Smith's Cloud is a high-velocity cloud of

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

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

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

'' Smith, who was an astronomy student at

Leiden University Leiden University (abbreviated as ''LEI''; ) is a Public university, public research university in Leiden, Netherlands. Established in 1575 by William the Silent, William, Prince of Orange as a Protestantism, Protestant institution, it holds the d ...

in the

Netherlands , Terminology of the Low Countries, informally Holland, is a country in Northwestern Europe, with Caribbean Netherlands, overseas territories in the Caribbean. It is the largest of the four constituent countries of the Kingdom of the Nether ...

Properties

Using the National Science Foundation's Robert C. Byrd

Green Bank Telescope The Robert C. Byrd Green Bank Telescope (GBT) in Green Bank, West Virginia, US is the world's largest fully steerable radio telescope, surpassing the Effelsberg 100-m Radio Telescope in Germany. The Green Bank site was part of the National Rad ...

, radio astronomers have found that Smith's cloud has a mass of at least one million

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es and measures long by wide in projection. The cloud is between and from Earth and has an

angular diameter The angular diameter, angular size, apparent diameter, or apparent size is an angular separation (in units of angle) describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the ''visual an ...

of 10 to 12 degrees, approximately as wide as the

Orion constellation Orion is a prominent set of stars visible during winter in the northern celestial hemisphere. It is one of the 88 modern constellations; it was among the 48 constellations listed by the 2nd-century astronomer Ptolemy. It is named after a hun ...

, 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 The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

at 73 ± 26

kilometer The kilometre ( SI symbol: km; or ), spelt kilometer in American and Philippine English, is a unit of length in the International System of Units (SI), equal to one thousand metres (kilo- being the SI prefix for ). It is the preferred mea ...

s per second. Smith's Cloud is expected to merge with the Milky Way in 27 million years at a point in the

Perseus arm The Perseus Arm is one of two major spiral arms of the Milky Way galaxy. The second major arm is called the Scutum–Centaurus Arm. The Perseus Arm begins from the distal end of the long Milky Way central bar. Previously thought to be 13,000 ligh ...

. 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 interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

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 In 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 matter halo ma ...

. 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 A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is so ...

, 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) ?