176 PART 2^ |^ THE STARS
Our Home Galaxy
From a dark location away from city lights, you can see the
Milky Way stretching across the sky. Th e winter Milky Way is
especially dramatic (■ Figure P-6a). It is actually the disk of the
galaxy that we live in—the Milky Way Galaxy seen from the
inside. Of course, no one has ever journeyed out into space to
look back and take a picture of our galaxy, but astronomers have
evidence that the nearby Andromeda Galaxy, shown in
■ Figure P-6b, looks much like our own. Our galaxy contains
roughly 100 billion stars, and we live about two-thirds of the way
from the center to the edge.cooked up inside the star, are returned to the interstellar P-3
medium.
Th e violence of a supernova explosion can fuse atoms
together to build elements heavier than iron. Gold, platinum,
uranium, and other elements heavier than iron are rare and
valuable because they are made only in the moments of a super-
nova explosion. Th e iodine atoms in your thyroid gland and
the gold atoms in your class ring were made in supernova
explosions.
Th e atoms of which you are made had their birth inside
stars. Th at process is common in the universe because stars are
common. Our galaxy contains billions of them.
The supernova remnant called the
Cygnus Loop is 5000 to 10,000
years old and 80 ly in diameter.Supernova 1006 is 1000
years old and 60 ly in
diameter.Supernova 1006 is 1000
years old and 60 ly in
diameter.X-ray imageX-ray imageX-ray imageX-ray imageX-ray imageX-ray imageCas ACas AVisual-wavelength
imageCassiopeia A (Cas A) is
about 300 years old and
about 10 ly in diameter.Cassiopeia A (Cas A) is
about 300 years old and
about 10 ly in diameter.Cas A was produced by
a type II supernova and
contains a neutron star.Visible light produced
by gas expanding into
surrounding interstellar
mediumSN 1006 was
produced by a
type a supernova.Jets of gas ejected in
opposite directions■ Figure P-5
How do astronomers know that supernova explosions eject gas back
into the interstellar medium? They see the remnants left behind by
these violent events. A supernova remnant is an expanding bubble of
hot gas created by a supernova explosion. As the remnant expands
and pushes into neighboring gas, it can emit radiation at many wave-
lengths. (Cygnus Loop: Mikael Svalgaard; SN1006: NASA/CXC/J. Hughes et al.;
Cas A: NASA/CXC/GSFC/U. Hwang et al.)