Astronomy

Can we see individual stars in other galaxies?

Can we see individual stars in other galaxies?


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Just wondering if we can examine individual stars from other galaxies, or if we are pretty much stuck with the billions that are in the Milky Way?


Yes, Edwin Hubble did that for the first time in 1919. Before that time, it was thought that the galaxies we can observe were just nearby gas nebulae located inside our Milky Way. But Hubble was able to resolve the nearby galaxies like the Andromeda nebula into individual stars. By measuring the brightness of so-called Cepheid variable stars, he was able to calculate the distance to the Andromeda galaxy. Here one uses the fact that the total power radiated by the star is related to the period of the brightness oscillations, so by observing such stars you can deduce the distance to these stars and hence the distance to the galaxy. But later it was found that there were two different types of Cepheid stars and the wrong relation had been used; the distances were actually about twice as large.


In addition to Count Iblis wonderful answer, consider that supernovae in other galaxies can sometimes be seen with low-powered binoculars. I seem to remember mention of a naked-eye extra-galactic supernova, but I cannot find the reference.


Can we see individual stars in other galaxies? - Astronomy

I am doing two distance learning courses - one in astronomy and one in cosmology. I've spent hours on the internet and rifling through books but haven't been able to answer a particular question. I have already handed the piece of work in (so you won't be helping me cheat) but I keep going over and over in my mind what on earth the answer was. Please can you help. It's about the Large Magellanic Cloud (LMC).

If the brightest stars in the LMC have an absolute magnitude of about -10.0 and the distance to the galaxy is 50kpc, then surely that would give the LMC an apparent magnitude of 8.49. If the naked eye has an approximate limiting magnitude of 6.0 then we shouldn't be able to see the LMC. Yet it is clearly a naked eye object. Why is this? The answer is probably something really simple, I'm sure I'll kick myself. The only things I could think of were the fact that there is a large concentration of stars occupying a small area. Or maybe something to do with the supernova remnants maybe from N132D of SN1987A.

Your first guess is completely correct!

It's true that if your eyes were good enough to resolve the individual stars in the Large Magellanic Cloud, then none of them would be bright enough for you to see because they are so far away. Lucky for you, though, your vision isn't perfect! Your eye is actually made up of tiny "pixels" (rods and cones, they're called), each of which takes in the light from a small region within your field of view. Each "pixel" records how much light is coming in from the region it's looking at, but it can't tell anything about how that light is distributed within the region. (This is one of the reasons that things start to look blurry when you get far away from them - all the individual details start to become smaller than the angular size of a "pixel".)

Anyway, galaxies are extremely far away, so the stars within them appear so close together that each "pixel" in your eye is looking at many different stars. The "pixel" simply adds up the light from all those stars and records that, so even though each star isn't bright enough to see on its own, you are still able to see the galaxy.

It is interesting to note, by the way, that even if you move a galaxy farther away, it won't appear fainter. The light you detect from each individual star will go down, but the stars will also become more bunched together, so each pixel in your eye will have more stars in it. These two effects cancel out, and what we refer to as the galaxy's "surface brightness" (which is basically what you are detecting with your eye) remains constant.

This may seem surprising, but it actually shouldn't be - you experience it in everyday life! For example, imagine a room with big white walls. As you get closer or farther away from each wall, its brightness doesn't appear to change, right? This is the same phenomenon as above - the wall gets fainter as you move away from it, but each pixel in your eye sees more of the wall, and the effects cancel out.

Of course, this can't continue forever - otherwise, we would be able to see galaxies out to the edge of the universe with the naked eye. Once a galaxy is far enough away so that the entire galaxy fits within one "pixel", then it will get fainter as it is moved even farther, just like what happens with a star.

This page was last updated January 28, 2019.

About the Author

Dave Rothstein

Dave is a former graduate student and postdoctoral researcher at Cornell who used infrared and X-ray observations and theoretical computer models to study accreting black holes in our Galaxy. He also did most of the development for the former version of the site.


Research Box Title

The combination of high resolution and infrared-detecting instruments on NASA's upcoming James Webb Space Telescope will reveal stars that are currently hidden even from the powerful Hubble Space Telescope. The wealth of additional star data will allow astronomers to investigate a range of questions, from star birth to star death, to the universe's elusive expansion rate. Early observations with Webb will demonstrate its ability to distinguish the individual light of stars in the local universe in a range of environments and provide astronomers with tools for making the most of Webb's powerful capabilities.

