglueck of the University of Munich, record the Pleiades, or “Seven Sisters,” as well as the “Summer Triangle,” composed of the three stars Vega, Deneb and Altair. A 14,000-year-old star map recording the Northern Constellation was also found in the Cueva di El Castillo in Spain. The art of the ancients in such places as Lascaux and Alta Mira, Spain, dating to the Paleolithic (17,000+ Before Present), or Adduara, Sicily (15,000-10,000 BCE), shows a high degree of intelligence, comparable to that of humans today. In discussing the ancients it should be kept in mind that, despite the impression given by strict, linear-evolutionary thinking, humans at least 100,000 years ago (a number that keeps being pushed back) possessed the identical cranial capacity as they do today. Instead of a bunch of grunting ape-men, there were likely individuals among them with IQ’s similar to modern geniuses. It is probable that, as today, there were human beings living in varying states of “civilization,” with some prehistoric humans wearing rough skins and living in caves, while other early humans created more advanced culture…. (30-31)
The Archaic Winter Solstice
In Prehistoric Lunar Astronomy, S.B. Roy postulates that various artifacts found deep in caves, such as the painting known as “Sorcerer with the Antelope’s Head” from Les Trois Freres caves in the French Pyrenees, are representative of…secret deposits [relating to the mysteries]. These caves were occupied during the Magdalenian period, 10,000-16,000 years ago, although [mythologist] Robert Graves dates the paintings to “at least 20,000 B.C.” Regarding possible rituals performed in these caves, some of which are very inaccessible and would therefore likely represent the place of a secret, esoteric initiation, Roy remarks that they would “necessarily be performed at a particular auspicious moment,” upon which their potency would depend. This auspicious moment would be dependent on the solar and lunar phases, as well as the seasons: “The ancient wise men looked up at the heavens to ascertain the proper timing, because the Moon was the most ancient timekeeper, says Yaska [1400 BCE]…” Such “auspicious moments” can be dated using these astronomical keys.
Roy posits that the antelope-headed “sorcerer” was “a figure marking the onset of a season.” The reasons for this assertion include that the “remote traditions” in the Rig Veda and in Vedic astronomy relate that the Stag’s head represents the star L-Orionis and the winter solstice at the new moon, as well as the summer solstice at the full moon. Roy concludes that the sorcerer figure “marked the winter solstice,” which was “a great day in the Ice Age of Europe.” Based on the astronomy, the figure dates to 10,600 BCE. Furthermore, this stag-headed sorcerer figure is similar to solar images on seals from the Indus Valley city of Mohenjo-Daro dating to the third millennium BCE.
Dating the migration of the European Magdalenian cave-dwellers to the recession of the “fourth glacial Wisconsin-Valders final sub-phase,” 10,000 years ago, Roy further states:
In Northern Europe and Asia, in latitudes of 60º and higher, where Slavonic languages now prevail, the winter was then long and dark. It was very cold. Everyone looked to the day of the winter solstice when the sun would turn North. The astronomers would know the date even though the sun itself was not visible. This was the great day, for the spring would now come.
Thus, the winter solstice was an important factor in human culture, particularly that of the cold, northern latitudes, at least 12,000 years ago. The winter solstice celebration that developed throughout much of the inhabited world has been handed down as “Christmas,” i.e., December 25 th , the birthday of the sun of God. “Christmas” is thus an extremely ancient celebration, predating the Christian era by many millennia…. (33-34)
Who Were The Ancient Gods?
The subject of what or who were the ancient gods has been the focus of much serious debate and wild speculation over the centuries. The reality is that the ancient gods were mainly astrotheological and/or based on natural, earthly forces. This fact is attested by numerous authorities over the millennia, including ancient writers reflecting upon their own religions and those of other known cultures. …[T]he ancient authorities who knew that the gods were astronomical, i.e., the sun, moon, stars and planets, and elemental, i.e., water, fire, wind, etc., or natural, i.e., rivers and springs, included Epicharmos (c. 540-450 BCE), Prodikos (5th cent. BCE) Caesar (100-40 BCE) and Herodotus (484?-425 BCE)…. (35-36)
The Precession of the Equinoxes
Another important factor in ancient astrotheology is the precession of the equinoxes, a phenomenon caused by the earth’s off-axis tilt, whereby the sun at the vernal equinox (spring) is back-dropped by a different constellation every 2150 or so years, a period called an “age.” One cycle of the precession, through the 12 signs of the zodiacal ages is called a “Great Year,” and is approximately 26,000 years long. According to orthodox history, the precession was only “discovered” in the second century bce by the Greek astronomer Hipparchus however, it is clear from ancient texts, traditions, artifacts and monuments that more ancient peoples knew about it and attempted to compensate for it from age to age. In Hamlet’s Mill, Santillana and Dechend demonstrate knowledge of the precession at much earlier times, stating: “There is good reason to assume that he [Hipparchus] actually rediscovered this, that it had been known some thousand years previously, and that on it the Archaic Age based its long-range computation of time.”… (39-40)
The Astrotheological Priesthood
The best-known astronomical priestly caste was that of the Assyro-Babylonian culture called the Chaldeans, who, with the demise of the Assyro-Babylonian empire, were eventually dispersed into other parts of the world, including Greece. After this development, the Chaldean occult science became less hidden and more known to the masses. From ancient authorities it is evident that the term “Chaldean” ceased to be descriptive of an ethnicity but came to be considered an appellation for the astrological priestly order, from which the Hebrew priesthood, among others, was in large part derived, although the biblical imitators never reached the sublimity of the original. Reflecting their widely held esteem, in On Mating with the Preliminary Studies (X, 50), the Jewish philosopher Philo Judaeus of Alexandria (c. 20 BCE-c. 50 CE) described the Chaldeans as understanding to an “eminent degree” what he called “astronomy” and further termed “the queen of all the sciences.”… (42-43)
Christian Attestation of Astrotheology
We have seen how various ancient, pre-Christian writers explained that their gods were astrotheological and that astrology was a predominant ideology or “science” in the Pagan world. Like the Pagans, the early Church fathers discussed the pervasive astrotheology, as they could hardly avoid it, since it was their competition. Naturally, when they did address it their comments were often condescending or disparaging. For example, in Against the Heathen, theologian St. Athanasius (c. 293-373) attempted to raise the Christian god above all the rest, establishing the ancient worship as astrotheological and relating that mankind “gave the honour due to God first to the heaven and the sun and moon and the stars, thinking them to be not only gods, but also the causes of the other gods lower than themselves…” (48)
Astrotheological Origin of Christianity
The Christian assault on astrology was furious and motivated by a desire for dominance and the replacement of the Pagan astrotheology with that of Christianity, with an eye to covering up the latter’s own astrotheological roots. The Christian fathers eventually were responsible for vicious persecution of “astrologers,” i.e., those Chaldeans and others who were priests of Pagan faiths. Arabic and Jewish universities and scholars kept astrology alive throughout the Middle Ages, despite continued persecution by Christians. As time went on, this “false doctrine,” which never disappeared from Europe but was condemned on the one hand and embraced on the other by Church authorities, began to resurface more overtly. Indeed, numerous emperors and popes “became votaries of astrology,” including “Charles IV and V, and Popes Sixtus IV, Julius II, Leo X, and Paul III,” as related by the Catholic Encyclopedia. “Among the zealous patrons of the art were the Medici,” CE continues, with Catharine de Medici popularizing astrology among the French and making Nostradamus her “court astrologer.” Popes Leo X and Clement VII retained the same court astrologer, Gauricus, who “published a large number of astrological treatises.” Moreover, during the Renaissance , CE further recounts, “religion…was subordinated to the dictation of astrology,” with the rise of each religion given astrological foundation… (52-53)
As is evident, the study and reverence of the heavens goes back many millennia, and has constituted in large part the original religious concepts developed by humanity. As is also clear, the ancients were well aware that they were worshipping the sun, moon, stars and “all the host of heaven.” Entire cultures were based upon astrotheology, and numerous magnificent edifices were constructed for its glorification. Indeed, the proscription by biblical writers shows how important and widespread was this worship of the cosmic bodies and natural phenomena. The Church fathers and other Christian writers also acknowledged this astrotheology and its antiquity, but denigrated it as much as possible. Why? What would a detailed investigation reveal about their own ideology? As demonstrated in The Christ Conspiracy and here, the knowledge about astrotheology would reveal the Christians’ own religion to be Pagan in virtually every significant aspect, constituting a remake of the ancient religion. Yet, this astrotheology devised by our remote ancestors over a period of millennia was symbolically and allegorically a treasure-trove. Hence, the restoration of this knowledge is not to be despaired but rejoiced. (56-57)
See also The Astrotheology Calendar Guide , now on Kindle, iPhone, iPod, iPad, Android, Blackberry, PC, Mac and more!
