Distances in Astronomy

Astronomers are constantly dealing with distance scales that are unfathomable to the average person.  As a result, there are many distance measures that only astronomers are familiar with.  For example, what exactly is a parsec?  It’s a pretty big distance, but how big exactly?  Here’s a quick overview of astronomical distances to help give a better sense of scale elsewhere in the website.

Within the Solar System, astronomers typically use a unit called the ‘Astronomical Unit‘ (AU) defined by the average distance between the Earth and the Sun.

  • 1 AU = the average distance between the Earth and the Sun
  • 2.5 AU = the distance from the Earth to Mars
  • 31.9 AU = the distance from the Earth to Pluto

Outside of the Solar System, the most common unit used is the ‘parsec‘ (pc).

  • 1 pc = 3.26 light years
  • 1.3 pc = the distance to the nearest star to Earth (Proxima Centauri)
  • 1 pc = 206265 AU (by definition)

Within our galaxy, astronomers typically refer to ‘kiloparsecs‘ (kpc).

  • 1 kpc = 1,000 pc
  • 7.6 kpc = the distance from the Earth to Sagittarius A* (the supermassive black hole at the center of the Milky Way galaxy)

When considering the distances between galaxies or cosmological distances, astronomers switch to ‘Megaparsecs‘ (Mpc).

  • 1 Mpc = 1,000,000 pc
  • 0.78 Mpc = the distance from the Earth to the Andromeda galaxy (the nearest galaxy to the Earth)

Finally, when referring to the radius or mass of objects, astronomers like to relate things to the Sun.

  • 1 solar mass = the mass of the Sun = 1.99 x 10^33 grams
  • 1 solar radius = the radius of the Sun = 6.96 x 10^10 cm
  • The total mass of the Milky Way galaxy = 3 x 10^12 solar masses (meaning the entire galaxy weighs 3,000,000,000,000 times more than the Sun!)

And, if all of those numbers still seem a bit overwhelming, just refer to this rather fantastic view of relative distance/size scales in astronomy:

http://xkcd.com/482/

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s