Wednesday, 21 March 2012

Neighbor Stars

There is an old trick question in astronomy, what is the nearest star? The correct answer is the Sun, unless one specifies "nearest to the Sun" or "nearest to the Solar System."

The closest star after the Sun is best known as Proxima Centauri, but masquerades under a variety of names, such as Alpha Centauri C, HIP 70890, GJ 551, and V645 Centauri. Just as we recently passed the centennial of the Titanic disaster, we are soon coming to the centennial of the discovery of Proxima Centauri. It was discovered in 1915 by the Scottish astronomer Robert T. A. Innes (1861-1933), working at the Union Observatory in South Africa, in 1915. But it was not recognized as the nearest star until 1928, when the American astronomer Harold Alden (1890-1964) made the first measurement.

Proxima Centauri is 4.243 light years from us, where a light year is 5,878,625,373,183.6 miles, a fine example of how astronomers avoided writers cramp by inventing the light year. Proxima is 270,000 times further away than the Sun is from Earth. Proxima Centauri is a very dim star, called a red dwarf. Unless one uses a telescope, it will never be visible. The dimness results from its small size (thus the dwarf part of its type), 126,000 mile diameter (14.5% of the Sun's 864,000 mile diameter) and cool temperature (red stars are the coolest). Where the normal, undisturbed temperature of the Sun's surface is around 9950 F (and possibly as low as 8100 F inside sunspots), Proxima's surface temperature is little more than 3000 F. However, in 1951 Harvard astronomer Harlow Shapley discovered Proxima has periodic flares that can increase temperatures in a limited region by a few thousand degrees. No planets have been found around this star (the first extensive search for planets is being planned by the Planetary Society), but they would have to be between 2.14 million and 5 million miles from the star for temperatures to allow water to be liquid. Of course, the flares would fry anything living that close.

With the Sun and Proxima both moving through space, we will be closest in 27,400 years, when Proxima will be only 2.90 light years away, and still too faint to see without a telescope, unless human eyesight has improved a lot by then.

Proxima Centauri is in an enormous orbit around Alpha Centauri, probably taking close to half a million years to go around once. Alpha is itself two stars. one very similar to the Sun, and one a bit smaller and cooler. The pair are among the brightest stars in the sky, but only visible from fairly far south. These two stars orbit each other in 80 years, ranging from one billion to over three billion miles apart. All three stars are believed to be around 4.85 billion years old (about 500 million years older than the Sun). Proxima, however, is expected to last for another 400 billion years, long after its companions or the Sun have burned out.

The next three closest stars are loners, like the Sun, and red dwarfs like Proxima Centauri. The first is called Barnard's Star in memory of the American astronomer Edward Emerson Barnard (1857-1923) who discovered it in 1916 while working at Lick Observatory in California. It is twice the age of the Sun, and 5.9 light years from us. There have been unsubstantiated claims of a planet going around Barnard's Star, but most astronomers think the claims are a mistake. The second is Wolf 359, so called because it is the 359th star in a list of red dwarfs drawn up by the German astronomer Maximilian Franz Josef Wolf (1863-1932). Barnard's is located in the constellation of Ophiuchus while Wolf 359 is in Leo. Third is Lalande 21185, from a catalog prepared by the French astronomer Jerome Lalande (1732-1807), located in Ursa Major. None of these stars can be seen without a telescope, and no planets are known for any of them although Lalande 21185 had an apparently erroneous claim made in1951. Wolf 359 is so small that it is only a third larger than Jupiter.

We finally come to a bright star, Sirius, 8.6 light years from us, and the brightest star in the night sky (although, strangely, some people think Polaris is the brightest, when it only ranks number 50). Sirius weighs twice what the Sun does, and is much hotter, with a surface temperature approaching 20,000 F. Hot stars have shorter life spans, and Sirius is not expected to last more than a billion years, with a couple hundred million yet to go. Sirius has a small very hot star going around it in a fifty year orbit. This companion star was the first white dwarf to be discovered, by the American telescope manufacturer Alvan Graham Clark (1832-1897), testing a new telescope in 1862. It is slightly larger than Earth, but weighs nearly as much as the Sun, 324,000 times the mass of the Earth. It is a star nearing the end of its life, having evolved faster than its companion.

The last three stars within ten light years of us are a pair of red dwarfs, Luyten 726-8, discovered in the constellation Cetus by a Dutch astronomer in 1948, and Ross 154, also a red dwarf. It is in the constellation of Sagittarius. No one has suggested any planets for these stars.

But in 2011 the WISE spacecraft found a brown dwarf in the constellation Lyra, just 9.6 light years away. With a surface temperature of 80 F, it could not warm any planet enough to support life, but no planets have been found around any brown dwarfs.

Thus within ten light years of Earth we have 12 stars (including the Sun) and one brown dwarf, with no confirmed planets except around the Sun. Most of the stars are small, cool and dim red dwarfs, leaving the Sun as one of just three larger stars. This seems to match what is found throughout our galaxy, that about three quarters of all stars are red dwarfs, six percent are white dwarfs, and only about five percent are similar in size and temperature to the Sun. But with over 200 billion stars in our galaxy, that leaves a lot that match the star we are in orbit around.

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