Largest black hole collision ever detected
Seven billion years ago, two truly huge black holes slammed together and formed one 142 times the mass of the sun.
Seven billion years ago, two large black holes crashed together and formed a massive new one. It is the largest black hole collision ever detected in space, and the new black hole formed in the crash is the largest of its kind ever detected. It's so large, in fact, that physicists weren't sure it could exist at all.
The ripples from that collision reached the two Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors in the U.S. and another in Italy called on May 21, 2019, after traveling through space for 2.5 billion years longer than the sun has existed. Those ripples revealed signatures of the merger of at least two black holes — one a black hole 85 times the mass of the sun and one 66 times the sun’s mass. When they collided, they formed a black hole 142 times the mass of the sun. The missing nine suns’ worth of matter got converted into energy in the collision, shaking the universe hard enough for LIGO and Virgo to detect and interpret. And that's how scientists learned that 85 solar mass black holes and monster 142 solar mass black holes can exist at all.
&ldquoThis [signal] doesn't look much like a chirp, which is what we typically detect," Nelson Christensen, a member of the Virgo collaboration, said in a statement. "It is more like something that goes 'bang.'"
That "bang" shook our region of space for only about a tenth of a second after traveling for a longer period of time than any gravitational wave ever detected before. But analyzing the shape of the wave, the researchers realized the type of black holes involved and exactly how big they were. .
As Live Science previously reported, until now black holes have fallen into two categories: stellar-mass black holes and supermassive black holes.
Stellar-mass black holes, formed from collapsing stars, don't get much larger than a few dozen times the mass of the sun. These are the black holes that LIGO and Virgo have previously detected merging.
At the other end of the spectrum are supermassive black holes, with masses millions or billions of times that of our sun and the gravity to grow whole galaxies around themselves. The Milky Way has one, as do most other comparable galaxies. But scientists aren't sure where they came from or how they formed they've never detected a merger of such monstrous black holes.
In between the supermassives and the stellar-mass black holes is a wide "mass gap," a range of masses where no black holes have ever been detected. One idea is that supermassives grow out of mergers of stellar-mass black holes. But if that is the case, then they would have to pass through this mass range. Stellar-mass black holes would combine to form mass gap black holes, which would combine and combine until becoming supermassive. But until now, scientists have never seen that happen.
"One of the great mysteries in astrophysics is &lsquoHow do supermassive black holes form?’" study co-author Christopher Berry, a Northwestern University physicist, said in a statement. "They are the million solar-mass elephants in the room. Do they grow from stellar-mass black holes, which are born when a star collapses, or are they born via an undiscovered means? Long have we searched for an intermediate-mass black hole to bridge the gap between stellar-mass and supermassive black holes. Now, we have proof that intermediate-mass black holes do exist."
This discovery actually revealed that black holes can exist in two separate mass gaps. The 85 solar mass black hole fits into the "pair instability" gap.
Lighter stars don’t collapse into black holes because the outward pressure from photons and gas in their cores keeps them puffed up to large volumes, researchers believe. But at very large masses, the energy in a star's core converts photons into pairs of electrons and antielectrons, which together produce less pressure than photons do. That means that when the star does collapse, the process is so fast and energetic that much of the mass gets blown away into space. So a 130 solar-mass star can collapse and form a 66 solar-mass black hole.
This "pair instability" gap ranges from 66 solar masses to 120 solar masses — a range where theoretical calculations suggest no black hole could directly form from a collapsing star. The 85 solar-mass black hole detected in 2019 falls squarely in that range. The most likely explanation is that it's a "second generation" black hole, formed from two smaller progenitors. It's possible that the 66 solar-mass black hole was a second generation type as well, the researchers wrote in the study.
The 142 solar mass black hole falls into a different, bigger mass gap. A black hole that’s more massive than 120 suns could theoretically have formed from very large collapsing stars. But no black hole in that range has ever been detected, and until now researchers weren't sure whether black holes of that mass could exist at all, now matter how they formed. But this new, 142 solar-mass black hole falls squarely in that range — between the stellar masses and the supermassives. It proves that black holes of that mass can exist.
And it's still possible, the researchers wrote, that this signal doesn't reveal very large black holes as researchers assumed.
"What if something entirely new produced these gravitational waves?" Vicky Kalogera, another Northwestern researcher, said in the Northwestern statement. "It's a tantalizing prospect. . For instance, perhaps the gravitational waves were emitted by a collapsing star in our galaxy. The signal also could be from a cosmic string produced just after the universe inflated in its earliest moments — although neither of these exotic possibilities matches the data as well as a binary merger."
Gravitational wave astronomy is still so new that it's impossible to be certain. As LIGO, Virgo and future detectors gather more data from new events, the researchers wrote, the picture should become clearer. And this event, with its promise of super-big black holes, might eventually become easier to explain.
A paper describing the discovery was published today (Sept. 2) in the journal Physical Review Letters. Another paper exploring its implications was also published today in The Astrophysical Journal Letters.
Ancient rings surrounding the Sun could have divided the solar system
The clear divide between the inner and outer solar system is the legacy of a ring structure that existed a very long time ago in the planet-forming disc that surrounded the Sun. That is the conclusion of Ramon Brasser at Tokyo Institute of Technology and Stephen Mojzsis at the University of Colorado Boulder, who have combined computer simulations of Jupiter’s formation with observations of the discs surrounding young stars.
The solar system is clearly divided between the inner rocky planets and asteroids, and the outer gas giants – with the border between the two regions lying between Jupiter and the asteroid belt. This difference can be quantified in terms of carbon – with the element being much more abundant in the outer part of the solar system than it is in the inner rocky planets and asteroids. The difference is so stark that astronomers now widely believe that material in the newly-formed Sun’s planet-forming circumstellar disc was similarly divided in terms of its composition.
