NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, recently found evidence of a lopsided supernova explosion. During the explosion, the star’s core went off in one direction while the shell of the star jetted off in the opposite direction. These findings are important for scientists, who have come one step closer to finding out why some supernovae collapse into black holes and others into neutron stars.
NuSTAR was launched in June 2012 and is NASA’s Explorer-class mission and it allows scientists to study high energy X-rays. One of the fundamental objectives of the mission was to understand the birth of elements when stars exploded and figure out what was actually happening during an explosion.
What Did NuSTAR Find
Supernova 1987A was the closest supernova to the Earth to be detected in almost 400 years. It was 168,000 light-years away and was first detected when light from its explosion reached Earth in 1987. This was the first neutrino source other than our sun to be seen by astronomers. Neutrinos are nearly massless subatomic particles produced during the decay of radioactive elements.
The Hubble Space Telescope has been studying SN1987A since 1990. The high-mass star lies in the Large Magellanic Cloud galaxy. What made it even more important was that it was the first time scientists could study the star’s death in detail.
The energy signature of a radioactive isotope of titanium, titanium-44, was detected. These isotopes are produced when the core of the star collapses, triggering a massive explosion, also called a Type-2 star explosion. When the unstable isotope decays and turns into calcium, it can be detected by NuSTAR, as it emits gamma rays at a specific wavelength. It glows in X-rays and is produced only in the event of an explosion.
Most of the titanium-44 from the explosion was seen moving away from the Earth. The Doppler shifts principle was used to map the isotope’s direction and velocity relative to the Earth, and heavy elements were observed moving at about 1.6 million miles an hour – speeds that are several times higher than what was expected from computer-simulated, spherically-symmetric models.
What This Actually Means
A better understanding of the NuSTAR findings will help scientists answer the question of black hole or neutron star. When stars are dying, their gravity is counterbalanced by the heat being produced in the star. Once the star runs out of fuel, the gravity can go out-of-whack, as there is nothing to counteract it. The inner layers of the star succumb to gravitational forces and collapse onto themselves. The outer shell of the star, then bounces off the core, creating a shock wave and resulting in an explosion.
Titanium is produced at the heart of an explosion, and thus traces the shape of the core’s fallout. When scientists first noticed SN1987A, they were seeing the outermost debris – the shell of the supernova which had lit up first as it traveled towards the Earth. Scientists then saw that the titanium, which was present in the core, was moving away from the Earth, which was surprising.
For years, simulations predicted that the core of a Type-2 supernova must change shape before exploding. The almost-spherical core would have to absorb neutrinos to become a turbulent, wobbly mass, which would help launch the explosion. NuSTAR finding these evidences of asymmetry is the best proof found so far to show the warped nature of a core-collapse supernova.
A supernova explosion can have three outcomes depending on the mass and size of the star – white dwarf, neutron star or black hole. For scientists, the NuSTAR findings has suggested an alternate. It has made it possible for an explosion to happen, leaving behind a black hole, not a neutron star, in a supernova with a higher degree of asymmetry.
By Anugya Chitransh
Photo by ALMA (ESO/NAOJ/NRAO)/Alexandra Angelich (NRAO/AUI/NSF) – Creativecommons Flickr License
Photo of SN1987A by X-ray: NASA/CXC/PSU/S.Park & D.Burrows.; Optical: NASA/STScI/CfA/P.Challis – Creativecommons Flickr License