"NASA's Hubble and Spitzer space telescopes have been transformative, opening the door to the infrared universe, beyond the realm of red visible light. Webb is a natural evolution of those missions, combining Spitzer's view of the infrared universe with Hubble's sensitivity and resolution," says Daniel Weisz of the University of California, Berkeley, the principal investigator on Webb's early release science (ERS) program on resolved populations of stars.

Webb's ability to resolve individual stars that are shrouded behind gas and dust in visible light will be applicable to many areas of astronomical research. The goals of this ERS program are to demonstrate Webb's capabilities in the local universe and create free, open-source data analysis programs for astronomers to make the best use of the observatory as quickly as possible. Data from the ERS programs will be available to other astronomers immediately, and archived for future research via the Barbara A. Mikulski Archive for Space Telescopes (MAST).

Insight into Dark Energy

Webb's ability to pick out details for more individual stars than we have seen before will improve distance measurements to nearby galaxies, which Weisz says will be crucial to one of the biggest mysteries of modern-day astronomy: How fast is the universe expanding? A phenomenon called dark energy seems to be driving this expansion. Various methods for calculating the expansion rate have resulted in different answers—discrepancies astronomers hope Webb's data can help reconcile.

"In order to do any of this science, calculating distances and then the universe's expansion rate, we need to be able to extract the light of individual stars from Webb images," Weisz says. "Our ERS program team will develop software that empowers the community to make those types of measurements."

The Stellar Lifecycle

Seeing more stars will mean more insight into their lifecycle. Webb will provide new views of the full range of stages in a star's life, from formation to death.

"Right now we are effectively limited to studying star formation in our own Milky Way galaxy, but with Webb's infrared capabilities we can see through the dusty cocoons that shelter forming protostars in other galaxies—like Andromeda, which is more metal-rich—and see how stars form in a very different environment," Weisz says.

Astronomer Martha Boyer , also on this observing-program team, is interested in the insights Webb will provide toward the end of the stellar lifecycle, when stars become bloated, red, and dusty.

"NASA's Spitzer Space Telescope showed us that dusty, evolved stars exist even in very primitive galaxies where they weren't expected, and now with Webb we will be able to characterize them and learn how our models of the star lifecycle line up with real observations," says Boyer, an instrument scientist on Webb's Near Infrared Camera (NIRCam ) team at the Space Telescope Science Institute in Baltimore, Maryland.

The Early Universe via the Local Neighborhood

Resolving and studying individual stars is necessary for understanding the bigger picture of how galaxies formed and function. Astronomers then can ask even bigger questions of how galaxies have evolved over time and space, from the distant, early universe to the Local Group—a collection of more than 20 nearby galaxies to which our galaxy belongs. Weisz explains that even though this observing program will be looking locally, there is evidence of the early universe to be discovered.

"We will have Webb study a nearby, ultra-faint dwarf galaxy, a remnant of the first seed-galaxies to form in the universe, some of which eventually merged to form larger galaxies like the Milky Way," Weisz says. "At great distances these types of galaxies are too faint for even Webb to see directly, but small, local dwarf galaxies will show us what they were like billions of years ago."

"We really need to understand the local universe in order to understand all of the universe," Boyer says. "The Local Group of galaxies is a sort of laboratory, where we can study galaxies in detail—every single component. In distant galaxies we can't resolve much detail, so we don’t know exactly what's going on. A major step towards understanding distant or early galaxies is to study this collection of galaxies that is within our reach."

As the Webb mission progresses, Boyer and Weisz expect that astronomers will use the tools their team develops in unexpected ways. They emphasize that developing the program was an effort of the entire local-universe astronomy community, and they plan to continue that collaboration once the data come in. Their observing-program team plans to host a workshop to go over the results of the program with other astronomers and tweak the software they’ve developed, all with the goal of assisting members of the astronomy community in applying for time to use Webb for their research.

"I think that is really important—the idea of working together to achieve big science, as opposed to a lot of us trying to compete," Weisz says.

The James Webb Space Telescope will be the world's premier space science observatory when it launches in 2021. Webb will solve mysteries in our solar system, look beyond to distant worlds around other stars, and probe the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

Leah Ramsay
Space Telescope Science Institute, Baltimore, Maryland

Christine Pulliam
Space Telescope Science Institute, Baltimore, Maryland


Relativistic Effects

Still other creationists have proposed different ways the problem can be explained by the strange effects of general relativity, the modern theory that time is relative to space and gravity. The speed with which time passes depends on your location. In most cases, the differences are miniscule. But proponents of this view argue that conditions were very different during Creation Week. Light could travel through much of the universe while only a day or so passed locally.

These compelling proposals also have some drawbacks, which are too technical to cover in a short article.


Galaxies

The Milky Way is named because it looks like a path in space that looks 'Milky'.

