Nova, Supernova, Hypernova, Kilonova: let’s take stock!

As the name suggests, a nova is initially detected as a new star. In reality, it is an existing star, which suddenly becomes much brighter: it becomes visible, whereas it was not visible before.

Further study of novas has made it possible to understand that these are always stars at the end of their lives, and that different mechanisms may be at work. The names used as discoveries have been made reflect ever-increasing amounts of energy released: nova, supernova, etc. But the most energetic phenomena are not necessarily the brightest in visible light: recent discoveries in gravitational waves have made it possible to highlight kilonovas, which had never been observed in visible light before.

Below we propose a summary table of the different phenomena and their characterization. The following texts A, B, C, and D are more detailed explanations of each of them.

Note: we have chosen to simplify the words nova and supernova. To be correct, we should write them in Latin and say novæ and supernovæ in the plural. We use novas and supernovas.

A. Nova – Explanation of the phenomenon

Stars are often in pairs: double or triple systems are very common in the universe. When a pair of stars reach the end of their lives, one of them can become a white dwarf. This is what will happen to the Sun at the end of its life. The first white dwarf was observed in 1862. It is Sirius B, the companion of the star Sirius. Its presence explains the oscillating motion of Sirius on the sky observed from Earth.

A white dwarf is a star at the end of its life in which nuclear reactions have died out, which has upset the balance between the forces of contraction and expansion, which held it in place. Indeed, a living star is in equilibrium: on the one hand, its mass tends to make it contract on itself; on the other hand, nuclear reactions, which release energy, tend to expand it.

When the nuclear reactions stop, there is no longer any dilating force. Only the force of gravity remains and the star contracts on itself and becomes extremely dense [a solar-type star goes through other stages, before becoming a white dwarf, but that’s not the subject here]. A white dwarf with the mass of the Sun has a diameter of the order of the Earth. A cubic centimeter of material weighs a ton.

Of course, we could not bring a cubic centimeter of this matter back to Earth: as soon as it came out of the extreme force of attraction that holds it, the matter would explode and dilute. Matter can only exist in this state if it is extremely confined by the gravitational force of attraction exerted on it.

The white dwarf has found a new equilibrium: the force of attraction is offset by the degeneracy pressure exerted by electrons and neutrons. Matter in this state is said to be degenerate, because the density is so high that the “normal” relationship between pressure, volume and temperature no longer applies. On the contrary, quantum effects apply at the macroscopic scale. The degeneracy pressure is also called Fermi pressure or quantum pressure.

If the other star in the pair is a red giant, which corresponds to the previous stage of stellar evolution, the difference in density between the two stars is enormous. In this case, the white dwarf, which is very dense, can tear matter away from its red giant companion, which is very sparsely dense and therefore does not retain its material well. The material thus torn “falls” towards the white dwarf in the form of an accretion disk. Once it falls on the white dwarf, the material is subjected to such temperature and pressure that a thermonuclear reaction is initiated, which releases a large amount of energy, radiated in the form of visible light. A new star seems to be born: it is a nova.

As the white dwarf was not destroyed in this event, it can continue to tear material from its companion and the phenomenon will occur again sometime later. This is called a recurrent nova. Thus, RS Ophiuchi has already been observed six times: in 1898, 1933, 1958, 1967, 1985 and in 2006.

In some cases, the accretion disk of material falling towards the white dwarf is unstable, so the temperature and luminosity within the disk increase periodically. The phenomenon is less luminous than a classic nova, which is called a dwarf nova.

B. Type Ia supernova – Explanation of the phenomenon

The initial conditions are the same as for a nova: a white dwarf is tearing material from its companion star. However, in the case of the type Ia supernova, the mass thus agglomerated is such that the force of gravity that tends to contract it on itself becomes greater than the resistance offered by the fermions that make up the star (degeneracy pressure, or Fermi pressure).

