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Unit 4 Chap. 19 Astronomy Test for Jaunty30
Terms in this set (17)
Death of stars
Low mass stars on the lower main sequence of the H-R diagram have extremely long lifetimes. Their entire original mass of hydrogen is available as fuel. When all the hydrogen of the star is used (fused to helium), it collapses quietly to a helium white dwarf.
Stars with masses of about 1/2 M
to about 8 M
follow roughly the evolution of a 1M* star.
Mass loss among red giants
-Stars just larger than 1.4 M* lose their extra mass through accelerated stellar wind.
-Stars with masses up to 8 or 9 M* often have their outer layers go unstable and explode. The result is a nova.
A nova is seen from Earth as a sudden brightening of an existing star. The explosion is very bright for a few days to weeks as the gas expands. Then it fades as the gas expands and cools. This process can be repeated every 3 or 4 hundred years until the star reaches the mass limit then it can go white dwarf.
More than half the stars in the sky are double stars and are close enough to share matter when one goes giant. The larger star goes giant 1st and dumps its extra mass to the small er main sequence star until it is under the mass limit then it quietly goes to white dwarf. When the now bloated 2nd star goes giant, it feeds back to the white dwarf where it is ejected by explosion and we see it as a nova.
Death of very large stars
A large star has a great number of shell fusion furnaces. The ´ashes´ from one furnace serves as fuel for the next. The inner most ash layer is iron.
A high-mass star can continue to fuse elements in its core right up to iron (after which the fusion reaction is energetically unfavored).
As heavier elements are fused, the reactions go faster and the stage is over more quickly.
A 20-solar-mass star will fuse carbon for about 10,000 years, but its iron core lasts less than a year.
ON the left, nuclei gain mass through fusion; on the right they lose it through fission.
Iron is the crossing point; when the core has fused to iron, no more fusion can take place.
Many of the elements are formed during the normal stellar fusion. left, 3 helium nuclei fuse to form carbon; right a couple of more complex fusion reactions. Some are made during the supernova explosion.
A supernova is a one-time event - once it happens, there is little or nothing left of the progenitor star.
There are two different types of supernovae, both equally common:
Supernova I, which is a supernova explosion around a core which implodes.
Supernova II, which is an explosion of the core resulting in the complete destruction of the star.
A supernova has not occurred in our part of the Milky Way since the invention of the telescope. So, we have not had the opportunity to study one up close. We have seen many in other galaxies as well as remnants in our galaxy.
A supernova in a distant galaxy is often brighter than the entire galaxy it is in.
Supernovae I arise in two ways. The first kind, a single star, SNI, come s from an explosion in the silicon layer around the iron core of a large star. The second kind come from interaction of large binary stars.
The iron is very reluctant to fuse. Sometimes the oxygen and silicon layers around the core become unstable and explode, imploding the iron to a neutron star.
Normally, a large star would die as a supernova. In a binary situations, however, it dumps its excess mass over to its smaller companion and becomes a white dwarf. The now very large companion finishes its life and goes giant, dumping it excess matter on the white dwarf. The now multi-layered star around is very unstable and explodes in a supernova imploding the white dwarf to a neutron star.
The classic results of a supernova I are the expanding debris of the explosion, a neutron star and a pulsar.
The crab nebula is the result of a supernova in 1054. It was observed and location recorded by the Chinese. We see the expanding debris of the explosion today at that location.
Often as a large star ages much of the fuel is used up and deposited as ´ash´ in the iron core. The inward pressure on the iron core is enormous, due to the high mass of the star. As the core continues to become more and more dense, the protons react with one anotehr to become neutrons + a flood of neutrons + much energy.
These local hot spots initiate fusion of the iron which triggers formation of all of the elements more massive than iron + more neutrinos and much more energy. The energy builds up in a cascade effect producing a gigantic explosion and the complete destruction of the star, known as a supernova II.
The classic results of a supernova II are:
-collapse of the iron core
-flood or neutrinos
-super explosion debris cloud
-complete disassembly of the star
While doing a theoretical study of supernovae, Zwysic and Baade in the 1930´s predicted the existence of neutron stars, but they had never been seen even with the 200 inch hale telescope on Mount Palomar. The first one found, much later, was associated with a Pulsar in the Crab nebula.
Neutron stars, although they have 1-3 solar masses, are so dense that they are very small. This image shows a 1-solar-mass neutron star, about 10 km in diameter, compared to Manhattan.
As the parent star collapses, the neutron core spins very rapidly, conserving angular momentum. Typical periods are fractions of a second. Again as a result of the collapse, the neutron star´s magnetic field becomes enormously strong.
in 1967 Jocelyn Bell led a group of graduate students at the University of Cambridge in England in a search for Radio Sources in the sky. They discovered a source that emitted extraordinarily regular pulses. After some initial confusion, it was realized that this was a neutron star, spinning very rapidly.
Why do neutron stars pulse?
Strong jets of matter and beams of light are emitted at the magnetic poles, as that is where they can escape. If the rotation axis is not the same as the magnetic axis, the two beams will sweep out circular paths. If the Earth lies in one of those paths, we will see the star blinking on and off.
The velocities of the material in the Crab nebula can be extrapolated back, using Doppler shifts, to the original explosion point.
This is the pulsar at the center of the Crab Nebula.
Summary of the death of stars.
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