With the naked eye it looks like a furry light irregular column in the sky. We only see this bright long area, we can't make out any stars that we could see with binoculars. With binoculars, the Milky Way&rsquos haziness is cleared a little and we can make out individual faint small stars.

Using a telescope we can see that the Milky Way comprises of countless stars which we can see clearly. Sweeping along the Milky Way this continues if we move the telescope away from the Milky Way a little, we will see less stars.

What we see of the Milky Way is actually the plane of our own galaxy.

All other stars we see in the night sky with the naked eye are part of the Milky Way and they are only our close neighbours.

The Milky Way is part of a group of 30 or so galaxies called the Local Group. This group is one of many in an even bigger group called a supercluster which is part of the Universe.


Over time, they somehow change their structure and their shape, going from spiral galaxies like the Milky Way to big, bloated, old galaxies that have an elliptical shape. These old galaxies somehow stop forming stars, and become what we call quiescent, or “dead”.

Can you see an American flag on the moon with a telescope? Even the powerful Hubble Space Telescope isn’t strong enough to capture pictures of the flags on the moon. But the Lunar Reconnaissance Orbiter, the unmanned spacecraft launched in 2009, is equipped with cameras to photograph the moon’s surface.


Can Astronomers actually see other galaxies rotating, other stars moving, and other such events in "real-time" or does space appear to stand still?

These are the remnants of the supernova that exploded in 1987. These are the locations of stars around a very massive, invisible object in the center of our galaxy. Barnard's Star is moving pretty fast, by star standards. In all of those you'll notice that things are changing over the course of several years. If you're lucky enough to be looking at a galaxy as a supernova explodes you'll see a very bright star appear and then fade over the next few weeks.

You won't see galaxies rotating or anything like that. That takes millions of years. If you watch Alpha Centauri for long enough, you'll see the two stars rotate around each other in 80 years.

Are the exposure levels in the last photo sequence different for no particular reason, or is it because the supernova was so bright that the exposure had to be reduced to so that it could be captured with more definition?

Other galaxies are too distant for us to see them change in any measurable fashion from how they looked when we discovered them. The angular velocity of the stars in the galaxy is just too small from this distance, and our resolution on our telescopes isn't high enough to resolve any position changes.

However, if you point the telescope at opposite ends of the galaxy spiral, you get opposing redshift and blueshift in the spectra, which tells us how fast on average the stars in that galaxy are moving.

In fact, we can see the shift in motion of stars in other galaxies! Well, one other galaxy - Andromeda, out closest neighbour. This was done by assembling two ultra high-resolution hubble space telescope mosaics of this spiral galaxy 5-8 years apart and comparing them statistically. While astronomers could not spot movement from individual stars (and did not look for rotation), they were able to find an average shift in the relative position of Andromeda's stars compared to background sources like galaxies of 12 μas yr–1, or 0.0000000033 degrees per year (paper here). This put to bed the question of whether Andromeda and the Milky Way will collide (they will). This has also been done for the Milky Way's dwarf satellite galaxies.

So no, we cannot wait for new stars & regions of other galaxies to move into view (they move and rotate far too slowly), but we can just about detect that super slow motion. I imagine this same "average velocity" technique used above will allow astronomers to independently measure the rotation rate of stars in Andromeda in the near future.

One event you can observe in real-time is the rotation of a pulsar. Pulsars are radio sources that rotate extremely quickly, such that when the emitter is oriented at the Earth we detect a burst of energy. The rotation period is usually on the scale of seconds, which means that you could absolutely sit there and watch the pulsar rotate. Pulsars are radio sources, so you couldn't actually "see" anything happening, but if you hook your telescope up to an oscilloscope or another plotting device youɽ see a spike in energy once per period. If you hooked an audio speaker up to the data source and your radio telescope was pointed at a strong pulsar youɽ hear a burst of static once per period (think of something similar to a geiger counter).

There's example graphs of pulsar data and a link to an audio recording of a pulsar on the following page: https://www.atnf.csiro.au/outreach/education/everyone/pulsars/index.html

Many pulsars are too weak to detect easily, and their observation might require hours of recorded data that is then analyzed for subtle patterns of RF energy.


Can we see individual stars in other galaxies? - Astronomy

We know quite a bit about other galaxies by using information about the Milky Way and applying it to them as well as using our own observations of other galaxies to figure out what is going on out there. The reverse is also true - characteristics of other galaxies can be applied to the Milky Way. Remember, we have a hard time seeing various parts of our own galaxy, so checking out other galaxies gives us an idea about what distant parts of our galaxy look like or what is probably happening in those places.