For some reason, carbon-rich material from the outer solar system has been prevented from migrating into the inner solar system. One explanation for this barrier is that it arose during formation of Jupiter. As the gas giant gathered mass, the theory proposes, it prevented carbon-rich dust and sub-metre “pebbles” from reaching the inner solar system.
However, Brasser and Mojzsis claim to disprove this hypothesis in their study. Through simulations recreating the conditions of the early solar system, they showed that Jupiter would not have accreted mass fast enough to create such a significant barrier.
For an alternative explanation, the duo looked to observations made by Chile’s ALMA telescope, which has observed a rich variety of ring structures in the circumstellar discs of many young stars. They propose that similar rings were likely to have existed around the Sun as the planets were forming. If so, they could have created regions of high-pressure gas and dust which would have been difficult, though not impossible, for carbon-rich objects to cross.
Giant exoplanet orbiting tiny star defies current theory of planet formation
If these ring structures lasted long enough, Brasser and Mojzsis argue that they could have fundamentally altered the structure of the solar system, preventing today’s terrestrial planets from acquiring more matter to become giants. They also believe that other high-pressure rings are likely to exist further out in the solar system, and that the gas giants may have formed as they fell into the lower pressure sinks that lay between the rings.
Ultimately, their work suggests the need for a fundamental rethink of Jupiter’s role in the solar system’s characteristic distribution of carbon. With the diversity of observations of circumstellar discs gathered by ALMA, they could also help astronomers to learn more about the formation of star systems other than our own.
When stars collide
The mechanism behind a luminous red nova is thought to be the merging of two stars. This can occur when stars within a binary system end up coming too close together -– possibly because their orbit decays or because one of the stars reaches old-age and as it swells to become a red giant star, the other star finds itself stuck inside the red giant’s outer envelope.
A similar fate awaits the Earth, when our sun reaches its red giant phase in around 5 billion years time.
When the stars eventually merge, material is dredged up from inside the stars and ejected into space. What’s left behind, matches Nova Vul 1670 fairly closely - a faint remnant embedded in cool gas that is rich in molecules and dust.
And what of that nova that we can currently see? Sagittarius is visible from Australia from around 2am until sunrise, when it will be high in the east. But it won’t be around for long, so try and see it while you can.
Sagittarius is found high in the east before sunrise from across Australia and just above the constellation is the planet Saturn. Museum Victoria/Stellarium
A Vast Alien Star Stream –“Unveiled Near the Sun”
Nyx, a vast new stellar stream discovered in the vicinity of the Sun, named after the Greek goddess of the night, may provide the first indication that a dwarf galaxy had merged with the Milky Way disk. These stellar streams are thought to be globular clusters or dwarf galaxies that have been stretched out along its orbit by tidal forces before being completely disrupted.
Among the oldest objects in the universe, the Milky Way is surrounded by about 150 globular clusters, formed about 11.5 billion years ago, 2.3 billion years after the Big Bang and shortly before the rate of cosmic star formation reached its peak, 10 billion years ago –a period known as “cosmic high noon.”
“If there are any clumps of stars that are moving together in a particular fashion, that usually tells us that there is a reason that they’re moving together,” says Lina Necib, who discovered the star stream. Necib a postdoctoral scholar in theoretical physics at Caltech, who studies the kinematics—or motions—of stars and dark matter in the Milky Way.
Starting at the Beginning of Time
Since 2014, researchers from Caltech, Northwestern University, UC San Diego and UC Berkeley, among other institutions, have been developing highly-detailed simulations of realistic galaxies as part of a project called FIRE (Feedback In Realistic Environments) that seeks to develop and explore cosmological simulations of galaxy formation. The simulations directly resolve the interstellar medium of individual galaxies while capturing their cosmological environment.starting from the virtual equivalent of the beginning of time, the simulations produce galaxies that look and act much like our own.
The researchers also incorporate data from Gaia space observatory was launched in December of 2013 by the European Space Agency. to create an extraordinarily precise three-dimensional map of about one billion stars throughout the Milky Way galaxy and beyond. Gaia’s stunning first dataset, published in 2016, cataloged more than a billion stars and contained distance and motion data for 2 million stars.
“It’s the largest kinematic study to date. The observatory provides the motions of one billion stars,” Necib,explained. “A subset of it, seven million stars, have 3-D velocities, which means that we can know exactly where a star is and its motion. We’ve gone from very small datasets to doing massive analyses that we couldn’t do before to understand the structure of the Milky Way.”
“Galaxies form by swallowing other galaxies,” Necib said, addressing the question of how the Milky Way became what we see today.. “We’ve assumed that the Milky Way had a quiet merger history, and for a while it was concerning how quiet it was because our simulations show a lot of mergers. Now, with access to a lot of smaller structures, we understand it wasn’t as quiet as it seemed. It’s very powerful to have all these tools, data and simulations. All of them have to be used at once to disentangle this problem. We’re at the beginning stages of being able to really understand the formation of the Milky way.”
Neural Networks– Huge Datasets of the “Fire” Galaxies
“Before, astronomers had to do a lot of looking and plotting, and maybe use some clustering algorithms. But that’s not really possible anymore,” Necib said. “We can’t stare at seven million stars and figure out what they’re doing. What we did in this series of projects was use the Gaia mock catalogs.”
The Gaia mock catalog, developed by Robyn Sanderson (University of Pennsylvania), essentially asked: ‘If the FIRE simulations were real and observed with Gaia, what would we see?’ Necib’s collaborator, Bryan Ostdiek (formerly at University of Oregon, and now at Harvard University), who had previously been involved in the Large Hadron Collider (LHC) project, had experience dealing with huge datasets using machine and deep learning. Porting those methods over to astrophysics opened the door to a new way to explore the cosmos.
“At the LHC, we have incredible simulations, but we worry that machines trained on them may learn the simulation and not real physics,” Ostdiek said. “In a similar way, the FIRE galaxies provide a wonderful environment to train our models, but they are not the Milky Way. We had to learn not only what could help us identify the interesting stars in simulation, but also how to get this to generalize to our real galaxy.”
The team developed a method of tracking the movements of each star in the virtual galaxies and labeling the stars as either born in the host galaxy or accreted as the products of galaxy mergers. The two types of stars have different signatures, though the differences are often subtle. These labels were used to train the deep learning model, which was then tested on other FIRE simulations.
After they built the catalog, they applied it to the Gaia data. “We asked the neural network, based on what you’ve learned, can you label if the stars were accreted or not?” Necib said. The model ranked how confident it was that a star was born outside the Milky Way on a range from 0 to 1. The team created a cutoff with a tolerance for error and began exploring the results.
This approach of applying a model trained on one dataset and applying it to a different but related one is called transfer learning and can be fraught with challenges. “We needed to make sure that we’re not learning artificial things about the simulation, but really what’s going on in the data,” Necib said. “For that, we had to give it a little bit of help and tell it to reweigh certain known elements to give it a bit of an anchor.”
Unveiling the Gaia Sausage
They first checked to see if it could identify known features of the galaxy. These include “the Gaia sausage”—an ancient and dramatic head-on collision between the Milky Way and a smaller object, dubbed the “Sausage” galaxy that was a defining event in the early history of the Milky Way that reshaped the structure of our galaxy, fashioning both its inner bulge and its outer halo.
