Astronomy

When will all other galaxies become not visible from Earth/Milky way?

When will all other galaxies become not visible from Earth/Milky way?


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I know that because of the universe expansion, at some point we will not see any other galaxies from Earth, and we will be able to see only our own galaxy.

Does anyone have an origin or time estimation when will it happen?


That will never happen. We will always be able to see the galaxies in the Local Group. We won't get separated from them by the expansion of space because the group is bound together gravitationally. But the rest of the galaxies will be beyond the cosmic light horizon in 150 billion years or so.

However, because of the mutual gravitational attraction it's likely that the Local Group will merge into a single giant galaxy. It appears that the Milky Way & the Andromeda galaxy are already on a collision course, and will merge in around 4.5 billion years, about half a billion to a billion years before the Sun starts its red giant phase. The full merger of the group will take a long time, Wikipedia says the median time is around 450 billion years.

Due to the Sun's gradually increasing luminosity, the Earth will be uninhabitable in a billion years or so, and it's likely to get swallowed by the red giant Sun at some stage. But I guess humans may survive elsewhere in the Solar System (or outside it).

For further details, please see Wikipedia's Timeline of the far future.


Milky Way

Among the constellations the Milky Way passes through are Carina, Crux (the Southern Cross), Sagittarius (where it is brightest), Scorpius, Aquila, Cygnus, Perseus, Cassiopeia, Auriga, and Gemini. In the direction of Cygnus is the Great Rift, a band of dark matter that lies along the Milky Way, dividing it into two forks. Another dark region is the Coalsack, in Crux. Once believed to be vast empty regions in space, these dark areas are now known to be clouds of dark matter blotting out the light behind them. Such nonluminous clouds of dust and gas, called dark nebulae nebula
[Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g.
. Click the link for more information. , obscure many parts of the sky from sight in the direction of the galactic center, the view is almost entirely obscured.

Size and Shape of the Milky Way

The Milky Way is a large barred spiral galaxy galaxy,
large aggregation of stars, gas, dust, and usually dark matter, typically containing billions of stars. Recognition that galaxies are independent star systems outside the Milky Way came from a study of the Andromeda Galaxy (1926󈞉) by Edwin P.
. Click the link for more information. comprising an estimated 200 billion stars (some estimates range as high as 400 billion) arrayed in the form of a disk, with a central elliptical bulge (some 12,000 light-years in diameter) of closely packed stars lying in the direction of Sagittarius. A compact radio source found there, Sagittarius A*, is believed to be a supermassive black hole black hole,
in astronomy, celestial object of such extremely intense gravity that it attracts everything near it and in some instances prevents everything, including light, from escaping.
. Click the link for more information. such black holes are believed to lie at the center of most spiral and elliptical galaxies. The central bulge is surrounded by a flat disk marked by six spiral arms that project from a dense, elongated concentration of stars, or bar, that runs through the bulge&mdashfour major and two minor&mdashwhich wind out from the nucleus like a giant pinwheel. Our sun is situated in one of the smaller arms, called the Local or Orion Arm, that connect the more substantial next inner arm and the next outer arm. The sun lies roughly 27,000 light-years from the center of the galaxy, and in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way. The diameter of the disk is c.120,000 light-years its average thickness is 10,000 light-years, increasing to 30,000 light-years at the nucleus.

Certain features of the region near the sun suggested that our galaxy resembles the Andromeda Galaxy Andromeda Galaxy,
cataloged as M31 and NGC 224, the closest large galaxy to the Milky Way and the only one visible to the naked eye in the Northern Hemisphere. It is also known as the Great Nebula in Andromeda. It is 2.
. Click the link for more information. . In 1951 a group led by William Morgan detected evidence of spiral arms in Orion and Perseus. Another bright arm stretches from Sagittarius to Carina in the southern sky. With the development of radio astronomy, scientists have extended a nearly complete map of the spiral structure of the galaxy by tracing regions of hydrogen that dominate the spiral arms. The development of telescopes that could be placed in orbit led by 2005 to confirmation that the Milky Way was a barred spiral galaxy, not a spiral one as had been believed.

