Stellar RemnantsWhite Dwarfs, Neutron Stars & Black Holes • These objects normally emit light only due to their very high temperatures. • Normally nuclear fusion has completely stopped. • These are very small, dense objects. • They exist in states of matter not seen anywhere on Earth. They do not behave like normal solids, liquids or gases. • They often have very strong magnetic fields and very rapid spin rates. Sirius & Sirius B - a White Dwarf Star
White Dwarfs • composed mainly of Carbon & Oxygen • formed from stars that are no more than 8 Solar masses • White Dwarfs can be no more than 1.4 Solar masses and have diameters about the size of the Earth (1/100 the diameter of the Sun). • If a White Dwarf is in a binary system and close enough to its companion star it may draw material off this star. This material can then build up on the surface of the White Dwarf. A White Dwarf pulling material off of another star in a binary system
White Dwarfs in Binary Systems • This material pulled off the companion star is mostly Hydrogen. • As it accumulates on the star it may become hot enough for nuclear fusion to occur. • The Hydrogen begins to fuse and the White Dwarf emits a bright burst of light briefly. • We see this on Earth as a nova. • This process can repeat as new material accumulates.
Another Kind of Supernova • If too much material accumulates the White Dwarf may collapse. • Rapid fusion reactions of Carbon & Oxygen begin. Carbon & Oxygen fuse into Silicon and Silicon into Nickel. • The energy from this event may cause the entire White Dwarf to explode leaving nothing behind. • This is called a supernova but it is a different process from that which occurs for massive stars.
Stellar Remnants and the Chandrasekhar Limit • Stellar remnants greater than 1.4 Solar masses cannot form White Dwarfs. • Objects this massive cannot support their own weight but collapse to form either Neutron Stars or Black Holes. • This maximum mass is called the Chandrasekhar Limit.
Neutron Stars • Except for a thin crust of iron atoms a neutron star is composed entirely of neutrons. • The gravitational forces inside a neutron star are too strong for atoms to exist. • Instead electrons get crushed into the protons in the atomic nucleus forming neutrons. • Neutron stars have very intense magnetic fields and very rapid rotation. Neutron Stars weigh more than the Sun and are as large a city.
Pulsars • Neutron stars can sometimes be directly observed. • Astronomers have discovered rapidly pulsating stars emitting strong, very regularly timed bursts of radio waves. • These types of neutron stars are called pulsars. • Pulsar bursts are as regular as some of the best clocks on Earth. As the beam from a pulsar sweeps past Earth we see a brief pulse.
The Discovery of Pulsars • In 1967 in Cambridge England, Jocelyn Bell, a graduate student in astronomy, discovered very regularly spaced bursts of radio noise in data from the radio telescope at Cambridge University. • After eliminating any possible man-made sources she realized this emission must be coming from space. • The regularity of these pulses at first made her and her co-workers think they had discovered alien life. • Later they realized these must be due to rapidly spinning neutron stars. Jocelyn Bell Burnell in front of the radio telescope used to discover pulsars.
Black Holes • For Main Sequence stars of mass greater than about 10 Solar masses the remnant of the star left behind after a supernova explosion is too large (about 3 Solar masses) to be a white dwarf or even a neutron star. • These remnants collapse to form Black Holes. • No light can escape from a Black Hole which is why it’s black. • We can only “see” Black Holes due to their effects on other objects.
Escape Velocity & Curved Space • There is a minimum velocity that an object needs to escape the gravitational pull of any asteroid, planet, star, etc. • This is the escape velocity and depends on the mass and radius of the object • For the Earth the escape velocity is about 11 km/sec. • Since a Black Hole has so much mass in so small a space its escape velocity is the speed of light 300,000 km/sec. All objects exert a gravitational pull on all other objects in the Universe. One way to picture gravity’s effect is by imagining space as a rubber sheet. Heavy objects bend this sheet more than light objects. Black Holes are like tears in this sheet.