The residual mass of the star at which the force of gravity starts to exceed the degeneracy pressure was calculated in 1930 by the Indian physicist Chandrasekhar. It is therefore called the Chandrasekhar limit. It is equal to 1.44 solar mass.

Around this limit, the temperature of the core, which is composed of carbon and oxygen, rises further, triggering carbon fusion. As the matter is degenerated, the increase in temperature resulting from the fusion of carbon hardly increases the pressure, which could counterbalance it, and the reaction gets out of control. Oxygen in turn fuses and releases silicon, hence its presence in the spectrum of light emitted.

The thermonuclear energy released in this way is greater than the gravitational energy that held the star in place: the white dwarf is destroyed in the explosion.

The energy released by a type Ia supernova is much greater than in the case of the nova, because it is the entire mass of the star that explodes. In the nova, only the material that has fallen on its surface explodes and the star is not destroyed by the explosion.

To sum up, a nova and a type Ia supernova correspond to the same global phenomenon: the spillage of material from a red giant to a white dwarf. In the nova, matter falls “gently” and fuses on the surface of the white dwarf. In the supernova, the entire core fuses together in an explosive thermonuclear reaction. The difference lies in the residual mass of the star: for a nova, the white dwarf does not exceed the Chandrasekhar limit, while the white dwarf that is at the origin of the supernova exceeds it.

Since the explosion of a type Ia supernova occurs as soon as the white dwarf reaches 1.44 solar mass, it always produces the same amount of energy. The absolute luminosity of the event is therefore perfectly known. Thus, when such an explosion is spotted, by measuring its apparent brightness, we know precisely its distance. Type Ia supernovas therefore make it possible to know the distance of the galaxies in which they occur. For this reason, they are sometimes referred to as “standard candles.” As they are very luminous, they allow us to know the distance of very distant galaxies.

Alternative scenario: it is also possible that the merger of two white dwarfs could create a type Ia supernova. This scenario was until now theoretical, but in 2015, a pair of white dwarfs whose merger is close to (in 700 million years, stilll) was detected. The two stars are getting closer because the system dissipates energy through the emission of gravitational waves. The two stars are approximately one solar mass. At the time of their merger, the total mass will exceed the famous Chandrasekhar limit and give rise to a type Ia supernova.

C. Core collapse supernova – Explanation of the phenomenon

This type of supernova is usually the most well-known to the public: a star at the end of its life explodes and creates a black hole. We will first describe the mechanism of this explosion and then see that it does not always leave behind a black hole: it can be a neutron star.

The explosion of the supernova

This is a very massive star: at least eight times the mass of the Sun. In this case, when the star has finished converting its hydrogen into helium, its mass is sufficient to concentrate the atoms of the heavier elements, so that fusion process can continue at equilibrium. The following table shows the order in which the elements will fusion:

All nuclear fusion reactions release energy, which creates conditions to trigger the fusion of the next element in the table, except iron. Indeed, iron is the most stable element and melting stops. The core becomes inert and is composed of “degenerate” matter, as we have seen in the description of the mechanism of the white dwarf. The density reaches one ton per cubic centimeter. But around the core, nuclear reactions continue and its mass increases until it exceeds the Chandrasekhar limit, which we saw when describing the mechanism of type Ia supernovas. Atoms no longer resist pressure: electrons are captured by protons, which creates neutrons and neutrinos. Neutrinos, which interact little with matter, are released into space. What remains are neutrons: the core has transformed into a neutron star.

The density of the neutron star is more than 500 million tons per cubic centimeter. It is the density of an atomic nucleus.

The energy released by gravitational collapse is greater than the fusion energy of the core.

The outer layers of the star are no longer held out by nuclear fusion and fall towards the core. But it has become so dense that they bounce off it and are violently ejected outwards. Indeed, at this density, the nuclear force is very repulsive. The outer layers are expelled at speeds of the order of 10 to 20% of the speed of light.