Astronomers Discover a River of Stars Streaming Across the NorthernSky (Forwarded)

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March 15, 2006

Astronomers Discover a River of Stars Streaming Across the Northern Sky

PASADENA, Calif. -- Astronomers have discovered a narrow stream of stars
extending at least 45 degrees across the northern sky. The stream is about
76,000 light-years distant from Earth and forms a giant arc over the disk
of the Milky Way galaxy.

In the March issue of the Astrophysical Journal Letters, Carl Grillmair,
an associate research scientist at the California Institute of
Technology's Spitzer Science Center, and Roberta Johnson, a graduate
student at California State University Long Beach, report on the
discovery.

"We were blown away by just how long this thing is," says Grillmair. "As
one end of the stream clears the horizon this evening, the other will
already be halfway up the sky."

The stream begins just south of the bowl of the Big Dipper and continues
in an almost straight line to a point about 12 degrees east of the bright
star Arcturus in the constellation Bootes. The stream emanates from a
cluster of about 50,000 stars known as NGC 5466.

The newly discovered stream extends both ahead and behind NGC 5466 in its
orbit around the galaxy. This is due to a process called tidal stripping,
which results when the force of the Milky Way's gravity is markedly
different from one side of the cluster to the other. This tends to stretch
the cluster, which is normally almost spherical, along a line pointing
towards the galactic center.

At some point, particularly when its orbit takes it close to the galactic
center, the cluster can no longer hang onto its most outlying stars, and
these stars drift off into orbits of their own. The lost stars that find
themselves between the cluster and the galactic center begin to move
slowly ahead of the cluster in its orbit, while the stars that drift
outwards, away from the galactic center, fall slowly behind.

Ocean tides are caused by exactly the same phenomenon, though in this case
it's the difference in the moon's gravity from one side of Earth to the
other that stretches the oceans. If the gravity at the surface of Earth
were very much weaker, then the oceans would be pulled from the planet,
just like the stars in NGC 5466's stream.

Despite its size, the stream has never previously been seen because it is
so completely overwhelmed by the vast sea of foreground stars that make up
the disk of the Milky Way. Grillmair and Johnson found the stream by
examining the colors and brightnesses of more than nine million stars in
the Sloan Digital Sky Survey public database.

"It turns out that, because they were all born at the same time and are
situated at roughly the same distance, the stars in globular clusters have
a fairly unique signature when you look at how their colors and
brightnesses are distributed," says Grillmair.

Using a technique called matched filtering, Grillmair and Johnson assigned
to each star a probability that it might once have belonged to NGC 5466.
By looking at the distribution of these probabilities across the sky, "the
stream just sort of reached out and smacked us.

"The new stream may be even longer than we know, as we are limited at the
southern end by the extent of the currently available data," he adds.
"Larger surveys in the future should be able to extend the known length of
the stream substantially, possibly even right around the whole sky."

The stars that make up the stream are much too faint to be seen by the
unaided human eye. Owing to the vast distances involved, they are about
three million times fainter than even the faintest stars that we can see
on a clear night.

Grillmair says that such discoveries are important for our understanding
of what makes up the Milky Way galaxy. Like earthbound rivers, such tidal
streams can tell us which way is "down," how steep is the slope, and where
the mountains and valleys are located.

By measuring the positions and velocities of the stars in these streams,
astronomers hope to determine how much Dark Matter the Milky Way contains,
and whether the dark matter is distributed smoothly, or in enormous
orbiting chunks.

[NOTE: An image supporting this release is available at
http://spider.ipac.caltech.edu/staff...OfStars_R2.jpg
(645KB)]