An image showing a wide array of galaxies. The "spikey" objects are stars in our own galaxy. Can you tell which object is closer and which is further away? Are you sure that all of the fuzzy objects are actually galaxies? Image credit: NASA/ESA and the Hubble Heritage Team STScI/AURA

What do other galaxies look like? To the eye they are fuzzy patches in the sky. With long exposures on film, various features such as spiral structure and star clusters are visible. Though it is possible to see trends in the general shapes of galaxies, they are all unique - sort of like snowflakes.

But when you go to a telescope and look out in the sky and you see these fuzzy things, how can you tell it is a galaxy? All little bits of fuzz look sort of similar through a telescope, though there are certain pattern to some of them. And just by looking at them with your eyes through a telescope doesn't really help. Even the invention of photography which helped astronomers get more detailed images of galaxies, wasn't enough to tell them what these things were and how far away they were. That was a major problem in astronomy around the beginning of the 20th century. There were all of these fuzzy things out there (which we called nebula, plural is nebulae) located all over the place. But what were they? We thought some of them were in our galaxy, but we were not sure which ones were and which ones weren't. Perhaps none of the fuzzy things were outside of our galaxy? Perhaps our galaxy is the only galaxy - who says there really are any other objects out there any ways? Or perhaps our galaxy is just like one of those fuzzy things - but like which type of fuzzy things? Does it have a spiral shape like some of the spiral nebulae? Or an oval shape like some of the nebula have? Or maybe some other kind of shape? According to the best data at the time, we didn't really know where we were located even in our own galaxy, so how was it possible to figure out what those darn fuzzy things were?

This is really a conundrum. How will the astronomers be able to solve this problem and save the day (well, at least solve this problem)? They didn't really solve the problem at first. Initially there was just a lot of talking about the problem and trying to show who was right, who was wrong, and who had the worst breath. One of the big discussions about the whole nebula problem took place in Washington D.C. in 1920 at the National Academy of Science. The two people involved in the debate were Heber Curtis and Harlow Shapley, so this debate is known as the Curtis-Shapley Debate. Now you've already run into Harlow Shapley , who was the fellow that figured out the distance to the center of our galaxy back in 1915, but some people had their doubts about his work. The other fellow, Heber Curtis (1872-1942), studied the spiral nebulae quite a bit, so he was really the expert when it came to those things. You have two guys - one an expert about the size of our galaxy, the other an expert on the spiral nebulae - debating each other about various things like what are those spiral nebulae? What is it really like out there? Just how big are the spiral nebulae? How far away are they? How big is our galaxy? What is the Universe like?

It's sort of a draw as to who won the debate. Since Shapley was an expert on the Milky Way, he was able to put forth his idea about the distance we are from the center - which was pretty big. This was correct. (Shapley actually thought that the Milky Way was about 100 kpc wide, which isn't really correct, but he was along the right track). Then he proposed that since the Milky Way was so huge (at least, bigger than anyone ever thought it could be), those spiral nebulae were probably not very far away objects. After all, our galaxy is so huge - or so he thought.

Curtis believed that the spiral nebulae were actually distant objects, not part of our Milky Way, and not even very close to it. He also thought that our galaxy is just like them in terms of size, shape and structure. He was correct on those points. Then he put forth that the Milky Way was not very large (he thought it was only about 10 kpc in size, which is much too small), since it was not large enough to contain the spiral nebulae. He sort of missed that one.

In a way, since each of them was an expert in one aspect of the problem, they each got one thing right. They didn't know that at the time, since just debating about something doesn't help figure out what the actual answer is. Shapley was generally considered the winner of the debate due to his charisma (imagine that, an astronomer with charisma!). The debate really didn't solve the problem. What did solve the problem was good, solid science, in this case when Edwin Hubble provided real evidence for the distances to galaxies in 1924. Hubble had an advantage over Curtis and Shapley, since he had the use of the new, big telescope on Mount Wilson in southern California. At that time, the big scope was the 100" (2.5 meter) telescope located there.

Hubble and the telescope operator, Milton Humason used this telescope to find and analyze Cepheid stars in the Andromeda Galaxy (of course, at that time, it was still known as the Andromeda Nebula). What good are Cepheids? - plenty good! If you remember from the previous section, Cepheids are those Red/Super Giant stars that pulsate and you can use them to determine distance. The longer the period, the greater the average brightness (the Leavitt Law) so if you can find a Cepheid with a period similar to one in our own galaxy, you can compare their apparent magnitudes (how bright they look to your eye), and the difference in brightness is directly related to their different distances. If you can find a Cepheid in a galaxy, you can find the distance to that galaxy. That is exactly what Hubble and Humason did. By using the Cepheid Leavitt Law to determine the distance to the Andromeda Galaxy (one of our closer neighbors), they found that it was 900,000 light years away! This distance was much greater than anyone even suspected, and this is one of the close ones! Actually, they were a bit off - it is actually about 2,250,000 light years away - our current distance estimate methods are a little bit better now.

