Last week, we turned our ever watchful gaze toward one of the largest single objects in the universe, red giant stars. This week, we will look at the other major class of giant stars, blue giants.
We learned that red giant stars are not true giants at all. They may be large, but they are not always massive stars. They may be even smaller than our Sun, but red giants are the bloated corpses of dying stars.
The blue giants, on the other hand, are true giants. Although they may be only as large as red giants, the blue giants are far more massive then our Sun, and burn far hotter. A typical blue giant star has a mass perhaps 10 times that of the Sun, and has a surface temperature of two to four times that of our local star.
The most famous of the blue giant stars is the star Rigel, in the constellation of Orion, the hunter. It can be seen as the left foot of Orion, if he were facing us here on Earth. Rigel is one of the brightest stars in the sky, as seen from Earth, and would be larger than the orbit of the planet Mars, were it to suddenly replace our Sun at the center of our solar system.
Why are blue giants so large? We said it was not because they are bloated, like red giant stars, but rather, it is due to the simple fact that they are large because they have a lot of material. With so much hydrogen gas to fuse in order to produce starlight, one might think that blue giant stars would have very long lifetimes. This is, in fact, not the case. The more massive a star is, the more quickly it burns it's fuel. In fact, the lifetime of a blue giant star is much shorter then that of the Sun.
Stars are classified by letters; the most massive stars are called O class stars. Slightly less massive ones are B class stars, then A, F, G, K, and finally M class for the least massive stars. This order of letters is often remembered by astronomers by the phrase "Oh, Be A Fine Girl (or Guy), Kiss Me". Our Sun is a typical G class star, somewhere in the middle, and producing largely yellow light. The blue giants are O and B class stars, and would make an unlikely place to find an intelligent alien civilization.
Why? It is due to what we mentioned earlier; the lifetimes of the stars. A large O class star has a lifetime of only about one million years; far too short of a time for life to arise on any nearby planets, much less intelligent life. Even the slightly less massive B class stars only have an expected lifetime of ten to eleven million years. A typical G class star, like our Sun, on the other hand, has an average lifetime of perhaps 10 billion years, more than enough time for an intelligent civilization to develop.
At the other extreme are the K and M type stars, known as red drawfs. An average M class star can live over 50 billion years, far longer than the 12 billion years or so since the big bang. So, we should expect red drawfs to be a likely hunting ground for other civilizations. Correct? Not likely. Red dwarfs are so cool, that any planets harboring any kind of life would have to be very close to the star. The best chance to find other life appears to be searching F and G class stars, like our Sun.
By far, one of the most interesting aspects of Blue giant stars is what happens when they die. When a star like our Sun dies, it swells up once into a red giant, and then, it finally collapses into a white drawf. A white dwarf is a star with the mass of the Sun, but only having a size about that of the Earth, and held from further collapse by the repulsion of the electrons inside the star. When a star has the mass of a blue giant, it can swell several times before it's final collapse. When the final collapse occurs, however, the great mass of the star is too much for the electrons to hold the star back from further collapse, and when a white dwarf forms, it too, collapses. The white dwarf, which was about the size of the Earth, collapses into a neutron star, which has a diameter about that of a typical large city.
Neutron stars are held from further collapse by the repulsion of the neutrons within them. The most massive type O stars, however, have a mass too great for even neutron repulsion to keep them from further collapse, and the star goes through a final collapse, forming a black hole; the most mysterious object in the universe. Not all blue giants will collapse into a black hole, but nearly all the black holes produced after the big bang, come from blue giant stars.
Clear skies, and good viewing.
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