Astronomy

How to tell a nebula from a galaxy?

How to tell a nebula from a galaxy?


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Not all galaxies are spiral in shape[1][2], some nebulae are huge[3] and nebula are the nursery of stars[4]. How to tell them apart?

Note

I have already compare the contents for galaxy and a nebula, they are very similar as both have stars, planets and dusts.


Galaxies are a large organised collection of stars (& nebulae) outside of our own galaxy, very distant. They shine by their own light.

Nebulae are clouds of gas & debris from, usually, a stellar explosion (Nova, supernova, etc), within our own galaxy, lit up by nearby stars (maybe internal ones) - but the gas is not glowing with it's own light, generally.

So a galaxy is going to be a bright clump of glowing stars, regular in shape - be that elliptical, barred or other, generally they will appear symmetrical & regular. A nebula will be much more amorphous & patchy, possibly with shapes from the clouds of gas (c.f. Horsehead, Orion), and you will be able to see individual stars in it - unlike a galaxy where they are too tightly packed & distant to make out individual stars.


That is a good question, and before it was known that Andromeda is a galaxy, it was indeed assumed to be a nebula.

What really gives away a galaxy is its distance. Even Andromeda, with is the closest galaxy to ours, is over 30 times farther away than any object within our galaxy. If you know the distance, you can tell pretty clearly if it's something within our galaxy, or a completely different galaxy farther out.

Just from looking through a telescope it can be really difficult to tell them apart.


See Messier 20, the Trifid Nebula

Visible light pictures show the nebula divided into 3 parts by dark, obscuring dust lanes, but this penetrating infrared image by the Spitzer Space Telescope reveals filaments of luminous gas and newborn stars. Image via APOD/ JPL-Caltech/ J. Rho (SSC/Caltech).

The Trifid Nebula (Messier 20 or M20) is one of the many binocular treasures in the direction of the center of our Milky Way galaxy. Its name means divided into three lobes, although you’ll likely need a telescope to see why. On a dark, moonless night – from a rural location – you can star-hop upward from the spout of the Teapot in Sagittarius to another famous nebula, the Lagoon, also known as Messier 8. In the same binocular field, look for the smaller and fainter Trifid Nebula as a fuzzy patch above the Lagoon.

To locate this nebula, first find the famous Teapot asterism in the western half of Sagittarius. The Teapot is just a star pattern, not an entire constellation. Nonetheless, most people have an easier time envisioning the Teapot than the Centaur that Sagittarius is supposed to represent. How can you find it? First, be sure you’re looking on a dark night, from a rural location.

Then, look southward in the evening from Earth’s Northern Hemisphere. If you’re in Earth’s Southern Hemisphere, look northward, closer to overhead, and turn the charts below upside down. Want a more exact location for the Teapot in Sagittarius? We hear good things about Stellarium, which will let you set a date and time from your exact location on the globe.

You’ll find M20 in a dark sky near the spout of the Teapot in Sagittarius. Notice the 3 westernmost (right-hand) stars of the Teapot spout … then get ready to star-hop! Use binoculars and go about twice the spout’s distance upward until a bright hazy object glares at you in your binoculars. That’s the Lagoon Nebula (Messier 8), which is actually visible to the unaided eye on a dark, moonless night. Once you locate the Lagoon Nebula, look for the smaller Trifid Nebula as a hazy object some 2 degrees above the Lagoon. For reference, keep in mind that a binocular field commonly spans 5 to 6 degrees of sky. Here’s more about the Teapot. Chart showing one of the most star-rich regions of the Milky Way galaxy, toward the galaxy’s center, in the direction of the constellation Sagittarius. If you look closely, you can pick out M20 on this chart. Chart via astronomy.com.

Whether the close-knit nebulosity of the Trifid and the Lagoon represents a chance alignment or an actual kinship between the two nebulae is open to question. Both the Trifid and Lagoon are thought to reside about 5,000 light-years away, suggesting the possibility of a common origin. But these distances are not known with precision, and may be subject to revision.