“It has a very specific signature,” she explained. “If the neural network worked the way it’s supposed to, we should see this huge structure that we already know is there.”
“The collision ripped the dwarf to shreds, leaving its stars moving in very radial orbits” that are long and narrow like needles, said Vasily Belokurov of the University of Cambridge and the Center for Computational Astrophysics at the Flatiron Institute in New York City who was not involved in the study. The stars’ paths take them “very close to the center of our galaxy. This is a telltale sign that the dwarf galaxy came in on a really eccentric orbit and its fate was sealed.”
The new research also identified at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.
“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Sergey Koposov of Carnegie Mellon University, not part of the Nyx discovery, who has studied the kinematics of the Sausage stars and globular clusters in detail about at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters..
The Gaia sausage was there, as was the stellar halo—background stars that give the Milky Way its tell-tale shape—and the Helmi stream, another known dwarf galaxy that merged with the Milky Way in the distant past and was discovered in 1999.
The Nyx model identified another structure in the analysis: a cluster of 250 stars, rotating with the Milky Way’s disk, but also going toward the center of the galaxy. “Your first instinct is that you have a bug,” Necib recounted. “And you’re like, ‘Oh no!’ So, I didn’t tell any of my collaborators for three weeks. Then I started realizing it’s not a bug, it’s actually real and it’s new.”
But what if it had already been discovered? “You start going through the literature, making sure that nobody has seen it and luckily for me, nobody had. So I got to name it, which is the most exciting thing in astrophysics. I called it Nyx. This particular structure is very interesting because it would have been very difficult to see without machine learning.”
The project required advanced computing at many different stages. The FIRE and updated FIRE-2 simulations are among the largest computer models of galaxies ever attempted. Each of the nine main simulations—three separate galaxy formations, each with slightly different starting point for the sun—took months to compute on the largest, fastest supercomputers in the world. These included Blue Waters at the National Center for Supercomputing Applications (NCSA), NASA’s High-End Computing facilities, and most recently Stampede2 at the Texas Advanced Computing Center (TACC).
“Everything about this project is computationally very intensive and would not be able to happen without large-scale computing,” Necib said.
Necib and her team plan to explore Nyx further using ground-based telescopes. This will provide information about the chemical makeup of the stream, and other details that will help them date Nyx’s arrival into the Milky Way, and possibly provide clues on where it came from.
The next data release of Gaia in 2021 will contain additional information about 100 million stars in the catalog, making more discoveries of accreted clusters likely.
More information: Lina Necib et al, Evidence for a vast prograde stellar stream in the solar vicinity, Nature Astronomy (2020). DOI: 10.1038/s41550-020-1131-2
The original text has been lost, but a reference in book by Archimedes, entitled The Sand Reckoner (Archimedis Syracusani Arenarius & Dimensio Circuli), describes a work in which Aristarchus advanced the heliocentric model as an alternative hypothesis to geocentrism:
You are now aware ['you' being King Gelon] that the "universe" is the name given by most astronomers to the sphere the centre of which is the centre of the earth, while its radius is equal to the straight line between the centre of the sun and the centre of the earth. This is the common account (τὰ γραφόμενα) as you have heard from astronomers. But Aristarchus has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the "universe" just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of the fixed stars, situated about the same centre as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface. 
Aristarchus suspected the stars were other suns that are very far away,  and that in consequence there was no observable parallax, that is, a movement of the stars relative to each other as the Earth moves around the Sun. Since stellar parallax is only detectable with telescopes, his accurate speculation was unprovable at the time.
It is a common misconception that the heliocentric view was held as sacrilegious by the contemporaries of Aristarchus.  Lucio Russo traces this to Gilles Ménage's printing of a passage from Plutarch's On the Apparent Face in the Orb of the Moon, in which Aristarchus jokes with Cleanthes, who is head of the Stoics, a sun worshipper, and opposed to heliocentrism.  In the manuscript of Plutarch's text, Aristarchus says Cleanthes should be charged with impiety.  Ménage's version, published shortly after the trials of Galileo and Giordano Bruno, transposes an accusative and nominative so that it is Aristarchus who is purported to be impious.  The resulting misconception of an isolated and persecuted Aristarchus is still transmitted today.  
According to Plutarch, while Aristarchus postulated heliocentrism only as a hypothesis, Seleucus of Seleucia, a Hellenistic astronomer who lived a century after Aristarchus, maintained it as a definite opinion and gave a demonstration of it,  but no full record of the demonstration has been found. In his Naturalis Historia, Pliny the Elder later wondered whether errors in the predictions about the heavens could be attributed to a displacement of the Earth from its central position.  Pliny  and Seneca  referred to the retrograde motion of some planets as an apparent (and not real) phenomenon, which is an implication of heliocentrism rather than geocentrism. Still, no stellar parallax was observed, and Plato, Aristotle, and Ptolemy preferred the geocentric model that was held as true throughout the Middle Ages.
The heliocentric theory was revived by Copernicus,  after which Johannes Kepler described planetary motions with greater accuracy with his three laws. Isaac Newton later gave a theoretical explanation based on laws of gravitational attraction and dynamics.
After realizing that the Sun was much larger than the Earth and the other planets, Aristarchus concluded that planets revolved around the Sun. But this brilliant insight, it turned out, "was too much for the philosophers of the time to swallow and astronomy had to wait 2000 years more to find the right path." 
The only known surviving work usually attributed to Aristarchus, On the Sizes and Distances of the Sun and Moon, is based on a geocentric world view. Historically, it has been read as stating that the angle subtended by the Sun's diameter is two degrees, but Archimedes states in The Sand Reckoner that Aristarchus had a value of half a degree, which is much closer to the average value of 32' or 0.53 degrees. The discrepancy may come from a misinterpretation of what unit of measure was meant by a certain Greek term in the text of Aristarchus. 
Aristarchus claimed that at half moon (first or last quarter moon), the angle between the Sun and Moon was 87°.  He might have proposed 87° as a lower bound, since gauging the lunar terminator's deviation from linearity to one degree of accuracy is beyond the unaided human ocular limit (with that limit being about three arcminutes of accuracy). Aristarchus is known to have studied light and vision as well. 
Using correct geometry, but the insufficiently accurate 87° datum, Aristarchus concluded that the Sun was between 18 and 20 times farther away from the Earth than the Moon.  (The true value of this angle is close to 89° 50', and the Sun's distance is approximately 400 times that of the Moon.) The implicit false solar parallax of slightly under three degrees was used by astronomers up to and including Tycho Brahe, c. AD 1600. Aristarchus pointed out that the Moon and Sun have nearly equal apparent angular sizes, and therefore their diameters must be in proportion to their distances from Earth thus, the diameter of the Sun was calculated to be between 18 and 20 times the diameter of the Moon. 
Can Stars Collide?
Imagine a really bad day. Perhaps you’re imagining a day where the Sun crashes into another star, destroying most of the Solar System.
No? Well then, even in your imagination things aren’t so bad… It’s all just matter of perspective.
Fortunately for us, we live in out the boring suburbs of the Milky Way. Out here, distances between stars are so vast that collisions are incredibly rare. There are places in the Milky Way where stars are crowded more densely, like globular clusters, and we get to see the aftermath of these collisions. These clusters are ancient spherical structures that can contain hundreds of thousands of stars, all of which formed together, shortly after the Big Bang.
