For a long time, astronomers thought that our home galaxy the Milky Way was the entire Universe. Our galaxy alone contains almost uncountably many stars and is incomprehensibly large, so it would be perfectly satisfactory if that was all there was. It contains such varied structures as star-forming nebulas, ancient globular clusters, dynamic spiral arms, and a central supermassive black hole.

However, around 100 years ago, astronomers started to suspect that the mysterious “spiral nebula” structures that they saw throughout the sky were actually other galaxies like our own. Since then, we have discovered innumerable galaxies in every direction, populating the Universe as far as we can see.

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Milky Way

Does the Sun orbit any fixed point in the Universe? (Beginner)

The point that the Sun orbits around depends on the reference frame you choose. Typically, when we talk about the Sun’s rotation about some point in the Universe, we mean that the Sun rotates about the center of our own Milky Way galaxy. The Sun is about 25,000 light-years from the center of our own Galaxy, and it is traveling in a roughly circular orbit about the Galactic Center at a speed of about 250 kilometers per second. It would take the Sun about 250 million years to complete one full revolution about the Galactic center. There are a lot of stars close to the Galactic Center and it’s actually pretty hard to observe all of them because they’re so far away, faint/small, and obscured behind intervening dust. But typically the object at the Galactic Center that people refer to a lot is the supermassive black hole (which weighs more than a million times the mass of our own Sun).

Let me also just briefly mention that while the Sun is orbiting around the center of the Milky Way galaxy, the Milky Way galaxy itself is also moving with respect to other galaxies in our own “Local Group” of galaxies (which includes, for example, our neighbors the Andromeda Galaxy and the Triangulum Galaxy). So you could in some sense also talk about the Sun’s orbit relative to the center of this “Local Group” of galaxies, or relative to other more distant reference objects (e.g., the “cosmic microwave background radiation” leftover from the Big Bang). But typically it makes more intuitive/practical sense to discuss the Sun’s orbit relative to the center of our own Galaxy.

How has the Milky Way Galaxy changed in the past 100 years? (Beginner)

Galaxies are very big and very old, so 100 years is not a very long time for a galaxy. The Milky Way is over 10 billion years old, so 100 years is just one millionth of 1 percent of its lifetime! Because this is so short, not that much changes drastically in that time, but here are a few things that do change:

The Sun will move over 400 billion miles, which is more than 33,000 times further than Pluto is from us. However, this is not very much compared to the size of the galaxy. In 100 years, the Sun will only orbit around 0.00004% of the galaxy (it takes about 250 million years to go all the way around)
About 700 new stars are formed out of the gas and dust in the galaxy, but most of these are much smaller than the Sun, so they’re much dimmer. All of these new stars would be surrounded by clouds of gas and dust too, so we probably wouldn’t be able to see them for a long time
About 1 star in the galaxy should explode in a giant supernova which would be bright enough to see in the night sky for weeks or months. These explosions are from very big and bright stars that run out of fuel to burn and then make an explosion brighter than all of the rest of the stars in the galaxy combined. Astronomers expect this to happen about once every 100 years, but we have not seen one since 1680, so we are currently overdue for a supernova
The supermassive black hole at the center of the Milky Way grows by about the mass of Jupiter. The black hole is always surrounded by a cloud of gas and dust that is slowly falling into it, and over 100 years, the amount of gas that falls in will weigh about the same amount as Jupiter, over 300 times bigger than the Earth

So there’s a lot that goes on in the galaxy all the time, but most of it is changing so slowly that it doesn’t change much over 100 years.

How has the Milky Way change over the past 100,000 years? (Beginner)

100,000 years is still relatively short in the life of a galaxy (still only about a thousandth of a percent of its whole lifetime). One notable thing is that the Milky Way is about 100,000 light years across, so if you started from one side and traveled all the way to the other side at the speed of light, it would take you about 100,000 years. Here are some other interesting milestones that happen over longer times:

5 million years: This is how long the biggest and brightest stars live. Bigger stars use up their fuel much much faster than smaller stars, so a really big star will only last a few million years before it explodes. This is about the shortest timescale that things can happen in the galaxy.
15 million years: About every 15 million years in the Milky Way, two neutron stars (dead remnants of exploded stars) will merge together and cause a big explosion full of heavy elements like gold
250 million years: This is about how long it takes for the Sun to go around the Milky Way. Over this time it travels around 150,000 light years, meaning light could make the journey about 1,000 times faster than the Sun does.
4 billion years: The Milky Way will run into our nearest neighbor galaxy, the Andromeda galaxy, in about 4 billion years. This will mix up all of the stars in the two galaxies and turn them into one big cloud of stars rather than two flat spirals.
5 billion years: This is about when the Sun will run out of fuel and die. It will expand to the size of the Earth’s orbit before blowing off its outer shell and becoming a small dead star about the size of the Earth.

