Could you tell me about the life cycle of galaxies? (Intermediate)

Edit by Michael Lam on August 2, 2016: Galaxy life cycles are not as well defined as the lives of stars, for example. A galaxy like the Milky Way formed in the early universe and then underwent periods of increased "activity", such as increased star formation. Eventually, when all of the free gas is turned into stars, after many generations, only stars that can live for really long periods of time will survive, and so galaxies with lots of young, hot, blue stars will eventually turn into galaxies with lots of old, cooler, red stars. Sometimes, galaxies can "grow" by merging with one another. See below for more information.

How old is the Milky Way? Is it a young galaxy or an old one, relative to other galaxies?

Do galaxies die -- do they all get swallowed up by an internal black hole sooner or later? If so, what is their average life span? What fraction of its likely life span has the Milky Way already 'lived through'?

Do they have life stages -- can one speak meaningfully of a galaxy passing through a childhood, adolescence, maturity, senescence, etc.?

These are all very good questions and I believe that a lot of astronomers are looking in to all of that at the minute. It used to be held that all the galaxies formed in a relatively short time period - the 'Epoch of Galaxy Formation'. This idea has pretty much been disproved and nowadays it is more popular to think of galaxies as forming from mergers of smaller galaxies and so on. This means that formation is an ongoing process so it is not easy to assign ages to galaxies. We could assign an age to the Milky Way by the ages of its oldest constituents. Globular Clusters are 13 billion years old, which is getting close to the supposed age of the universe. The likely fate of the Milky Way is not to be swallowed up by the central Black Hole, but to merge with another nearby galaxy, M31 in the constellation of Andromeda. The end result would most probably be a much larger elliptical galaxy.

It's very interesting what you said about merging of galaxies. When you say 'And so on' do you mean that literally? Eventually the number of galaxies in the universe will be 1?

I doubt that they would all merge into one, but the clusters of galaxies might merge into huge galaxies! This is all speculation of course.

Why not? What would choke off the urge to merge? If gravity can pull small galaxies together to make large ones, which couldn't it pull the large ones together as well? It's not as if the force has a maximum range.

Just because of the expansion of the universe taking the galaxies apart. The accepted knowledge at the moment is that there isn't enough matter the halt it, so galaxies on opposite sides of the universe would never be able to get close together.

I see what you are saying but is it true? You have two very large galaxies very far apart. They are closing with an acceleration of x. At the same time the universe is expanding with an acceleration of y. If x is greater than y then eventually the galaxies will combine. If not, not. But my understanding is that y is decreasing while any real x is certainly increasing (as the masses move towards each other).

Why will x increase as the galaxies get farther apart with the expansion of the universe?

It seems that as a practical result there is at least a distinct possibility that all the stars will end up in the same galaxy.

That is not likely to happen as I described above.

The expansion of the universe lowers the rate of increase in x, but never brings it to zero, let alone reversing it. Over long enough periods x will always get arbitrarily large; *perhaps* larger than any plausible value of y.

I don't see why you think the gravitational attraction between 2 bodies (or x) will increase as they get further apart!

Absolute changes in the distance separating any two bodies have a contracting, or closing, component and an expanding, or distancing, component. In the case we are discussing, the contracting component (or velocity) is a function of gravitational attraction. This function is an exponential; thus, over time, no matter what happens, the contracting component or velocity will get arbitrarily large. (All the expansion of the universe does is lower the rate of increase, lengthening the time needed to achieve any given closing velocity. But the bodies will always get to any given velocity, given enough time, because the closing acceleration is always positive.) I see no reason why eventually the closing velocity shouldn't overwhelm the expanding velocity, which of course doesn't mean no such reason exists.

To start with, gravitational attraction goes like the inverse square of distance - it's not an exponential! I'd also be interested to know where you got these ideas from. The universe will contract - and all the galaxies merge if there is enough matter in it to halt the Hubble Expansion. Otherwise the Universe will expand forever and take the matter with it. The distance between galaxies will increase so that the gravitational attraction between them will decrease. They would not merge in this case. Current observational evidence points towards a flat universe - ie. one which is just on the threshold where it will expand forever. The galaxies cannot all merge into one in that universe.

I sometimes read that the universe might either expand forever or contract back to a point. Perhaps it might do both: expanding forever so far as space is concerned while all the mass in the universe falls to a common location.

Space is in some way 'tethered' to the matter in it. If there is enough matter in the universe for it to overcome the expansion of the universe and contract to a point then the universe will also contract to a point. You can't have one without the other.

Really! This is predicted by what theory or body of data?

How do you propose to define space without involving matter?

We're really just talking about density fluctuations. Even if 9999999+ of the matter in the universe was to end up in .00000001- of the space (or whatever number of nines and zeroes are needed to capture the single-galaxy proposition) the rest of the universe would still be defined by very low densities of matter -- one hydrogen atom per cubic megameter, or whatever.

The universe is too 'clumpy' The clusters of galaxies which are gravitationally bound will probably each merge into a single galaxies, but each of these clusters will expand away from the others if the universe is open or flat. I'm sure thay any Astronomer you talk to will tell you that it is extremely unlikely (they are scientists after all so should never say never!) that all the galaxies would merge into one unless the universe is closed.

I'm convinced! Thanks for the analysis....

You're welcome! :)

This page was last updated on August 2, 2016

About the Author

Karen Masters

Karen Masters

Karen was a graduate student at Cornell from 2000-2005. She went on to work as a researcher in galaxy redshift surveys at Harvard University, and is now on the Faculty at the University of Portsmouth back in her home country of the UK. Her research lately has focused on using the morphology of galaxies to give clues to their formation and evolution. She is the Project Scientist for the Galaxy Zoo project.

Twitter:  @KarenLMasters
Website:  http://icg.port.ac.uk/~mastersk/

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