In 2017, LIGO detected gravitational waves from a merger of two neutron stars. Supernovae have allowed us to calculate distances to faraway galaxies and proved theories of expansion of the universe. The Zwicky transient facility set up at Palomar Observatory scans over one-third of the northern sky every night to detect transient sources.
This is the first article in the series of Stellar Explosions. Stay tuned to explore these ideas with upcoming blog posts.
Less than a century ago, the astrophysicists themselves did not use the term supernovae; Until it was coined in 1934 in a first-ever research paper dedicated to exploding stars. In this first-ever research paper about supernovae, Fred Zwicky and Walter Baade also laid theoretical foundations for neutron stars and cosmic rays. We will look at the crucial and foundational discoveries made by Zwicky and Baade, and understand, what else and what more have astrophysicists achieved since then.
Crab Nebula - A Supernova remnant (image credits: European Space Agency)
There exist two distinct types of new stars or novae, which might be distinguished as common novae and supernovae. No intermediate objects have so far been observed.1 The term novae meant new stars. Years back, people saw these novae as guest stars, lasting for a few days and then fading to darkness.
Novae
We now know three different types of novae explosions, all of which involve accreting white dwarf in a binary system. Accretion adds mass onto a white dwarf, and heating of this mass through thermal energy of white dwarf leads to the ignition of thermonuclear reaction in the ever-changing outer layers of a white dwarf. The white dwarfs burn hydrogen or helium in their outer shells to produce these explosive events.
Artistic rendition of accreting white dwarf (image credits: David A. Hardy)
Supernovae
As for supernovae, they are classified into various classes based on elements detected in their spectra and the evolution of their light curves. The two primary classification are Core-Collapse Explosion and Thermonuclear Explosion (SN Ia).
Core-Collapse Supernovae
Very massive stars fuse elements starting from hydrogen and go on to form an iron core, surrounded by layers of silicon-sulphur, oxygen-neon-magnesium, carbon-oxygen, helium and hydrogen. The fusion of iron is endothermic. Thus the star has no more fuel to burn. Energy generation source is lost, and gas loses energy to exert enough outward pressure. The hydrostatic equilibrium is massively distorted. The collapse under gravitational force is inevitable. The Large Blue Variables, Red and Blue giants and supergiants are progenitor systems for core-collapse supernovae.
Onion shell like structure of an evolved star
Thermonuclear Explosions
Astrophysicists are still working out the exact progenitor system involved in SN Ia. There is a confusion on whether it is
- two colliding white dwarfs ,or
- white dwarfs accreting mass from its binary and eventually crossing Chandrasekhar limit that leads to Thermonuclear explosion.
The significant insight we have is that the SN Ia is highly asymmetric explosions. The polarization of light obtained from these explosions imply that they cannot be spherically symmetric. However, the simulations tell that both merging white dwarfs or accreting white dwarf progenitor should lead to spherical explosions
Supernovae is found not only in the nearer systems, but apparently all over the accessible range of nebular distances.2 Here ‘nebular’ actually refers to ‘galactic’. Back in the 1930s, before Edwin Hubble observed these systems and calculated their redshifts and distances, visible galaxies like andromeda were thought to be distant nebulae or stellar systems. Current day transient facilities enable us to look at explosions that occurred more than 10 billion years ago.
The most distant supernova discovered by Hubble Space Telescope
Supernovae are a much less frequent phenomenon than common novae.3 Even with limited technology, this observational study hit a bull’s eye. In our galaxy, over 20-30 novae are observed in a century, whereas we may witness only one or two supernovae or none at all during 100 years. Ordinary novae are simply expulsion of mass by accreting white dwarfs. A single accreting white dwarf could throw off materials very often sometimes. The number of white dwarfs is far larger than the number of massive stars. It’s the average star, which turns into white dwarfs. The majority of stellar systems are also binary systems. Accounting all these, a much higher rate of novae is expected and indeed observed.
Massive stars need to burn much hydrogen to generate enough energy to support that considerable mass against gravitational collapse. They exhaust their mass quickly and evolve towards explosions. This means massive stars - with their much shorter lifetimes are found only in younger, star-forming galaxies.
White dwarfs, the progenitors of Novae, are corpses of average sun-like stars. They are present in older as well as young star-forming galaxies. Hence, Novae can occur in all kinds of galaxies. The thermonuclear explosions are also frequent. However, core-collapse explosions are limited to younger, star-forming galaxies.