Within one of these clusters, stars average about a light year apart, and at their core, they can get as close to one another as the radius of our Solar System. With all these stars buzzing around for billions of years, you can imagine they’ve gotten up to some serious mischief.
Within globular clusters there are these mysterious blue straggler stars. They’re large hot stars, and if they had formed with the rest of the cluster, they would have detonated as supernovae billions of years ago. So scientists figure that they must have formed recently.
How? Astronomers think they’re the result of a stellar collision. Perhaps a binary pair of stars merged, or maybe two stars smashed into one another.
Professor Mark Morris of the University of California at Los Angeles in the Department of Physics and Astronomy helps to explain this idea.
“When you see two stars colliding with each other, it depends on how fast they’re moving. If they’re moving at speeds like we see at the center of our galaxy, then the collision is extremely violent. If it’s a head-on collision, the stars get completely splashed to the far corners of the galaxy. If they’re merging at slower velocities than we see at our neck of the woods in our galaxy, then stars are more happy to merge with us and coalesce into one single, more massive object.”
There’s another place in the Milky Way where you’ve got a dense collection of stars, racing around at breakneck speeds… near the supermassive black hole at the center of the galaxy.
This monster black hole contains the mass of 4 million times the Sun, and dominates the region around the center of the Milky Way.
Supermassive black holes are enormously dense objects buried at the hearts of galaxies. Image credit: NASA/JPL-Caltech
“The core of the Milky Way is one of those places where you find the extremes of nature. The density of stars there is higher than anywhere else in the galaxy,”Professor Morris continues. “Overall, in the center of our galaxy on scales of hundreds of light years, there is much more gas present than anywhere else in the galaxy. The magnetic field is stronger there than anywhere else in the galaxy, and it has it’s own geometry there. So it’s an unusual place, an energetic place, a violent place, because everything else is moving so much faster there than you see elsewhere.”
“We study the stars in the immediate vicinity of the black hole, and we find that there’s not as many stars as one might have expected, and one of the explanations for that is that stars collide with each other and either eliminate one another or merge, and two stars become one, and both of those processes are probably occurring.”
Stars whip around it, like comets dart around our Sun, and interactions are commonplace.
There’s another scenario that can crash stars together.
The Milky Way mostly has multiple star systems. Several stars can be orbiting a common center of gravity. Many are great distances, but some can have orbits tighter than the planets around our Sun.
When one star reaches the end of its life, expanding into a red giant, It can consume its binary partner. The consumed star then strips away 90% of the mass of the red giant, leaving behind a rapidly pulsating remnant.
What about when galaxies collide? That sounds like a recipe for mayhem.
Surprisingly, not so much.
“That’s actually a very interesting question, because if you imagine two galaxies colliding, you’d imagine that to be an exceptionally violent event,’ Professor Morris explains. “But in fact, the stars in those two galaxies are relatively unaffected. The number of stars that will collide when two galaxies collide is possibly counted on the fingers of one or two hands. Stars are so few and far between that they just aren’t going to meet each other with any significance in a field like that.”
The Mice galaxies merging. Credit: Hubble Space Telescope
“What you see when you see two galaxies collide, however, on the large scale, is that the tidal forces of the two galaxies will rip each of the galaxies apart in terms of what it will look like. Streams of stars will be strewn out in various directions depending on the precise history of the interaction between the two galaxies. And so, eventually over time, the galaxies will merge, the whole configuration of stars will settle down into something that looks unlike either of the two initially colliding galaxies. Perhaps something more spheroidal or spherical, and it might look more like an elliptical galaxy than the spiral galaxy that these two galaxies now are.”
Currently, we’re on a collision course with the Andromeda Galaxy, and it’s expected we’ll smash into it in about 4 billion years. The gas and dust will collide and pile up, igniting an era of furious star formation. But the stars themselves? They’ll barely notice. The stars in the two galaxies will just streak past each other, like a swarm of angry bees.
So, good news! When you’re imagining a worse day, you won’t have to worry about our Sun colliding with another star. We’re going to be safe and sound for billions of years. But if you live in a globular cluster or near the center of the galaxy, you might want to check out some property here in the burbs.
History of Modern Astronomy
Instruments and observing methods were restricted to positional measurements of celestial bodies, and this did not change through the middle ages. The view of the universe that days was the geocentric system established by Greek astronomer Ptolemy around 120 AD: A sphere with fixed stars on it rotates daily around the spherically shaped Earth, with Sun, Moon, and planets being guided around Earth by a complicated machinery of epicycles many had even forgotten about the Earth's spherical shape.
The events that brought astronomy to the state of modern science were (a) the introduction of the heliocentric system, and (b) the invention of the telescope around 1600.
The Heliocentric System
After Copernicus, Danish astronomer Tycho Brahe (1546-1601) proposed a hybrid model of Moon and Sun orbiting the Earth and the other planets moving around the Sun, still needing epicycles for acurate description of their orbits. Strangely, he kept the idea that the sky and all planets encircle a static Earth daily, and got in conflict with Nikolaus Baer who thought Earth was rotating. Tycho also established the nature of comets as objects of translunar space and not atmospheric phenomena, as had been postulated by Aristotle, by measuring a lower limit of the distance of several times the Lunar distance for one comet, and observed a supernova in 1572, thus proving that the stellar skies are not so unchangable as people had believed previously.
- The orbit of each planet is an ellipse with the Sun in one focus.
- The radius vector from Sun to planet sweeps equal areas at each time, meaning that the planet moves faster when closer to the Sun.
- The squares of the revolution periods are proportional to the cubes of the mean distances from the Sun for all planets.
It was finally left to Galileo to give evidence for the heliocentric model with his telescopic discoveries of the moons of Jupiter and the phases of Venus. However, he got in serious trouble with the Roman Inquisition for his advocation of the Copernican system, and the Church authorities kept the old geocentric system of Ptolemy as their doctrine for a long time.
The first rigorous proof of the Earth's motion around the Sun came finally over a century later in 1729, when James Bradley discovered the aberration of light from the stars, a small apparent displacement caused by the combination of Earth's motion with the finite velocity of light (which had to be discovered previously, see below). The other predicted effect, stellar parallaxes, had to wait for their discovery until 1838, when Friedrich Wilhelm Bessel discovered the parallax of star 61 Cygni.
Early Telescopes and Telescopic Discoveries
In the same year 1610, Nicholas-Claude Peiresc (1580-1637) discovered the Orion Nebula M42 around the star Theta Orionis. Simon Marius, who had independently discovered the four brigt Jovian moons about the same time as Galileo, and who gave them their names, found and described the Andromeda "Nebula" M31 in 1912 (this was actually an independent rediscovery as it had been longly found visually by Al Sufi in 964 AD).
As mentioned, Johannes Kepler had proposed another telescope type, consisted of two convex lenses, published in 1611 such an instrument was first constructed by Christopher Scheiner between 1613 and 1617. The Keplerian telescope became the dominant design of all major post-17th century refractors.
The first reflecting telescope was constructed by Isaac Newton in 1668, based partially on a design created in 1663 by James Gregory (1638-75) one reason was the intention to overcome chromatical aberration. In 1672, Jacques Cassegrain (1652-1712, also known as Guillaume or N. Cassegrain) proposed the telescope type named after him, but probably never constructed any the first known Cassegrain telescope was built by James Short (1710-68). Other designs, or telescope types, were proposed about that time, such as a first idea of a schiefspiegler telescope of a Pater Zahn in 1685, but did not get any importance then.
