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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.

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.
  1. 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.
  2. 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.
  3. 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!
  4. 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.
  5. 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