Not only did Hubble figure out that those fuzzy spiral nebulae (like Andromeda) were actually very distant, separate objects, he started a whole new field of astronomy, the search for objects that could reveal distances to remote galaxies. To find the distances to far away galaxies, it is necessary to use objects whose brightnesses we know fairly well (or some other property that is well defined) and also objects that are bright enough to be seen at a great distance. Objects that fit both these criteria are referred to as Standard Candles . That's just sort of a cute nickname for bright, well behaved objects. What sort of things are Standard Candles? Here are some examples of them -

  • Cepheids - The best thing to use, very bright, fairly common, reliable thanks to the Leavitt Law
  • RR Lyrae - Like Cepheids, but not as useful, since not as bright. Generally used only for nearby galaxies
  • Supernovae - very bright, pretty useful, but can be tricky since there are two types of supernovae, and some can be rather abnormal
  • Planetary Nebulae - these are pretty hot and produce a lot of UV light, so they can be seen distinctly from other things in the galaxy pretty reliable as distance indicators
  • Novae - pretty bright, but not all novae are alike, so not as reliable for accurate distances
  • OB stars - by looking at the brightest stars you can sometimes get good distances, but these are only in certain types of galaxies and are often in regions of star formation that have quite a bit of dust
  • H II Regions - not so great, since they can have different sizes
  • Globular Clusters - if you can't see individual stars, use a whole cluster, but not too good, since clusters are not exactly alike - different sizes, brightnesses
  • Brightness of the entire galaxy - assuming that all galaxies have the same brightness is not too good, since they come in a range of brightnesses - often just the brightest galaxy in a group is used
  • and other methods.

Once an astronomer can determine how far away a fuzzy blob in the sky is using a Standard Candle, they'll know if the fuzzy blob is in our galaxy (a few thousand parsecs away) or is a distant galaxy (millions or more parsecs away).

Astronomers are able to get distances for galaxies within about 1 billion light-years fairly reliably, but there tend to be greater and greater uncertainties in the values for greater and greater distances. If you were to look up all of the distances to even nearby galaxies (like Andromeda or the Large Magellanic Cloud), you'd see a range of values, not just one single value. For very distant galaxies (say, more than 1 billion light years away) the distances that we derive are much more imprecise and can always be improved. This is one of the reasons that bigger and better telescopes are being built all of the time, to measure more and better distances. We need to know how far away things are so we can figure out what the Universe is like.

Galaxy Names

Before people knew that galaxies were separate objects outside our Milky Way, many of the brighter or more prominent ones were cataloged in with the groups of various fuzzy things in the sky, which includes stuff like planetary nebulae, star forming regions, globular clusters and other non-galaxy things. When you look at the names of some of these objects today, you can see a galaxy listed right next to a globular cluster or a planetary nebula. Later catalogs were compiled which tried to include only galaxies or were put together by specialized telescope surveys, so some names are really screwy. In general you'll only run across galaxies that are listed in the common catalogs. There are of course galaxies that have proper or cute names like Andromeda, the Large Magellanic Cloud, the Antenna galaxies, etc., but these are pretty rare. More often you'll see a name from one of the catalogs like the following -

  • Messier catalog (M1, M2, etc.), which has listed for the Andromeda Galaxy M31. There are about 110 objects in this catalog.
  • New General Catalog (NGC 1, NGC 2, etc.), which all start with NGC, and in this catalog the Andromeda Galaxy is known as NGC 224. There are nearly 8000 objects in this catalog.
  • Index Catalog (IC 1, IC 2, etc.), sort of an extension of the NGC catalog, and a galaxy in this system could be called something as exotic as IC 3242. There are about 5500 objects in this catalog.

From this little discussion, you can see how original and creative astronomers can be with naming galaxies. Actually, with there being thousands of galaxies visible to most large telescopes, giving them all individual names like "Bob" and "Becky" would be a bit too difficult. You must remember that the catalogs mentioned above are just the more commonly used ones, and these catalogs include a lot of non-galaxy objects. The Andromeda Galaxy is so commonly surveyed and studied that it has more than 20 different catalog names or designations - talk about overkill!