Surrounding the galaxy is a large spherical halo of globular star clusters star cluster,
a group of stars near each other in space and resembling each other in certain characteristics that suggest a common origin for the group. Stars in the same cluster move at the same rate and in the same direction.
. Click the link for more information. and individual stars that extends to a diameter of about 130,000 light-years this is called the stellar halo. The galaxy also has a vast outer spherical region called the corona, or dark halo, which is as much as 1.9 million light years in diameter and, in addition to dark matter which accounts for most of the Milky Way's mass, includes some distant globular clusters, the two nearby galaxies called the Magellanic clouds, and four smaller galaxies.

Stellar Populations and Galactic Evolution

The stars, gas, and dust that make up the Milky Way can be grouped into two broad stellar populations stellar populations,
two broadly contrasting distributions of star types that are characteristic of different parts of a galaxy. Population I stars are young, recently formed stars, whereas population II stars are old and highly evolved.
. Click the link for more information. that suggest how the galaxy evolved. The spiral arms and central plane of the Milky Way contain the interstellar gas, cosmic dust, and bright young stars categorized as Population I. The halo, spaces between the spiral arms, and central core of the galaxy contain the older, less spectacular stars that are categorized as Population II. This distribution can be explained by an evolutionary model in which an enormous cloud of gas and dust began to condense to form what are now Population II stars. The remaining gas and dust then collapsed, either suddenly or in stages, into the relatively thin disk in which Population I stars were (and still are being) formed.

Like other galaxies, the Milky Way is growing by absorbing small satellite galaxies. It is currently merging with the Large and Small Magellanic Clouds, a process that will be completed in about 100 million years. In 2003 a previously unknown galaxy was found to be colliding with the Milky Way. Its distinctive red stars are slowly being pulled into the Milky Way, and the dwarf will soon lose all its structure. Called the Canis Major dwarf galaxy after the constellation in which it lies, it is about 25,000 light years away from the solar system and 42,000 light years from the center of the Milky Way. This is closer than the Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the Milky Way. Several other galaxies are also, apparently, on a collision course with the Milky Way. The biggest and most spectacular collision will be with the Andromeda Galaxy. In about 2 billion years, massive tidal gravitational effects will tear spiral arms apart and start to shred the pinwheels from the outside in. The result will be an elliptical rather than a spiral Milky Way.

Bibliography

See E. J. Alfaro and A. J. Delgado, ed., The Formation of the Milky Way (1995) G. L. Vogt, The Milky Way (2002).


New collection of stars, not born in our galaxy, discovered in Milky Way

Astronomers can go their whole career without finding a new object in the sky. But for Lina Necib, a postdoctoral scholar in theoretical physics at Caltech, the discovery of a cluster of stars in the Milky Way, but not born of the Milky Way, came early -- with a little help from supercomputers, the Gaia space observatory, and new deep learning methods.

Writing in Nature Astronomy this week, Necib and her collaborators describe Nyx, a vast new stellar stream in the vicinity of the Sun, that 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.

The discovery of Nyx took a circuitous route, but one that reflects the multifaceted way astronomy and astrophysics are studied today.

FIRE in the Cosmos

Necib studies the kinematics -- or motions -- of stars and dark matter in the Milky Way. "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."

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). These simulations include everything scientists know about how galaxies form and evolve. Starting from the virtual equivalent of the beginning of time, the simulations produce galaxies that look and act much like our own.

Mapping the Milky Way

Concurrent to the FIRE project, the Gaia space observatory was launched in 2013 by the European Space Agency. Its goal is to create an extraordinarily precise three-dimensional map of about one billion stars throughout the Milky Way galaxy and beyond.

"It's the largest kinematic study to date. The observatory provides the motions of one billion stars," she explained. "A subset of it, seven million stars, have 3D 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."

