October 2015 musings on cosmic age

[Jonathan Barlow Gee]

The cosmos is 13.8 billion LY old. [1] The sun is 4.5 billion LY old, [2] and will burn on for another 8.5 billion LY. [3] The cosmos will be 22.3 billion LY old when the sun burns out. The Andromeda galaxy is 2.5 million LY away from earth. [4] When we see the light reaching us now from the Andromeda galaxy, it is the light that left the Andromeda galaxy 2.5 million LY before now. If the average life-span of a main-sequence star is around 50 billion LY, and if the average star in the Andromeda galaxy appears now to be at least as old as our own sun (4.5 billion LY), and if the Andromeda galaxy is 2.5 million LY away from us in space-time, then the average star in the Andromeda galaxy would have been around 4.5 billion LY old around 2.5 million LY ago; this means that, when we see the stars in Andromeda, they are not 4.5 billion LY old stars at a distance from us of 2.5 million LY away from us, but instead: 2.5 million LY ago, the stars we see now were (on average) around 4.5 billion years old, but that, by now, these same stars have aged 5.75 billion LY. This means that, for the average star in the Andromeda galaxy, the cosmos will only be 21.05 billion LY old when stars (alike our sun) in the Andromeda galaxy begin to burn out, but that we will not see this happening until 2.5 million LY later (around 7.95 billion LY from now, or 55 million LY before our own sun will burn out).

[1] ""The current measurement of the age of the universe is 13.799±0.021 billion years""

- https://en.wikipedia.org/wiki/Age_of_the_universe

[2] ""Our sun is 4,500,000,000 (four and a half billion) years old.""

- http://spaceplace.nasa.gov/sun-age/en/

[3] ""Stars that are 10 times the mass of the Sun will last about 100 million years. Stars with about the Sun's mass last about 13 billion years, and stars about one tenth the mass of our Sun last 100 billion years or longer. The average star has a mass of about half that of our Sun, so that means that the average star in our galaxy will live about 50 billion years or so.""

- http://www.astronomycafe.net/qadir/q607.html

[4] ""2.537 million light years = Andromeda Galaxy, Distance to Earth""

cf. ""a list of known galaxies within 3.59 megaparsecs (11.7 million light-years) of our Solar System, in ascending order of distance.""

-  https://en.wikipedia.org/wiki/List_of_nearest_galaxies

cf. ""This list contains all known stars and brown dwarfs at a distance of up to 5 parsecs (16.3 light-years) from the Solar System. In addition to the Solar System, there are another 54 stellar systems currently known lying within this distance. These systems contain a total of 56 hydrogen-fusing stars (of which 46 are red dwarfs), 14 brown dwarfs, and 4 white dwarfs.""

- https://en.wikipedia.org/wiki/List_of_nearest_stars_and_brown_dwarfs