The World won't end and neither will Betelguese

hs-1996-04-a-web
http://hubblesite.org/newscenter/archive/releases/1996/04/image/a/format/web/
NASA/STSci

Betelgeuse is a constant companion in the night sky for those of us in the northern hemisphere. It is the brightest star in the constellation, but its fame or infamy is due to a common fear that it will explode and destroy the Earth in a couple of days. Short answer is it won’t happen. There is zero chance it will destroy the Earth and practically zero chance we will see Betelgeuse explode in our lifetimes much less in the next month or so.

I’ll ignore the nonsense about the Mayan calendar ending except to say that the calendar probably isn’t ending, just starting a new cycle, like when we start a new year in our calendar. What I am interested in is Betelgeuse the future exploding star. Betelgeuse is a red supergiant star, meaning that it emits a lot more light than the Sun, about one hundred thousand times more. It is about ten to twenty times the mass of the Sun and Betelgeuse is near the end of its life. The Sun is main sequence star where it generates energy by fusing hydrogen atoms to helium in its center. Betelgeuse has fused all of the hydrogen in the center and has moved on, now fusing helium or maybe carbon. When stars evolve to this phase of life, time is running out; stars like Betelgeuse will live for a few hundred thousand years. This seems long but the Sun is more than four billion years old.

Betelgeuse will continue to age and fuse carbon, when it runs out of carbon it will fuse neon and then oxygen and so on until the core is made of iron. Iron is a poor element for nuclear reactions. When hydrogen atoms fuse together to make helium, energy is released, but for iron atoms to fuse together, energy has to be added to make them fuse. In that case the star becomes unstable and explodes. That is Betelgeuse’s fate and when it explodes we’ll see it. When Betelgeuse explodes it will emit roughly ten billion times more light than the Sun and will appear to use to be one hundred thousand times brighter than it appears now. Since Betelgeuse is about six hundred lightyears away then it will appear to be a bit dimmer than the full moon, at least for a week or so until the supernova begins to get darker and fade away. That’s pretty darn bright. We would be able to see the supernova during the day. But that is about it.

It has been speculated that Betelgeuse will blow up this year and that it could destroy the Earth. I don’t know how it could destroy the Earth, it is too far for a normal supernova to do any damage, if it were a special type of explosion called a gamma-ray burst, and if the pole of Betelgeuse were pointed directly at us then maybe. But, Betelgeuse is not pointing pole on to us, and it is not likely that Betelgeuse will be a gamma-ray burst. Most work suggests that Betelgeuse is less than 20 times more massive than the Sun and gamma-ray bursts are believed to occur in binary star systems or stars more massive. So no destruction there.

We can also be sure that Betelgeuse will not explode for a while. Recent research by a colleague of mine showed that Betelgeuse must be only recently a red supergiant. In his article (Mackey et al. 2012, ApJ, 751L, 10), he modelled observations of a double bow shock that is observed about Betelgeuse. A bow shock is formed by a wind from a star as it speeds through the interstellar gas and dust as the star itself is also moving, like a wake formed as a boat moves through water. It was shown that the double bow shock can form only if Betelgeuse is rapidly evolving from a hot blue supergiant and only just become a red supergiant. If Betelgeuse were a red supergiant for more than a ten to thirty-thousand years then we would see only the one bow shock. This means that Betelgeuse must have a little while to go before it can become a supernova.

So Betelgeuse will not destroy us nor explode in the next few months or years or centuries.
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Stellar Astrophysics in Lijiang

HPIM0922
I returned a week or so ago from a couple of conferences, one in Lijiang, China, and one in Naples, Italy. I’ll write about the latter conference in a later post. The conference in Lijiang was titled The 9th Pacific Rim Conference on Stellar Astrophysics and featured a number of presentations and posters on various aspects of stellar astronomy from star formation to Cepheids to black holes. This conference was also interesting because it coincided with an international summer school organized by the International Astronomical Union.

In the conference, I spoke about the star Betelgeuse and a new method to measure its mass based on observing the change in the amount of light from the center of the stellar disk to the edge. This is because the change of light depends on the structure of the outer layers of the star, and more important on what fraction of the stellar radius is the depth of the atmosphere. If we can measure this atmospheric extension then we can measure the mass of star. I applied the method to observations of Betelgeuse to determine its mass. I gave a second talk on the Cepheid mass discrepancy, see my earlier post. In this talk, I discussed if winds could explain the mass discrepancy for Cepheids in our galaxy and the discrepancy of Cepheids in the Large Magellanic Cloud. I found that winds can explain the result for Galactic Cepheids but was inconclusive for the Magellanic Cloud Cepheids. Still more work to be done.

Other talks were much more interesting. There was a talk showing that the Earth and Sun currently in the middle of a bubble in the interstellar medium where there is less gas. The bubble was created by supernovae that exploded a long time ago. Another talk discussed the habitability of planets orbiting a very low mass, cool star called an M dwarf. This is important because these stars are among the most common in the universe and therefore most extrasolar planets are likely to orbit these stars. One of my favorite was an talk discussing observations of a type of binary stars called Algols, where the speaker showed a plot suggesting that in these systems the binaries will merge and form one star. It was a bit of a subtle plot but very cool.

The most interesting part of the conference was meeting the students from the summer school. These students came from many different parts of Asia, including Vietnam, Nepal, and even North Korea. It was a unique opportunity to learn about these cultures and astronomy in their home countries. I really liked the meeting and look forward to the 10th Pacific Rim Meeting on Stellar Astrophysics.
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