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The Question

(Submitted June 16, 1997)

I have just became aware of the enormity of Betelgeuse. Do we know of stars that are larger than this? If so, approximately how many are of this scale are larger? Does a star have to be of this size to be a candidate for a black hole?

The Answer

Betelgeuse (also known as alpha ori) is a very large star, an M supergiant. This is because it has evolved far from the state in which stars spend most of their lives, known as the main sequence. For stars on the main sequence, which includes our Sun, there is simple proportionality between size and mass, and also a simple scaling for luminosity. For evolved stars the situation is less simple. Betelgeuse is more than 1000 times larger than the Sun, and 50000 times as luminous, but only about 20 times as massive. Most of the light from Betelgeuse comes out in the infrared, however, which is very different from the Sun. One consequence of the advanced evolutionary state of Betelgeuse is that it probably was much more massive when it was on the main sequence, and has already lost a significant fraction of its mass (probably more than half) in a stellar wind.

There are many stars that are as massive as Betelgeuse is now, and probably many that are as massive as Betelgeuse was when it was on the main sequence. Of the 100,000,000,000 (100 billion=10^11) or so stars in our galaxy, it is estimated that approximately 1% have main sequence masses greater than 30 times that of the Sun, which is where Betelgeuse may have started out. A very crude estimate is that such stars spend 1% of their lives as supergiants, which would suggest 10,000,000 stars similar to Betelgeuse in our galaxy.

In spite of this fact, there are very few stars which are visible to the naked eye which are as large as Betelgeuse. This is simply a consequence of the fact that we can distinguish bright stars in only a small fraction of the galaxy. Another one is Mira, in the constellation Cetus. Mira is probably larger than Betelgeuse, so large that it is thought that the outer layers of the star are barely held together by gravity. Mira is known to pulsate and eject its outer layers, probably in large part because of its weak gravity. Possibly the most massive known star is eta carina, which may have been 150 times as massive as the Sun when it first formed, and may be 50 - 60 times as massive as the Sun currently. In the 1830s eta carina underwent a tremendous outburst during which it became a brilliant naked eye object and ejected an amount of gas with mass approximately equal to the mass of the Sun.

It is likely that the minimum main sequence mass for a star which will eventually make a black hole is 8 - 10 times the mass of our Sun. This is quite a bit less than Betelgeuse had when it was on the main sequence, and there are many such stars in our galaxy.

I hope this helps!

Tim Kallman for Ask an Astrophysicist

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