Both the Trifid and Lagoon are vast cocoons of interstellar dust and gas. These are stellar nurseries, actively giving birth to new stars. The Trifid and Lagoon Nebulae are a counterpart to another star-forming region on the opposite side of the sky: the Great Orion Nebula.

Trifid Nebula via the Hubble Space Telescope. Image via NASA/ESA.

The Trifid Nebula (M20) is at RA: 18h 02.6s Dec: -23 o 02′

Bottom line: The Trifid nebula (M20) is located in the direction of the center of the Milky Way galaxy. If you have an extremely dark sky, you can see the nebula on a moonless night as a fuzzy patch in the Milky Way. Binoculars show more … and a telescope still more.


Images

Images are, perhaps, the most familiar way that astronomers look at light from cosmic objects.

Here are two very different images. On the left is an image of dalmatian on the right is an image of a galaxy. (Credit: NASA's Imagine the Universe/Whitlock NASA/STScI)

The two images above have one thing in common: they are optical light images. This is what the objects look like with our eyes (or, in the case of the galaxy, with our telescope-aided eyes). But an image can be made out of any kind of electromagnetic radiation. You just have to have the right kind of detector to 'see' the kind of radiation you want to study.

For example, let's say we want an infrared image of a parrot. We can do that with an infrared camera. Comparing the optical light to infrared, we would see something like this:

Image of a parrot in visible light (left) and infrared light (right). The area under the parrot's wings is very bright in infrared. Infrared shows us where things are warm or hot. The feathers under the parrot's wings are very thin, so more heat escapes the parrot from there. (Credit: NASA/IPAC via the Cool Cosmos site)

The infrared image above shows what we would see if our eyes were sensitive to infrared light instead of optical light. Heat from our bodies (and a parrot's body) is emitted as infrared light. The parrot is warm-blooded, so it radiates it's own heat, making it much warmer than it's surroundings. This can be seen in the image above, since the parrot shines much brighter in infrared light than its surroundings.

Astronomers do the same thing with light across the electromagnetic spectrum. They can use detectors of radio, infrared, optical, ultraviolet, X-ray and gamma-ray light to create images of stars and galaxies and other cosmic objects.

An image is just a way for scientists to plot or draw light. Most images show the brightness of an object in the spatial domain, i.e., how many photons are coming from a specific location in space. Three properties of an image – size, brightness, and resolution – are the most important properties of an image to a scientist. From size, we learn about astronomical scales, like how big the Moon is. From brightness, we learn the amount of energy that an object is producing, and then we may be able to figure out HOW it is producing that energy. The ability of a detector to tell one location from a nearby location is called spatial resolution. Higher resolution lets us know things like whether or not a planet has rings or if there are two stars close to each other, versus one star by itself.

Looking at images of the same object made with different parts of the electromagnetic spectrum is a very important tool for scientists. Each different wavelength of light tells astronomers something unique about that object. By studying all different wavelengths and creating models that explain everything those images show, astronomers can see the "the big picture" of what is really going on with that object.

The Crab Nebula in different wavelengths: radio (top left) shows where free electrons are interacting with a magnetic field, optical (top right) shows where hydrogen is in the nebula, ultraviolet (lower left) shows cooler electrons, and X-ray (lower right) shows wehre very hot electrons are.
(Credit: Radio from NRAO visible from Malin/Pasachoff/Caltech ultraviolet from Hennessy et al, 1992, ApJL X-ray from CXC.)

The figure above, shows four images of the Crab Nebula. The radio image tells about the magnetic fields and free electrons in the Nebula. The optical image tells about the hydrogen in the Nebula and more about the free electrons moving in the magnetic field of the pulsar. The UV image tells about the cooler electrons, while the X-ray image tells about the very hot electrons coming from the collapsed central object in the Nebula.


Star Forming Nebula

How Do Stars Form in Nebulas?

Stars are born in clouds of gas and dust. One such stellar nursery is the Orion Nebula, an enormous cloud of gas and dust many light-years across. Turbulence from deep within these clouds creates high density regions called knots. These knots contain sufficient mass that the gas and dust can begin to collapse from gravitational attraction. As it collapses, pressure from gravity causes the material at the center to heat up, creating a protostar. One day, this core becomes hot enough to ignite fusion and a star is born.

