Red Giants, 13 November 2013!Most of what astronomers know about the phases of red giants come from 1.calculations rather than observation. All of these calculations have problems because computer models are too approximate.!There are no examples of stars going through some of these phases. !A.This class will skip these stages. !B.Stars on the main sequence smaller than the sun should theoretically become 2.black cinders when they run out of available fuel. !They do not have enough energy to reach a new level of hydrogen fusion. !A.Astronomers have no observational evidence of their deaths, because their B.lives are longer than the existence of our solar system. !Any black cinders that exist would be unobservable, and observation data C.has never seen any. !This data does not prove much: it agrees with both astronomers who a.theorize that black cinders do not exist and those who say they do exist but cannot be seen. !Stars more massive than the sun (spectral types OBAF) become red giants. !3.When these stars run out of available hydrogen to fuse into helium, their A.cores shrink, causing their density, pressure, and temperature to increase. These factors allow the star to fuse three heliums and get 1 carbon and energy in the form of a gamma ray. !There are many steps between three heliums and carbon plus energy, a.but you do not have to know them.!This process is called the "triple alpha process" because the nickname b.for a helium nucleus is an "alpha particle," and three of them are involved at the start. !A star undergoing the triple alpha process is no longer on the main 1.sequence, but it is still a star.!These dying stars still fuse hydrogen into helium in their outer A.layers (shells), which makes them stars, but not in their cores. !Stars on the main sequence are defined by doing hydrogen to a.helium fusion in their cores. !The triple alpha process goes on in the core of a star for about 2 2.percent of its career. !The triple alpha process will eventually create carbon core under 3.immense temperature and pressure. This means that the cores of these stars might become a gaseous form of diamond. !The energy these stars get from the triple alpha process cause their outer B.layers to swell outward, and they become red giants. !Because these stars distribute their energy over so many square meters, a.their surface temperature is cooler than that of main sequence stars. !These stars are so bright (brighter than they were on the main b.sequence) that they look white to us. !Fusing helium marks a high point in a red giant's luminosity. !1.All of these stars are known as red giants because they redden, but 2.some of them are yellow, orange and white!Red giants are so large, that if a red giant like Betelgeuse was in the position C.of our sun, its outer layers would engulf Mercury, Venus, Earth up to Mars. !Red giants are mostly found in globular clusters. !D.When main sequence stars and red giants are in the same place (like a a.globular cluster), the red giants are so luminous that you don't even recognize the main sequence stars. !Main sequence stars look like haze in the background of red giants.!1.Globular Clusters: "globular cluster" is not a scientific term, it just means b.a cluster in the shape of a glob.!These clusters have a lot of stars in them, none fewer than a hundred 1.thousand stars.!The smallest globular clusters have more stars than the largest A.open clusters. !The biggest globular clusters have a few million stars. !B.All of a globular cluster's stars are very close together, perhaps a light 2.year apart in a cluster 50-100 light years wide. !These stars are not in contact, but they occasionally merge.!A.There are a bunch of things about globular clusters that are yet to be 3.explained.!They differ from theoretical models in places.!A.Some examples of globular clusters: !4.M13 in the northern hemisphere !A.47 Toucani and M79 in the southern hemisphere:!B.UV light from these clusters comes from dead and dying stars a.which give off more light than main sequence stars. !Omega centauri: !C.There are more than a million stars in this cluster which can be a.seen distinctly with a telescope 20 and wider. !This cluster is in the southern hemisphere.!b.The HR diagram for a globular cluster contains a bottom part of the 5.main sequence. Any star more massive than that part has used up all of its fuel and turned into a red giant. !Different star clusters have have a different amounts of the main A.sequence depending on their age. !Stars have longer life spans as they go down the main sequence.!a.The biggest blue giants turn off the main sequence and 1.become white or yellow giants in a few million years.!Less massive stars with longer lifespans will still be on the A.main sequence at this point.!The faintest red dwarfs don't even reach the main B.sequence by the time the blue giants have reached and left the main sequence.!It takes longer for a small star to join the main a.sequence because it doesn't have much gravity or mass to spark fusion, unlike massive blue giants. !This means that astronomers will never find a clusters b.whose stars are all on the main sequence.!If you assume all the stars in a cluster were made at the same b.time, then the farther up the main sequence the HR diagram of the cluster goes, the younger the cluster is. !The youngest star clusters astronomers have imaged support 1.that all of the stars in a cluster condense at once. !The greater amount of stars still on the main sequence, the 2.younger the star cluster.!Open clusters are fairly young, and have all their main sequence B.stars except the blue giants. !All known clusters still have stars whose lifespans are 13.8 billion C.years or more on their main sequence, which supports the idea that the universe is 13.8 billion years old.!When a star's helium runs out, it doesn't have enough outward pressure E.from radiation to counteract the inward pressure from gravity. This causes its core to contract again. What it does then depends on the total mass of the star. ! Red giants in certain areas are unstable and cannot reach stability. They a.are called variable stars. !In an effort to reach stability, these stars contract. However, when 1.they contract, they become opaque, and the energy inside of them generated from gravitational pressure cannot escape. The star must then expand to the point where it is no longer opaque, and can let that energy out. When that energy escapes, the star can no
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