October 2000

The Average Sun

In the Nightly Observing Program, guests often ask me, "How bright is the Sun, compared to other stars?"

The classic answer is "Average."

As always, there are details (and that's where the fun is). As brightness goes, the Sun is in the middle of the range. Stars at the extremes can be 100,000 times brighter or 100,000 times dimmer than the Sun.

But without a doubt, the Sun is an unusually bright star.

Here's why: Dim stars are more common. Superbright stars are actually very rare. Stars like Rigel, Betelgeuse, Deneb, and Epsilon Canis Majoris make up about a hundred millionth of the total. Stars this bright are so rare that there are only a few thousand in the entire galaxy, and they are spread thousands of lightyears from each other in space. Of these four, Betelgeuse is closest, about 500 lightyears away.

Now, let's talk about dim stars. If we examine the space within about fifteen lightyears (actually, five parsecs) of the Sun, we'll find 61 stars. Of those, 46 are less than one percent as bright as the Sun! Thirteen are less than one ten-thousandth as bright as the Sun. These dim little guys are red dwarfs, or class M stars, and they make up the majority of the stars in our galaxy.

They are so dim, they can't be seen in the night sky. When you look up at the constellations, you are seeing the small minority of bright stars which can be seen over much greater distances. The closest star to the Sun, Proxima Centauri, is a red dwarf (so is the fourth closest, and fifth, and seventh, eighth, ninth, etc).

The Sun is an average star, yes, but those dimmer than it vastly outnumber those brighter than it. So I like to think of it as a bright star.

Here's a not-to-scale sketch of those 61 stars. The Sun is at upper left, and the big blue one is Sirius (about 23 times brighter than the Sun). Within fifteen parsecs, there are four stars brighter than the Sun and 56 stars dimmer. That's a pretty bright Sun.

Steve White
Nightly Observing Program
Kitt Peak Visitor Center

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Telescope Operators/Guides for Nightly Program:
Adam Block (ablock@noao.edu) Lead Observer
Steve White (swhite@noao.edu)
Flynn Haase (fhaase@noao.edu)
Richard Barchfield (richardb@noao.edu)

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Updated: 08/17/2000