by S. Prasad Ganti
April’s guest lecture was about supernovae and their remnants. It reminded me of something special about them. Although they seem to be esoteric objects in the Universe, far, far away from the earth, and far away from our solar system, they are responsible for two of the major scientific understandings mankind accomplished in the twentieth century.
First, stars are being born all the time in different galaxies in different parts of the Universe. And stars die too. After exhausting their fuel, which is mainly hydrogen, stars die different kinds of death, mainly depending on their mass. Stars having a mass above 1.4 times the mass of our Sun end up as a supernova leading to either a neutron star or a black hole. The limit of 1.4 times the mass of our Sun is called Chandrasekhar limit, named for an astrophysicist Subramanian Chandrasekhar. The X-ray space telescope Chandra is named for him also. Our own Sun will end up as a white dwarf, taking a different path than its heavier cousins.
In the supernova phase, the explosion of the star is very violent, outshining everything else in the galaxy for a brief while. The first understanding that we obtained during the mid twentieth century was that heavier elements are cooked in these stellar explosions. In a process known as nucleosynthesis, elements heavier than hydrogen are made. In a star, hydrogen is compacted by the action of gravity to raise its temperature enough to create a nuclear fusion. The hydrogen atoms fuse to create helium and liberate vast amount of energy. The resulting helium is lighter than the input hydrogen atoms. The difference in mass is the creation of energy we see in form of light and other radiation from the Sun as per Einstein’s famous equation e=mc2.
Similarly, helium burns to produce higher elements like lithium, carbon, etc. Elements all the way up to iron are produced in the stars. The conditions in the stars, although extreme, are not enough to form elements heavier than iron. Something more violent is required for heavier elements like gold, uranium, etc. Supernovae are those extremely violent phenomena producing the heavier elements. That is why it is said that our wedding rings got cooked in a supernova! The nuclear fusion in the stars and the supernovae explain all the chemical elements we see around us.
The second understanding we obtained was the size of our Universe and the existence of other distant galaxies. Based on the observed spectra, the supernovae can be classified into different types. Type 1 contains hydrogen and type 2 does not. The presence of hydrogen shows up as a line at certain wavelength in the spectra. Type 1 is further classified into 1a, 1b and 1c. Type 1a are of interest to us. They show the presence of an ionized silicon in the spectra.
All the type 1a supernovae produce the same amount of brightness so they can be used as standard candles, which means that by measuring the brightness as perceived on earth, we can estimate the distance of such supernovae from us. Each galaxy has a supernova about once in hundred years. Observation of type 1a supernovae helped us understand the different galaxies and galaxy clusters in our Universe, even the remote ones. Eventually, this partly led to determining the size of our Universe.
Earlier, Edwin Hubble used a different kind of standard candle called Cepheid variables to determine that Andromeda was a different galaxy. At large distances, individual stars are no longer distinctly visible. Hence Cepheid variables from distant galaxies cannot be viewed or measured. The supernovae could be used as distant standard candles in such cases.
Supernovae are not abstract concepts. They really enhanced our understanding of our Universe.