Not all of the material in the collapsing cloud ends up as part of a star &mdash the remaining dust can become planets, asteroids or comets &hellip or it may remain as dust. Scientists running three-dimensional computer models of star formation predict that the spinning clouds of collapsing gas and dust may break up into two or three distinct blobs. This would explain why the majority the stars in the Milky Way are paired or in groups of multiple stars.

These gas disks illustrate an early stage of planetary formation. The red glow in the center of each disk is a young, newly formed star. As they evolve, the disks may go on to form planetary systems like our own. Credit: Mark McCaughrean (Max-Planck-Institute for Astronomy), C. Robert O'Dell (Rice University), and NASA NEWS RELEASE: 1995-45 >

Astrophysicists have used detailed observations and computer simulations to understand the lifecycles of stars, their chemistry, the nuclear processes within them and the nature of the gas and dust &mdash called the interstellar medium or ISM &mdash out of which stars form. Hubble probes the intricate complexity of these environments, and it has unveiled stars and planetary systems in the making.

The chemical makeup of stars, revealed through spectroscopy, depends on the material in which they originate. In the early universe, stars were formed from matter that lacked most elements except for hydrogen and helium. The other chemical elements have been and still are being created in the interior of stars through nuclear fusion processes. That new material is eventually recycled into subsequent generations of stars and planets.


How to tell a nebula from a galaxy? - Astronomy

The Helix Nebula (also known as NGC 7293 or Caldwell 63), is a planetary nebula located in the constellation Aquarius. [Credit: NASA, ESA, C. R. O’Dell (Vanderbilt Uni), M. Meixner & P. McCullough (STScl)

The word nebula - plural nebulae - is Latin for cloud. Viewed in 17th and 18th century telescopes, they were just cloudy bits of sky, and most astronomers paid little attention to them. Charles Messier (1730-1817) only catalogued them so that they wouldn't be mistaken for comets.

William Herschel (1738-1822) and Caroline Herschel (1750-1848) were the first to take the nebulae seriously, cataloguing nearly 2500 of them from southern England. William's son John Herschel (1792-1871) later added nebulae of the southern hemisphere. Yet except for realizing that some nebulae were star clusters, more powerful telescopes were needed to learn more. Birr in Ireland saw such a telescope in 1845, built by William Parsons (1800-1867), the Earl of Rosse. He was the first to discover spiral structure in some of the nebulae.

Some of the “nebulae” turned out to be star clusters and some were distant galaxies, but what about true nebulae, the clouds of gas and dust in the spaces between stars?

Diffuse nebulae
The material in the nebulae is so tenuous that an industrial vacuum on Earth is denser. There is nonetheless a lot of matter in them because they're spread out over many light years. The Orion Nebula, for example, is about 150 light years across – that's at least 25 times the diameter of our Solar System.

Emission and reflection nebulae
Since gas and dust aren't luminous, nebulae were difficult to study. However, we now know that emission nebulae and reflection nebulae are made visible by the light from nearby stars. As shown in the header image, hydrogen gas in an emission nebula glows red when it's energized by ultraviolet light from nearby bright young stars.

A reflection nebula appears blue because dust scatters the blue light from a bright neighboring star, but the red part of the spectrum isn't much affected. The Witch Head Nebula is a good example of a reflection nebula. [Photo credit: NASA]

Dark nebulae
Dark nebulae are a third type of nebula. They are characterized by the thick dust that hides their interiors and obscures background objects. Yet they can be seen when their dark shapes stand out against a luminous background, for example, the Horsehead Nebula.

Dark nebulae haven't been a historic feature of western astronomy. The Herschel catalogs didn't include them, even though William Herschel did note the existence of "holes in the sky”. Yet Australian aboriginal astronomy does include dark nebulae. One of the best known is the Flying Emu, here imaged by Barnaby Norris. It's near the Southern Cross and Scorpius, and the dark Coalsack Nebula is its head. Terrestrial emus, of course, don't fly, being large flightless cousins to ostriches.