In 1733, Chester Moore Hall invented the achromatic lens system by joining a crown glass lens and a flint glass lens, which allowed for minimizing chromatic aberration. John Dollond (1706-61) and others began to produce fine quality refractors with these achromatic objective lenses in 1757, while his eldest son, Peter Dollond (1730-1820), developed the achromatic triplet lens in 1765, placing convex lenses of crown glass on either side of a biconcave flint glass lens.
William Herschel (1738-1822) invented his kind of telescope, using one tilted mirror only, around 1780 he built a number of large telescopes after this principle, including an 48-inch constructed in 1789.
- Two massive bodies attract each other with a force F proportional to the product of their masses (m1 and m2) divided by the square of their distance r:
Studying the motion of Jupiter's moons, Ole (or Olaus) Roemer found in 1675 that they are observed in slightly deviating positions from what theory predicts, as the distance of Earth and Jupiter varies. He concluded that light was propagating with finite velocity.
The study of motion of solar system bodies was further stimulated by proving Edmond Halley's (1656-1742) prediction of the return of comet Halley in 1758 through Johann Georg Palitsch's (1723-1788) rediscovery, and other comet observations, the discovery of planet Uranus by William Herschel (1738-1822) in 1781 [a prediscovery observation of Uranus had been made by Flamsteed in 1690], and the discoveries of the first minor planets, the first being Ceres discovered in 1801 by Giuseppe Piazzi (1746-1826). Methods for determining orbits from few observations were developed by Carl Friedrich Gauss (1777-1855) and Wilhelm Olbers (1758-1840).
The ultimate fame of celestial mechanics was achieved by the discovery of planet Neptune in 1846 by Johann Gottfried Galle (1812-1910) and Heinrich d'Arrest () after mathematical predictions by Urbain Leverrier (1811-1877) in France and John Couch Adams (1819-1892) in England Neptune had been seen but not recognized in prediscovery observations by Galileo in 1612, and by Challis () in 1845 when checking Adams' predictions.
The Discovery of the Stellar Universe
Evidence to support this view came from the discovery of "new" and variable stars, and of proper motions of stars by Edmond Halley in 1718.
The search for stellar parallaxes (and thus distance determinations) was longly unsuccessful, because the parallaxes are so small (and the distances so large). While lookin for this, James Bradley (1693-1762) discovered the aberration of light in 1725-26 (published 1729) from observations of the star Gamma Draconis (Eltanin), and in 1847 the Earth's nutation, a small deviation of Earth's axis caused by the Moon with a period of 18.6 years. Bradley correctly gave an upper limit of 1 arc second for the stellar parallax and thus a lower limit of 1 parsec (3.26 light years) for the distance of this star. Also, the great observer William Herschel was unsuccessful in this thread for all his life, and it remained to Friedrich Wilhelm Bessel to finally find the parallax of 0.3 arc seconds and thus the distance of 11.1 light years for 61 Cygni in 1838 (the nearest star, Alpha Centauri, is at 4.3 light years) Bessel had selected this star for its large proper motion of 5.21 arc seconds per year (still the fifth-largest known). Almost simultaneously, Wilhelm Struve in Pulkova found the parallax of 0.12 arc seconds for Alpha Lyrae (Vega, at 27 light years) and Thomas Henderson at the Cape Observatory that of Alpha Centauri (0.745 arc seconds).
Special types of stars had been detected: Binary and multiple stars as well as variables. Giovanni Batista Riccioli of Bologna discovered the nature of Mizar (Zeta Ursae Majoris) as a double star in 1650. In 1656, Christian Huygens found that the star Theta in the Orion, in the Orion Nebula M42, was actually a group of stars he discovered three, the fourth Trapezium star was found in 1673 by Abbe Jean Picard (according to de Mairan), and independently by Huygens in 1684. Robert Hooke discovered Gamma Arietis in 1664 or 1665. Next, in the southern hemisphere Alpha Crucis (1685 by Father Fontenay at the Cape of Good Hope) and Alpha Centauri (1689 by Father Richaud from Pondicherry, India) were identified as double. In 1718, Gamma Virginis was found, and in 1719, James Bradley found the companion at Castor (Alpha Geminorum). A first catalog of 80 entries was compiled by Christian Mayer in 1779 and published in 1781 in Bode's "Jahrbuch für 1784", compiled with an 8-foot mural quadrant at power 60 to 80. Real systematic research was started in 1779 by William Herschel, who listed already 269 double stars in his early 1782 catalog, and about 700 in his 1785 catalog he extended this number in later publications.
While suddenly occurring "new stars" (novae and supernovae) had been occasionally recorded through the centuries from various cultures, it was only Tychos supernova of 1572 and Kepler's from 1604, as well as the nova-like outburst of P Cygni in 1600, discovered by W.J. Blaeu, that became generally known for western astronomers. Variable stars of other type were discovered, namely Mira (Omicron Ceti) in 1596 by David Fabricius (1564-1617) and Algol (Beta Persei) around 1669 by Geminiano Montanari (1632-87), though ancient naming suggests that already the ancients had noted and were alarmed by Algols variability as it was called Ras Al Ghul or "Demon's Head" by the Arabs and Rosh ha Satan or "Satan's Head" by the Hebrews. Another nova occurred in Vulpecula in 1670, and Edmond Halley discovered the variability of peculiar Eta Carinae in 1677, Gottfried Kirch that of Chi Cygni in 1687, and J.-D. Maraldi that of R Hydrae in 1704, making up a total of 9 variable stars known in 1781 (in addition, John Flamsteed has perhaps seen, but not noticed, the supernova that created Cassiopeia A in 1667).
Roth lists the number of known variables as follows: 12 by 1786, 18 by 1844, 175 by 1890, 393 by 1896, 4,000 by 1912, 22,650 by 1970, and 28,450 by 1983.
Besides stars, star clusters and "nebulae" (all appearing as nebulous patches in the small telescopes of 17th and early 18th century observers) can be found in the sky these are nowadays summarized under the term Deep Sky Objects. As described in more detail in the history of the discovery of the Deepsky objects, some few of these objects had been known since ancient times, but most of them have been discovered only with the aid of telescopes. Notable firsts: 1610 Galileo discovers that the Milky Way is made of stars and the nature of Praesepe (M44) as (open) star cluster 1610 Peiresc discovers the first (bright diffuse) gaseous nebula, the Orion Nebula M42 1665 Abraham Ihle discovers the first globular cluster, M22 in Sagittarius 1731 John Bevis discovers the first supernova remnant, the Crab Nebula (M1) 1749 Le Gentil discovers M32, the first telescopic galaxy 1751-2 La Caille discovers M83, the first galaxy beyond the Local Group, and the first extragalactic deepsky object, the Tarantula Nebula (NGC 2070) in the Large Magellanic Cloud 1764 Messier discovers the first planetary nebula, the Dumbbell Nebula M27 1778 Messier discovers M54 which is now known to be the first discovered extragalactic globular cluster 1779 Darquier discovers the Ring Nebula M57 and first compares a planetary nebula with planets 1781 Messier and Méchain discover the Virgo Cluster of Galaxies (then assumed to be a cluster of nebulae) The understanding of the nature of these objects, however, had to wait for new observational methods and better understanding of physics.
