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Do galaxies die?

  1. Sep 21, 2005 #1
    I read about this somewhere (can't remember) where it stated that the rate of starbirth has decreased as opposed to when our galaxy was younger. So if it is decreased is it still decreasing and if it stops completely (in any galaxy) does the galaxy itself just "die"?
     
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  3. Sep 21, 2005 #2

    SpaceTiger

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    It depends on one's definition of the death of a galaxy. In some senses, we're surrounded by dead galaxies -- ellipticals. Most of these galaxies stopped forming stars long ago and are now left with an old population of stars rolling around in a halo of dark matter. The only activity they experience will be the occasional collision, or perhaps accretion onto a central black hole. It's probably not the case, however, that these galaxies died by the process you're describing. Current popular theories suggest that spiral galaxies may evolve into elliptical galaxies after one or more major mergers. In these events, the gas in the spiral is gravitationally perturbed by the collision and begins vigorously formings stars. This rapid star formation, in turn, pumps a lot of energy into the galaxy, causing much of its gas to flow out in a galactic wind. There are still a lot of holes in our understanding of this process, but we think we have the basic picture right.

    So is it possible for a galaxy to run out of gas gradually (much as you're describing)? We think so. In fact, this may be the origin of the S0 Hubble type -- a galaxy that's shaped like a spiral but, like an elliptical, has no gas for forming stars. It's also possible that they only underwent minor mergers that weren't strong enough to distort their shape very much.

    As for our Milky Way, its star-forming "death" will likely occur by a collision with Andromeda on timsescales of billions of years, long before it would run out of gas at the current star formation rate.

    These events do not mean the destruction of the galaxy, so most astronomers don't actually think of them as a death...more like a transformation. The more literal interpretation of death (that is the disruption or collapse of the galaxy) will depend upon the cosmological model. Any way you slice it, however, that would be a long way off.
     
  4. Sep 21, 2005 #3
    Do we know the current rate of star formation in the milky way?
     
  5. Sep 21, 2005 #4

    SpaceTiger

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    It's about three solar masses per year.
     
  6. Sep 21, 2005 #5
    sorry to press you with questions, but how do we know that, that is the rate?
     
  7. Sep 21, 2005 #6

    SpaceTiger

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    As one might suspect, there are some complications (and questionable assumptions) that go into this calculation, but there are also a lot of ways it can be done. For example, one can simply count the number of stars of the various types (particularly bright young stars). Since stellar models relate the star's spectral type and its age, we can get a rough idea of the rate of star formation simply from these observations. In addition, one can directly observe the sites of active star formation (giant molecular clouds) and look for the signatures of recent starbirth.

    There's much more to it than this, but I don't think I could do the topic justice in the time I have.
     
  8. Sep 23, 2005 #7
    Sorry for two kidish question what you may expect from a med doc.
    1-What actualy is gas?
    2-when glaxies are moving away then why andromeda will collide with milky way?
     
  9. Sep 23, 2005 #8

    Janus

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    Mostly hydrogen
    Not all galaxies are moving away from us. The closer galaxies, called the local group, are close enough to each other that the gravitational attraction between them is strong enough to oppose the action caused by the expansion of the Universe. The galaxies in thelocal group basically orbit their common center of gravity. Right now the Andromeda and Milky way galaxies are on a collision course with each other.
     
  10. Sep 24, 2005 #9

    Phobos

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    To further what Janus said, many galaxies tend to gather in clusters due to their gravitational attraction. Those clusters move apart from each other due to the expansion of space. Given the billions of galaxies within the visible universe, you can say, on average that all galaxies are moving away from the Milky Way, but actually our "Local Group" has about 30 galaxies (2 big ones, many small ones) held together by gravity.

    Perhaps in the far future the accelerating expansion of space will eventually pull apart our Local Group.
     
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