First, some useful unit conversion:

- 1 light year = 9.4606 x 10
^{15}meters = 5.879 x 10^{12}miles - 1 parsec = 3.0857 x 10
^{16}meters - 1 mile = 1609.3 meters
- 1 mile = 5280 feet = 63,360 inches
- 1 Fermi unit = 10
^{-15}meters - 1 Planck Length = 1.616 x 10
^{-35}meters

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 ...

- 7.837 x 10
^{-4}gigaparsecs

- 7.837 x 10
^{-1}megaparsecs

- 7.837 x 10
^{2}kiloparsecs

- 7.837 x 10
^{5}parsecs

- 2.555 x 10
^{6}light years

- 9.326 x 10
^{8}light days

- 2.238 x 10
^{10}light hours

- 1.616 x 10
^{11}astronomical units [average earth-sun distance]

- 1.343 x 10
^{12}light minutes

- 3.473 x 10
^{13}solar radii [6.96 x 10^{8}meters]

- 7.018 x 10
^{13}Earth-moon distances [344,400 km]

- 8.057 x 10
^{13}light seconds

- 3.794 x 10
^{15}earth radii [6.37 x 10^{6}meters]

- 1.653 x 10
^{18}yodjana [14,624 meters, ancient Indian unit]

- 2.309 x 10
^{18}royal atour [10,470 meters, ancient Egyptian unit]

- 4.347 x 10
^{18}nautical leauges [3 UK nautical miles]

- 4.523 x 10
^{18}mil [5344.2 meters, old Swedish mile]

- 5.006 x 10
^{18}land leagues [4828 meters]

- 6.154 x 10
^{18}ri [3927.27 meters, old Japanese unit]

- 1.305 x 10
^{19}nautical miles [1852.0 meters]

- 1.502 x 10
^{19}statute miles [1609.3 meters]

- 1.629 x 10
^{19}millarium [1483.5 meters, ancient Roman mile]

- 2.417 x 10
^{19}kilometers

- 1.104 x 10
^{20}cable's lengths [219 meters]

- 1.201 x 10
^{20}furlongs [660 feet]

- 1.201 x 10
^{21}chains [66 feet]

- 4.806 x 10
^{21}rods [1/4 chain; 16.5 feet]

- 6.568 x 10
^{21}roeden [3.680 meters, old Dutch unit]

- 1.322 x 10
^{22}fathoms [6 feet]

- 2.417 x 10
^{22}meters

- 1.972 x 10
^{22}paces [30 in per pace]

- 2.644 x 10
^{22}yards [Henry VIII's arm]

- 4.355 x 10
^{22}cubits [0.555 meters, old Hebrew unit]

- 7.931 x 10
^{22}feet [1/3 yard]

- 1.201 x 10
^{23}surveyor's (Gunther's) links [2/3 foot]

- 2.379 x 10
^{23}hands [1/3 foot]

- 9.517 x 10
^{23}inches [1/4 hand]

- 2.417 x 10
^{24}centimeters

- 2.855 x 10
^{24}barleycorns [1/3 inch, UK]

- 2.417 x 10
^{25}millimeters

- 2.417 x 10
^{26}diameters of a typical human hair [about 100 microns or 0.1 millimeters]

- 4.395 x 10
^{28}wavelengths of visible light [roughly 5500 Angstrom units]

- 2.417 x 10
^{28}microns

- 2.417 x 10
^{31}nanometers

- 2.417 x 10
^{32}Angstrom units [atomic diameters; 10^{-10}meters]

- 4.567 x 10
^{32}Bohr radii [5.29177249 x 10^{-11}meters]

- 8.577 x 10
^{36}classical electron radii [2.818 x 10^{-15}meters]

- 2.417 x 10
^{37}Fermi units [nuclear diameters; 10^{-15}meters]

- 1.496 x 10
^{57}Planck Lengths [quantum distance; 1.616 x 10^{-35}meters]

- 1.496 x 10
^{57}light Planck times [the Planck time is the time it takes light to travel one Planck length]

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.