Galaxy Characteristics

Once it was determined that many of those fuzzy things were actually quite distant galaxies, astronomers had to classify them. Why? We are sort of compulsive about doing things like this, but really it is because we could learn more about them if we could group them together in a way that was scientifically meaningful. This is sort of like how we can group stars into bins like Main Sequence, Red Giant, and so forth. We know that objects in such groups share common characteristics, and we can use that information to learn more about galaxies that we don't see very well or that are too distant to measure all of their characteristics with much certainty.

Figure 1. The Hubble Tuning fork diagram showing the different forms that galaxies come in. The main groups are the ellipticals, the spirals and the barred spirals. A transitional form is the Lenticular type (labeled S0). Anything that can't be placed on the Tuning fork due to unusual structures is simply labeled as Irregular.

What do they look like? Do they all look the same? No, of course not that would be too easy. Most often we classify galaxies based upon their appearance, since that is the most easily observed feature. The basic classification scheme that is used is known as the Hubble Tuning Fork Diagram (I wonder what clever astronomer thought that up). Yes, good old Eddy Hubble set down the framework for the primary classification scheme. There are some other schemes used, and there have been slight alterations to the guidelines that Hubble used, but it is pretty much still the same thing used today. It should be noted that this scheme is based only on appearance - the shapes of galaxies. It doesn't account for how they got into those shapes or the differences in sizes that exist.

Not only do the galaxy shapes vary, but also the content of the galaxies varies - different types of galaxies can have quite different types of stars in them and different environments. This can result in galaxies having different colors, different things happening (or not happening) in them, different ages, different evolutions, and so on. Remember the different stellar populations -

  • Population I - hot, young stars present, which are chemically like the Sun their presence indicates that current star formation is going on
  • Population II - old stars dominate, metal deficient compositions, no new or significant star formation currently occurring

Remember, galaxies are very far away, so you generally can't see individual stars, but you can see large groups of stars. That is why we talk about stellar populations, since the characteristics of groups of stars is what we are able to measure. Let's start checking out the different types of galaxies that are out there.

Ellipticals


Figure 2. Several different elliptical galaxies are shown. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.

As the name implies, these are elliptical in shape, though some are not very elliptical at all but look like circles. To distinguish the different shapes we use a numerical designation along the lines of E0, E1, E2. all the way up to E7. The "E" is for elliptical, while the number describes the degree of ovalness. The number is found by measuring the long (a) and the short (b) axis, and taking those values and putting them into the following formula
10(a-b)/a

Figure 3. The method used for defining the different elliptical galaxies is illustrated here. The longer axis length is compared to the shorter length and a number based upon this value is used to distinguish the range of elongation. In the first case both axes are the same length so the type is E0, while in second the value of 4.7 is found using the formula, which becomes 5, making that elliptical an E5.

Ellipticals tend to look rather yellowish or orangish. This indicates that they are made up of mainly Population II stars. Observations of them show that there is no new (or significant) star formation occurring. There is not much star formation occurring, which means that there must not be a lot of gas and dust in them, since this is what stars are made from. This also gives us a clue concerning how they were made - but I'll get to that later. If you were to look at how the stars in elliptical galaxies move, you'd tend to see rather random motions (sort of how globular clusters move around our galaxy).

Figure 4. A group of galaxies with a large cD (Giant Elliptical) in the center of the group. Image from the Hubble Space Telescope.

The biggest of the ellipticals are often just called Giant Ellipticals and these are the largest of all galaxies. They get a special designation rather than the E designation they are labeled as cD galaxies - don't ask me why they're called that, they just are. These tend to be very spherical in shape, so I guess they don't need the "E" designation scheme - but there are other features that make them distinct. They can have masses of up to 10 trillion solar masses (10 13 Msolar). They are so big that they tend to be found in the center of groups or clusters of galaxies. It is likely that these big brutes weren't always that big but have gotten bigger over time by eating up little galaxies that got too close to them (what we call Galactic Cannibalism - really, we do).

On the other end of the scale, one finds the Dwarf Ellipticals and Dwarf Spheroidals . These are among the smallest of all galaxies, typically with masses around a few million solar masses (10 6 Msolar). Dwarf Ellipticals and Spheroidals can be best described as galaxy groupies, since they tend to hang around much larger galaxies. If you look at a picture of the Andromeda Galaxy, you'll see two little dwarf galaxies (one is an elliptical the other spheroidal) around it. Due to their wide range of masses, ellipticals are sort of hard to figure out. Sometimes it is hard to determine if you are looking at a nearby dwarf elliptical or a distant larger elliptical.

Spirals


Figure 5. Several different spiral galaxies. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.

