Astronomy
Black Holes
Weblecture
Black Holes
Pulsars
The discovery of the neutron in 1932 led physicists to the realization that under certain circumstances, a proton and electron will merge. Walter Baade and Fritz Zwicky predicted that under the conditions of pressure and temperature in the core of massive stars, electron degeneracy would fail. Electrons and protons merge until only densely-packed neutrons are left. Such a core would not support thermonuclear fusion, but would still be very hot.
The discovery of pulsars in 1967 by Jocelyn Bell Burnell, who was working as a graduate student under Antony Hewish at New Hall College, Cambridge. Initially, her results were considered an instrument anomaly, but eventually she was able to convince others that here data was correct: a single celestial object was generating pulses 1.33 apart, and it could not be man-made.
A second pulsar discovery led them to tie their observations to a model proposed only a year earlier by Franco Pacini that a rotating neutron star with a magnetic field would generate radiation that could be focused by the magnetic field into beams. For several years, astronomers were puzzled by observations of pulsation from some neutron stars but not others, and eventually realized that the direction of rotation and the formation of a dust cloud "torus" around many neutron stars could alter observations: many characteristics observed depend on the angle between the magnetic poles of the star and the line of sight to the observer.
Nevertheless, there are multiple types of neutron stars, as shown in this diagram:
Jet Propulsion Laboratory
The beams are constantly ejected into space along the magnetic axis, but the star itself may around a different axis. The beams therefore act like a lighthouse, sweeping around space and intersecting with some observers at short regular intervals. The rapid rotation is explained by the conservation of angular momentum as the stellar mass contracted after the collapse of electron degenerate pressure from the core. Some observers, of course, never see the beam at all, if they are not in the line-of-sight to intersect with a beam-sweep.
High gravity white dwarf and neutron stars also attract surrounding gases, which collapse onto the stellar surface, heat up from the internal pressure of the star, and eventually ignite in a brief hydrogen thermonuclear reaction, creating nova and bursters that can repeat the cycle as long as the local gas clouds remain dense enough. Pulsars are often connected with nebulae from the original stellar collapse event, as well as subsequent burster nebulae ejections.
Black Holes: The End of Massive Stars
Study the diagram of the stages through which sub-solar mass, solar-mass, and high mass stars pass in this diagram. The diagram compares the evolutionary paths of these stellar types, plus very high mass stars whose end is a black hole. You may need to greatly enlarge the diagram in order to study different parts.
Escape Velocity
We have discussed before the concept of escape velocity: the velocity an object must have on leaving the surface of a planet or other massive body in order to escape the body's gravitational field. The potential energy due to position in a gravitational field for a spherical mass such as a planet is If all of this potential energy is converted to kinetic energy, then the object can escape the gravitational field: By solving this equation for velocity, we can determine the "escape velocity required:
Note that the escape velocity depends entirely on the mass of the attracting object, not on the mass of the escaping object! The escaping object could even be massless, such as a photon of light. But light has a finite speed so if it starts at a distance R from an object which is massive enough that the escape velocity is greater than the speed of light (vesc > c), it will be bent back by the object's gravitational field. The object becomes "a black hole", something which can absorb any radiation or matter falling upon it, but from which nothing, not even light, can escape.
While such objects were predicted by Newton's theory of universal gravity in the 17th century, the full implications of such an object were not realized until Einstein explained his theory of relativity in the early 20th century.
Relativity
Einstein's theory of relativity depends on the idea that all measurements are made relative to an observer, who may be moving or accelerating through space, if observed from another viewpoint. As long as two observers move at constant velocities (the velocities do not have to be the same, but there can be no acceleration or deceleration), their observations will be equivalent. We can use the velocity of one observer to convert the measurements made by the second observer into the first observer's frame of reference. However, both observers will measure the speed of light as the same constant velocity (2.9998 * 108 m/s), regardless of their relative velocities. In order for this constant to hold, their perceptions of space and time must be different. A "stationary" observer watching a spaceship move by at a significant fraction of the speed of light will measure the object as shorter than a passenger on the spaceship, and will experience the passage of time as longer than the passenger on the spaceship. Length contraction and time dilation have implications for mass and energy as well: mass increases as speed increases.Optional Readings
- For further information, explore the Hubble Telescope site on Black Holes: Gravity's Relentless Pull for both its Physics and Astronomy topics.
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