William Herschel was the first who tried to make a physical model of the stellar universe on observational foundations, and therefore invented the method of stellar statistics to derive a first model of the Milky Way as an island universe (or galaxy). Previously, Johann Lambert (1728-77), Thomas Wright (1711-86), and Immanuel Kant (1724-1804) had hypothesized, on religious and philosophical grounds, that the Milky Way might be a thin flat system of stars, presumably a disk, and of some "nebulae" being other systems of the same kind (however, all their objects are really part of our Galaxy, mostly globular clusters). Herschel also determined the motion of the solar system with respect to the neighboring stars with remarkable good acuracy, and supposed that other "milky ways" should be in the universe, among them the nearest, the "Andromeda Nebula" M31. However, he significantly underestimated both the size of our Galaxy and the distance to M31, which he assumed to be at 2,000 times the distance of Sirius, and most of his other "milky way" candidates were nebulae within our Galaxy.
Catalogs of celestial objects
In the 18th century, better instruments allowed the compilatin of more acurate and larger catalogs. A milestone was the Bonner Durchmusterung, created 1852-59 under Friedrich Wilhelm Argelander (1799-1875), which contains positions and magnitudes for 320,000 stars. This catalog was extended southward by the Cordoba Durchmusterung, compiled 1885-1892. Other important catalogs compiled visually include the Harvard Revised Photometry and the Potsdammer Durchmusterung, both published 1907.
The pioneering work of photographic photometry was Karl Schwarzschild's (1873-1916) Göttinger Aktinometrie, compiled 1904-1908.
When spectroscopy came up, a first classification of 316 stars was published by the Italian Father Angelo Secchi (1818-1878) in 1867. A more comprehensive compilation of spectral classification was the Henry Draper Catalogue published 1918-24 at Harvard Observatory and containing data of 225,300 stars.
In 1818, Joseph Fraunhofer (1787-1826) was the first to take a good spectrum of the Sun and discovered 576 dark lines in it he labelled the more prominent lines with letters A to K. He later discovered that the light from Moon and planets show the same spectral features as the solar spectrum, that the spectra of star differ from this spectrum, and developed the diffraction grating, one of his had 3,625 lines per centimeter.
In 1832, David Brewster showed that cold gasses produce dark absorption lines in continuous spectra. In 1847, John W. Draper found that hot solids emit light in continuous spectra while hot gasses produce line spectra. In 1859, Gustav Robert Kirchhoff (1824-87) and Robert Bunsen (1811-99) discovered that each chemical element (and compound) shows a characteristic spectrum of lines, which are at the same wavelengths in emission and absorption spectra. Thus, the chemical composition of a light source (including celestial bodies) can be determined from spectral analysis Kirchoff published a study of the chemical constitution of the Sun im 1859.
Anders Jonas Angstrom (1818-74) published his map of the Solar spectrum with identifiication of lines corresponding chemical elements in 1863.
In 1864, British amateur William Huggins (1824-1910) published his investigations of spectra of stars and nebulae (thereby finding the gaseous nature of diffuse and planetary nebulae). The same year, Giovanni Batista Donati showed that comet spectra contain emission lines. The first spectrogram (photo of a spectrum) of a star, Vega (Alpha Lyrae), was obtained in 1872 by American amateur Henry Draper (1837-82).
Christian Doppler (1803-53) had discovered that moving bodies show shifted spectral lines, so that radial velocities can be determined spectroscopically with high acuracy. William Huggins stated in 1868 that because of this effect, spectral lines of moving celestial objects should appear shifted. The first measurements of this effect were obtained in 1888 by Hermann Carl Vogel (1841-1907).
Of the early spectral classifications schemes, that of Edward Charles Pickering (1846-1919) and Annie Cannon (1863-1941), used in their Henry Draper Catalogue, was finally adopted by the IAU. Astronomical Photography The first photo of the Moon was obtained in 1841 by J.W. Draper on Daguerre plates. The first solar eclipse photos were obtained on July 18, 1851. W.D. Bond obtained the first photos of stars in 1857. The major breakthrough was finally the invention of dry photographic plates by R.L. Maddox in 1871 which made durable photos possible.
The power of photography for every branch of astronomy was quickly demonstrated early pioneering work was done by Isaac Roberts, Edward Emerson Barnard (1857-1923) and Max Wolf (1863-1932) especially for the Milky Way, star clusters, and nebulae. Better large telescopes During the 18th and early 19th century, small refractors and larger metal mirror reflectors (up to Lord Rosse's 72-inch Leviathan of 1845) were the telescopes available for observers.
Telescope optics was notably improved by Fraunhofer when he developed the achromatic objective in 1824, which led to the construction of larger refractors up to the Yerkes 102 cm.
The reflector techniques was significantly improved by the invention of glass mirrors by Steinheil in 1857 who built a 10 cm reflector, succeeded by Faucault's 33-cm and Lassell's 60-cm glass mirrors. Almost all big telescopes of the 20th century are reflectors with glass mirrors. The first telescope exceeding Lord Rosse's Leviathan of 1845 in aperture was the 100-inch Mount Wilson telescope constructed 1917, followed by the Palomar 200-inch in 1948, and the limitedly successful 6.1-meter Selenchukskaya telescope in 1976.
Milky Way, Nebulae, and Stellar Systems
In 1845, William Parsons, third Earl of Rosse (1800-67) discovered the spiral pattern of M51, and later of M99 and 13 other "nebulae" which were since known as "spiral nebulae".
The essential event marking the discovery of gaseous nebula came when William Huggins observed their spectra in 1864 and found them to be emission line spectra. Now there was a simple and unique criterion distinguishing them from star clusters, which like the stars composing them, show a continuous spectrum (with overlaid absorption and sometimes emission lines). Spiral "nebulae", however, show continuous spectra like stars.
It was known since Herschel that the Milky Way forms a system of stars one of which is our Sun. Since Kant and Herschel, it was speculated that there might be other similar stellar systems some believed Rosse's spiral nebulae could be candidates. By 1900, Easton proposede a model of the Milky Way as a spiral nebula. Another fraction of astronomers, including astrophotographer Isaac Roberts who interpreted his photo of the Andromeda "nebula" M31, thought these nebulae were solar systems in formation (with the companions M32 and NGC 205 [M110] supposed as forming Jovian planets).
Stellar statistical methods, invented by Herschel and improved by H. von Seliger and J. Kapteyn, indicated that the Solar System was, presumably by chance, situated close to the center of the Milky Way Galaxy. In 1904, interstellar reddening and absorption were found nevertheless, it was longly believed to be a minor effect only.
In 1912, Vesto M. Slipher of Lowell Observatory discovered the nature of the nebulae in the Pleiades star cluster M45 as reflection nebulae. In 1914, he found that the spiral and some elliptical "nebulae" are moving at very high radial velocities so that their membership in the Milky Way got questionable, and in 1915 he determined the rotational velocity of the edge-on "nebula" M104 to be about 300 km/s. The view that spiral "nebulae" might be galaxies like our Milky Way was stressed by Heber D. Curtis of Lick Observatory, also on the basis of nova observations and as absorption could explain why spirals "avoid" to be seen near the galactic plane, but opposed in particular by Adriaan van Maanen (1884-1946) who erroneously believed to have found internal proper motions in spirals which would have indicated observable rotation.
