M1 - The Crab Nebula
In 1054, Chinese astronomers took notice of a “guest star” that was, for nearly a month, visible in the daytime sky. The “guest star” they observed was actually a supernova explosion, which gave rise to the Crab Nebula, a six-light-year-wide remnant of the violent event.Messier 1 is the only supernova remnant listed in Messier’s
catalogue and the most famous object of its kind in the night sky. The nebula
has a total luminosity 75,000 times that of the Sun and lies at a distance of
6,500 light years from Earth.
A rapidly spinning neutron star (the ultra-dense core of the
exploded star) is embedded in the center of the Crab Nebula. Electrons whirling
at nearly the speed of light around the star’s magnetic field lines produce the
eerie blue light in the interior of the nebula. The neutron star, like a
lighthouse, ejects twin beams of radiation that make it appear to pulse 30
times per second as it rotates.
The explosion of the progenitor star produced a large shell
of filaments that has continued to expand ever since and will eventually
disperse and disappear into the surrounding space.
The filaments of the Crab Nebula are what is left of the
progenitor star’s atmosphere and they mainly consist of ionized hydrogen and
helium, along with other elements including oxygen, carbon, iron, nitrogen,
sulfur and neon. The temperatures of the filaments are typically in the range
from 11,000 to 18,000 K.
The Crab Nebula was discovered by the English astronomer
John Bevis in 1731. Bevis added the object to his sky atlas Uranographia
Britannica, which was completed in 1750 but never published.
Charles Messier discovered the nebula independently on
August 28, 1758 while looking for a bright comet and entered it as the first
object in his catalogue on September 12.
His entry read, “Nebula above the southern horn of Taurus,
it doesn’t contain any star; it is a whitish light, elongated in the shape of a
flame of a candle, discovered while observing the comet of 1758.”
Messier at first believed that the Crab Nebula was Halley’s
Comet, which was predicted to return that year, but then noticed that the
object was not moving.
The discovery gave him the idea to compile a catalogue of
objects that observers could easily confuse with comets because of their cloudy
appearance.
It was the French astronomer and mathematician Alexis
Clairaut who had predicted the return of Halley’s Comet in late 1758, saying it
would likely appear in the constellation of Taurus, which was why Messier was
searching for it in this area of the sky.
In the first publication of his catalogue, Messier credited
himself for the discovery. John Bevis was eventually acknowledged years later,
after he had sent a letter to Messier in June 1771.
M1 was named the Crab Nebula after William Parsons, the 3rd
Earl of Ross made a drawing of the object in 1844. Messier 1 lies near the
southern horn of the celestial Bull. It is located 1 degree northwest of the
bright star Zeta Tauri. The star can easily be found by first locating
Aldebaran, the brightest star in Taurus, and then following the line of the
V-shape that Aldebaran is part of, to Zeta Tauri. Aldebaran can be located by
following the line formed by the three stars of Orion’s Belt. It is the first
bright star that appears on that imaginary line.
Zeta Tauri forms a square with three much fainter stars. The
Crab Nebula is located in the vicinity of the square and it appears as a faint
patch of light in binoculars. The Crab Pulsar is 16th magnitude and can only be
seen in larger telescopes (20 inches or so) in very good viewing conditions,
with clear skies and no light pollution.
The Crab Nebula’s filaments and structure may become
apparent in 16-inch
telescopes under good conditions, while smaller telescopes,
starting with 4-inch aperture, only reveal some detail in the shape of the
remnant. In smaller instruments, M1 looks like a comet without a tail.
The Crab Nebula Pulsar was one of the first pulsars to be
discovered and it provided evidence for the theory that pulsars were formed by
supernova events.
The progenitor star of was identified in 1942 by the
German-American astronomer Rudolf Minkowski, who discovered that it had a very
unusual optical spectrum.
In 1967, the region around the star was identified as one of
the brightest gamma-ray sources in the night sky.
The mass of the neutron star is believed to be in the range
from 1.4 to 2 solar masses.
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