The Crab Nebula is the remains of a supernova that exploded July 4,
1054 AD. Actually, it was observed by the Chinese as a new, bright
star on that date, but since we now know that it was a supernova
6000 light years away it actually exploded at about 5000 BC and the
from the explosion merely reached us in 1054 AD.
Figure 1. Left part of constellation Taurus, where the
lines signify the ends of the "bull's horns" at the stars Zeta Tau
(lower) and Elnath (upper). The Crab Nebula is located within the
circle just above Zeta Tau (not visible in this image). [Canon 10D, 50
mm EFL, 3x30s; 2005.01.25].
Figure 2. Zoom factor 13. The star Zeta Tau is the bright
star in the lower-left corner, and the Crab Nebula is barely visible as
a "smudge" in the middle. [Canon 10D, 200 mm EFL, f/5.6, 11x30s;
Figure 3. Field-of-view is 70x47 'arc (the full moon would fit neatly inside this FOV). [Celestron 14-inch, HyperStar prime focus, f/1.9, SBIG ST-8XE CCD, CFW8, RGBL, each exposured 12x30sec; 2001.11.05; Hereford, AZ]
Figure 4. FOV = 11 x 15 'arc. Unsaturated star widths (FWHM) = 3.5 "arc. Faintest stars have V-mag = 21.2. Meade 14-inch LX200GPS. [LRGB: L=R+V, R = 35 min, V = 27 min, B = 47 min; 2008.02.08 UT; Hereford Arizona Observatory].
A supernova differs from a nova in a significant way. A supernova explodes when it becomes unstable after consuming hydrogen at the star's center. An implosion is followed by an explosion, blasing most of the star's material outward. With most of the star's mass lost to space, the remaining core collapses further to a neutron star, spinning fast due to the "ice skater" effect (conserving angular momentum during the collapse). At the center of the Crab Nebula is neutron star rotating 30 times per second, referred to as a "pulsar." The escaping, outward-moving material is luminous due to its encounter with interstellar material. A supernova is a one-time event for the star that explodes, and its inherent brightness is immense.
A nova, on the other hand, is one star of a binary pair that has stellar material "dumped" on it by its companion, and the infalling material briefly raises the temperature of the surface and creates an outpouring of light energy which gradually subsides. The same nova may undergo repeated outbursts, as it is never destroyed by any of its brightenings. A nova is a repeating event for the affected star, and its brightness for each event is inherently much less than for that of a supernova.
This site opened: November 15, 2001. Last Update: February 8, 2008