The discovery of Nyx involved combining these two major astrophysics projects and analyzing them using deep learning methods.

Among the questions that both the simulations and the sky survey address is: How did the Milky Way become what it is today?

"Galaxies form by swallowing other galaxies," Necib said. "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."

Applying Deep Learning to Gaia

A map of a billion stars is a mixed blessing: so much information, but nearly impossible to parse by human perception.

"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 catalogues."

The Gaia mock catalogue, 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 labelling 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 catalogue, 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."

They first checked to see if it could identify known features of the galaxy. These include "the Gaia sausage" -- the remains of a dwarf galaxy that merged with the Milky Way about six to ten billion years ago and that has a distinctive sausage-like orbital shape.

"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 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.

First Sighting: Nyx

The 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, the Greek goddess of the night. 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).

The researchers used clusters at the University of Oregon to train the deep learning model and to apply it to the massive Gaia dataset. They are currently using Frontera, the fastest system at any university in the world, to continue the work.

"Everything about this project is computationally very intensive and would not be able to happen without large-scale computing," Necib said.

Future Steps

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 catalogue, making more discoveries of accreted clusters likely.

"When the Gaia mission started, astronomers knew it was one of the largest datasets that they were going to get, with lots to be excited about," Necib said. "But we needed to evolve our techniques to adapt to the dataset. If we didn't change or update our methods, we'd be missing out on physics that are in our dataset."

The successes of the Caltech team's approach may have an even bigger impact. "We're developing computational tools that will be available for many areas of research and for non-research related things, too," she said. "This is how we push the technological frontier in general."


Dust at Cosmic Dawn: Clues from the Milky Way’s Center

Title: Old Supernova Dust Factory Revealed at the Galactic Center
Authors: Ryan M. Lau, Terry L. Herter, Mark R. Morris, Zhiyuan Li, Joseph D. Adams
First Author’s institution: Cornell University
Status: Published in Science

The center of our galaxy is enshrouded in interstellar dust — a mixture closer to smoke than dust bunnies here on Earth. This veil makes it difficult to study the interior the Milky Way using visible light. Dust lurks in other galaxies too. In distant galaxies in the young Universe, dust is ubiquitous. How were these vast reservoirs of dust created so quickly? The authors of today’s paper use observations of dust near the center of the Milky Way to argue that supernovae may have been responsible for our Universe’s dusty dawn.

When a star goes supernova, it ejects material rich in heavy elements, like carbon and oxygen. These heavy elements are the building blocks for interstellar dust. The problem is, after the supernova explodes, the expanding supernova remnant collides with surrounding gas and sends a reverse shockwave back through the expanding shell. This shockwave can destroy much of the newly formed dust as hot gas rips apart the dust grains atom by atom, in a process called sputtering. In order to show that dust can survive this interaction in the early Universe, the authors targeted a region of our Milky Way that is likely the closest analogue to the earliest galaxies: the galactic center.

The authors observed the remnant of a supernova (called the Sgr A East supernova remnant) that exploded 10,000 years ago, just 5 parsecs from our galaxy’s supermassive black hole. In the galactic center, the density of the ambient gas is higher than the galactic average, similar to the dense environment in the earliest galaxies. The age of this remnant also ensures that any dust remaining must have survived the reverse shockwave triggered when the remnant collided with this dense surrounding gas.

Warm dust emits blackbody radiation most strongly in the infrared. Using the SOFIA observatory – a modified Boeing 747! – the authors detected infrared emission coming from dust in the supernova remnant. To be sure that this dust is actually inside the supernova remnant, the authors compiled observations at other wavelengths (Figure 1). The authors used radio, X-ray, and submillimeter observations to show that the dust emission is located near the center of the remnant and is not associated with any nearby cold gas clouds.

