March 1, 2001
Photo No: H2001-07
NGC 4013: A Galaxy on the Edge
NASA's Hubble Space Telescope has snapped this remarkable view
of a perfectly "edge-on" galaxy, NGC 4013. This new Hubble
picture reveals with exquisite detail huge clouds of dust and
gas extending along, as well as far above, the galaxy's main disk.
NGC 4013 is a spiral galaxy, similar to our own Milky Way, lying
some 55 million light-years from Earth in the direction of the
constellation Ursa Major. Viewed pole-on, it would look like a
nearly circular pinwheel, but NGC 4013 happens to be seen edge-on
from our vantage point. Even at 55 million light-years, the galaxy
is larger than Hubble's field of view, and the image shows only
a little more than half of the object, albeit with unprecedented
Dark clouds of interstellar dust stand out in the picture because
they absorb the light of background stars. Most of the clouds lie
in the plane of the galaxy, forming the dark band, about 500
light-years thick, that appears to cut the galaxy in two from
upper right to lower left. A similar effect can be seen in our
own sky. If one views the Milky Way by going well away from city
lights, dust clouds in the disk of our own galaxy appear to split
the glowing band of the Milky Way in two.
When light passes through a volume containing small particles (for
example, molecules in the Earth's atmosphere, or interstellar dust
particles in galaxies), it becomes fainter and redder. By studying
the color and the amount of light absorbed by these distant clouds
in NGC 4013, astronomers can estimate the amount of matter in them.
Individual clouds contain as much as one million times the amount
of mass in our Sun.
Dark interstellar clouds are believed to be where new stars are
formed. Later, when the dust disperses, the young stars become
visible as clusters of blue stars. NGC 4013 shows several
examples of these stellar kindergartens near the center of the
image, lying in front of the dark band along the galaxy's equator.
The extremely bright star near the upper left corner, however, is
merely a nearby foreground star belonging to our own Milky Way,
which happens to lie in the line of sight to NGC 4013.
This Hubble Heritage picture was constructed from Hubble images
taken in January 2000 by Dr. J. Christopher Howk (Johns Hopkins
University) and Dr. Blair D. Savage (University of
Wisconsin-Madison). Images taken through three different
filters have been combined into a color composite, covering
the region of the nucleus of the galaxy (behind the bright
foreground star at the upper left), and extending along one
edge of the galaxy to the lower right.
Image credit: NASA and The Hubble Heritage Team (STScI/AURA)
Acknowledgment: J.C. Howk (Johns Hopkins University) and
B.D. Savage (University of Wisconsin-Madison).