Focus On The Crab Nebula
For any telescope
In July of 1054 AD a bright
star appeared in the sky where before there had been none. Chinese
Astronomer/Astrologers carefully recorded it's appearance to the northwest
of the 3rd magnitude star known to us as Zeta Tau (marking the eastern
horn of Taurus, the Bull). This star became as bright as magnitude
-6 and according the their records was visible during daylight for 23 days,
and visible at night for almost two years.
The appearance of this
new star was not unique, although only about a dozen or so such events
have been recorded in human history. The ancient Chinese called it
a "Guest Star." Modern astronomers refer to it as a supernova.
A supernova is one of more cataclysmic events in the Universe and marks
the extraordinary death of a very massive star. Such massive stars
live their lives on the edge, fusing their elements at an enormous rate.
In addition to causing the star to shine brightly, the outflow of fusion
energy provides pressure to hold up the outer layers of the star against
gravity; without this flow of energy the gaseous star would begin to collapse
into an ever more dense ball. Compared to the low mass stars which
can fuse their elements for billions of years, the very massive stars live
short, bright lives. In the end they can no longer fuse enough energy
to hold themselves up and the collapse begins. In less than a second
the core of such a star can collapse from the size of the earth to a mere
20km in diameter! As suddenly as it began, the collapse is just as
suddenly halted. The remaining falling matter rebounds, exploding
back outward in a flurry of nuclear reactions, producing for a brief moment
in time as much light as all of the rest of it's parent galaxy! The
expanding material explodes into the interstellar medium as a strong wind,
moving outward at about 22,000 km/hour.
Nearly
700 years after the supernova of 1054, John Bevis of England discovered
a faint nebula at the same location in his small telescope. Later
Charles Messier independently discovered this nebula, at first mistaking
it for a comet (the expected return of comet Halley). It is said
that the discovery of this object resulted in his decision to begin his
own catalog; the nebula appears as his first entry (Messier 1). Of
course, at that time the supernova of 1054 was unknown to both of these
men, largely because no records of the event had survived in Europe.
An 1844 drawing of the
nebula made by Lord Rosse using the 36-inch reflector at Birr Castle indicated
intricate filaments. This drawing eventually led to the name
"Crab Nebula."
In 1921 J.C. Duncan of
Mt. Wilson Observatory compared photographs of the nebula taken 11 eleven
years apart. He found that the nebula was expanding and measured
the rate. He estimated that at the present rate of expansion the
nebula had begun expanding about 900 years earlier. At about the
same time Knut Lundmark made the connection between the position
of the nebula and the supernova of 1054. It was soon apparent that
the two were somehow connected. In the decades that followed
our understanding of stars and their evolution grew by leaps and bounds,
eventually leading to a basic understanding of the supernova phenomenon.
In
1948 the Crab was discovered to be a bright radio source (Taurus A).
It was later discovered to be a bright source of x-rays as well.
In 1968 a quickly pulsating radio source was found near the center of the
Crab. These pulsing radio sources, or Pulsars, had only been recently
discovered and the discovery of the Crab pulsar generated a great deal
of interest. This pulsar was found to pulsate 30 times per
second. Astonishingly, it was recognized that this pulsation was
due to the rotation of the ultra-compact core of the star that had exploded
in 1054. This core was now a neutron star, a star made entirely of
neutrons tightly packed together. Like an ice skater who brings in
her arms, the rotation of the core had been sped up greatly as it collapsed.
The Crab pulsar is also known as the variable star CM Tau, and appears
as a 17th magnitude star near the center of the nebula.
Observing the Crab
The Crab nebula (M1,
NGC 1952) is often one of the first objects observed by newbie Skyhounds,
along with M42, the Ring Nebula, M31, and M13. Some may be disappointed
because it does not show the many delicate and colorful filaments seen
in long exposure photographs. But that may be a good lesson to learn
early, because at the eyepiece few objects show the brilliant color and
detail seen in photographs. Much of the wonder of the night sky zoo
comes from the mind's eye -- from knowing how extraordinary the object
is and that you are seeing it with your own eye. Perhaps Todd Gross
described the Crab best when he wrote "Enjoyable viewing, but won't knock
your socks off."
Walter Scott Houston wrote
that "The Crab Nebula usually shows in small telescopes as a featureless
gray ghost." He goes on to say that his "4-inch Clark refractor has
revealed hints of the nebula's ragged edge that appears so prominently
in photographs" and that "these edge serations are usually apparent in
a 12-inch telescope and easy in a 17-inch."
Observations made with
large telescopes in the 1800's revealed the filaments for the first time,
reportedly becoming visible at magnifications in excess of 500X.
I didn't find that many modern observations of this nebula in larger instruments,
perhaps because it is assumed that the Crab is essentially featureless.
Most observers, including myself, describe the Crab as "mottled"
in large instruments rather than as filamentary. I wonder if we aren't
using enough magnification? I'm looking forward to trying the Crab
again in the 18-inch this month at higher magnification, perhaps with an
OIII filter as well. I'd be greatly interested to hear the results
of others!
The Crab Nebula
| Right Ascension |
Declination |
Magnitude |
Size |
Con |
Distance |
|
05h34m30.0s
|
22o01'00"
|
8.4
|
8'
|
Tau
|
6300 ly
|
Look for the Crab ~1o
to the north east of Zeta Tau. North is up and east is to the left.
Stars are shown to 9th mag.
The View in a 6-inch at
50x. North is down and east is to the right.
Millennium
Star Atlas Vol I Chart 158
Sky
Atlas 2000 Chart 5
Uranometria
2000 Vol I Chart 135
Herald-Bobroff
Astroatlas B-05 C-22 |
|
