Longevity of Galaxies

[csmyth3025]

Is there any estimate of how long a galaxy such as the Milky Way can last - assuming it is not disturbed by another galaxy?

This topic might be covered elsewhere but I couldn't find it when I searched the Topics.

My thinking here is that a galaxy is a dynamic, gravitationally bound structure. Over the course of many billions of years it seems to me that the interaction of its constituent parts (stars, gas, dust, etc.) would cause it to gradually lose angular momentum - sort of like the accretion disk around a black hole. I'm wondering if this process is so slow that no galaxies have had enough time since they were first formed to actually collapse. Still, I'm guessing that they can't be a sort of cosmic version of a perpetual motion machine.

I may be entirely mistaken about galactic structures so feel free to correct me.

Chris

 

[csmyth3025]

I've found the answer to my own post in the Wikipedia article on galaxies, the relevant portion of which is:

At present, most star formation occurs in smaller galaxies where cool gas is not so depleted.[87] Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms.[92] Elliptical galaxies are already largely devoid of this gas, and so form no new stars.[93] The supply of star-forming material is finite; once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.[94]

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (10^13–10^14 years), as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.[94][95]

Chris

 

[neilsox]

I can't confirm the wikipedia article, but it does seem reasonable generalities. I'll wild guess some numbers and perhaps others will tell us where my numbers are unreasonable. 8 billion years ago, the milky way galaxy = ours was only partially organized, but perhaps there were 50 billion stars on main sequence, mostly in older small galaxies that would soon be a part of our galaxy. Star birth rate was perhaps 10 per year, but about that many left main sequence annually, so we started 7 billion years ago with about the same number = 50 billion main sequence stars. We added 25 stars per year until quite recently, but we have dropped back to ten per year for the past billion years and maybe only 5 stars per year for the coming billion years. Perhaps 30 billion stars left main sequence in the recent billion years, so the total has decreased to about 200 billion stars in our galaxy at present. since the new stars are forming less often. In another 8 billion years, our Sun and most of the more massive stars will be off main sequence, but about 130 billion class m stars = all of them plus 20 billion class k stars will still be on main sequence. Likely no stars will be visable naked eye from the location of Jupiter, except the the white dwarf that was our sun. It may look about as bright as Siris (the brightest star) does now, but much of its energy output will be ultraviolet and Xrays. There is about an even chance that our Earth will be absorbed into the red giant 5 to 7 billion years from now. In 100 billion years, all or most of the class k and more massive stars will be off main sequence, but all or nearly all the class m stars will still be main sequence. As the Wikipedia article infrared some of the smaller galaxies will still be making new stars, but nearly all will be significantly dimmer than at present. Neil