R and T Coronae Borealis: Two Stellar Opposites

By: Alan MacRobert July 17, 2006 0

This finder chart (click on it for better version) covers about as much sky as the field of view in a typical pair of 7-power binoculars. How bright will you find R and T Coronae Borealis tonight? The italic numbers next to stars are their visual magnitudes to the nearest tenth (with the decimal point omitted), for comparison purposes. North is up and east is left.

Sky & Telescope diagram.

Rising up the eastern evening sky in springtime, and high overhead in summer, is the little constellation Corona Borealis next to Bootes (Northern Hemisphere spring and summer, we're talking). Aside from its distinctive naked-eye shape, Corona Borealis holds two claims to fame: a bright star that unpredictably turns faint, and a faint one that on rare occasions turns bright.

R Coronae Borealis spends most of its time at magnitude 6.0, but every few years it suddenly drops to magnitude 14 or fainter. It gradually recovers in the following months, though often suffering relapses along the way. The fade-outs are believed to be caused by clouds of carbon particles — sooty smoke — condensing above the star’s carbon-rich atmosphere.

T Coronae Borealis behaves oppositely: it’s a repeating nova. It spends decades on end simmering at 10th magnitude, then blazes to 2nd or 3rd magnitude. Its last eruptions came in 1866 and 1946. The next could happen any time.

Take a look with binoculars using this map. You’ll probably find R bright and T invisible. (Comparison-star magnitudes are given, with the decimal point omitted.) Look often, and pretty soon you won’t need the map to check on what these stellar opposites might doing whenever you step out for an evening's starwatching.

About Alan MacRobert

Alan M. MacRobert became an avid Sky & Telescope subscriber in 1966 at age 14, joined the editorial staff in 1982, and is now a senior contributing editor, semi-retired. He played a role in practically every part of the magazine and the company's other products for more than a generation, both on the amateur-observing side and the science-reporting side. In 1994 a book collection of his observing how-tos and telescopic sky tours was published as Star Hopping for Backyard Astronomers. He has produced This Week's Sky at a Glance online every week since 1989.