Astronomy-Ch 18

What is the white dwarf limit? o Why does this limit exist fundamentally?

1.44Msun. Theoretical calculations show that electron speeds would reach the speed of light in a white dwarf with a mass of about 1.4 times the mass of the SunBecause neither electrons nor anything else can travel faster than the speed of light, no white dwarf can have a mass greater than

What are gamma-ray bursts? Where do they come from?

gamma ray bursts:extremely energetic explosions that have been observed in distant galaxies. They are the brightest electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several hours. Observations show that at least some gamma-ray bursts are produced by supernova explosions. Others may come from collisions between neutron stars.

In this chapter, multiple stellar corpses are discussed. Now why they are 'dead' and the rough sizes and masses that lead to these bizarre objects. o Review Cosmic Context Part V Balancing Pressure and Gravity

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Which type of stars go through this phase?

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What is a neutron star? What prevents a neutron star from collapsing?

the ball of neutrons created by the collapse of the iron core in a massive star supernova. Typically just 10 kilometers in radius yet more massive than the Sun, neutron stars are essentially giant atomic nuclei made almost entirely of neutrons and held together by gravity.neutrons rather than electrons that are closely packed, so we say that neutron degeneracy pressure supports them against the crush of gravity.

What is a white dwarf? What prevents a white dwarf from collapsing?

White dwarfs are the remaining cores of dead stars.A typical white dwarf has the mass of the Sun compressed into a volume the size of Earth. Electron degeneracy pressure supports them against the crush of gravity.

What is a pulsar? What distinguishes a pulsar from a neutron star?

a neutron star that beams radiation along a magnetic axis that is not aligned with the rotation axis. They are formed in the exact same way as a neutron star, except they keep some of their angular momentum, but as the radius is much smaller than the star, its rotational speed is increased. The pulsar has two axes, a rotational axis and a magnetic axis. The magnetic axis is where the beams are emitted from, while the pulsar rotates around the rotational axis.(?) Eventually, a pulsar's spin slows so much and its magnetic field becomes so weak that we can no longer detect it. In addition, some spinning neutron stars may be oriented so that their beams do not sweep past our location. We therefore have the following rule: All pulsars are neutron stars, but not all neutron stars are pulsars.(?)

What are gravitational waves, and why are they important?

disturbances in the curvature of spacetime, generated by accelerated masses, that propagate as waves outward from their source at the speed of light. Ripples' in space-time caused by some of the most violent and energetic processes in the Universe. Just like two closely orbiting white dwarfs, two neutron stars orbiting close together will emit gravitational waves, ultimately causing the two neutron stars to spiral together and merge (see Figure S3.22). The total energy released during this cataclysmic merger can be even greater than that of a massive star supernova, and it happens in an environment rich in neutrons. As a result, neutron-star mergers can produce a blend of rare elements that differ from those made in a massive star supernova. Most of the neutron-star matter remains gravitationally bound to the system, but some of it is ejected into interstellar space and then reincorporated into new star systems. Models of these mergers indicate that they produce most of the gold, platinum, and rare-earth elements present in the universe

What is a white dwarf supernova? How does this compare and contrast to a core collapse supernova?

Carbon fusion suddenly begins as a white dwarf in close binary system reaches white dwarf limit, causing total explosion.The carbon detonation that creates a white dwarf supernova is quite different from the iron catastrophe that leads to a supernova at the end of the life of a high-mass star.Astronomers can distinguish between the two types of supernova by studying their light. Both types shine brilliantly, with peak luminosities about 10 billion times that of the Sun but the luminosities of white dwarf supernovae fade quickly during the first few weeks and then decline more gradually, while the decline in brightness of a massive star supernova is often more complicated (Figure 18.5). In addition, spectra of white dwarf supernovae lack hydrogen lines (because white dwarfs contain very little hydrogen), while these lines are prominent in the spectra of most massive star supernovae.

What would happen if you jumped into a black hole?

Down he falls, clock in hand. He watches the clock, but because he and the clock are traveling together, its time seems to run normally and its numerals stay blue. From his point of view, time seems to neither speed up nor slow down. When his clock reads, say, 00:30, he and the clock pass through the event horizon. There is no barrier, no wall, no hard surface. The event horizon is a mathematical boundary, not a physical one. From his point of view, the clock keeps ticking. He is inside the event horizon, the first human being ever to vanish into a black hole. Back on the spaceship, you watch in horror as your overly curious friend plunges to his death. Yet, from your point of view, he will never cross the event horizon. You'll see time coming to a stop for him and his clock just as he vanishes from view because of the huge gravitational redshift of light. When you return home, you can play a video for the judges at your trial, proving that your friend is still outside the black hole. Strange as it may seem, all this is true according to Einstein's theory. From your point of view, your friend takes forever to cross the event horizon (even though he vanishes from view because of his ever-increasing redshift). From his point of view, it is but a moment's plunge before he passes into oblivion.The truly sad part of this story is that your friend did not live to experience the crossing of the event horizon. The force of gravity grew so quickly as he approached the black hole that it pulled much harder on his feet than on his head, simultaneously stretching him lengthwise and squeezing him from side to side (Figure 18.14). In essence, your friend was stretched in the same way the oceans are stretched by the tides, except that the tidal force near the black hole is trillions of times stronger than the tidal force of the Moon on Earth [Section 4.5]. No human could survive it. the larger size of a supermassive black hole makes its tidal forces much weaker and hence nonlethal. Your friend could safely plunge through the event horizon. Unfortunately, anything he saw or learned on his continuing plunge toward oblivion would be known to him alone, because there would be no way for him to send information back to you on the outside.

What is a black hole? Know your black hole terminology and properties. Why are these objects not on the HR diagram?

object whose gravity is so powerful that not even light can escape it.Beyond the neutron star limit, no known force can resist the crush of gravity. According to the General Theory of Relativity, gravity crushes all the matter into a single point known as a singularity. The boundary between the inside of a black hole and the universe outside is called the event horizon The "surface" of a black hole is the radius at which the escape velocity equals the speed of light. This spherical surface is known as the event horizon. The radius of the event horizon is known as the Schwarzschild radius. mass,electric charge,angular momentum Theory tells us that mass is one of only three basic properties of a black hole. The second property is electric charge, but this is relatively unimportant: If a black hole had any positive or negative charge, it would quickly attract oppositely charged particles from its surroundings, making it electrically neutral. The third property of a black hole is its angular momentum. Conservation of angular momentum dictates that black holes should rotate rapidly when they form in the collapse of a rotating star. Much like an ice-skater pulling in his arms, a collapsing stellar core should rotate faster and faster as it shrinks in size. The fact that black holes emit no light might make it seem as if they should be impossible to detect.

How do neutron stars differ from white dwarfs? Why are these objects not on the HR diagram?

pulsars must be neutron stars because no other massive object could spin so fast. A white dwarf, for example, can spin no faster than about once per second without being torn apart, because a faster spin would mean its surface would be rotating faster than the escape velocity.while novae occur when hydrogen fusion ignites on the surface of a white dwarf in a close binary system, x-ray bursts arise from the ignition of helium fusion on the neutron star in a close binary system.They arent bright enough(not high enough L)

What is the Schwarzchild radius? Know how to use this equation in both formats and the standard units necessary for each version. o See Mathematical Insight 18.1

rad of the event horizon