Figure 1: Multi-wavelength view of the region around the supernova remnant. The dust emission is indicated in yellow contours. The X-ray emitting hot gas (purple) does not overlap the dust emission, indicating that the surviving dust is in a cooler, denser part of the remnant. The submillimeter emission (green) indicates that the dust emission is not coming from a nearby cold molecular cloud.

All of the observations seem to point to the same conclusion: dust has survived in this supernova remnant. The authors go on to investigate the temperature and structure of the dust.

By comparing the intensity of the infrared emission in different wavelength ranges, the authors made a map of the color of the region (Figure 2), translating color to dust temperature using Planck’s Law. The dust in this region is being heated by radiation from the central star cluster around the black hole. But the dust in the supernova remnant has a strangely high temperature it’s at 100 Kelvin compared to 75 Kelvin dust at the same distance from the star cluster. The authors posit that this high temperature could be due to three factors.

Figure 2: Observed dust temperature in and around the supernova remnant. The supernova remnant is the blob at 100 K located above center. The circles indicate the expected dust temperatures assuming that the central star cluster (yellow star) is heating the dust. The large dust grains (orange and red circles) are not heated as easily as the small dust grains (green circle). The remnant dust emission agrees most closely with the small dust grain model (green circle).

First, there could be other nearby stars providing additional heating. But these would be expected to show up as point sources in the infrared images, which are not seen. Second, collisions of dust with electrons could heat the dust, but the authors determine that the density and temperature of the electrons in the area are not sufficient for collisional heating to be important. Third, the authors propose that the dust is composed of smaller dust grains than usually assumed. Just as chopping up an onion allows it to be cooked more quickly, breaking grains of dust into smaller pieces allows them to be heated more easily.

The authors posit that the reverse shockwave in the supernova remnant fragmented the dust grains which survived at all, reducing the average dust grain size by a factor of 10. The authors also show that modeling such a mixture of dust grain sizes can reproduce the observed infrared fluxes at a range of wavelengths.

Adding up the infrared emission observed in this remnant, a total of 0.02 solar masses of dust has survived 10,000 years after the supernova. Depending on how dust production is modeled, between 7% and 20% of the dust produced in the supernova survived the shockwave in this remnant. This survival rate could be boosted even further in denser environments, as we expect for the earliest galaxies. The authors conclude that supernovae could have contributed much of the dust observed in the early Universe.


Astronomers Discover 27 New Supernova Remnants in Milky Way

An international team of astronomers has detected 27 new supernova remnants using a data release of the GaLactic and Extragalactic All-sky MWA survey (GLEAM) from the Murchison Widefield Array (MWA), a low-frequency radio telescope at the Murchison Radio-astronomy Observatory (MRO) in Western Australia.

These are the 27 newly-discovered supernova remnants. The MWA radio images trace the edges of the explosions as they continue their ongoing expansion into interstellar space. Some are huge, larger than the full Moon, and others are small and hard to spot in the complexity of the Milky Way. Image credit: Natasha Hurley-Walker, ICRAR & Curtin / GLEAM Team.

The GLEAM survey maps the sky using radio waves at frequencies between 72 and 231 MHz and has a resolution of two arcminutes (about the same as the human eye).

“It’s the power of this wide frequency range that makes it possible for us to disentangle different overlapping objects as we look toward the complexity of the Galactic center,” said Dr. Natasha Hurley-Walker, an astrophysicist at the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR).

“Essentially, different objects have different ‘radio colors,’ so we can use them to work out what kind of physics is at play.”

Using the MWA images, Dr. Hurley-Walker and colleagues discovered the remnants of 27 massive stars that exploded in supernovae.

These stars would have been eight or more times more massive than our Sun before their dramatic destruction thousands of years ago.

This MWA radio image shows a new view of the Milky Way, with the lowest frequencies in red, middle frequencies in green, and the highest frequencies in blue. Huge golden filaments indicate enormous magnetic fields, supernova remnants are visible as little spherical bubbles, and regions of massive star formation show up in blue. The supermassive black hole at the center of our Galaxy is hidden in the bright white region in the center. Image credit: Natasha Hurley-Walker, ICRAR & Curtin / GLEAM Team.

