Astronomy Notes Chapter 13.notebook. April 11, 2014

Transcription

1 All stars begin life in a similar way the only difference is in the rate at which they move through the various stages (depends on the star's mass). A star's fate also depends on its mass: 1) Low Mass Stars (less than 8 suns) Once helium fusion ends, the core is filled with several elements, mostly carbon and oxygen. Helium shell fusion occurs in a shell around the core while hydrogen shell fusion occurs in a layer outside of that. The star expands once more to form a red supergiant. Most red supergiants are variable and have strong stellar winds (much mass loss). The core contracts as fusion byproducts settle into it, but there is not enough pressure (not enough mass) to initiate fusion of heavier elements. Apr 17 9:44 AM Apr 17 9:53 AM Hydrogen and helium shell fusion continues outside the core. Helium shell flashes begin to occur about every 100,000 years or so. With each flash, some of the star's outer layers are expelled out into space, forming a planetary nebula. Eventually, nothing is left except for the hot carbon core. This hot core is known as a white dwarf it is very hot and bright, but has no nuclear reactions occuring in it. This is the Sun's eventual fate. These nebulae have a variety of shapes, depending on how the layers were expelled. Apr 17 10:27 AM Apr 17 10:33 AM Cores with masses below 1.4 solar masses (Chandrasekhar Limit) collapse until electrondegeneracy pressure stops them from collapsing further. White dwarfs are extremely dense about 5 tons per teaspoon. Eventually, the white dwarf cools and forms a giant crystal (diamond?) and is called a black dwarf. Binary stars are usually composed of stars of unequal mass. This means that they age at different rates. One star usually will reach the white dwarf stage long before the other. When the second star reaches the red giant phase, it swells up and fills its Roche lobe matter (hydrogen) pours onto the white dwarf. Apr 17 10:50 AM Apr 17 11:47 AM 1

2 As the hydrogen flows onto the surface of the white dwarf, its temperature and density increase. Eventually, the temperature and pressure reach a point where fusion occurs and the layer is blown off into space in a gigantic explosion. This type of event is called a nova. The process then begins again so most novae reoccur. 2) High Mass Stars (greater than 8 solar masses) In high mass stars, helium fusion is not the endpoint. These stars are massive enough that gravity continues to compress the helium core. Eventually carbon fusion begins in the core. Apr 17 11:50 AM Apr 19 9:09 AM Following carbon fusion, oxygen fusion begins. Fusion of various elements procedes in the core one after another each step goes faster than the step before it: Reaction Timescale Hydrogen 10 million years Helium 1 million years Carbon 300 years Oxygen 200 days Silicon 2 days Each type of fusion adds a new shell to the star where fusion of lighter elements is occuring. Apr 19 9:20 AM Apr 19 9:30 AM Silicon fusion produces many types of new elements, but the end product is iron. Unlike the fusion of lighter elements, iron fusion does not release energy (it absorbs it) so core fusion stops. Gravity now compresses the core until it is about the size of the earth. The core compression is stopped by electron degeneracy pressure. Fusion continues in the star's outer layers so iron continues to rain down onto the core its mass continues to increase. When the core reaches the Chandrasehkar Limit (1.4 solar masses), electron pressure is no longer enough and the core collapses rapidly. Apr 19 9:34 AM Apr 19 9:40 AM 2

3 The core shrinks from about the size of the earth down to about 50 miles across in one second or so. The star's outer layers are now unsupported so they fall inward too. The core temperature climbs (1/10 second) until it reaches 5 billion kelvins. Gamma rays are now produced with enough energy to break the iron down in protons, neutrons, and electrons (photodisintegration). The enormous pressure in the core now forces the electrons into the protons forming neutrons. Many neutrinos are also produced, carrying energy away from the core. The core is now at nuclear density (density of a neutron) and its collapse suddenly stops. The core "bounces" and rebounds outward where it collides with the infalling outer layers. Apr 19 10:12 AM Apr 19 10:38 AM The shockwave produced travels upward through the star, blowing its outer layers off in a huge explosion known as a supernova. Supernovae release unimaginable amounts of energy their output is equal to that of all other stars in the galaxy combined. The star is sometimes completely destroyed, but usually the neutron core survives. So much energy is released in the explosion that all elements heavier than iron are formed (nucleosynthesis) and spread into space. There are two types of supernovae: 1) Type I a binary white dwarf has its mass pushed past the Chandrasekhar Limit by infalling gases from its companion. 2) Type II the type already described Supernova 1987A video Apr 19 10:43 AM Apr 19 10:48 AM When stars with masses between 8 and 25 solar masses explode, their remaining cores are composed of neutrons. They are known as neutron stars for this reason. When the neutrons are highly compacted, they must move rapidly to keep from having identical energies (Pauli Exclusion Principle). Because of this motion, they exert pressure that is greater then EDP. The existence of neutron stars was first proposed in 1933, but many people did not think that they could actually exist. In 1967, radio pulses were detected coming from space with a rate of one every seconds (Jocelyn Bell). At first, it was thought that perhaps an alien civilization had been detected. It was later determined that it was a rapidly rotating neutron star now known as a pulsar. Apr 23 10:18 AM Apr 23 10:22 AM 3

4 Pulsar Anatomy How do pulsars occur? As the star's core collapses, it speeds up to conserve angular momentum this gives it a very rapid rotation rate. Neutron stars have an "atmosphere" of lighter elements (hydrogen through iron) which are electrically charged. They also have very powerful magnetic fields. Apr 23 10:59 AM Apr 23 10:28 AM The star's rapid rotation accelerates the charged particles along the star's magnetic field lines and out into space above the north and south magnetic poles. Since the star's magnetic poles do not coincide with the rotational poles, the effect is a "beam" of highly charged particles sweeping out into space as the star rotates. If the beam sweeps by the earth, we see a pulsar. Pulsars slow down as they age since their rotational energy is what powers the energy beam. Apr 23 10:33 AM Apr 23 10:52 AM Two binary pulsars are known one with a period of only 8 hours. They are so close that Newton's Laws no longer work only Einstein's do. Some pulsars orbit normal stars and so are eclipsed periodically. Some of these are X ray pulsars which have companions whose gases flow onto the pulsar. Apr 23 10:58 AM Apr 23 10:45 AM 4

5 Due to the intense gravitational field, these gases hit the pulsar at very high speeds (lots of energy) and so cause the pulsar to emit x ray beams. The upper limit for a neutron star is about 30 solar masses. Apr 23 10:47 AM 5

6 Attachments Sounds of Pulsars APOD Planetary Nebulae