Black Holes -- from Eric Weisstein's World of Physics Black Holes & Neutron Stars - Neutron stars are very dense and spin very fast and are typically only 10-15 km in radius. Because neutron stars form from burnt-out stars, they do not glow. The collapse of the star causes the matter to be converted into mostly neutrons, hence the name neutron star. Some neutron stars emit radio waves that pulse on and off. These stars are called pulsars. Pulsars don't really turn radio waves on and off--it just appears that way to observers on Earth because the star is spinning. What happens is that the radio waves only escape from the North and South magnetic poles of the neutron star. If the spin axis is tilted with respect to the magnetic poles, the escaping radio waves sweep around like the light beam from a lighthouse. Far away on Earth, radio astronomers pick up the radio waves only when the beam sweeps across the Earth.   Time-Lapse Movies of Crab Nebula   Sounds of Crab Nebula Many people think black holes continually suck in everything like great big cosmic bathtub drains. And what the heck are neutron stars? Understanding the nature of black holes and neutron stars--how they form, what they're like, and how we know they are there--can lead to a better understanding of how our Universe works. The Life and Death of Stars - Main sequence stars are stars, like our Sun, that fuse hydrogen atoms together to make helium atoms in their cores. For a given chemical composition and stellar age, a stars' luminosity, the total energy radiated by the star per unit time, depends only on its mass. Stars that are ten times more massive than the Sun are over a thousand times more luminous than the Sun. However, we should not be too embarrassed by the Sun's low luminosity: it is ten times brighter than a star half its mass. The more massive a main sequence star, the brighter and bluer it is. For example, Sirius, the dog star, located to the lower left of the constellation Orion, is more massive than the Sun, and is noticeably bluer. On the other hand, Proxima Centauri, our nearest neighbor, is less massive than the Sun, and is thus redder and less luminous. Lives and Deaths of Stars - Stars live for a very long time compared to human lifetimes. Your great, great grandparents saw the same stars as you will see tonight (if it's clear). Our lifetimes are measured in years. Star lifetimes are measured in millions of years. Even though star timescales are enormous, it is possible to know how stars are born, live, and die. Neutron Stars - For those with serious interest in neutron stars and other compact objects, an excellent reference is "Black Holes, White Dwarfs, and Neutron Stars", by Stuart Shapiro and Saul Teukolsky (1983, John Wiley and Sons). Neutron Stars - If the core mass is between 1.4 and 3 solar masses, the compression from the star's gravity will be so great the protons fuse with the electrons to form neutrons. The core becomes a super-dense ball of neutrons. Only the rare, massive stars will form these remnants in a supernova explosion. Neutrons can be packed much closer together than electrons so even though a neutron star is more massive than a white dwarf, it is only about the size of a city. The neutrons are degenerate and their pressure (called neutron degeneracy pressure) prevents further collapse.   Time-Lapse Movies of Crab Nebula   Sounds of Crab Nebula Black Holes - If the core remnant has a mass greater than 3 solar masses, then not even the super-compressed degenerate neutrons can hold the core up against its own gravity. Gravity finally wins and compresses everything to a mathematical point at the center. The point mass is a black hole. Only the most massive, very rare stars (greater than 10 solar masses) will form a black hole when they die. As the core implodes it briefly makes a neutron star for just long enough to produce the supernova explosion. Fix: Chapter 19 The Evolution of Stars Fix: Chapter 20 White Dwarfs, Neutron Stars, and Black Holes Recent Supernovae Latest Supernovae Gamma-Ray Bursts: Introduction to a Mystery     © Copyright 2008 - Samuel J. Wormley   by swormley1@mchsi.com