“One of the newly-discovered supernova remnants lies in such an empty region of space, far out of the plane of our Galaxy, and so despite being quite young, is also very faint,” Dr. Hurley-Walker said.

“It’s the remains of a star that died less than 9,000 years ago, meaning the explosion could have been visible to Indigenous people across Australia at that time.”

Two of the supernova remnants discovered are quite unusual ‘orphans,’ found in a region of sky where there are no massive stars, which means future searches across other such regions might be more successful than astronomers expected. Other supernova remnants discovered in the research are very old.

“This is really exciting for us, because it’s hard to find supernova remnants in this phase of life — they allow us to look further back in time in the Milky Way,” Dr. Hurley-Walker said.

N. Hurley-Walker et al. 2019. New candidate radio supernova remnants detected in the GLEAM survey over 345° < l < 60°, 180° < l < 240°. Publications of the Astronomical Society of Australia 36: E045 doi: 10.1017/pasa.2019.34

N. Hurley-Walker et al. 2019. Candidate radio supernova remnants observed by the GLEAM survey over 345° < l < 60° and 180° < l < 240°. Publications of the Astronomical Society of Australia 36: E048 doi: 10.1017/pasa.2019.33

N. Hurley-Walker et al. 2019. GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey II: Galactic plane 345° < l < 67°, 180° < l < 240°. Publications of the Astronomical Society of Australia 36: E047 doi: 10.1017/pasa.2019.37


Peering into the past

A time machine would be an ideal tool for answering these questions: we could view the universe throughout its history, seeing how galaxies such as our own form and grow.

Luckily enough, such time machines are available. Telescopes allow us to peer into the past. Because the speed of light is finite, we see distant objects as they were, not as they are and the further we look, the further we look into the past.

With Hubble Space Telescope, Spitzer Space Telescope and giant telescopes on Earth, astronomers have been able to view much of the history of the universe.

Images from Hubble reveal thousand of galaxies, many billions of light years away, in just a tiny patch of the night sky.

Connecting these snapshots into a coherent history is a challenge for astronomers. We are not watching individual galaxies over the course of time, but taking samples of populations separated by billions of years of history.

New telescopes and instruments will revolutionise the field in the coming decade, and Australia will play a leading role.

HERMES, a new instrument for the Anglo-Australian telescope, will survey the stars of the Milky Way with unprecedented precision.

Australia is also a partner in the Giant Magellan Telescope, a leviathan that will utilise seven mirrors that are 8-metres in diameter. This telescope will provide views of the distant universe with detail greater than Hubble.

The Australian Square Kilometre Array Pathfinder, now under construction in Western Australia, will identify galaxies vigorously forming stars when the universe was young.

Question will remain, but in the coming decade astronomers may well answer the big questions about the origin and evolution of our home galaxy, the Milky Way.


Location

The Milky Way and the Andromeda Galaxy are a binary system of giant spiral galaxies. Together with their companion galaxies they form the Local Group, a group of some 50 closely bound galaxies. The Local Group is part of the Virgo Supercluster.

The Milky Way is orbited by two smaller galaxies and a number of dwarf galaxies in the Local Group. The largest of these is the Large Magellanic Cloud with a diameter of 20,000 light-years. It has a close companion, the Small Magellanic Cloud. The Magellanic Stream is a peculiar streamer of neutral hydrogen gas connecting these two small galaxies. The stream is thought to have been dragged from the Magellanic Clouds in tidal interactions with the Galaxy. Some of the dwarf galaxies orbiting the Milky Way are Canis Major Dwarf (the closest), Sagittarius Dwarf Elliptical Galaxy, Ursa Minor Dwarf, Sculptor Dwarf, Sextans Dwarf, Fornax Dwarf, and Leo I Dwarf. The smallest Milky Way dwarf galaxies are only 500 light-years in diameter. These include Carina Dwarf, Draco Dwarf, and Leo II Dwarf. There may still be undetected dwarf galaxies, which are dynamically bound to the Milky Way. Observations through the zone of avoidance are frequently detecting new distant and nearby galaxies. Some galaxies consisting mostly of gas and dust may also have evaded detection so far.

