How Far Away is M31?

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 10² kiloparsecs
  
• 7.837 x 10⁵ parsecs
  
• 2.555 x 10⁶ light years
  
• 9.326 x 10⁸ light days
  
• 2.238 x 10¹⁰ light hours
  
• 1.616 x 10¹¹ astronomical units [average earth-sun distance]
  
• 1.343 x 10¹² light minutes
  
• 3.473 x 10¹³ solar radii [6.96 x 10⁸ meters]
  
• 7.018 x 10¹³ Earth-moon distances [344,400 km]
  
• 8.057 x 10¹³ light seconds
  
• 3.794 x 10¹⁵ earth radii [6.37 x 10⁶ meters]
  
• 1.653 x 10¹⁸ yodjana [14,624 meters, ancient Indian unit]
  
• 2.309 x 10¹⁸ royal atour [10,470 meters, ancient Egyptian unit]
  
• 4.347 x 10¹⁸ nautical leauges [3 UK nautical miles]
  
• 4.523 x 10¹⁸ mil [5344.2 meters, old Swedish mile]
  
• 5.006 x 10¹⁸ land leagues [4828 meters]
  
• 6.154 x 10¹⁸ ri [3927.27 meters, old Japanese unit]
  
• 1.305 x 10¹⁹ nautical miles [1852.0 meters]
  
• 1.502 x 10¹⁹ statute miles [1609.3 meters]
  
• 1.629 x 10¹⁹ millarium [1483.5 meters, ancient Roman mile]
  
• 2.417 x 10¹⁹ kilometers
  
• 1.104 x 10²⁰ cable's lengths [219 meters]
  
• 1.201 x 10²⁰ furlongs [660 feet]
  
• 1.201 x 10²¹ chains [66 feet]
  
• 4.806 x 10²¹ rods [1/4 chain; 16.5 feet]
  
• 6.568 x 10²¹ roeden [3.680 meters, old Dutch unit]
  
• 1.322 x 10²² fathoms [6 feet]
  
• 2.417 x 10²² meters
  
• 1.972 x 10²² paces [30 in per pace]
  
• 2.644 x 10²² yards [Henry VIII's arm]
  
• 4.355 x 10²² cubits [0.555 meters, old Hebrew unit]
  
• 7.931 x 10²² feet [1/3 yard]
  
• 1.201 x 10²³ surveyor's (Gunther's) links [2/3 foot]
  
• 2.379 x 10²³ hands [1/3 foot]
  
• 9.517 x 10²³ inches [1/4 hand]
  
• 2.417 x 10²⁴ centimeters
  
• 2.855 x 10²⁴ barleycorns [1/3 inch, UK]
  
• 2.417 x 10²⁵ millimeters
  
• 2.417 x 10²⁶ diameters of a typical human hair [about 100 microns or 0.1 millimeters]
  
• 4.395 x 10²⁸ wavelengths of visible light [roughly 5500 Angstrom units]
  
• 2.417 x 10²⁸ microns
  
• 2.417 x 10³¹ nanometers
  
• 2.417 x 10³² Angstrom units [atomic diameters; 10^-10 meters]
  
• 4.567 x 10³² Bohr radii [5.29177249 x 10^-11 meters]
  
• 8.577 x 10³⁶ classical electron radii [2.818 x 10^-15 meters]
  
• 2.417 x 10³⁷ Fermi units [nuclear diameters; 10^-15 meters]
  
• 1.496 x 10⁵⁷ Planck Lengths [quantum distance; 1.616 x 10^-35 meters]
  
• 1.496 x 10⁵⁷ 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.

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