The short nights of late spring don’t give much time to do astronomy. Despite this there are some interesting constellations to be explored! One of my favourites is Corona Borealis – the Northern Crown. At this time of the year it is high in the southern sky when it gets dark. If you’ve not seen this little constellation before, well, it’s easy to find by star hopping from The Plough to the brilliant orange star Arcturus and then onto Corona Borealis. Like this:
It an easily recognisable constellation – just a small semi-circle of stars. And one of the few that actually resembles the object it is meant to represent:
The brightest star in the constellation is called Alphekka (or Gemma). It’s a second magnitude star about 75 light-years away. Alphekka is an eclipsing binary star and it falls *very* slightly in brightness every 17 days as its unseen companion star eclipses it.
The Northern Crown is well known to astronomers for a couple of its odd variable stars! The first is known as T Corona Borealis, or more informally, The Blaze Star. It is normally invisible to the unaided eye but it erupted dramatically in 1866 and 1946, becoming the brightest star in the constellation. The Blaze Star is a recurrent nova; a white dwarf star being fed gaseous material by a red giant companion.
A critical point is aperiodically reached and the material ignites in a thermonuclear explosion visible from Earth. After a few months the star has returned to its usual dim self.
The second oddball star is called R Corona Borealis (it doesn’t have a more exciting name). Binoculars or a telescope are needed to see the star because it usually hovers on the border of naked eye visibility at magnitude 6. At irregular intervals of months or years this star dramatically fades by a factor of 30 – requiring a very big telescope to see. The best model to explain the unusual behaviour of the star suggests that carbon dust (soot!) builds up in the star’s atmosphere preventing light getting out and causing it to fade. On the inside of the star the radiation cannot escape and the pressure rises until the carbon dust is blown out again causing the star to return, albeit temporarily, to normal brightness.
I downloaded a more detailed finder chart from the AAVSO showing the location of R CrB:
The numbers next to the stars are actually brightness labels (so 46 means magnitude +4.6). Naked eye limit from a dark sky are those labelled with numbers less than about 65. Here’s a light curve showing how the light from the star has brightened and dimmed over the last 20 years:
R CrB tumbled in brightness in 2007 and has been hovering near minimum brightness since. The star will eventually recover dramatically – will it be in the next few weeks or months? Time will tell so watch out!