How Far Away is M31?


First, some useful unit conversion:

M31 and the Milky Way are approaching each other at about 100 km/sec, relative to their centers of mass. That speed will of course increase, as the two largest galaxies in the Local Group get closer to each other. Our sun will become a red giant in about 5 billion years, eventually forming a planetary nebula and a white dwarf star of about 0.45 (current) solar masses. It's a good guess that the two big events, the sun becoming a red giant, and the galaxies colliding, will probably happen at about the same time. It seems unlikely that we will be around for the big show, but then again, you never know ...


Now let's use 2,555,000 light years as the distance to M31 [Holland, 1998; Stanek & Garnavich, 1998]. That's ...


References:

As of August 2004, these papers remain the current expositions on the distance to M31, and are still commonly referenced.

The Distance to the M31 Globular Cluster System
Holland, Stephen
Astronomical Journal 115: 1916-1920 (May 1998)

Distance to M31 with the Hubble Space Telescope and Hipparcos Red Clump Stars
Stanek, K.Z. & P.M. Garnavich
Astrophysical Journal Letters 503: L131-L134 (August 20 1998)

As a standard for definitions of units, I am using Scientific Unit Conversion, Francois Cardarelli, Springer, 1999 (2nd ed) for all of the obsolete & unusual units. The linked NIST values are used for the Planck length & Planck time.


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