Are the galaxies in the Local Group orbiting something? (Intermediate)

Saying that things in the Local Group “orbit” each other is kind of a stretch, but we are definitely gravitationally bound to the rest of the galaxies in the Local Group. The Solar System and the Milky Way are other examples of gravitationally bound systems, but unlike those systems, we don’t expect the Local Group to have stable orbits. One reason for this is that galaxies are really large relative to the size of the Local Group, especially when compared to the planets in the Solar System or stars in the Milky Way. The diameter of the Milky Way is a few percent the size of the whole Local Group, whereas the Earth is about 0.0001% the size of the Solar System. This means that the galaxies in the Local Group are much more likely to run into each other when moving around randomly, and that is indeed what we expect to happen. In about 4 billion years, the Milky Way and the Andromeda Galaxy will collide and merge into one giant galaxy that some people call “Milkdromeda”. Because galaxies are relatively large and fuzzy, they can also be ripped apart easily (unlike planets or stars).

Another reason that the Local Group behaves differently than the Solar System is that there is no one object like the Sun that has a bunch of smaller things orbiting around it. Instead, we have the Milky Way and the Andromeda Galaxy that are about the same size as each other and together make up more than 99% of the mass of the Local Group, with only the other small galaxy Triangulum and some small dwarf galaxies making up the rest of the mass. When Milkdromeda forms, there might be enough stability in the Local Group for some of the dwarf galaxies to orbit around it with relative stability, but they likely won’t have very circular or flat orbits unlike the Solar System or a disk galaxy.

The question of “where does it all stop” is also an interesting one, and it is one that has only been resolved relatively recently. If you look at all of the nearby galaxies and track where they are going, you can map out the flows of what is moving towards what to make a map of what is gravitationally bound together. The Local Group will ultimately hold itself together, but as a whole it is falling into the nearby Virgo Cluster that we are on the outskirts of, and ultimately the Virgo Cluster is falling into the center of a much more massive supercluster called Laniakea. None of these things can really be called “orbits” since they’re not stable or regular. Really, it’s all just gravitational collapse, merging galaxies into one giant snowball of stars.

There’s one more complication to all of this though, and that is the expansion of the Universe. Though our galaxy is moving in the direction of Virgo and Laniakea, we will never actually get there because our speed is slower than the speed of the Universe’s expansion. So in the time it takes for us to move 100 light years towards Virgo, the fabric of spacetime will expand by more than 100 light years between us and the destination, meaning that we have covered no net distance. SO ultimately, we will remain stranded here in the Local Group where the gravitational pull of the Andromeda galaxy will pull us into one large elliptical galaxy, all while getting increasingly more isolated from the rest of the Universe.

Globular Clusters

If globular clusters are so old, why can't we see dust clouds from their exploded stars? (Intermediate)

This is a great question! The canonical picture for the formation of a globular cluster is that all the stars in the cluster did indeed form at the same time. So when someone says that a globular cluster is 13.1 billion years old, that means that each and every single star in the cluster is also 13.1 billion years old. It’s actually more complicated than this with there being evidence that some globular clusters have two or more generations of stars (e.g., some stars are 13.1 billion years old and other stars are 3 billion years old, etc.), and that some globular clusters are actually the remnant nuclei of formerly more massive galaxies that were stripped of their outer layers of stars. But… let’s stick to the canonical picture for simplicity: all the stars in a globular cluster are the same age and extremely old.

As for why we can’t see dust clouds from the exploded stars, this is actually a mystery that astronomers actively think about. Because globular clusters are so tightly packed with stars, any interstellar dust or gas is thought to simply ‘evaporate’ out of the cluster. This is one reason why it’s hard for new stars to form in globular clusters (you need interstellar gas to form new stars). The other problem is that if there was gas/dust in the clusters, because there might not be that much of it, it becomes hard to detect observationally (especially given how bright the underlying combined starlight is). Finally, while exploding stars can generate interstellar dust and gas clouds, most massive stars will explode within a hundred million years — if globular clusters are 13.1 billion years and all the stars are the same age, the massive stars would’ve exploded a long time ago and their gas/dust probably would’ve escaped the cluster. But you may still expect interstellar gas/dust as the byproduct of normal stellar evolution or due to interactions between the tightly packed stars, so it’s interesting that there are so few/no detections.