Starbirth
Stars can form from the vast accumulation of matter in these nebulae. Usually, the process begins with a disturbance that starts the gravitational collapse of the material. Since this occurs in different parts of the nebula, stars form in groups. Such an area of developing stars is often called a stellar nursery.

Most of the matter in nebulae is primordial hydrogen, which means that it formed shortly after the Big Bang. Heavier elements are made in stars, so nebulae are now enriched with elements from previous generations of stars. In fact, two further types of nebula are actually formed from dying stars.

Planetary nebulae
Planetary nebulae aren't related to planets. William Herschel gave them the name because they showed a planet-like disk in his telescope. Bigger and better telescopes, as well as an understanding of stellar evolution, has changed our view of these nebulae. But the name remains.

When a medium-sized star runs out of hydrogen fuel, it swells into a red giant and throws off the outer layers of its atmosphere. It will happen to the Sun several billion years from now. This can produce a number of interesting shapes, but it often occurs fairly symmetrically, leaving something like the Little Ghost Nebula. This is the sort of nebula that would have looked much like a planet in an 18th century telescope.

Supernova remnants
If a star is many times more massive than the Sun, when it finally runs out of nuclear fuel, it explodes as a supernova, releasing vast amounts of energy. For a time, a supernova shines as brightly as an entire galaxy. In such extreme conditions, some of the heaviest chemical elements are forged. Then, although the core of the star collapses into a neutron star or black hole, the outer layers form a nebula called a supernova remnant. Probably the most famous one is the Crab Nebula (Messier 1), which is the remnant of a supernova witnessed by Chinese astronomers in 1054.

With the advent of infrared telescopes, nebulae became a particularly promising field of study. For example, they can tell us something about the chemical elements of which we and our world are made. Astronomers are finding complex organic molecules in nebulae, suggesting that they might also have something to tell us about the origins of life in the Galaxy.

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Deep-sky astronomy: a beginner’s guide

A beginner's guide to viewing the faint objects of the distant cosmos.

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Published: August 19, 2019 at 8:29 pm

Do you wish you could spot a nebula or star cluster but don’t know where to start? Observing these faint deep-sky objects is not as tricky as you may think. You can view them from the comfort of your garden or local dark-sky site and you needn’t spend a fortune on the latest equipment.

A deep-sky object is an astronomical object that is outside our Solar System and includes galaxies, nebulae and clusters.

Once you know Orion from Andromeda you will find that deep-sky targets are not as elusive as you first thought.

It’s worth buying a good observing book or star map, or downloading a stargazing app to help guide you around the sky.

You can locate objects by ‘star-hopping’ from nearby obvious features, so getting to know the constellations will help.

Consider joining a group on social media to ask for advice or get involved with a local astronomical society.

These societies often have observing sessions that you can join in, even if you don’t have your own equipment.

Astronomers love sharing their knowledge and practical hints and tips for getting the best from the night sky, no matter your ability or observing preference.

The most limiting factor in your quest to see the deep sky is probably light pollution, the thorn in every astronomer’s side.

The darker the sky, the fainter the objects you’ll be able to see, so try to get to a dark-sky site if you can.

But remember, not all light pollution is man-made.

The full Moon can easily drown out even the brightest of deep-sky objects so take the lunar cycle into consideration when you are planning an observing session.

You’ll also have to consider the time of year.

Earth’s rotation and annual trip around the Sun will mean that what you observed in the winter months may not be visible during the summer.

For example, the Orion Nebula will only be visible to us in the Northern Hemisphere from November to February.

On observing night, give your eyes time to adjust to the dark.

Make sure you have a red torch so you can use a star chart or locate any equipment you have taken with you while preserving your night vision.

To get the best out of viewing faint objects use a technique called averted vision, where you don’t look directly at the object and instead use the periphery of the eye’s retina, which is more sensitive to dim light.

Remember that the image you’ll see, even in a telescope, won’t look like the glossy images in books and magazines.

