Bengaluru: A group of Indian astronomers were part of an international collaboration that traced the origin of the unusual supernova SN 2015dj, which was observed in 2015.
Supernova refers to the explosion of a massive star towards the end of its life, which is a powerful and luminous phenomenon and can often be brighter than the galaxy it is located in.
The astronomers from the Aryabhatta Research Institute of Observational Sciences (ARIES) in Nainital — an autonomous institute under the Department of Science & Technology — calculated the mass of the star that resulted in the supernova and the geometry of its ejection.
They successfully traced the origin of the type 1b supernova — which is a rare type of supernova — to a binary star system (two stars orbiting each other). This system comprised a type of massive and rare star called a Wolf-Rayet star, with a mass of about 13 to 20 times that of the sun, and another one with a mass much less than that of the sun.
The findings were published in The Astrophysical Journal last month.
Type 1b supernova
Supernovae are of two primary types — type 1 and type 2. Type 1 lacks hydrogen absorption lines in their spectra and is further categorised into type 1a, type 1b and type 1c. Absorption spectrum shows the kind of radiation or gases that get absorbed by a body. Type 2, meanwhile, contains hydrogen absorption lines.
Type 1a occurs in a binary star system where one star is a white dwarf — a remnant of a supernova — which devours the other star.
Type 1b and 1c supernovae occur when the core of a massive star collapses in itself, and also has an absorption line of silicon that 1a lacks.
Type 1b supernova occurs first by evolution of a massive star with layers of gas. The outermost gas is hydrogen, followed by an inner layer of helium, carbon, neon, oxygen, silicon with iron in the core.
When the star reaches a point in its life where it starts to lose mass because of the stellar wind (flow of gas from stars) or charged particles emanating from inside the star, it can start to ‘shed’ its outer layer of hydrogen and helium.
Stars that result in a type 1b supernova typically shed their outermost hydrogen layer while type 1c progenitors also shed the subsequent helium layer.
As iron accumulates in the core of the star, fusion — the process that powers a star — comes to a stop. This also ends the outward pressure from the core of the star, making it unstable. The star then starts to collapse under its own gravity, releasing a thermal shock that we see as a supernova.
Type 1b and 1c can also occur in binary systems when two stars of low mass are very close to each other, and one is absorbing material from the other.
Wolf-Rayet stars or WR stars, one of which was part of supernova SN 2015dj, are a rare group of stars with unusual spectral signatures.
They are some of the hottest stars in the universe, with surface temperatures ranging between 20,000 degree Kelvin and 2,00,000 degree Kelvin. These stars have shed their outer hydrogen layer and are also highly luminous or bright because of their temperature.
Some of the most massive stars known are WR stars and many are at the centre of planetary nebulae, ejecting gas and dust into space around it.
(Edited by Rachel John)