Astronomers predict that T Coronae Borealis, also known as the Blaze Star, will soon turn nova. Though not quite a supernova-scale explosion, it will still be incredibly powerful. In this article, we explain how novae come to life.
Novae and Supernovae
For the better part of 2024, astronomers have waited for T Coronae Borealis to erupt and turn nova. While this has yet to happen, astronomy enthusiasts don’t lose hope. But to understand what exactly will occur in our sky, we need to first understand what novae stars actually are.
Over hundreds of years of observation, we have learned that one truly consistent thing about our sky is the permanence of stars. But even back then, ancient astronomers would witness incredible phenomena: stars that seemed to have spawned out of nowhere and then gradually vanished over the next few weeks or months.
The first scientific description of this occurrence, dating to 1572, was authored by Danish astronomer Tycho Brahe. It was him who coined the name “nova”. Over the following centuries, astronomers continued to observe similar occurrences and eventually discovered that, in reality, these phenomena are very diverse, varying both in concept and scope.
Subsequently, powerful eruptions that collapse stars and form neutron stars or black holes were given the name supernovae. It was these collapsing stars that Tycho Brahe recorded so long ago.
But observers also noted another type of eruption, this time smaller in scale. Eventually, they were simply designated novae. This is what T Coronae Borealis will soon become. Naturally, the concept of novae, just like supernovae, doesn’t mean that a new star just pops out of nothing. Rather, it describes the process when a particular object, which was previously too dim to notice, sparks with brightness so powerful it’s impossible to ignore.
How Novae Work
All new novae occur in close binary systems. Each of the system’s components has the initial mass equal to our Sun. As the star with the larger mass evolves, it loses all of its nuclear fuel and, while bypassing its red giant phase, transforms into a white dwarf. At the same time, the other star gradually balloons in size, transferring matter from its surface onto its tiny, but truly massive companion.
The transferred gas then forms an accretion disk around the white dwarf. The gravity of this extremely hot dead star pulls the gas to its surface, forming a layer of hydrogen. In this state, hydrogen reacts with plasma — the matter that makes up most of the star’s mass.
This, in turn, heats the newly formed hydrogen shell and causes it to expand against the pull of gravity. At some point, the two forces balance out, and the hydrogen shell reaches hydrostatic equilibrium. If this were the object’s permanent state, we would have also detected supersoft X-ray emissions typical of onion-shell stars.
Scientists have indeed discovered several such objects. Within them, all processes typically occur at constant rates. Once the objects accumulate enough matter, both temperature and density reach criticality, triggering a runaway thermonuclear reaction which leads to explosion.
The white dwarf’s shell immediately erupts. Its luminosity increases by thousands, even millions, meaning that its absolute magnitude rises by 6-16, or about 12 magnitudes on average.
Types of Novae
All novae eruptions occur more or less the same way, by brightening over several days, peaking, and then gradually dimming. The nova classification is based on the rate at which their magnitude decreases.
Those that dim by 3 magnitudes over 100 days are called fast novae. The ones whose brightness decreases over 150 or more days are called slow novae.
Another subtype is the very slow novae, also known as symbiotic. Their relatively high luminosity could last for years and decades after the initial outburst. In this case, the red giant supplies a constant source of energy for the white dwarf’s thermonuclear eruption. At the same time, the nova regularly shifts between its quiescent (or dormant) and outburst phases.
In addition, there are several types of outbursts that resemble novae, but aren’t formally classified as such. They all occur in close binary systems consisting of a white dwarf and a red giant, but the actual process differs from that of regular novae eruptions.
The first type is a dwarf nova. In this case, an eruption is chiefly defined by the star’s accretion disk, formed with matter from the red giant. At some point, as temperature shifts, the matter changes its viscosity and subsequently collapses on the surface of the white dwarf. This triggers a thermonuclear explosion. As a rule, this type of eruption is weaker than a classic nova, but on the other hand, it can recur as often as every few weeks.
Another variety of nova-like objects is a pole star. A system of this type comprises a red dwarf that acts as a donor star, and a white dwarf with an extremely powerful magnetic field. Pole stars are notable for the fact that their white dwarves rotate in sync with the entire system. Their strong magnetic field rejects any matter transfer from the red dwarf, so they never form accretion disks or pull that gas to their surface. Instead, the gas is forced to travel along the magnetic lines towards the star’s poles. Once there, it eventually reaches mass criticality and explodes.
Recurrent Novae
There is an important caveat to all this. Unlike supernova explosions, nova outbursts don’t destroy stars. Instead, the stars return to almost the exact state they had prior to eruption. Which begs the question: what happens if the red giant renews its efforts of transferring matter? The answer to this is obvious: we’d have another nova outburst on our hands.
Recurring novae have indeed been observed many times. After all, T Coronae Borealis belongs to this exact class. Astronomers have so far recorded two of its outbursts: in 1866 and 1942. But there is evidence of it erupting much earlier, which is why there is so much anticipation for the star’s imminent third explosion.
As a rule, recurring novae brighten every few decades. In between these events, such systems show no significant change in luminosity beyond the regular activity, namely the white dwarf acquiring a hydrogen shell and other related processes.
Though recurrent novae are defined as their own class of objects, scientists are becoming more and more certain that all novae will inevitably turn recurrent at some point. Some of them take centuries to erupt again, so we simply haven’t had the time to witness it.
Moreover, scientists know of quite a number of binary systems that resemble quiescent novae despite showing no sign of eruption on record. It’s very possible that these stars will explode at some point in the future, and we only need to wait them out.
Theoretically, nothing prevents novae from erupting as many times as they see fit, as long as they get enough fuel from their donor stars, and the white dwarves remain within the Chandrasekhar limit. If a white dwarf exceeds this limit, it destabilizes and, instead of the usual nova outburst, it undergoes a supernova explosion, turning into a neutron star.
