Roger's Equations Blog

Roger's Equations

This blog is all about science and technology (with occasional math thrown in for fun). The goal of this blog is to try and pass on the sense of excitement and wonder I feel when I read about these topics. I hope you enjoy the posts.

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Smith's Cloud - Mass of a Million Suns With No Stars

Posted December 16, 2015 9:30 AM by Bayes

A Failed Dwarf Galaxy?

Smith's Cloud is a high-velocity cloud of hydrogen gas located in the constellation Aquila. It was discovered in 1963 by Gail Bieger, an astronomy student at Leiden University.With a mass of over a million suns and roughly 10,000 lightyears long by 3,000 lightyears wide it is located about 45,000 lightyears from Earth.

Too dim to be seen with the unaided eye, if it were visible, it would be roughly the size of the constellation Orion (20 times the diameter of the moon). I'm always struck by the whole "if visible this would be huge in the sky thing" My favorite is the Andromeda galaxy, that if it were brighter, would appear like in the picture to the right (but since it is so dim only the core of the galaxy can be seen).

The cloud is currently moving towards the Milky Way at 73 kilometers per second and is expected to merge with it in 27 million years. It is estimated to have passed through the Milky Way 70 million years ago. Given that it was not ripped apart, it is speculated that it has a dark matter halo and may be a failed Dwarf Galaxy.

Here is a pretty cool article about Smith's Cloud. Apparently some of what was generally held to be true that I've detailed above are now being questioned:

Prodigal gas cloud was born in Milky Way and is crashing back in

Call it the comeback kid. A massive cloud of gas crashing into the Milky Way probably started its life in our galaxy, according to new observations from the Hubble Space Telescope. Located 40,000 light years from Earth, Smith's Cloud is 11,000 light years long, 2500 light years across and has a mass millions of times that of our sun. If we could see it, it would span 30 times the diameter of the full moon from tip to tail. But because it is made mostly of hydrogen gas, it is nearly invisible. It was discovered in the early 1960s when astronomer Gail Smith detected radio waves emitted by its hydrogen atoms. In 2008, other astronomers reported that the cloud is moving towards the Milky Way and will crash into its disc in 27 million years. "This cloud has always stood out as something of an oddball," says Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland.

Some thought Smith's Cloud was fresh gas falling into the Milky Way, or a starless galaxy carryinggas and shrouded in dark matter. Either way, the cloud should contain few elements heavier than hydrogen and helium, which are made by stars. Now Fox and his colleagues have used Hubble to observe ultraviolet light from three active galactic nuclei that lie billions of light years beyond Smith's Cloud. By measuring how this light filters through the cloud, they can work out the heavy elements in its composition. They found that sulphur atoms in the cloud absorb so much ultraviolet light that its sulphur-to-hydrogen ratio is half that of the sun, suggesting stars have polluted it (Astrophysical Journal Letters, in press). This means the cloud is hardly a pristine relic from the dawn of time. Instead, it is probably material cast out of the galaxy's disc that is now falling back in.

Article Continues Here

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