Supernovae, initially, are quite ordinary stars whose masses are not greater than 1033 gram to 1035 gram.45
This statement came from generally observed masses of stars and is quite precise. The stars we have surveyed have a mass between 1032 gram to 1035 gram, and it’s known that systems more massive or lighter than this do not and cannot exist as stars. Stars with mass less than ~0.8 solar mass form white dwarf with helium core . The one’s with a bit higher mass form a carbon-oxygen core, the most common type. A little more massive star could form a stable Oxygen-Neon-Magnesium core. S. Chandrasekhar derived the maximum mass for a white dwarf, about 1.4 solar mass. Throughout the evolutionary phase, stars of mass up to ~10 solar mass are observed to shed off their outer layers or eject mass in violent runaway reactions to reduce mass to form a stable white dwarf. Stars with a mass higher than ~10 solar mass, evolve towards core collapse explosions.
\[\begin{aligned} L_{\odot} &= 3.78 \times 10^33 \text{ergs/sec} \\ L_{V} &= 6.3 \times 10^{7}\ L_{\odot} = 2.38 \times 10^{41} \text{ergs/sec} \\ E_{V} &= 1.19 \times 10^45 \text{ergs} \end{aligned}\]At maximum the visible radiation \(L_{V}\) emitted per second is equal to that of \(6.3\times 10^{7}\) suns.6
This value of energy is so close to the correct values, that one may think Zwicky and Baade, had indeed unraveled the complete physics involved in stellar explosions. Nonetheless, they had obtained correct answers by applying wrong assumptions.
We now know the stupendous amount of energy that powers the light curves of a supernova comes from
- Radioactive decay of unstable isotopes of nickel and cobalt (and few other iron-group isotopes) in the outer atmosphere of exploding star,
- Ejected mass moving outwards at velocities of 10,000 km/s,
- Shock heating and interaction with surrounding circumstellar medium.
Light curve of supernovae explosion (image credits: Chaisson and McMillan)
The radioactive decay is the primary power source for thermonuclear supernovae. The circumstellar interaction is expected to emit radiation radio and x-ray band if the explosion is due to an accreting white dwarf.
In the case of a core-collapse process, a massive stellar core collapse to a mere ~10km radius, releasing a humongous amount of gravitational energy. Gravity initially pushes the outer layers and the core into an implosion. At a point, the inner iron core becomes so dense that it is no more iron, but a sea of free protons and electrons. The protons capture these electrons and form neutrons plus neutrinos. The neutrons go on collapsing till they are smashed together to nuclear densities. Then at a point, the infalling neutrons overshoot nuclear density and then rebounds. This is the rebound that generates a shock wave and throws off the infalling outer layers outwards. The neutrinos carry away the gravitational energy and support the shock wave in powering the explosion. What we observe is a marvelous Supernova.
The expanding layers traveling at speeds of 10,000 km/s and interacting with the circumstellar medium produces emissions almost all over the electromagnetic band. A major fraction of the energy is carried away by ~1057 neutrinos generated during the collapse. In 1987 the Super Kamiokande neutrino observatory detected about dozen of neutrinos. These neutrinos were traced to a naked eye transient that occurred in Large Magellanic Cloud, and it confirmed the neutrino-generation process in a core-collapse explosion.
The 1987 supernova in Large Magellanic Cloud (image credits: Anglo-Australian Observatory)
About the final state of supernovae practically nothing is known.[^7] Though we know the energies generated in thermonuclear and core-collapse supernovae, the underlying theories about how these energies are generated lack details. Once the discrepancy in the progenitor of SN Ia is solved, a more crude theory of how the thermonuclear explosion indeed occurs can be developed.
For the core-collapse explosion, astrophysicists have not worked out the exact details of neutrino transport. The theory of rebound, and how the gravitational implosion turns into a colossal explosion is incomplete. Observational advances and computational simulation of neutrino energies and transport are needed to work out these remaining details.
A supernova represents the transition of an ordinary star into a neutron star, consisting mainly of neutrons. Such a star may possess a very small radius and an extremely high density. As neutrons can be packed much more closely than ordinary nuclei and electrons, the “gravitational packing” energy in a cold neutron star may become very large, and, under certain circumstances, may far exceed the ordinary nuclear packing fractions. A neutron star would therefore represent the most stable configuration of matter as such.
This is the most extraordinary statement made by Baade and Zwicky in the 1934 paper. The first-ever paper on explosion started with coining the term- Supernova and went to predict what kind of stellar corpse would remain at the end of the explosion. The observational proof for the neutron star would come much later in the late 1960s when Jocelyn Bell would detect the periodic radio emission from a pulsar.
Baade and Zwicky presented brilliant and revolutionary ideas in their 1934 paper. Since then, astrophysicists have come a long way to figure out the fundamental processes involved in explosions.
References:
- Baade, W., and F. Zwicky. “On Super-Novae.” Proceedings of the National Academy of Sciences, vol. 20, no. 5, May 1934, p. 254-59.
- Baade, W., and F. Zwicky. “Remarks on Super-Novae and Cosmic Rays.” Physical Review, vol. 46, no. 1, July 1934, pp. 76–77.
- Srinivasan, Ganesan. Life and Death of the Stars. Springer Science & Business Media, 2014.
- Branch, David, and J. Craig Wheeler. Supernova Explosions. Springer, 2017.
- Alsabti, Athem W., and Paul Murdin, editors. Handbook of Supernovae. Springer International Publishing, 2016.