Other Galaxies

What are your favorite galaxies? (Beginner)

Brian DiGiorgio’s response:

I study galaxies for a living so I have some particular galaxies that I am personally attached to, but there are also some that I just think are cool.

The Milky Way: hard to argue with this one. It’s the only galaxy we know of that harbors life (because we live in it), but overall it’s an extremely average galaxy. It’s about the average size for a galaxy in the Universe and its shape is entirely normal for galaxies its size at this point in the life of the Universe. Despite us being inside it, we know more about other nearby galaxies than we do about the Milky Way because we can’t really see the other side of it due to all the stars and dust in the way. The Milky Way may be exceptional in the fact that it has a relatively large dwarf galaxy orbiting around it called the Large Magellanic Cloud though, so aliens might think we are interesting as well.
Messier 82: This is a nearby galaxy that you can see relatively easily with a backyard telescope if you know where to look. Despite being relatively small, it is pretty bright because it is forming huge amounts of new stars at its center. These new stars, caused by an interaction with its neighbor galaxy Messier 81 that stirred up all of the gas inside of it, are shining so brightly that they are blowing hydrogen gas out of the galaxy in big red cones, which we can see well because the galaxy is oriented edge-on relative to us, Overall, a very cool and colorful galaxy.
Messier 51: This is another bright nearby galaxy that is undergoing an interaction with a neighboring galaxy. In this case, it is a larger spiral galaxy that is swallowing a smaller galaxy that is orbiting it. This results in a strong spiral structure in the galaxy (much more prevalent than in most noninteracting galaxies) that can easily be seen if you take a long exposure picture of it through a telescope. There was one time that I was able to look through a telescope 3 feet across and see the spiral arms with my own eyes though, and that was very exciting!
8138-12704: This is a galaxy that means a lot to me because it is the galaxy that I use very often for my research. I write code to figure out how galaxies are rotating, and this galaxy (one of over 10000 observed as part of the MaNGA galaxy survey that I work on) is nice because it has very regular rotation. On the page above, you can see that below the picture of the galaxy is a red and blue hexagon in the shape of the galaxy. Each point within this hexagon is an individual measurement of how fast the galaxy is moving towards or away from you at that point, and these data points combine to make a very nice set of observations that match theoretical models well. Whenever I write a new version of my code, I test it on this galaxy.
8078-12703: This is another MaNGA galaxy that I like for the opposite reason as the one above. You can see that even though this is a spiral galaxy, it has a big straight bar in the middle of it that connects the two spiral arms to the center of the galaxy. This bar messes with how the galaxy is rotating and makes it so theoretical models don’t fit its rotation very well (you can see this in the “emline gvel ha 6564” red and blue hexagon that may or may not be shown at the bottom of the page. My current research project is to write a new galaxy rotation model code to describe this galaxy (and others like it) well, so I have been looking at this galaxy a lot.

Madelyn Broome’s response:

It’s hard to pick a favorite, so I will give my top two. The first is one that you have likely heard of: our nearest galactic neighbor, the spiral galaxy Andromeda. Not only is it close enough to allow us to do some pretty cool science using observations of Andromeda, it is headed on a collision course with the Milky Way! We see lots of evidence for these kinds of mergers – where two galaxies are colliding and combining – throughout the universe, so it is neat to know that, in 4.5 billion years, our Milky Way will likely become part of a much bigger elliptical galaxy.

Also, at just 2.5 million light years (10 billion billion miles) away, we can see Andromeda with the naked eye on a dark night.

My second favorite is a bit odder of a galaxy. It doesn’t have a common name like Andromeda, but, instead is just called by its scientific name: APM 08279+5255. It is home to the largest and farthest collection of water in the universe. The galaxy is more than 12 billion light years away and has the equivalent of 140 trillion oceans worth of water in it. The water isn’t liquid like on Earth, but is in the form of water vapor (steam) and most of it is orbiting around the active galactic nucleus, which is the supermassive black hole at the center of the galaxy that is eating up material and throwing out massive amounts of energy. Because of how bright the galaxy appears as it gives off all of that energy, we call that type of galaxy a quasi-stellar object, or, quasar, since the galaxies looked like a bright stars to early astronomers. The amount of water in APM 08279+5255 is mind boggling and it shows that water can be found throughout the universe.

