The Sun is nearing its most northerly point at the solstice on the 20th, meaning that Britain’s nights are brief and becoming increasingly twilit if we, too, venture northwards. We do have three bright planets, though – Jupiter, which sinks towards our W horizon during the first half of the night, and Mars and Saturn which are the two brightest objects low in the S as it does so.
High above Mars and Saturn is the semicircle of stars that make up Corona Borealis, the Northern Crown. It is here that a remarkable star may be preparing to brighten dramatically to justify its reputation as the Blaze Star.
One way to find Corona is first to locate the Plough high in the W as the twilight dims, then extend a curving line along its handle to the conspicuous star Arcturus in Bootes. Corona lies some 20° above and left of Arcturus. Said to represent the crown of Ariadne in Greek mythology, it is also a beggar’s bowl in Arab tradition and the den of the Great Bear for some native Americans.
Its principal star is identified as Alphecca on our chart, a name that derives from the Arabic for “the bright star of the broken ring”. Also known as Gemma, for jewel, it shines at mag 2.2 from a distance of 75 light years.
T Coronae Borealis, or T CrB for short, lies just beyond the SE edge of the crown and is our Blaze Star. In 1866 it brightened to the second magnitude to rival Alphecca before dropping below naked-eye visibility barely a week later.
It repeated the performance in 1946, this time reaching the third magnitude. In doing so, it became the brightest-known member of the rare class of variable stars called recurrent novae. It is thought to consist of a red giant star and a white dwarf, with hydrogen tumbling from the giant to build up on the surface of the dwarf. Eventually enough accumulates to trigger a violent but short-lived nuclear fusion explosion, and the system flares up to naked-eye visibility in our sky.
Following its two observed outbursts, T CrB seemed to return to normal, settling down to become a telescopic object near, or just below, the tenth magnitude. For much of the past year, though, it has been brighter than mag 10 and it has turned noticeably bluer. The suggestion is that it is getting ready to “blow” again. Its next eruption may be a decade away, but it might come tomorrow, so watch out.
Another extraordinary variable star, R CrB, has been likened to a nova in reverse. Normally it shines at mag 5.9, making it the brightest star within the semicircle and just inferior to Pi to its north. Sometimes, though, it dips sharply to as faint as the 15th mag, making it a difficult telescopic target, before recovering after a few weeks or months. We believe that these episodes occur when clouds of soot condense in its atmosphere to block its light. Strangely, it has yet to recover from a fade in July 2007 and, despite a false dawn early last year, remains stubbornly dim.
Interesting double stars for telescope-users include Sigma and Zeta, both with stellar separations of 6 or 7 arcsec. Binoculars show Nu to be a wide double of equal stars, as is Mu in the neighbouring constellation of Bootes.
