Post #22by AVBursch » 16.06.2009, 08:36
Before sounding the alarm on what could possibly happen, I think that it's worth noting that there are alternative explanations for why Betelgeuse has shrunk during the last 15 years. These are:
1 -- Long-term pulsation behavior.
2 -- The star isn't always spherical, creating the illusion of changing size.
3 -- The star could be in the process of transforming into a blue supergiant.
1 -- This is the most likely explantion. We already know of many stars that pulsate in size, shrinking and growing and back again. Cepheid variables and Mira variables are well-known types of stars that exhibit this type of behavior. With that said, it is possible that Betelgeuse is exhibiting a Cepheid-like behavior pattern, one where the size of the star changes but the visual luminosity does not. This could be a type of pulsating behavior that goes on a 50 - 60 year cycle that hasn't been recognized as such because astronomers have been able to resolve the star as a disk only for the last 15 years.
2 -- Betelgeuse has a relatively low escape velocity (107 km/sec) and a low surface gravity (0.0092 m/s^2). This means that the star won't always have a spherical shape. Unlike planets and dwarf planets, which are relatively rigid, stars are made of plasma, which is much more "fluid-like". This is important because random oscillations of the star's surface can create the appearance of changing size. Couple that with the known 17-year rotation period, and it's easy to imagine that a bulge in the star may have possibly rotated out of view as seen from Earth, making the star appear to be smaller.
3 -- This is the least likely explanation. However, it is plausible. The star that exploded as SN 1987A had a ring around it, indicating that the star was a red supergiant that puffed out a ring of gas before turning into a B3 supergiant. Then there is Sher 25, a B1 supergiant that has a ring of gas around it. It all comes down to how fast such changes can happen.
It is worth noting that a red supergiant that is burning helium and a red supergiant that is burning silicon will appear virtually identical to each other. Even in the spectral signature, the stars would appear identical. It all comes down to what is going on within the core of the star. It may be possible for astronomers to probe all the way to the core in the future, thus allowing for a much more accurate forecast on when the star will explode.
