An hour or so after sunset during the summer in Northumberland. Twilight is draining from the sky. The first star to appear is Arcturus – a brilliant orange star high in the south. As the sky darkens further – look to the left and a little higher in the sky. You should make out a small semi-circle of stars. This is Corona Borealis – the Northern Crown.
The Northern Crown is well known to astronomers for a couple of its odd stars!
The first is known as T Corona Borealis, (T CrB for short) 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. T CrB is a nova and one of the few recurrent novae in the Milky Way that we know of. Novae consist of white dwarf (a compact, inert stellar remnant) which is accreting gaseous material on its surface from an evolved companion star (in this case, a red giant). A critical point is aperiodically reached and the accreted material ignites in a thermonuclear runaway and material is ejected at high speed from the surface of the white dwarf. During previous eruptions the nova brightened to magnitude +2: rivalling Alphecca, the brightest of the stars the Northern Crown. Eventually the nuclear runaway shuts down and the nova returns to its usual dim self and the process repeats. In recent years there have been signs that T CrB is brightening as it did in the run up to the 1946 eruption. We may not have to wait long to see this star blaze again.
The second odd star is called R Corona Borealis (R CrB, nicknamed the “reverse nova” for reasons we’ll get into). Binoculars or a telescope are needed to see the star because it usually hovers on the border of naked eye visibility. At irregular intervals of months or years this star dramatically fades become perhaps 4000 times dimmer – requiring a very big telescope to see. This behaviour is almost the opposite of how the Blaze Star behaves. What’s going on here?
R CrB is a yellow giant star which has ended its hydrogen burning and evolved away from the main sequence. The core is now burning helium to make carbon, oxygen and nitrogen. Convection dredges up these nuclear products from the core and transports them to the cooler outer layers of the star. The carbon is thought to condense out at suitably low temperatures and densities to form a dark soot. This builds up in the outer atmosphere preventing visible light getting out and causing it the star to fade. As time passes the radiation can’t escape the interior and the pressure rises. Eventually the carbon is expelled from the star causing it to return, albeit temporarily, to normal brightness. Despite the nickname there is nothing nova-like about this mechanism. Also, this change in brightness is only seen at visible wavelengths; the star remains at fairly constant brightness in the infrared.
The constellation chart at the top shows the locations of T CrB and R CrB. More accurate finder charts are shown below courtesy of the AAVSO.