Spirals show a much greater range of structure than ellipticals, so their classification is a bit more complex. First there is the letter "S" designating the galaxy as a spiral. Then there are the cases where there is a Bar going through the center of the galaxy. If so, you need to add a "B" to the designation. Then there are the other characteristics - how big the bulge is compared to the entire galaxy, and how tightly wound up the arms are. There is a tendency that when the bulge is large the arms are wound up pretty tightly, and when the bulge is really small the arms are really spread out. The letters a, b, c and d are used to categorize this characteristic. The various designations for spirals are Sa, Sb, Sc, Sd, SBa, SBb, SBc and SBd. Some people are a bit indecisive about a galaxy being in a particular group, so sometimes a spiral can be designated as a Sab or Sbc, since they're not sure which group it belongs in.

Spirals are easy to identify since they have a spiral structure or flat disk shape (if seen edge on). Of course, they have the spiral arms due to the star formation that is occurring there, but remember, there is material between the arms it is just not as exciting or as easy to see as the arms. The arms stand out so well because they have all of those hot, big stars to light them up as well as the H II regions in the area. The masses of spirals are typically a few billions to a trillion solar masses. There is the added complication that they aren't made of the same stellar populations. The populations of stars vary depending upon where you are looking - in the disk you find Population I stars and in the bulge and halo you find Population II. This is why in color pictures of some spirals you see the disk looking bluish while the bulge looks yellowish-orangish.


Figure 6. Classic examples of Barred spirals. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.

Barred Spirals share pretty much the same characteristics as spirals except for that extended bulge. It is sort of like someone has taken the normally circular bulge shape and stretched it out. The arms then start up on the ends of the bar. Due to this added structure, the arms in barred spirals tend to be wound up a little bit more tightly than in regular spirals. It is now thought that the Milky Way Galaxy is a barred spiral perhaps it could be classified as a SBb or maybe even an SBc.

Figure 7. A barred spiral, NGC 6744. It is possible that this is what our Milky Way galaxy looks like. Notice how the arms are not very distinct and poorly defined. It is also thought that the Milky Way galaxy has a bar similar to this one. Image credit: ESO.

Lenticular Galaxies

S0 and SB0 types, also known as Lenticular Galaxies , are sort of crosses between a spiral and and elliptical. They are best described as having a flying saucer shape, since they have a disk and a bulge like a spiral galaxy but no spiral arms. The bulge is often pretty big! They don't have any spiral structure, so they don't have much star formation going on (remember, that's why we have spiral structure). The lack of star formation indicates a lack of gas and dust out of which to make stars. Thus, S0 galaxies have mainly Population II stars in them. If they have a bar, then the SB0 designation is used (make sure you don't get the letters out of order for this designation!) In general S0 galaxies are pretty rare.


Figure 8. Lenticular (or S0) galaxies. These look like a cross between an elliptical and a spiral. You might think of them as a type of spiral galaxy without any spiral structure. The one on the left has a dusty plane, but that is unusual for these galaxies. The one on the right is in the same orientation but shows no dusty structure. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.

Irregulars

As with any classification system, there have to be a bunch of objects that don't fit in. For galaxies, these are the Irregulars . Amongst the more famous irregular galaxies are the two neighboring galaxies to the Milky Way, the Large and Small Magellanic Clouds ( LMC and SMC ). They sort of have a bar-like structure, but there isn't anything else there - no spiral structure, no defined bulge, nothing.


Figure 9.Some typical irregular galaxies. The Large Magellanic Cloud is in the middle, and the Small Magellanic Cloud is on the right. Copyright Association of Universities for Research in Astronomy Inc. (AURA), all rights reserved.

It is thought that if there was some structure to an irregular galaxy at some time, like spiral arms or bars, then those parts of the galaxy could have been stripped off due to collisions or other gravitational interactions with larger galaxies. Galaxies don't have to actually get too close for there to be tidal disruptions of the galactic structure. As previously mentioned, it is possible for one large galaxy to strip off the gas and dust from a small, nearby galaxy and to suck it up. The bigger galaxy is basically eating away the star forming material (gas and dust) from its hapless victim. This is known as Galactic Cannibalism (and is best served with fava beans and a nice chianti). Irregular galaxies tend to be associated with rather tumultuous events, so they tend to have a lot of star formation going on in them, but this isn't true for all of them, since there can be a wide range of stellar populations (I and II) in different irregulars. Generally, Population I is what is seen. Irregular galaxies form from previously normal galaxies, so they tend to have a wide range of masses. They're just irregular!

