Smith's Cloud is a high-velocity cloud of hydrogen gas located in the constellation

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at Galactic coordinates ''l'' = 39°, ''b'' = −13°. The cloud was discovered in 1963 by Gail Bieger, ''

née A birth name is the name of a person given upon birth. The term may be applied to the surname, the given name, or the entire name. Where births are required to be officially registered, the entire name entered onto a birth certificate or birth re ...

'' Smith, who was an astronomy student at

Leiden University Leiden University (abbreviated as ''LEI''; nl, Universiteit Leiden) is a Public university, public research university in Leiden, Netherlands. The university was founded as a Protestant university in 1575 by William the Silent, William, Prince o ...

in the Netherlands.

Properties

Using the National Science Foundation's Robert C. Byrd Green Bank Telescope, radio astronomers have found that Smith's cloud has a mass of at least one million

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass ...

es and measures long by wide in projection. The cloud is between and from Earth and has an angular diameter of 10 to 12 degrees, approximately as wide as the

Orion constellation Orion is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous and recognizable constellations in the night sky. It is named after Orion, a hunter in Greek mythology. It ...

, or about 20 times the diameter of the full moon, although the cloud is not visible to the naked eye. The cloud is apparently moving towards the disk of the Milky Way at 73 ± 26 kilometers per second. Smith's Cloud is expected to merge with the Milky Way in 27 million years at a point in the Perseus arm. Astronomers believe it will strike the Milky Way disk at a 45° angle, and its impact may produce a burst of

star formation Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

or a supershell of neutral hydrogen. Projecting the cloud's trajectory backwards through time, it is estimated that it had passed through the disk of the Milky Way some 70 million years ago. To have survived this previous encounter, astronomers have suggested that it is embedded inside a massive

dark matter halo According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matte ...

. The fact that it survived this previous encounter means that it is likely to be much more massive than previously thought, and may be a candidate for being a dark galaxy. In this scenario it would be a failed dwarf galaxy, with the ingredients to form a stellar galaxy, but few if any detectable stars. However, chemical abundance measurements from the Hubble Space Telescope argue against this hypothesis; these measurements show that the Smith Cloud has an average metallicity of one half of the solar value, indicating that its gas originates in the Galaxy, not from an extragalactic source. The cloud's orbit and metallicity are both consistent with an origin in the outer disk of the Milky Way. The mechanism by which this gas was released is not known.

References

External links

* * {{Aquila (constellation) High-velocity clouds Dark galaxies Aquila (constellation) ?