In our recent post about Alnilam, the middle star in Orion’s belt, several people indicated that they wanted to see similar information on other stars that make up this treasure trove of a constellation. Betelgeuse seems like a good choice, especially since we mentioned it in the Alnilam post. The huge red star that marks Orion’s right shoulder (we see it on the left, since Orion is facing towards us) is interesting in and of itself, and we can make some comparisons to Alnilam to show some similarities and differences between the two.
Betelgeuse, like Alnilam, is a supergiant. With an estimated age of about 8.5 million years, it is older, and thus farther along in its short life cycle than the belt star is. It has swollen into a red supergiant, its surface cooling in the process – at 3,500 Kelvin, it is seven times cooler than Alnilam. With an estimated mass about 20 times that of the Sun, it is indeed a big star, but that is only half the mass of Alnilam. The red supergiant shines with an energy of 125,000 times that to the Sun. Very bright, but still only one-third of the prodigious energy output of Alnilam. The solar wind carrying material away from Betelgeuse blasts away a million times more matter than the Sun does – a large number indeed, but far less than Alnilam.
It may sound like Betelgeuse is coming out the red-headed stepchild in this comparison, but that would be selling this Orion standout short. Very short. Here is where we turn the tables, and show you what makes Betelgeuse special.
Betelgeuse is big. Really big. Maybe monstrous would be a better term for it. Its diameter is about 1,000 times that of the Sun, or 33 times that of Alnilam. If you placed it at the center of our solar system, it would be about the same size as the orbit of Jupiter. It is so big that it was the first star other than our Sun to have its angular size measured, way back in 1920. In 1996, it was the first star, again, other than the Sun, to have its atmosphere directly imaged by the Hubble Space Telescope (in the very image accompanying this post). Not quite big enough for you? In 2013, observations at radio wavelengths detected the extended atmosphere of material that Betelgeuse has thrown off into space, some of it reaching almost 7.4 billion kilometers (4.6 million miles) away from the star. Using our solar system comparison, we’re now talking about the orbit of Pluto.
Betelgeuse is a winner in visual magnitude, shining at 0.42 compared to 1.69 for Alnilam. That makes it the second brightest star in Orion, and the ninth brightest in the night sky. Although it is cooler than its blue friend, it is only half the distance from us (650 light-years vs. 1,340) and it has a much greater surface area. In addition, Betelgeuse is a variable star, so that 0.42 apparent magnitude is sort of an average, or a snapshot. It is estimated that less than 100 years ago, it was as bright as 0.20 magnitude.
If we want to use racing analogies, we can chalk up a couple of more wins for the red supergiant. On the slow end of the spectrum, Betelgeuse wins when it comes to rotation. Our Sun spins once on its axis in about a month. It takes Betelgeuse 8.4 years to to accomplish the same feat, thanks to the conservation of angular momentum. And when it comes to the ultimate finish line, Betelgeuse, farther along in its life cycle than Alnilam is, will explode as a supernova first. When? Maybe tonight. Maybe a million years from now. The best current consensus is that it will likely by in 100,000 years, give or take. Until then, enjoy the view.
Go ahead, say it: “Beetlejuice! Beetlejuice! Beetlejuice!”
Image credit: Andrea Dupree (Harvard-Smithsonian CfA), Roland Gilliland (STScI), NASA and ESA