New jewel in the crown

If you head out after midnight this month, you can’t miss bright orange Arcturus, some 25 degrees above the northern horizon.

The fourth brightest star in the heavens marks the foot of Bootes, the Herdsman.

If the sky is clear down to the horizon, below and to the right of Arcturus, you should be able to pick out the distinct semi-circle of seven stars that comprise the constellation Corona Borealis, the Northern Crown. According to legend, Corona Borealis was said to be the crown worn by Princess Ariadne of Crete when she married the god Dionysus. The constellation’s brightest star is sometimes called Gemma, the Latin name for the jewel, a fitting description for this celestial coronet that adds a touch of beauty to our night sky.

From our part of the world, Corona Borealis never climbs exceptionally high; in Dunedin, it gets to just 17 degrees above the horizon. Despite its relatively low altitude, there’s a reason to keep your eye on this constellation for the next few months.

That’s because sometime between now and September, astronomers are predicting a significant event in the constellation Corona Borealis. An eighth star is expected to appear in the crown, a rare occurrence caused by an explosion in a star system called T Corona Borealis. This system, located more than 2600 light-years from Earth, is binary, with one component being a cool red giant and the other a white dwarf, smaller than Earth.

Usually, T CrB is invisible to the naked eye, but roughly once every 80 years, an explosion occurs, which makes the star rapidly increase in luminosity and blaze brilliantly for a few days before sinking back into invisibility.

Astronomers are predicting another explosion because T CrB has previously been observed at least twice: on February 9, 1946, and on May 12, 1866.

Stellar explosions that can be seen with the naked eye are rare. Stellar explosions that can be seen with the naked eye and predicted with some degree of accuracy are rarer still; to date, only five are known in the sky.

So, keep your eyes peeled for the next few months. You may get to witness a once-in-a-lifetime stellar explosion.