In this section, I look at black holes resulting from the collapse of a star; that is, stellar mass black holes. I include information on massive and super massive black holes in the Galaxies section, and primordial black holes in the section on the Big Bang. A black hole is a region of space from which nothing that enters beyond the event horizon can escape; essentially, the escape velocity is greater than the speed of light. John Michell first proposed the idea of a body so massive that even light could not escape in 1783, but this assumed that the mass would reduce the SPEED of light to zero, preventing itsí escape, rather than the fact that lightís ENERGY is reduced to zero.
Black holes have been identified both from the radiation emitted by in falling gas that becomes heated to very high temperatures, and from their affect on the orbits of nearby stars. Observations of galaxies seem to show that most of them contain a massive black hole at their core, the Milky Way included. It was Subrahmanyan Chandrasekhar, in 1930, who calculated that a white dwarf star weighing more than about 1.4 solar masses would collapse, but the astronomer Arthur Eddington insisted that nature would not permit such an occurrence. In fact, a white dwarf between weighing more than approximately 1.4 solar masses will collapse into a neutron star, and one more than approximately three solar masses will collapse into a black hole, although this value is uncertain and depends on a number of assumptions.
So what lies inside a black hole? According to classical physics, it is a singularity of infinite density in an infinitely small volume. In fact, physicists generally agree that General Relativity breaks down at the point where quantum mechanical effects start to assume importance. A solution requires the marriage of Relativity with Quantum Mechanics. String theory makes some predictions that avoid the singularity, but this has yet to be proved.
We have seen how a star runs out of fuel and starts to collapse. Depending on the stars final mass, after having blown off the surface layers in various ways, the result could be a white dwarf, a neutron star, or a black hole. If the remnant is more than about 3 solar masses, neutron degeneracy pressure is unable to prevent further collapse, and it forms a black hole. Over time, the black hole can draw in matter from the gas and dust surrounding it as well as from the surface of nearby stars, even consuming stars entirely and growing larger all the while. Two black holes can coalesce to form one larger black hole. A number of black hole candidates have been identified all of which are part of X-ray binary systems in which the black hole candidate has a visible partner from which it draws matter via an accretion disk. V1487 Aquilae (or GRS 1915+105) is an example which is about 40,000 light-years away, and has a companion about the same mass as the Sun.
Astronomy & Cosmology -
Stars - Life & Death of Stars
NASA first recorded a burst of X-rays from XTE J1550-564, which is about 17,000 light years away, in 1998. Later Chandra observations detected first one jet of high-energy particles moving to the left of the source, then another to the right. After four years, the two jets were over three light years apart with the left jet slowing down and disappearing. The diagram shows how the black hole pulls matter off the surface of a companion star onto its accretion disk, which then produces the jets of particles as matter from the accretion disk falls into the black hole.
Cygnus X-1 is the first identified black hole, and remains the most famous. It is photographed here in X-Rays by the Chandra X-Ray telescope. Recent observations (June 18th 2011) with the Very Long Baseline Array of radio telescopes shows that it is about 6,000 light-years away, and comprises a black hole of about 14.8 solar masses in a close orbit with a blue supergiant (type O) star of about 20 solar masses. Gas flows from the normal star, and some is captured by the black hole into its accretion disk. Material from the accretion disk then spirals into the black hole, releasing gravitational energy in the form of powerful X-Rays.
V1487 Aquilae is an X-ray binary system with a black hole and a normal star in orbit around each other, nearly 36,000 light years away. The black hole, GRS 1915+105, is the heaviest stellar mass black hole discovered so far at between 10 and 18 solar masses. It is a microquasar that appears to be rotating on its axis 1,150 times/second.