These are created using long exposures and extensive post processing.

With your eyes they will appear as faint, fuzzy or foggy shapes against the night sky.

Don’t be put off the thrill of finding a distant nebula or star cluster – located thousands of lightyears away – with your own eyes is genuinely exciting and will leave a lasting impression.

You can help preserve those memories by recording your observations in a logbook or by making a sketch.

Using your eyes to view deep-sky objects

This may sound impossible but it’s not. Your observing options will be limited and you may have to drive somewhere dark, but it’s achievable.

The brighter the object and the darker the sky the better the object will look.

The Andromeda Galaxy (or M31, its catalogue number) and open star cluster, the Pleiades, M45, are the most observed deep-sky objects with the naked eye.

The former is best observed away from light pollution, but the Pleiades can be observed even in towns you should be able to see a fuzzy patch with six to eight bright stars.

Other targets include the Beehive Cluster, M44, and the Hyades.

Using binoculars to view deep-sky objects

Binoculars are cheap, so make a good starting point before taking the plunge with a scope, but they also broaden the list of what you can see.

Locating deep-sky objects with binoculars will be easier than trying to initially find them with a telescope since they have a wider field of view.

Targets like the Pleiades, Melotte 111, Melotte 186, and the Hydra’s Head are far more suited to binoculars than telescopes.

Looking through binoculars will transform the Pleiades.

Even small, compact 10×30 binoculars will bring the fainter stars into focus and the star count will increase greatly.

Bode’s Galaxy, M81, is a popular target for binocular users. You can also catch the Cigar Galaxy, M82, just to its north.

These galaxies are best viewed through bigger binoculars such as 15x70s and above, but the larger the binocular lens, the heavier they will be and may require a tripod.

Using a telescope to view deep-sky objects

For many deep-sky objects you will need a scope at least 6 inches in diameter to see more than a faint blur. But don’t let that put you off.

Despite the limitations of deep-sky viewing without high-tech equipment, you will not be disappointed when it comes to choosing and observing one of the hundreds of objects out there.

Watching satellites and meteors glide by while you are searching for objects to tick off your list is always a treat.

If you are in a dark-sky spot then gaze up at the Milky Way viewing a snippet of our home Galaxy is a remarkable thing!

Six deep-sky sights for visual observing

With the entire Universe in front of you, one of the biggest dilemmas of any observing session is deciding what to look at, so here’s our top selection of deep-sky objects you can see visually.

The Pleiades, M45

A fantastic open star cluster, easily found in the night sky. Commonly known as the Seven Sisters, this sparkling cluster contains hot blue stars formed within the last 100 million years.

Where Located within the constellation of Taurus, star hop from Orion to locate the red star Aldebaran. Further on from Aldebaran, your eyes will fall upon this fuzzy cluster of stars.

When November to February

How to observe Naked eye or binoculars

Difficulty Easy

Orion Nebula, M42

A gigantic stellar nursery 1,500 lightyears from Earth, composed of dust and gas. At its centre lies the Orion Trapezium, four stars shaping the Nebula.

Where The constellation of Orion, below Orion’s Belt. This is one of the most prominent constellations in the winter sky. The Nebula can be found within the Sword of Orion.

When January

How to observe Naked eye, binoculars
or telescope

Difficulty Easy

The Great Globular Cluster, M13

Comprised of hundreds of thousands of stars that are tightly bound by gravity and orbiting a galactic core. Billions of years old, M13 is almost as old as the Universe.

Where The constellation of Hercules. Forming the body of Hercules are four fairly bright stars called the Keystone of Hercules. M13 sits between the top and bottom right-hand stars.

When May to July

How to observe Binoculars or telescope

Difficulty Moderate to difficult

Andromeda Galaxy, M31

A beautiful spiral galaxy located 2.5 million lightyears from Earth. It is the closest galaxy to our own, the Milky Way and 220,000 lightyears in diameter, containing a black hole at its centre.

Where Located within the constellation of Andromeda. Star hop to it using Cassiopeia. It looks like a faint, oval fuzzy star to the naked eye.

