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Searching out the nearest stars.

ALPHA CENTAURI is known to every science-fiction fan and sixth-grade sky buff as the nearest star after the Sun. Many can also tell you that Alpha Centauri is triple and that the truly nearest star is Alpha Centauri C, nicknamed Proxima.

But only a tiny fraction of sky observers have ever seen it.

Proxima Centauri is an 11th-magnitude red dwarf separated from Alpha Centauri A and B by 2|degrees~ 11'. This is such a wide separation that Proxima is completely lost in the Centaurus Milky Way unless you have a map of photograph like the one at right.

Also working against Proxima's visibility, of course, is the far-southern declination of the whole Alpha Centauri system, about -61|degrees~. This means none of its stars can be seen north of central Florida or southern Texas, latitude 29|degrees~. Skywatchers a little south of there can spot the zero-magnitude AB pair skimming the southern horizon at around sidereal time |14.sup.h~ |40.sup.m~. This comes within a few minutes of 2:00 a.m. local mean time (LMT) on April 1st, midnight LMT May 1st, and 10:00 p.m. LMT June 1st.

Even in southern Florida and Texas Proxima is out of reach. It's so faint that it needs to be many degrees above the horizon to be seen, meaning you have to be still farther south.

If you do find yourself far south with a telescope, the Alpha Centauri system becomes a tremendous spectacle. The dazzling A and B components, magnitudes 0.0 and 1.3, are currently separated by 18" and closing up in their 80-year orbit; they'll be 14" apart in 2000. The stars are spectral types G2V and K1V, pale yellow and pale orange, shining with 1.5 and 0.5 times the Sun's luminosity, respectively. A distance determination by Karl Kamper and Adriaan Wesselink in 1978 placed the pair 4.34 |+ or -~ 0.03 light-years away.

The same study confirmed Proxima to be a bit closer, 4.22 |+ or -~ light years. It is 27,000 times fainter than component A, which is also a main-sequence dwarf -- a dramatic illustration of the great diversity in luminosity found even among middle- and lower-main-sequence stars. An M5.5V star, Proxima glows pure orange.

Ormond C. R. Warren of the Wanganui Astronomical Society in New Zealand enjoys showing Proxima to the public with the 9 1/2-inch Cooke refractor of the Ward Astronomical Observatory. He centers Proxima in the hairs of an antique bifilar micrometer so viewers can tell which faint star it is. "Sometimes during public nights I show visitors the bright AB components after first viewing this object," he writes. "Such a comparison gains gasps."


Those of us stuck at northerly latitudes have a consolation prize. The second nearest star system, 6.0 light-years away, is Barnard's star in Ophiuchus, a single 9.5-magnitude red dwarf of type M4. Like Proxima, it first gained attention through its enormous proper motion. In 1916, only a year after Proxima was discovered, Edward Emerson Barnard found this object, which crosses the sky at 10".31 per year (or 1|degree~ in 350 years). TABULAR DATA OMITTED This is still the record for the fastest proper motion known. The finder chart on page 71 shows the path of Barnard's star for a century.

Although Barnard's star is eight times as luminous as Proxima, it is still only 1/2,300 as bright as the Sun. It rises into good view in the eastern sky by about 2 a.m. local daylight time in April, midnight in May, and right after darkness falls in June.

The next nearest star, Wolf 359 in Leo, is a real challenge at magnitude 13.5. It puts out only 1/18 the light of Barnard's star, making it the dimmest dwarf in the sky that's generally visible to amateurs.

Much easier, on the other hand, is the red dwarf Lalande 21185 in the hind feet of Ursa Major. This is one of the few red dwarfs visible in binoculars.

The dozen nearest stars, including every star within 10 light-years, are tabulated above. The data are mostly from a table by Alan H. Batten in the 1993 Observer's Handbook of the Royal Astronomical Society of Canada.
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Title Annotation:Celestial Calendar; positions of Proxima Centauri, Barnard's Star, Wolf 359 and Lalande 21185 in the sky
Author:MacRobert, Alan M.
Publication:Sky & Telescope
Date:Apr 1, 1993
Previous Article:Leo announces spring.
Next Article:A daylight occultation of Venus.

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