"We were blown away by just how long this thing is," said Carl Grillmair of the California Institute of Technology. Grillmair and Roberta Johnson, a graduate student at California State University Long Beach, reported the discovery in the March issue of the Astrophysical Journal Letters.

"As one end of the stream clears the horizon this evening, the other will already be halfway up the sky," Grillmair said.

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, and is powered by a process called tidal stripping, resulting when the force of the Milky Way's gravity becomes markedly different on one side of the cluster compared to the other. This phenomenon tends to stretch the cluster, which normally is 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 outward and away from the galactic center fall slowly behind.

The same forces create ocean tides, the researchers explain, though in the tides� case it is the difference in the Moon's gravity from one side of Earth to the other that stretches the oceans. If the gravity at the Earth�s surface was very much weaker, then the oceans would be pulled from the planet - just like the stars in NGC 5466's stream.

Despite its huge size, the stream has never been seen before 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 brightness of more than 9 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," Grillmair said.

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

Grillmair said the new stream may be even longer than can be seen, because Earth-based astronomers are limited at the southern end by the extent of the currently available data. "Larger surveys in the future should be able to extend the known length of the stream substantially, possibly even right around the whole sky," he added.

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 3 million times fainter than even the faintest stars that we can see on a clear night.

Grillmair said such discoveries are important for understanding what makes up the Milky Way galaxy. Just like earthbound rivers, such tidal streams can reveal 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 also hope to determine how much dark matter the Milky Way contains, and whether dark matter is distributed smoothly or in enormous orbiting chunks.

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