When November

How to observe Naked eye, binoculars or telescope

Difficulty Easy to moderate

Bodes Galaxy, M81

This large, bright galaxy is a popular pit stop for amateurs. Composed of interstellar dust, its spiral arms are associated with star-forming regions. Nearby the Cigar Galaxy, M82, is busy producing new stars at a very high rate.

Where Near the Big Dipper, an asterism in the constellation of Ursa Major. Draw a short imaginary diagonal line up from its right-hand star to find M81.

When March to May

How to observe Binoculars or telescope

Difficulty Moderate

The Dumbbell Nebula, M27

M27 is a planetary nebula located around 1,200 lightyears away. Despite the name, the nebula has nothing to do with planets and was instead created by a dying star throwing off its outer layers.

Where From Albireo at the beak end of Cygnus, The Swan, draw a line through mag. +4.58 star 13 Vulpeculae and extend it by a quarter as much again to find the Dumbbell Nebula.


One Step Back, Two Steps Forward

Astronomers have long known that planetary nebulae come about in the dying days of less-massive stars — including, one day, the Sun. Such stars shed their outer layers, making the glorious displays we see from Earth, even as the stellar cores collapse into white dwarfs.

But what astronomers have struggled to understand is how the presumably symmetrical winds coming off aging stars make such beautiful asymmetric shapes. Some ideas have included the effects of stellar companions, though more recent research has shown that even giant planets can affect the winds expelled by stars like the Sun.

To understand the evolution of planetary nebulae, Leen Decin (KU Leuven University, Belgium) and colleagues went a step backward in stellar evolution, to asymptotic giant branch (AGB) stars. These cool, red giants can expand to the size of Earth’s orbit, and they shine thousands of times brighter than the Sun. Yet they’re running out of fuel, reduced to fusing carbon in their cores. (Later in their lives, AGB stars can appear as ruby-red carbon stars.)

AGB stars lose up to 0.01% times the Sun’s mass every year, beginning the process of stripping away the stars’ outer layers. But the high rates of mass loss have also obscured what’s happening closer to the star.

The AGB phase is short-lived, and that has also complicated our understanding of the end stages of low-mass stellar evolution. Depending on their individual masses, AGB stars are only 100,000 to 20 million years away from becoming planetary nebulae. The process of making the planetary nebula takes a few thousand years, a blink of the astronomical eye, and the nebula itself only lasts some 20,000 years more before it disperses into interstellar space.


Stars are categorised by a number of characteristics:

One of these classification is by surface temperature called “Spectral Classes”.

These seven major groups range from the coolest stars which are designated as “M” and up to the hottest stars which are designated as “O”.

The above figure shows that the amount of light they emits from Bright to Dim.

Stars are also classified by the amount of light they emit or luminosity called “Luminosity Classes”.

The above figure shows the Nine Luminosity classes from the bottom of the White Dwarfs to the top of Hyper giants.

It’s looks like the “White Dwarfs”.

But no matter their luminosity or surface temperature, all stars eventually burn through their hydrogen fuel and die out.

Less massive stars such as our sun release their “Stellar Material” into space leaving behind a White Dwarf surrounded by a planetary “Nebula”.

The above picture look behind the White Dwarf surrounded by a planetary “Nebula”.

More massive stars insteady blast matter into space in a Bright “Supernova” leaving behind an extremely dense body called a “Neutron Star”.

After the Supernova explosion leaving behind an extremely dense body to form a Neutron Star.

But the most massive stars, stars that are at least three times our suns mass collapse into themselves and create a bottomless well of gravity and form a “Black Hole”.

After the neutron star it forms like Black Holes and the stars are die out.

But from the remnants of stars, heavier elements are cast into the universe and it is this star dust that form the seedlings of life itself.

It’s impossible to know how many stars exist, but astronomers estimate that in your milky way galaxy alone their are about 300 Billion.

From Stephen Hawking is discovered that “Hawking Radiation” the Black Holes are evaporated eventually and the Matter is Completely Erased.