In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they circle the Galaxy, causing vibrations at certain frequencies when they pass through its edges. [22] Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, by taking into account dark matter, the movement of these two galaxies creates a wake that influences the larger Milky Way. Taking dark matter into account results in an approximately twentyfold increase in mass for the Galaxy. This calculation is according to a computer model made by Martin Weinberg of the University of Massachusetts, Amherst. In this model, the dark matter is spreading out from the galactic disc with the known gas layer. As a result, the model predicts that the gravitational effect of the Magellanic Clouds is amplified as they pass through the Galaxy.

Current measurements suggest the Andromeda Galaxy is approaching us at 100 to 140 kilometers per second. The Milky Way may collide with it in 3 to 4 billion years, depending on the importance of unknown lateral components to the galaxies' relative motion. If they collide, it is thought that the Sun and the other stars in the Milky Way will probably not collide with the stars of the Andromeda Galaxy, but that the two galaxies will merge to form a single elliptical galaxy over the course of about a billion years. [23]

Velocity

In the general sense, the absolute velocity of any object through space is not a meaningful question according to Einstein's Special Theory of Relativity, which declares that there is no "preferred" inertial frame of reference in space with which to compare the Galaxy's motion. (Motion must always be specified with respect to another object.)

Many astronomers believe the Milky Way is moving at approximately 600 km per second relative to the observed locations of other nearby galaxies. Most recent estimates range from 130 km/s to 1,000 km/s. If the Galaxy is moving at 600 km/s, Earth travels 51.84 million km per day, or more than 18.9 billion km per year, about 4.5 times its closest distance from Pluto. The Galaxy is thought to be moving towards the constellation Hydra, and may someday become a close-knit member of the Virgo cluster of galaxies.

Another reference frame is provided by the Cosmic microwave background (CMB). The Milky Way is moving at around 552 km/s [24] with respect to the photons of the CMB. This can be observed by satellites such as COBE and WMAP as a dipole contribution to the CMB, as photons in equilibrium at the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.

Relative to Earth

The Sun (and therefore the Earth and Solar System) may be found close to the inner rim of the Galaxy's Orion Arm, in the Local Fluff, at a hypothesized distance of 7.62±0.32 kpc from the Galactic Center. [25] [26] [27] [28] The distance between the local arm and the next arm out, the Perseus Arm, is about 6,500 light-years. [29] The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone.

The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (dampening) term.

It takes the Solar System about 225–250 million years to complete one orbit (a galactic year), [30] and so it is thought to have completed about 20–25 orbits during its lifetime or 0.0008 orbit since the origin of humans. The orbital speed of the solar system is 220 km/s, i.e., 1 light-year in ca. 1400 years, and 1 AU in 8 days. [31]


Antlia 2: Enormous Dwarf Galaxy Discovered in Orbit around Milky Way

Astronomers using data from ESA’s Gaia satellite have discovered a new dwarf satellite galaxy of the Milky Way. The dwarf galaxy, named Antlia 2, is located roughly 130,000 light-years from Earth in the constellation Antlia. It has avoided detection until now thanks to its extremely low density as well as a perfectly-chosen hiding place — behind the Milky Way’s bright central disk.

The Large Magellanic Cloud, the Milky Way Galaxy and Antlia 2 (from left to right). Image credit: V. Belokurov / Marcus and Gail Davies / Robert Gendler.

As structures emerged in the early Universe, dwarf galaxies were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor.

But compared to the other known Milky-Way dwarf satellites, Antlia 2 is immense: it is as big as the Large Magellanic Cloud, and a third the size of the Milky Way itself.