It is worth mentioning that the two irregular galaxies that are best known are the LMC and SMC (Large and Small Magellanic Clouds). These are not visible from Iowa, but only from fairly far south of the equator. It is possible to see these two nearby galaxies with the naked eye. Since they are so close, they have been studied very extensively - though you have to go to Chile, Australia, or South Africa to study them at all. Because these galaxies appear to be interacting with our own, and it is likely that the Milky Way collided with them in the past, they show a great deal of star formation currently going on. New images showing the hot gas in these galaxies are visible, and can be seen here - for the LMC and the SMC. Features such as hot gas from star formation, novae and old supernovae is clearly visible. The part of the galaxies that we tend to see with our eyes is generally much smaller.


Is it possible to see Stars in other galaxies with modern telescopes/detection devises?

Do we have the equipment to look into other galaxies, or is it simply too far away to see anything besides the galactic disk?

If so does anyone have a picture of an extra galactic star?

Yes, we can more or less see individual stars in the Andromeda Galaxy at a few pixels resolution. For other galaxies I don't know of photos existing.

Here is hubble's view of he Andromeda galaxy:

We can see individual stars in Andromeda only starting half way away from it's galactic center, as the center is too dense to highlight them. Even with our best resolution we end up with blurry 5 pixel stars, so there is not much to look at, but it's cool in that you can tell apart individual red giants and white dwarfs.

The larger stars you see are in our own galaxy.

How on earth could we be able to see stars that are in andromeda 2 million ligth years away? I have very hard time believing individual stars could spawn multiple pixels. Here is the best picture we have of the star with biggest angular size aside from the sun

Its a red Giant 200 ligth years away. The biggest star we know of is 5 times larger by diameter. Such a star in the andromeda galaxy would appear 2000 times smaller in diameter than the picture I linked and thus cover 4 million times less pixels.

So no, the blobs in your picture is not individual stars. Even the foreground milkiway stars in your picture would not seem bigger than one pixel to us. They just appear bigger because of the glare.


Astronomy: 'Microlensing' technique might reveal planets in other galaxies

We know that planets orbit stars in our own galaxy, but what about in other galaxies? It seems reasonable that other galaxies should be similar to our own Milky Way, but how can we be sure?

The mediocrity principle, from the field of philosophy, states that an object sampled at random from a larger set is most likely to come from the most common category. For example, in a set of 100 balls, with 98 of them green and two red, a ball chosen at random is likely to be green. Of course, it&rsquos still possible you could draw a red one.

This principle has been used to argue that Earth, our sun and our solar system are not special. If life evolved here on Earth, then the mediocrity principle suggests there is life on planets orbiting other stars.

The problem with the principle of mediocrity is that it&rsquos not always correct. For example, if there is intelligent life elsewhere in our galaxy, where is it? Some people believe that aliens have visited (or are visiting) Earth, but there is no scientific evidence of it. That suggests there might be something special about Earth or our solar system.

Two decades of evidence has shown that there are planets around many stars, called exoplanets. The most common observation is that a star will have a single, Jupiter-size planet that orbits close to the star, but in recent years, it was found that some stars have Earth-like exoplanets, along with other planets similar to our own solar system.

If planets are common in our own galaxy, and the laws of physics are the same everywhere in the universe, then it follows that there should be planets in other galaxies.

However, observations of other galaxies are difficult, because of the distance. To give a sense of scale, the nearest star is about 4 light-years away, and the center of the Milky Way galaxy is about 25,000 light-years away. On the other hand, the nearest galaxy is about 2.5 million light-years away, and most galaxies are much farther.

Using a standard telescope, it&rsquos impossible to see individual stars in another galaxy, unless it&rsquos during an event like a supernova. Different techniques are needed.

A recent paper in The Astrophysical Journal Letters suggests that planets may be prevalent in other galaxies. Using a technique called microlensing, astronomers can use other massive objects such as black holes to magnify the light coming from faraway sources.

Albert Einstein first proposed that massive objects can bend light. But there are other ways to bend light, such as sending it through a material such as glass. A magnifying glass uses the principle of bending light, also called refraction, to focus light. If your eyes can&rsquot distinguish the headlights of a car a mile away, then using a lens (such as in binoculars) can help. Similarly, if you want to distinguish details of stars in a faraway galaxy, bending the light can help. This is what a black hole can do, acting like a huge lens.

Of course, the problem is to get the black hole perfectly aligned between us and the galaxy in the background. That doesn&rsquot happen often, but it does happen.

Using microlensing, the new paper reports the presence of iron in the spectrum of light from a background quasar. Based on frequent shifts of the iron emission light, this suggests that planets likely exist in the background galaxy.

While some doubts remain about the interpretation of these data, it is a first step. Future observations using the microlensing technique are needed to be sure.

Kenneth Hicks is a professor of physics and astronomy at Ohio University in Athens.