This time-lapse, assembled from images taken by the Hubble Space Telescope over a period of two and a half years, shows the evolution of the light echo coming from the supernova SN 2014J.
The supernova was located in Messier 82, the „Cigar Galaxy“, about 3.5 mpc (11.5 million light-years) away from earth. The light echo is caused by light scattering of the interstellar dust cloud. When it was discovered in January 2014, it was one of the closest supernovae seen for decades and, due to its relative closeness and unusual brightness, of great interest for scientists and amateur astronomers.
The closeness of the supernova allowed astronomers to study it in much more detail than usual and might help us to understand how supernovae form and evolve. Moreover, SN2014J was, as its optical spectrum showed, a Type Ia supernova. These supernovae are used for determining distances in space, understanding them better may, therefore, help clarify the shape of the Universe.
A supernova is a large explosion that takes place at the end of a star's life cycle and produces heavier elements. When a supernova happens, the star expels all its, or at least a great majority of it, matter at speeds up to 10% the speed of light. In the case of SN2014J, the supernova produced a debris that originally expanded at up to 20,000 kilometres per second. The expelled matter can be seen as a supernova remnant.
Supernovae are classified according to their light curves, their absorption lines and the chemical elements they emit. If a supernova has a strong ionised silicon absorption line, it‘s a type Ia supernova; otherwise, it’s classified as Type Ib (this type has helium absorption lines) or Ic (doesn’t have helium absorption lines). The same classifications are used for type II supernovae. Type Ib, Type Ic, and Type II supernovae are the result of the core-collapse of a massive star.
A type Ia supernova occurs in a binary star system. Here, two stars are orbiting the same point. One of the two stars, usually a white dwarf, „consumes“ matter from its companion star. Due to this, the white star accumulates too much matter; its mass becomes greater than the Chandrasekhar limit, and this results in a supernova. The Chandrasekhar limit is the maximum mass of a stable white dwarf star, which is 1.4 times the mass of our sun. White dwarfs resist gravitational collapse primarily through electron degeneracy pressure. The Pauli exclusion principle disallows two electrons with the same spin to occupy the same energy state in the same volume. This forces them into higher energy states, once the lowest energy level is filled. When they are in a higher energy state, the electrons are travelling at faster speeds. These faster moving electrons create a pressure called electron degeneracy pressure. When the mass of the white star approaches the Chandrasekhar limit, its gravity attempts to squeeze the star into a smaller volume. This forces electrons into a higher energy state and therefore travelling at higher velocities. When the mass of the white dwarf is smaller than the Chandrasekhar limit, the electron degeneracy pressure in the star's core generates an equilibrium to the star's gravitational self-attraction, this prevents it from collapsing. At the Chandrasekhar limit, the electrons are travelling close to the speed of light, but the pressure exerted by them becomes insufficient to support the star, thus causes it to collapse and to evolve into a different type of stellar remnants, such as a neutron star or a black hole.