What makes the newly-discovered galaxy even more unusual is how little light it gives out.

Compared to the Large Magellanic Cloud, Antlia 2 is 10,000 times fainter. In other words, it is either far too large for its luminosity or far too dim for its size.

“This is a ghost of a galaxy. Objects as diffuse as Antlia 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data,” said lead author Dr. Gabriel Torrealba, an astronomer at Taiwan’s Institute of Astronomy and Astrophysics.

Dr. Torrealba and co-authors searched the data from the Gaia mission’s second data release for Milky Way satellites by using RR Lyrae stars — old and metal-poor, typical of those found in dwarf galaxies.

“RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disk, we weren’t totally surprised,” said co-author Dr. Vasily Belokurov, researcher at Cambridge’s Institute of Astronomy.

“But when we looked closer at their location on the sky it turned out we found something new, as no previously identified object came up in any of the databases we searched through.”

Using the 3.9-m Anglo-Australian Telescope, the astronomers measured the spectra of more than 100 red giant stars in the ghostly object they spotted and confirmed that it was real — all the stars were moving together.

The team was also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.

“The simplest explanation of why Antlia 2 appears to have so little mass today is that it is being taken apart by the Galactic tides of the Milky Way,” said co-author Dr. Sergey Koposov, a scientist at Carnegie Mellon University.

“What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.”

G. Torrealba et al. 2018. The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2. MNRAS, in press arXiv: 1811.04082


A new VISTA through the Milky Way

This small extract from the VISTA VVV survey of the central parts of the Milky Way shows the famous Trifid Nebula to the right of centre. It appears as faint and ghostly at these infrared wavelengths when compared to the familiar view in visible light. This transparency has brought its own benefits — many previously hidden background objects can now be seen clearly. Among these are two newly discovered Cepheid variable stars (detailed image below) &mdash the first ever spotted on the far side of the galaxy near its central plane. Image credit: ESO/VVV consortium/D. Minniti A new image taken with ESO’s VISTA survey telescope reveals the famous Trifid Nebula in a new and ghostly light. By observing in infrared light, astronomers can see right through the dust-filled central parts of the Milky Way and spot many previously hidden objects. In just this tiny part of one of the VISTA surveys, astronomers have discovered two unknown and very distant Cepheid variable stars that lie almost directly behind the Trifid. They are the first such stars found that lie in the central plane of the Milky Way beyond its central bulge. The positions of the newly discovered faint Cepheids are marked on this annotated version. Image credit: ESO/VVV consortium/D. Minniti As one of its major surveys of the southern sky, the VISTA telescope at ESO’s Paranal Observatory in Chile is mapping the central regions of the Milky Way in infrared light to search for new and hidden objects. This VVV survey (standing for VISTA Variables in the Via Lactea) is also returning to the same parts of the sky again and again to spot objects that vary in brightness as time passes.

A tiny fraction of this huge VVV dataset has been used to create this striking new picture of a famous object, the star formation region Messier 20, usually called the Trifid Nebula because of the ghostly dark lanes that divide it into three parts when seen through a telescope.

This picture compares a new view of the Trifid Nebula in infrared light, from the VVV VISTA survey (top) with a more familiar visible-light view from a small telescope (bottom). The glowing clouds of gas and dust are much less prominent in the infrared view, but many more stars behind the nebula become apparent, including the two newly discovered Cepheid variable stars. Image credit: ESO/VVV consortium/D. Minniti/Gábor Tóth The familiar pictures of the Trifid show it in visible light, where it glows brightly in both the pink emission from ionised hydrogen and the blue haze of scattered light from hot young stars. Huge clouds of light-absorbing dust are also prominent. But the view in the VISTA infrared picture is very different. The nebula is just a ghost of its usual visible-light self. The dust clouds are far less prominent, and the bright glow from the hydrogen clouds is barely visible at all. The three-part structure is almost invisible.

