by Dr. Whitney Shane, MIRA’s Charles Hitchcock Adams Fellow |
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Stars The Crab Nebula, which combines a whole zoo of astrophysical phenomena in a single object, is conveniently located for observation in the evening winter sky. It is appropriate, if quite accidental, that it is number 1 on Messier’s list. We have already mentioned it as an example of a supernova (Fall ’97) and a radio source (Fall ’99). We shall now look at some more properties of this fascinating object. The outer part of the nebula (the legs of the crab) is composed largely of irregular filaments which shine mainly in the light of hot hydrogen atoms. They are moving outward from the center of the nebula as a result of the explosion. As in most supernovae, they are probably heated mainly by collisions with the surrounding interstellar gas, but radiation from the hot inner parts of the nebula may also contribute. Among the measurable properties of these filaments are (1) their true velocities along the line of sight, (2) their apparent velocities perpendicular to the line of sight (the expansion velocity of the nebula), and (3) the apparent size of the nebula. Combining 1 and 2, and assuming that the true and apparent velocities are the same, gives us an estimate of the distance, which is 5000 to 6000 light years. Tricks like this are often used to determine astronomical distances. Combining 2 and 3 and extrapolating back to the origin, making some assumption about slowing due to the interstellar medium, we can estimate the age. In this way we can identify the Crab Nebula with a supernova observed by the Chinese in 1054. It is odd that there is no record of this very prominent event having been observed in Europe. In the inner part we see a smooth structure which shows no spectral lines, indicating that it is at any rate not hot gas. It radiates at all wavelengths from X-rays to radio, and the radiation is polarized. This can be explained as the result of electrons moving with almost the speed of light and being deflected by a magnetic field. We call this synchrotron radiation because it was first recognized in particle accelerators of this type. The electrons must get their energy from a central engine, which we can also observe. This central engine is a pulsar, and that we can observe it at all is a very convenient coincidence. Pulsars radiate only perpendicular to their rotation axes, so the observer must be very close to the equatorial plane in order to see anything. Apparently the sun is located in just the right place. The pulsar (which does not pulsate) is probably a neutron star rotating about 30 times per second. It is very hot and expels particles (mostly electrons and protons) from its atmosphere in the direction of the magnetic field. These are accelerated by the rotation. As they near the speed of light they radiate the pulses that we observe (also a form of synchrotron radiation). As they move out further they feed the smooth structure described above. Thus we see that the whole nebula is powered largely by rotation of the neutron star in its center. This loss of rotation energy causes the pulse frequency to decrease, an effect which we can measure in most pulsars. Planets Venus will be visible in the morning sky in the south-east at the beginning of the quarter, but it will soon vanish into the twilight and not be seen again until September. Mars will be in the southwestern sky during the evening hours. As the quarter progresses it will sink further in the west and become more difficult to observe. Jupiter is also moving into the evening sky where it will be well placed for observation at the beginning of the quarter but less so later on as it moves further west. Saturn remains north and east of Jupiter, and thus better placed for
observation, as they gradually move closer together during the quarter. Saturn is
stationary on January 13. Eclipses |
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 Go back to Winter 1999 Newsletter index | MIRA home mira@mira.org © 1999 MIRA Last updated 2/13/01 et |
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