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WHAT ARE PULSARS? a tutorial in stellar astronomy

(This tutorial contains Java animations).

Glossary: Electromagnetic Spectrum, Main Sequence, Neutron Star, Pulsar, Supernova

Pulsars (PULSating stARS) are among the most exotic objects found in the galaxy. They are the peculiar relics of massive stars that have ended their lives in a tremendously powerful explosion called a supernova. A pulsar appears to flash on and off many times a second. The diagram at left simulates an "on" pulse. That's a curious thing - how can a star turn itself on and off?

During their lifetimes, all stars continuously perform an energy balancing act. The heat and energy generated in a star's core want to make it expand, while the star's gravity wants to make it contract. The perfect balance between the two can keep a star shining stably for billions of years. A star in this phase of life is said to be "on the main sequence."

Eventually, however, a star runs out of fuel in its center. When this happens, there is no longer a generation of heat and energy in the interior, and nothing is present to counteract the self-gravitation of the star. For very massive stars (more than 10 times bigger than our sun) the sudden, catastrophic, gravitational collapse of the star results in the supernova explosion. The Crab Nebula (at right), is the remnant of a supernova which exploded in the year 1054. Click on the picture for a Hubble Space Telescope view of the Crab (160KB).

After a supernova explosion, all that's left of the original star is the core - called a neutron star. Neutron stars are very small by astronomical standards. Our own Sun's radius is 100 times bigger than the radius of the Earth. However, the typical radius of a neutron star is thought to be only about 10 kilometers (6.25 miles). At the same time, a neutron star contains up to 1.5 times as much matter as the Sun, making the density of these objects tremendous. A teaspoon of neutron star material weighs about a billion (1,000,000,000) tons. This much matter in such a small space creates an enormous gravitational field, so powerful, in fact, that it can bend light!

Neutron stars also have very large magnetic fields. The magnetic field on Earth, which makes compasses point north, is a trillion (1,000,000,000,000) times weaker than the typical neutron star magnetic field. The magnetic field is so strong that it causes most of the light and radiation that the neutron star emits to be concentrated into cones of emission, like beams from a lighthouse. In fact, the key to a pulsar is the combination of the extraordinary magnetic field and the rotation of a neutron star. If the neutron star is spinning, like the Earth rotates on its axis, and if the Earth happens to lie in the path of the beams, we see a pulse of light each time a beam sweeps across the earth. The center of the Crab Nebula (above, right) contains a pulsar which rotates an amazing 33 times per second!


The following animation shows the effect of a neutron star like the Crab pulsar. In this animation, the Earth lies directly in the path of the beam. Try speeding up and slowing the animation to get a feel for the effect of the pulsar. If your browser does not support Java, look at this sequence of images instead.

Web Links NASA Science Headlines - space science research on the web The strongest magnets in the galaxy? - are magnetars, a very weird kind of pulsar Burst and Transient Source Experiment - research information from NASA's all-sky gamma-ray burst instrument Compton Gamma Ray Observatory - mother satellite for NASA gamma-ray astronomy Hubble Space Telescope web site with pix, news, and more.

The incredibly strong gravitational and magnetic fields of a pulsar make it an excellent laboratory for the study of physical processes in extreme conditions. A pulsar may be seen in gamma rays, X-rays, visible light, radio waves or other bands of radiation. There are many unanswered questions about exactly how different pulsars produce the radiation that we see. At the Marshall Space Flight Center, astronomers are attempting to answer some of these questions. One of the instruments used is the Burst and Transient Source Experiment, an instrument on board the Compton Gamma Ray Observatory, a NASA satellite.