* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Chapter 2 Cosmic tombstones
Survey
Document related concepts
Magnetohydrodynamics wikipedia , lookup
Nucleosynthesis wikipedia , lookup
White dwarf wikipedia , lookup
Accretion disk wikipedia , lookup
Main sequence wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Kerr metric wikipedia , lookup
Nuclear drip line wikipedia , lookup
Astrophysical X-ray source wikipedia , lookup
Hawking radiation wikipedia , lookup
First observation of gravitational waves wikipedia , lookup
Transcript
Chapter 2 Cosmic tombstones What’s left from the death of a massive star? Stellar graveyard: neutron stars • • • • • • Neutron stars are compact objects supported by pressure of neutrons created in the process of a stellar collapse A typical neutron star has a ~10 km radius, a ~1.4 M mass, it spins several times a second and has a huge, frozen-in magnetic field A neutron star is formed, when a massive stellar core (> 1.4 M ) collapses, because it cannot support itself against its own weight. The continuing collapse is stopped by pressure of “neutron gas” produced from protons and electrons. Collapse leaves a newly formed neutron star very hot (millions of degrees K) During the collapse, the dying star shrinks dramatically. This leads to fast spin and large magnetic field of a leftover neutron star A dice-sized piece of a neutron star would weigh some 100 million tons on Earth, which corresponds to the density of the atom Until 1967 neutron stars were purely theoretical objects The discovery of pulsars • Pulsars were serendipitously discovered by Jocelyn Bell (grad student) and Anthony Hewish (her boss) at Cambridge U. in England • They appeared as faint, rapidly and very regularly (periodically) pulsating radio sources • Although the pulses are almost as regular as the ticking of atomic clocks, they do slow down by a few billionths of a second per day Figuring out the nature of pulsars • • • • Little Green Men? too many of them, all with different periods, pulses not emitted from planets Normal stars or white dwarfs? cannot spin that fast, too big for rapid pulsing Clue #1: very short pulse duration (few milliseconds) pulses must be emitted by an object much smaller than a white dwarf Clue #2 : young pulsars found in supernova remnants direct evidence that they must be leftovers from supernova explosions How do they pulse and shine? • • • • • A rotating pulsar generates two narrow cones of radio emission propagating along the magnetic axis Since the magnetic axis and the rotation axis are not aligned, the pulsar operates like a lighthouse an observer sees periodic pulses of radio emission According to the leading theory, this emission is produced by fast particles propagating along the magnetic field lines Particles are extracted from the neutron star surface by a huge electric field Some supernova remnants are powered by the pulsar spindown energy and continue to glow Black holes: where do they come from? • • For stellar cores more massive than 2-3M, neutron gas cannot stop the core collapse and it continues to form a singularity at which mass density and gravity become infinite. Astronomers call such an object a black hole Black holes can be deduced from a simple argument involving mass and the escape velocity there must be a mass large enough to make escape from its gravity impossible (escape velocity would have to be greater than the speed of light) How exotic are the black holes? • • • • • • Full description of black holes requires Einstein’s General Relativity and its concept of space-time curvature caused by gravity When a collapsing mass becomes small enough, gravity curves space around it to the point that it closes upon itself This forms a boundary of the black hole called the event horizon that has a radius depending on its mass Events that take place inside this radius cannot be seen by an outside observer Any object that has mass can in principle be squeezed enough to become a black hole Our Galaxy has a 3 million solar mass black hole at its center! Black hole and you… • • • Falling into a black hole looks different for an outside observer, who will see an infalling object slowing down and practically coming to a halt, never crossing the event horizon this is due to a clock slowdown in curved space-time Objects in the background of a black hole would be difficult to observe because light loses energy in the gravitational field Tidal force would squeeze you laterally and stretch you longitudinally, you would also become extremely hot and emit x-rays and γ-rays