Smith's Cloud is a

high-velocity cloud High-velocity clouds (HVCs) are large collections of gas found throughout the galactic halo of the Milky Way. Their bulk motions in the local standard of rest have velocities which are measured in excess of 70–90 km s−1. These clouds of ...

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

gas located in the constellation Aquila at

Galactic coordinates The galactic coordinate system is a celestial coordinate system in spherical coordinates, with the Sun as its center, the primary direction aligned with the approximate center of the Milky Way Galaxy, and the fundamental plane parallel to an a ...

''l'' = 39°, ''b'' = −13°. The cloud was discovered in 1963 by Gail Bieger, '' née'' Smith, who was an astronomy student at Leiden University in the

Netherlands ) , anthem = ( en, "William of Nassau") , image_map = , map_caption = , subdivision_type = Sovereign state , subdivision_name = Kingdom of the Netherlands , established_title = Before independence , established_date = Spanish Netherl ...

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 Radi ...

, radio astronomers have found that Smith's cloud has a mass of at least one million solar masses 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 distance describing how large a sphere or circle appears from a given point of view. In the vision sciences, it is called the visual angle, and in optics, it is ...

of 10 to 12 degrees, approximately as wide as the Orion constellation, or about 20 times the diameter of the

full moon The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This means ...

, 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 is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

at 73 ± 26

kilometer The kilometre ( SI symbol: km; or ), spelt kilometer in American 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 now the measurement unit used for ex ...

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 lig ...

. Astronomers believe it will strike the Milky Way disk at a 45° angle, and its impact may produce a burst of star formation 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. 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 A dark galaxy is a hypothesized galaxy with no, or very few, stars. They received their name because they have no visible stars, but may be detectable if they contain significant amounts of gas. Astronomers have long theorized the existence of dark ...

. 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) ?