Star of the Month
T Coronae Borealis
Position: 15 hrs 59 min 30.2 sec +25 degrees 55 min 13 sec
Due south at 23:24 (BST) on 15 June 2022
T Coronae Borealis
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
This is yet another red giant star, but one with a twist, namely a white dwarf companion which is just over half an astronomical unit distant or roughly the distance of Mercury from the Sun. Their mutual orbit takes 228 days and they lie 3,000 light years away from us. The white dwarf is surrounded by an accretion disk which gathers material from its bloated companion. Originally the two stars would have been F type main sequence stars with somewhat more mass than our Sun. The heavier of the two stars aged more rapidly, after becoming a red giant, it then exploded and became a white dwarf. Meanwhile the lighter star has evolved into a red giant and will one day also become a white dwarf. At present the red giant’s luminosity fluctuates like most red giants, between magnitude 9.9 and 10.6 with a period of about four or five months. However in a semi-regular cycle, the white dwarf accumulates enough material to undergo a thermonuclear explosion with a resulting massive increase in brightness.
There have been two recorded outbursts, one in 1866 when it reached magnitude 2 and in 1946 with a peak of magnitude 3. In 2015, T Coronae Borealis displayed a sustained brightening to 9.2 and while it has dimmed since, it has remained unusually active. A similar brightening occurred in 1938 eight years before the 1946 outburst, so a third outburst may be imminent. Keep your eye on T CrB, you might be the first to see it! T Coronae Borealis is just below Epsilon Coronae Borealis, magnitude 4.1 and at the eastern end of the “crown”, but as there are many stars in this region, it is difficult to spot. Either use goto or this chart here. However if it “blows” and reaches magnitude 3, it will be unmistakable. There is no star brighter than magnitude 7 in the triangle formed by Epsilon Coronae Borealis (4.1), Delta Coronae Borealis (4.6) and Pi Serpentis (4.8) to their south-east. It would also be the only bright star in the line between 2.2 magnitude Alpha Coronae Borealis or Alphekka, also called Gemma, and 2.8 magnitude Beta Herculis or Kornephoros.