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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
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
• 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
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