In 1912, Henrietta Leavitt found the period-luminosity relation of Cepheid variables in the Magellanic clouds. Using this relation, Harlow Shapley, in 1918, determined distances in the Milky Way, and in particular of globular clusters, which he found centered aroung a location in Sagittarius: He concluded that the center of the Galaxy should be located there, with the solar system lying in an outer region of Milky Way however, as he significantly underestimated the influence of interstellar absorption, he overestimated the size of the Milky Way by a factor of about 3.
In 1924, Edwin Hubble resolved the outer part of the Andromeda "Nebula" M31 into stars and found novae and Cepheid variables, thus establishing its nature as an external star system or galaxy.
In 1926, Bertil Lindblad and Jan Oort developed the theory of kinematics and dynamics of the Milky Way Galaxy.
In 1929, Hubble derived his distance - redshift relation for galaxies, indicating the expansion of the universe.
In 1930, Robert Julius Trumpler (1886-1956) of Lick observatory found from investigations of open clusters that the interstellar absorption had been signficantly underestimated, and the Milky Way Galaxy was correspondingly smaller. In 1937, interstellar molecules (CO_2) were found as absorption lines.
In 1943, Carl Seyfert discovered that certain galaxies (now called Seyfert Galaxies) have "active" nuclei with peculiar nonthermal spectra. In 1944, Walter Baade discovered that the stellar population in different regions of galaxies varies and there are two different stellar populations: Young Population I in spiral arms and irregular galaxies, and old Population II stars in elliptical (and lenticular) galaxies, globular clusters, and the bulges and nuclei of spiral galaxies.
In 1951, the 21-cm radio radiation of neutral hydrogen was discovered. Observations of the Milky Way in this wavelength provided first direct evidence of the spiral structure of our Galaxy.
In 1952, Baade found that Cepheids of two classes exist: Type I Cepheids ("classical" Delta Cephei stars) which are members of population I and Type II Cepheids (W Virginis stars) which are 4 to 5 times fainter. This discovery implied that the intergalactic distance scale had to be revised, moving the galaxies to more than double distance away, and thus removing discrepancies of Milky Way size compared to external galaxies. Since, the distance scale had been subject to minor modifications on various occasions, last due to revision of the Cepheid distances found by the astrometric satellite Hipparcos in early 1997.
In 1963, the first quasar was discovered by Maarten Schmidt.
Observations in the Invisible Light and Space Astronomy
In 1931, K.G. Jansky discovered radio radiation from the Milky Way. In 1939, G. Reber found this radiation concentrated within the galactic plane and toward the galactic center. In 1942, J.S. Hey and J. Southward found the first extragalactic radio radiation.
Individual radio sources were identified in the early 1950s, and the first radio galaxies in 1954.
With upcoming space missions, astronomy became possible in those parts of the electromagnetic spectrum for which Earth's atmosphere is not transparent. In 1960, cosmic X-rays (fronm the Solar corona) were observed for the first time by an aerobee rocket. In 1965, the first cosmic X-rays were discovered (E.T. Byram, H. Friedman, T.A. Chubb) U.S. satellite Uhuru discovered 160 X-ray sources in 1970.
In 1963, radio astronomers discovered the first quasar (M. Schmidt), and in 1967, the first pulsar (J. Bell and A. Hewish).
Since, astronomical satellites have become a powerful tool to investigate astronomical objects in every spectral range for more detail, look at the list of orbiting astronomical observatories (astronomy satellites).
Immigrant sun: Our star could be far from where it started in Milky Way
A long-standing scientific belief holds that stars tend to hang out in the same general part of a galaxy where they originally formed. Some astrophysicists have recently questioned whether that is true, and now new simulations show that, at least in galaxies similar to our own Milky Way, stars such as the sun can migrate great distances.
What's more, if our sun has moved far from where it was formed more than 4 billion years ago, that could change the entire notion that there are parts of galaxies -- so-called habitable zones -- that are more conducive to supporting life than other areas are.
"Our view of the extent of the habitable zone is based in part on the idea that certain chemical elements necessary for life are available in some parts of a galaxy's disk but not others," said Rok Ro&scaronkar, a doctoral student in astronomy at the University of Washington.
"If stars migrate, then that zone can't be a stationary place."
If the idea of habitable zone doesn't hold up, it would change scientists' understanding of just where, and how, life could evolve in a galaxy, he said.
Ro&scaronkar is lead author of a paper describing the findings from the simulations, published in the Sept. 10 edition of the Astrophysical Journal Letters. Co-authors are Thomas R. Quinn of the UW, Victor Debattista at the University of Central Lancashire in England, and Gregory Stinson and James Wadsley of McMaster University in Canada. The work was funded in part by the National Science Foundation.
Using more than 100,000 hours of computer time on a UW computer cluster and a supercomputer at the University of Texas, the scientists ran simulations of the formation and evolution of a galaxy disk from material that had swirled together 4 billion years after the big bang.
The simulations begin with conditions about 9 billion years ago, after material for the disk of our galaxy had largely come together but the actual disk formation had not yet started. The scientists set basic parameters to mimic the development of the Milky Way to that point, but then let the simulated galaxy evolve on its own.
If a star, during its orbit around the center of the galaxy, is intercepted by a spiral arm of the galaxy, scientists previously assumed the star's orbit would become more erratic in the same way that a car's wheel might become wobbly after it hits a pothole.
However, in the new simulations the orbits of some stars might get larger or smaller but still remain very circular after hitting the massive spiral wave. Our sun has a nearly circular orbit, so the findings mean that when it formed 4.59 billion years ago (about 50 million years before Earth), it could have been either nearer to or farther from the center of the galaxy, rather than halfway toward the outer edge where it is now.
Migrating stars also help explain a long-standing problem in the chemical mix of stars in the neighborhood of our solar system, which has long been known to be more mixed and diluted than would be expected if stars spent their entire lives where they were born. By bringing in stars from very different starting locations, the sun's neighborhood has become a more diverse and interesting place, the researcher said.
Such stellar migration appears to depend on the galaxy having spiral arms that twist their way through the galaxy, as are present in the Milky Way, Ro&scaronkar said.
"Our simulated galaxy is very idealized in the formation of the disk, but we believe it is indicative of the formation of a Milky Way-type of galaxy," he said. "In a way, studying the Milky Way is the hardest thing to do because we're inside it and we can't see it all. We can't say for sure that the sun had this type of migration."
However, there is recent observational evidence that such migration might be occurring in other galaxies as well, he said.
Ro&scaronkar noted that the researchers are not the first to suggest that stars might be able to migrate great distances across galaxies, but they are the first to demonstrate the effects of such migrations in a simulation of a growing galactic disk.
The findings are based on a few runs of the simulations, but it is expected additional runs using the same parameters and physical properties would produce largely the same results.
"When you swirl cream into a cup of coffee, it will rarely look exactly the same twice, but the general process, and the resulting taste, is always the same," said Wadsley, the team member from McMaster University.
The scientists plan to run a range of simulations with varying physical properties to generate different kinds of galactic disks, and then determine whether stars show similar ability to migrate large distances within different types of disk galaxies.
Materials provided by University of Washington. Note: Content may be edited for style and length.