In 1572, Danish astronomer Tycho Brahe, along with observers around the world, noticed a new star in the constellation Cassiopeia. Rivaling Venus as the brightest object in the night sky after the Moon, the unexpected guest remained visible for nearly two years before fading from view. The appearance of a new star was nothing short of revolutionary; astronomers long agreed that the celestial sphere was unchanging. A transient star challenged this assumption and suggested that the heavens were perhaps more dynamic than originally thought.
The celestial intruder came to be known as a “nova” – after Tycho Brahe’s extensive manuscript on the phenomenon – where he referred to the object simply by the latin term for a new star: “stella nova”. While no one could have known it at the time, the new star was actually not new at all but rather a very old star experiencing a cataclysmic event.
Most of the stars in the sky are not singular, like our Sun, but rather members of binary star systems – two stars locked in orbit around one another. Of this pair, one star is often more massive than its companion and therefore burns through its nuclear fuel at a faster rate. The heavier star reaches the end of its life before its sibling. When it does, the star inflates to become a red giant and sheds its outer layers into space leaving behind the hot, dense stellar core. The naked core, known as a “white dwarf“, continues to orbit its stellar neighbor while taking the next few billion years to passively cool from a white-hot 100,000 degrees.
In the case of close binary pairs, the white dwarf can actually siphon matter off of its companion. The intermingled gravitational fields of the two stars builds a bridge of predominately hydrogen gas spanning the distance between them. Gas from the still active star flows through this gravitational funnel and spills on to the surface of the white dwarf. The mass of the white dwarf controls the rate of mass accretion; if the rate is high enough, then something quite spectacular can happen.
White dwarfs are an example of a rather exotic type of matter that physicists call “degenerate“. Most gasses expand when you heat them up: the increasing temperature causes the molecules to zip around faster which in turn increases the gas pressure. At extreme densities – like those seen in the cores of stars – the gas behaves rather peculiarly. The pressure is no longer a slave to temperature: turn up the heat and the gas does not expand.
This counterintuitive behavior is crucial to what happens next on the white dwarf’s surface. As hydrogen from the donor star comes crashing down, the crushing force of gravity compresses it into a degenerate state and the intense radiation heats the gas to many millions of degrees. But having now obtained the properties of a degenerate gas, the newly acquired hydrogen shell doesn’t expand in response to the rapid heating, but holds its pressure steady. And now things get interesting.
At temperatures exceeding 16 million degrees Celsius, conditions on the surface mimic those deep in the cores of stars and a thermonuclear explosion is the result. Hydrogen is rapidly fused into helium and the resulting release of energy blows the outer hydrogen shell off the surface of the white dwarf at speeds exceeding ten million kilometers per hour. In mere days, the white dwarf can increase its brightness by 100,000 times. It can then take months – or in some cases years – for the star to slowly fade from view.
Our Milky Way Galaxy experiences several dozen novae each year, only about ten of which are visible from Earth. A few are even visible to the naked eye. The most recent nova to be seen without the aid of binoculars or telescopes did so in the constellation Scorpius, reaching its peak brightness on February 17, 2007. Even more rare are what astronomers call “recurrent nova”. Once the white dwarf has blown the hydrogen shell into space, the gas can slowly start building up again. These are novae that flare up repeatedly, sometimes once a year, sometimes once every couple of decades. In our whole galaxy, only ten novae are known to be recurrent.
In the centuries since the new star of 1572, astronomers have come to realize that the event which gave novae their name was not what we know now as a nova. Up until early in the 20th century, a nova referred to any rapid brightening of a star. But there are many reasons stars suddenly flare into view. What Tycho saw was actually something far more powerful – a type of, aptly named, supernova. The supernova of 1572 was not the result of a flash of hydrogen burning on a white dwarf shell, but rather was caused by the complete detonation of a white dwarf. By stealing gas from a nearby companion slowly enough, the progenitor of the supernova delayed a nova flash while gradually increasing its temperature and pressure and eventually igniting hydrogen fusion throughout the interior of the once dead stellar core. With out the counterbalancing force of the rest of the star to throttle these reactions, the now violently active core obliterated itself in one of the most powerful explosions the Universe can produce.
The event of 1572 ushered in a new era of astronomy, one in which the constancy of the celestial sphere could no longer be assumed. Novae – and their more powerful supernovae cousins – are constant reminders that we, in fact, live in a highly dynamic and energetic Universe.