In the new image, as if to compensate for the fading of the nebula, a spectacular new panorama comes into view. The thick dust clouds in the disc of our galaxy that absorb visible light allow through most of the infrared light that VISTA can see. Rather than the view being blocked, VISTA can see far beyond the Trifid and detect objects on the other side of the galaxy that have never been seen before.

By chance this picture shows a perfect example of the surprises that can be revealed when imaging in the infrared. Apparently close to the Trifid in the sky, but in reality about seven times more distant [1] , a newly discovered pair of variable stars has been found in the VISTA data. These are Cepheid variables, a type of bright star that is unstable and slowly brightens and then fades with time. This pair of stars, which the astronomers think are the brightest members of a cluster of stars, are the only Cepheid variables detected so far that are close to the central plane, but on the far side of the galaxy. They brighten and fade over a period of eleven days. The Trifid Nebula, or Messier 20, lies in northwestern Sagittarius &mdash a Northern Hemisphere summer constellation rich in deep-sky objects (labelled red). AN graphic by Ade Ashford [1] The Trifid Nebula lies about 5,200 light-years from Earth the centre of the Milky Way is about 27,000 light-years away, in almost the same direction, and the newly discovered Cepheids are at a distance of about 37,000 light-years.


New collection of stars, not born in our galaxy, discovered in Milky Way

Astronomers can go their whole career without finding a new object in the sky. But for Lina Necib, a postdoctoral scholar in theoretical physics at Caltech, the discovery of a cluster of stars in the Milky Way, but not born of the Milky Way, came early - with a little help from supercomputers, the Gaia space observatory, and new deep learning methods.

Writing in Nature Astronomy this week, Necib and her collaborators describe Nyx, a vast new stellar stream in the vicinity of the Sun, that 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.

The discovery of Nyx took a circuitous route, but one that reflects the multifaceted way astronomy and astrophysics are studied today.

Necib studies the kinematics -- or motions -- of stars and dark matter in the Milky Way. "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."

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). These simulations include everything scientists know about how galaxies form and evolve. Starting from the virtual equivalent of the beginning of time, the simulations produce galaxies that look and act much like our own.

Concurrent to the FIRE project, the Gaia space observatory was launched in 2013 by the European Space Agency. Its goal is to create an extraordinarily precise three-dimensional map of about one billion stars throughout the Milky Way galaxy and beyond.

"It's the largest kinematic study to date. The observatory provides the motions of one billion stars," she explained. "A subset of it, seven million stars, have 3D 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."

The discovery of Nyx involved combining these two major astrophysics projects and analyzing them using deep learning methods.

Among the questions that both the simulations and the sky survey address is: How did the Milky Way become what it is today?

"Galaxies form by swallowing other galaxies," Necib said. "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."

Applying Deep Learning to Gaia

A map of a billion stars is a mixed blessing: so much information, but nearly impossible to parse by human perception.

"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 catalogues."

The Gaia mock catalogue, 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 labelling 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 catalogue, 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."

They first checked to see if it could identify known features of the galaxy. These include "the Gaia sausage" -- the remains of a dwarf galaxy that merged with the Milky Way about six to ten billion years ago and that has a distinctive sausage-like orbital shape.

"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 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 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, the Greek goddess of the night. 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).

The researchers used clusters at the University of Oregon to train the deep learning model and to apply it to the massive Gaia dataset. They are currently using Frontera, the fastest system at any university in the world, to continue the work.

"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 catalogue, making more discoveries of accreted clusters likely.

"When the Gaia mission started, astronomers knew it was one of the largest datasets that they were going to get, with lots to be excited about," Necib said. "But we needed to evolve our techniques to adapt to the dataset. If we didn't change or update our methods, we'd be missing out on physics that are in our dataset."

The successes of the Caltech team's approach may have an even bigger impact. "We're developing computational tools that will be available for many areas of research and for non-research related things, too," she said. "This is how we push the technological frontier in general."

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