Details: The Crab Nebula (also called
Messier
1, M1, or
NGC 1952) lies in the direction of the constellation Taurus
and was one of the first pulsars ever identified. It is also the
first object ever associated with an historical supernova. The
Earl of Rosse William Parsons drew the nebula in 1842 using a 36
inch telescope; the drawing resembled a crab, giving it its
name. While it was discovered by English astronomer John Bevis
in 1731, it wasn’t until the early 20th century that astronomers
discovered that it seemed to be expanding which you can see
below. That indicated a relatively recent supernova, and indeed
it was, because, on July 4th, in the year 1054, Chinese
astronomers recorded a new star in the same part of the sky as
the Crab Nebula. That new star was bright enough to be seen
during the day. A sudden new “star” recorded in ancient history,
usually meant either a comet or a
nova. In fact,
the word “nova” means new star. A nova happens whenever an old
star suddenly brightens as it throws off material into space,
often a result of collecting this material over time from a
nearby younger star. A nova does not usually destroy the star.
A supernova, however, occurs when a star reaches the end of
its life and explodes in one of the most violent events in the
universe. A supernova’s original star (called its progenitor) is
then greatly changed or even destroyed in the process. Most
supernova become a
neutron
star or if massive enough, a black hole. A neutron star is a
gigantic atomic nucleus, tens of kilometers across. It is held
together by gravity since it contains only neutrons, many formed
from protons and electrons combining during the supernova in
such extreme pressures of the explosion. Neutron stars are
therefore so dense that a teaspoon of their material weighs
several billion tons.
The Crab Nebula is very close to
the
ecliptic
(the plane in which the earth, moon, and all planets orbit our
sun). Consequently, many solar system objects come visually
close to this nebula during their orbits as seen from earth. On
January 4, 2003, Saturn passed in front of the Crab. In the
image above, another solar system object is visible*; it even
came very close to transiting M1, a day or so after this image.
The object identifies as
Asteroid
3409 Abramov, a stony
Koronian asteroid about 11 kilometers in diameter. Its
orbits between Mars and Jupiter in the outer asteroid belt once
every 5 years. Ironically, the asteroid is not much smaller than
the Crab Pulsar. However, at an average of 400 million
kilometers away from us, 3409 Abramov is quite close compared to
the Crab Nebual at 6500 light years away or 150 million times
further away. Click on the above image, zoom into the asteroid
line caused by it moving during many exposures. Note the gaps in
the line caused by both processing and a delay of about 1 minute
between each exposure.
It is rare and exciting to see any changes, even movements in
astronomical objects outside of our solar system. Most objects,
even in our own galaxy, are so far away, they move or change too
slowly to see any difference during our entire lives. However,
incredibly, the above time lapse video is two images taken
exactly 13 years apart - the first in October 2008 and the
second in October 2021, a long time lapse indeed! Several things
are noteworthy in the video. First, the gas and dust in the
explosion can be seen expanding considerably*, in fact, the
shell is moving outwards at nearly 1000 miles per second or 0.5
percent the speed of light. The nebula has, therefore, expanded
nearly 1 trillion miles in the 13 years between images.
Reversing this rate gives a nice calculation of the explosion
origin at about 1000 years ago as seen by the ancient Chinese.
Second, note the blue-white cloudy material inside and
throughout the nebula moving during its expansion. Third, note
the movement of the yellow-orange star in the lower left corner
of the image. The star is Tycho 1309-1640-1 and exhibits what is
called proper motion since it is relatively nearby to us
compared to all the other stars in this image. Proper motion is
the movement of stars, each in their own path around our galaxy.
Every star has its own proper motion; we can see this ones
clearly over 13 years due to both its and our sun’s movement.
Looking forward to the next timelapse image in a decade or so!
* Hover mouse over an image to animate or see annotations.
When stars collapse to form neutron stars they often spin faster
and faster, like a figure skater pulling their arms in during a
spinning performance. This collapse and fast spin may form a
pulsar in
which charged particles, accelerated to relativistic speeds,
cause a kind of wind and a standing shock wave. The Crab Nebula
contains such a pulsar as indicated in the above and below
images. It was originally a normal star with a size of over a
million kilometers in diameter, however, now, due to its
catastrophic supernova, what remains of the star is only 28-30
kilometers in diameter, yet spins at over 30 times each second.
In the image below you can more clearly see this pulsar and the
surrounding shock waves caused by the pulsar’s wind. The shock
waves appear elongated, but are actually more circular since
the pulsar spin is at an angle to us. This pulsar lights up the
crab nebula with a ghostly bluish-white color, and the red
filaments are the remnants of the original star’s atmosphere
containing ionized helium and hydrogen along with some heavier
elements. Also visible below is a mysterious jet of material
creating a tail of sorts. What causes the jet is still a mystery
as it is not related to the pulsar spin or shock waves.
Finally, the below image is a zoomed out version of the above to show more of the surrounding stars. It also shows
the expanding materail by hovingering a mouse pointer over the image.
