The editors of Astronomy deem the discoveries concerning supernova 1987A as the 7th most important astronomy story of 2015.
In February 1987, a brilliant new point of light shone in the southern sky. This turned out to be the explosive blast marking the death of a star and earned the name Supernova 1987A. Lying just 168,000 lightyears from Earth, it is the closest supernova to explode since astronomers developed the tools to study such a blast. And that proximity makes it a perfect laboratory to watch how supernovae evolve. Several discoveries published in 2015 reveal changes to the blast site and uncover secrets of the explosion first seen 30 years ago.
SN 1987A is recognized by its ring of bright nodules, like shining diamonds along a band. These brilliant spots mark where the blast’s shock wave is slamming into previously shed material. While astronomers had seen the diamonds brightening for the past 15 years, new observations show them fading for the first time. This means the blast’s shock wave is passing through the ring of material, breaking it apart. Visible-light observations made by Stockholm University’s Claes Fransson and colleagues using the Hubble Space Telescope show the ring is fading while spots outside of the ring are beginning to light up. [The image above depicts Supernova 1987A observations that show the fading ring of debris.] X-ray images from the Chandra S-Ray Observatory also show the rights light changing. David Burrows, who has been watching SN 1987A evolve for 17 years, says the blast’s high-energy light is plateauing.
Another 2015 study focused on SN 1987A’s guts. The image below shows x-ray observations mapping titanium-44 created during Supernova 1987A. They show that the explosion was a lopsided event, with the bulk of material streaming away from Earth. This is discussed below.
When a star at least 10 times the Sun’s mass explodes at the end of its life, the energies, temperatures, and pressures are so high that the supernova produces a range of heavy chemical elements. One of those is titanium-44 (Ti44), which is an unstable radioactive isotope. “The isotope is produced deep in the core of the explosion, and its properties –mass, ejection speeds, and distribution—directly reflect the physics in the core,” says Steve Boggs of the University of California, Berkeley.
Like all elements, Ti44 glows with specific colors of light (it spectrum), so if astronomers look for those colors, they can learn where that material is. But none of Ti44 colors had been visible to astronomers until a recent x-ray telescope, the Nuclear Spectroscopic Telescope Array (NuSTAR), opened its eyes and began collecting data.
The element’s distribution is clumpy and uneven, implying that the explosion was off-center [as shown in the image]. This is the second supernova remnant the team has been able to probe; the other is Cassiopeia A. Both explosions were asymmetrical, Boggs’ team says, which means now astronomers have to rethink the theoretical models of these blasts.
Most computer models have assumed a symmetrical blast, but the new studies prove something more complex is happening.
 See Astronomy (44, 1, 2016, p. 26)