You can learn more about the quasar APM 08279+5255 from the NASA website: https://www.nasa.gov/topics/universe/features/universe20110722.html

Why aren't there planet-like structures in disk galaxies? (Intermediate)

The idea of some large object in a galaxy analogous to a planet is another interesting one that I have considered before, but although the orbital dynamics of galaxies and solar systems may be similar in a lot of ways, there are a few key differences to think about. The most important question to be asking here is what this large object would be. The obvious first thought is a black hole, but the problem with black holes is that they aren’t actually very good at colliding with things. A planet is not very dense, so it has a relatively large surface area with which to sweep its path compared to a black hole. It’s surprisingly difficult to get something to go into a black hole since you have to shoot it basically directly at the center or else it just orbits around again. So a black hole trying to “clear its orbit” would likely just end up with a cloud of stars that it had kind of accreted orbiting around it, which is what is present in the core of every large galaxy. A closer analog of a planet within a galaxy is a giant cloud of dust: heavy enough to hold itself together and diffuse enough to have a large radius.

The real problem though is time. The Milky Way takes about 250 million years to complete one orbit, meaning that in the entire life of the Universe, it could have only completed around 50 orbits. Orbits require many many more orbits than that to really be considered “clear”. We have observed young solar systems that are hundreds of thousands or millions of years old and they are still just mostly uniform disks of matter, so we can’t expect a galaxy that is only 50 “years” old to be totally settled down. Earth was still being pounded by large numbers of asteroids almost a billion years into its existence.

What causes the formation of spiral arms and bars in some galaxies? Are there magnetic fields holding them in place> (Advanced)

Spiral arms are understood to be patterns that move through stars as waves rather than real permanent structures. The best way to think about this is as the wake of a boat travelling through water. As the boat cuts through the water, it sends out a wave that disturbs the surrounding water, but it doesn’t really move that much of the water out of the way. Rather, it sends a pulse of energy through the surface of the water, temporarily giving more energy to each water molecule (making it raise up) before each molecule passes the energy along to the next one. Spiral arms are analogous to this in that the individual stars don’t travel along with the disturbance, they are just temporarily affected by the passing energy. Long ago in each spiral galaxy, a disturbance (like a dwarf galaxy or a large dust cloud) passed through and created a wake of gas density with its gravity. These densities tend to amplify themselves over time before they stabilize in a self-perpetuating spiral arm structure. As the density waves pass through the disk of the galaxy, they concentrate the gas that is floating around between stars, making it condense down into bright new stars. This is why the stars in a spiral arm appear blue and why there are regions of red ionized hydrogen scattered throughout them that indicate the presence of new bright stars.

The question of bars is slightly more complicated. Bars arise due to disturbances just like the spiral arms (the nature of which are not totally settled in the scientific literature), but they move at a separate speed. Bars are in the centers of galaxies in a region where the galaxy is rotating like a “solid body”. Solid body rotation is essentially like that of a record on a record player: the outer parts of the bar are rotating faster than the inner parts of the bar, meaning that the straight line bar is never messed up. This is in contrast to spiral arms, that mostly live in the regions in galaxies where the galaxy rotates at the same speed regardless of the radius, meaning the outer parts of the galaxy lag behind because they have more distance to travel. This is illustrated in the diagram below:

The result of these two structures living in different rotational regimes in the galaxy means that bars tend to complete one rotation faster than the spiral arms do. So even though bars often look like they are directly feeding into two nice spiral arms, the truth is more complicated. The reality is that the spiral arms themselves are transitory, meaning they can reform anew if the density wave changes shape. As one end of the bar rotates past the reach of a spiral arm, the other one will likely be coming into its neighborhood at the same time, meaning the spiral arms just switch which arm they are attached to. So though they look causally connected, the connection between the spiral arms and the bar is really just opportunistic at any given time.

Magnetic fields were one mechanism proposed for spiral and bar formation in the 1950s and 60s, but unfortunately the math doesn’t work out. The typical strength of magnetic fields in spiral galaxies is only enough to hold together structures on the scale of about 1 kiloparsec, which is only a few percent the size of the galaxy. Structures much larger than that tend to be torn apart by the rotation of the galaxy and the random motions of stars. Smaller structures like star forming regions or spurs off of spiral arms can be held together by magnetism though.