7/9. What happens to a star depends almost completely on the mass of the star. Mass Categories: Low-Mass Stars 0.2 solar masses and less
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1 7/9 What happens to a star depends almost completely on the mass of the star. Mass Categories: Low-Mass Stars 0.2 solar masses and less Medium-Mass Stars 0.2 solar masses up to between 2 and 3 solar masses. High-Mass stars from 2 to 3 solar masses on up Main Sequence Lifetime of a Star The lifetime of a star is given the energy E available to the star divided by the luminosity L of the star: Put this in terms of solar units the energy available to the star is proportional to its mass (recall E = mc 2 ) E = M. Recall that, in solar units, L = M 3.5, so we get Example: Range of lifetimes of stars. Lowest Mass Star: M = 0.08 M S Highest Mass Star: M = 100 M S
2 Since a star s evolution depends so much on its mass, we should note that the mass of a star changes over time. For instance, due to the solar wind, the Sun will lose about 0.1% of its mass over its main sequence existence. Once a star evolves off the main sequence, its mass changes more drastically. Some stars will lose as much as 90% of their mass over their total lifetimes. Post Main Sequence Evolution: Low-Mass Stars 1. Convective throughout deliver heat from core to surface purely by convection. 2. Convection mixes the material of the star is the material carries the heat. 3. Once the H is gone in the center of the star, it is gone everywhere in the star. 4. The star is now all He and can t get hot enough to fuse He into other elements. 5. Once the H is gone, fusion stops. 6. No longer outward radiation pressure; the star begins to gravitationally contract again. 7. Eventually, this contract stops due to Pauli Repulsion. (Aside: Pauli Exclusion Principle To explain the structure of the periodic table of the elements why different elements have different chemistry hypothesized that electrons could not share the same quantum state. Also true of protons, neutrons, and neutrinos among other particles.) Further contraction of the star would cause electrons to occupy the same quantum states can t happen. Matter in this state is called degenerate electrons occupy all available quantum states. 8. The star is now about the size of the Earth and has become quite hot because of its contraction. 9. It is a white dwarf. 10. The white dwarf sits in space radiating its heat away and eventually becomes a black dwarf. More Massive Stars:
3 Either the star has a radiative core and convective envelope (medium-mass stars such as the Sun) or the star has a convective core and radiative envelope (most massive stars). In either case, there is no mixing between the material in the core and the material in the envelope. Eventually, the hydrogen is used up in the core but, when this happens, there is still H in the envelope. When the H is used up in the core, the core begins to contract. When the core contracts, it gets hotter this causes H fusion into He to start in the envelope. The outflow of energy from this region, because of its low density, causes the envelope to expand. The star expands and cools becomes a red giant. In the lower mass stars among the medium mass star, the core will never get hot enough to fuse He into heavier elements. The outflow of energy in the envelope eventually expels the envelope into space, leaving behind the core of the star. The core of the star contracts until stopped by Pauli repulsion becomes a white dwarf. Our own Sun will get hot enough to fuse He into heavier elements carbon and oxygen. It will spend a short amount of the time on the He main sequence. The Sun will not get hot enough to fuse carbon and oxygen into heavier nuclei. The Sun will also expel its envelope and end up as a white dwarf made of carbon and oxygen. More massive stars will be able to fuse carbon and oxygen into heavier elements. More and more massive stars will eventually form white dwarfs of heavier and heavier elements. Until we get to iron that s the next chapter. We can test these ideas by examining star clusters. Two types of star cluster: 1. Open few stars a few 10's to a few 1000's stars tend to be young where stars are born size about 10 pc irregular shape
4 2. Globular Star Cluster Many stars 100's of thousands to millions of stars. stars tend to be old may be the oldest structures in the universe sizes 20 to 30 pc name from their spherical shape like a globe Assume that all the stars in a cluster were created at about the same time. We should see different mass stars in clusters at different stages of evolution. Expect to see for an open cluster: 1. No low-mass stars on the main sequence not enough time to evolve that far. 2. Only the most massive stars very few will have evolved off the main sequence. 3. Draw HR diagram for an open cluster. 4. The turn-on point, where stars are just evolving on to the main sequence can be used to deduce the age of the cluster. Expect to see for a globular cluster. 1. Even the lowest mass stars will have had time to evolve onto the main sequence. 2. The massive stars will have had time to evolve off the main sequence. 3. Draw HR diagram for a globular cluster. 4. Turn-off point tells us about the age of the cluster. Chapter 10 The Deaths of Stars We have already discussed the deaths of less massive stars end as white/black dwarfs. Here we discuss the end states of the most massive stars. Chandrasekhar discovered that a white dwarf can be no more massive than 1.4 solar masses if it is, not even Pauli repulsion can prevent further contraction. We will now discuss what happens to stars whose end states have masses greater than the Chandrasekhar limit.
5 The most massive stars, after leaving the main sequence, will continue to fuse elements until they get to iron. Iron is the most stable of nuclei produced inside a star no energy can be generated from fusing iron into heavier elements. When the core of the star becomes iron, fusion stops. In a fraction of a second, the core collapses. The core rebounds like a compressed spring that produces an explosion that tears the star apart supernova. A supernova can be brighter than the galaxy of which it is apart. A type II SN such as this one can be as bright as 600 million Suns. One nearby can be bright enough to shine out in the daytime. A SN within 50 ly of the Earth could destroy all life here on Earth. What is left at the center? Depends on the mass of the star. If the main sequence mass is less than 40 solar masses, the remnant will be between 1.4 solar masses and 2 to 3 solar masses. Since the mass is greater than the Chandrasekhar limit, not even the Pauli repulsion of the electrons can prevent further contraction. The electrons are forced into protons, converting them to neutrons. We end up with a ball of neutrons with a few electrons and protons floating around that continues to contract. But neutrons also must obey the Pauli principle object contracts until further contraction would cause neutrons to occupy the same quantum states. We end up with a neutron star.
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