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Barnard's Runaway Star
© Copyright 2001, Jim Loy
In 1916, Edward Emerson Barnard discovered what soon
became known as Barnard's Runaway Star, in the constellation Ophiuchus. One of
the star's many official names is Barnard's Star. Besides the Sun, Alpha
Centuri and its two companion stars are the nearest stars to us (1.34 parsecs
or 4.36 lightyears for Alpha Centauri A and B, and 1.30 parsecs and 4.32
lightyears for Proxima Centauri). After them, Barnard's Star is next closest
(1.83 parsecs or 5.96 lightyears); see Known Space. It
is a red dwarf (type M4, slightly variable), too dim to be seen without a
telescope. See the diagram on the left. Barnard's star has the largest proper
motion of any star. That means that it is moving fastest, across our line of
sight. There are two reasons for this. It has a large space motion (actual
motion through space), and it is very close to us. It appears to move 10.358
seconds of arc per year, almost straight north (355.6 degrees). It will move
the apparent width of the full moon in 174 years. Other stars appear to move,
but not nearly that fast. Large proper motion is how many of the nearest stars
have been discovered.
All of the nearby stars appear to wobble with a period of 365.25 days,
because of the motion of the earth around the sun. This is their parallax, and
is the most accurate way of determining the distance to such a star. The far
distant stars have no measureable parallax, and other methods must be used to
determine their distances. See What Is A Light-Year?
In 1963, Peter van de Kamp announced the discovery of a planet orbiting
Barnard's star, with a period of about 6 months. After the the star's proper
motion and parallax had been subtracted off, there was still a small wobble in
the position of the star. Such a wobble is how some double stars had been
discovered; an unseen companion star makes the brighter main star appear to
wobble back and forth among the background stars, as the two stars orbit each
other. Barnard's Star's companion would be about 1.6 times the mass of Jupiter,
making it a planet, not a star. And the star's wobble was much less than that
of a double star. The evidence came from studying a large number of
photographs. Well, it now seems that Barnard's Star does not have a planet, at
least there is now no evidence of one. It seems that the position of the star
was slightly in error. And so, subtracting off the parallax motion (because of
the Earth's motion) was only partly successful. There was a small residue of
the parallax that was not subtracted off. And that produced an apparent wobble
with a 6 month cycle. Many stars have been found to have planets (74 planets
orbiting 66 different stars, as of the end of 2001). But Barnard's Star is no
longer among them.
Stars have many names (and numbers) depending upon which star catalog you
look in. Here are some of the names of Barnard's Star: NSV 9910, CCDM
J17578+0441A, CSV 7737, GAT 12, GSC 00425-00184, IRAS 17553+048, LHS 57, UBV
15269, VVO6, AC2000 146626, Ci20 1067, 1E 1755.3+0438, GCRV 10392, HIC 87937,
JP11 18, LTT 15309, USNO 347, Zkh 269, BD+04 3561, CSI+04-17554, G 140-24, GJ
699, HIP 87937, LFT 1385, MCC 799, PLX 4098, USNO 876.
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