Crab Nebula
In the year 1054 A.D., Chinese astronomers
were startled by the appearance of a new star,
so bright that it was visible in broad daylight
for several weeks. Today, the Crab Nebula is
visible at the site of this violent stellar
explosion. In this view, NASA's Hubble Space
Telescope has zoomed in on a portion of the
Crab to reveal its detailed structure.
Located about 2 kpc (6,500 ly) from Earth in
the direction of the constellation Taurus, the
Crab Nebula is the remnant of a star that began
its life with about 8-10 times the mass of our
Sun. Such a massive star consumes its nuclear
fuel so rapidly that it lives only about 50
million years before exploding as a supernova.
For this star, the end came on July 4, 1054.
The explosion was witnessed as a naked-eye "Guest
Star" by Chinese astronomers, and is also depicted
in rock paintings of native Americans in the
southwestern United States.
This image was obtained by Hubble's Wide Field
and Planetary Camera 2. Images taken with five
different color filters, totaling over 10 hours
of exposure time, have been combined to construct
this false-color picture. Resembling an abstract
painting, the image shows ragged gaseous shreds
of the original star that are expanding away
from the explosion site at over 1,500 km/s (3.4
million mph). The colorful network of filaments
is the material from the outer layers of the
star that was expelled during the explosion.
The core of the star has survived the explosion
as a "pulsar," visible in the Hubble image as
the lower right of the two moderately bright
stars near the center. The pulsar has about
1.4 times the mass of the Sun, crammed by gravity
into an object only about 10 miles in diameter.
This incredible object, a "neutron star," is
even more remarkable because it spins on its
axis 30 thirty times a second. The spinning
pulsar heats its surroundings, creating the
ghostly diffuse bluish-green synchrotron cloud
in its vicinity, including a blue arc toward
the upper right of the neutron star.
The picture is somewhat deceptive in that the
filaments appear to be close to the pulsar.
In reality, the yellowish green filaments toward
the right side of the image are closer to us,
and approaching at some 350 - 800 km/s. The
orange and pink filaments toward the top of
the picture, including the "backwards question
mark", is material behind the pulsar, rushing
away from us at 200 - 1000 km/s.
The various colors in the picture arise from
different chemical elements in the expanding
gas, including hydrogen (orange), nitrogen (red),
sulfur (pink), and oxygen (greenish-blue). The
shades of color represent variations in the
temperature and density of the gas, as well
as changes in the elemental composition.
These chemical elements, some of them newly
created during the evolution and explosion of
the star and now blasted back into space, will
eventually be incorporated into new stars and
planets. Astronomers believe that the chemical
elements in the Earth and even in our own bodies,
such as carbon, oxygen, and iron, were made
in other exploding stars billions of years ago.
