“The glowing entrails of an exploding star, thought to have faded over time, now appear to be lighting up again, a new Hubble Space Telescope photo reveals. NASA released the new Hubble image of the well-known star explosion, called Supernova 1987A, today (June 10). The photo shows the closest supernova explosion witnessed in almost 400 years. This has allowed astronomers to study it in unprecedented detail as the outburst evolves.”
About 165,000 years ago, a massive star in a small galaxy called the Large Magellanic Cloud (LMC), orbiting the Milky Way distantly, was in its death throes. After a few million years of furiously burning hydrogen and releasing the resulting energy as heat and light, its hydrogen fuel burned out. It began burning the helium that was effectively the “ashes” of the hydrogen burning phase; this too was soon exhausted. The ashes of each successive fusion cycle became the fuel for the next as the nuclear reactions raced up the periodic table, until all that was left was iron. Iron is unusual, however, in that rather than releasing energy in nuclear reactions, it actually absorbs it. In a matter of seconds as the iron built up, the star’s core was robbed of the energy needed to support it against the gravitational crush of overlying layers of material. After existing in a state of physical equilibrium for millions of years, with the energy of nuclear fusion pushing back against gravity, the power source was gone. The outer layers of the supergiant stat collapsed onto the Earth-sized ball of iron at the center of the star; as the density of this material quickly increased, those outer layers rebounded outward, suddenly destroying the star.
The news took thousands of years to travel the distance to Earth. On the morning of 24 February 1987, it was heralded by the unexpected arrival of neutrinos, detected in laboratories around the world. The nearly massless particles produced in the catastrophe of the core collapse of the star raced out at the speed of light, while the light and radiation of the ensuing explosion were optically “trapped” in the expanding shell of debris. A few hours later, a brilliant “new” star appeared toward the LMC, the brightest supernova to be seen from Earth in 400 years. Called “SN 1987A” (“supernova 1987A”, as it was the first supernova discovered in 1987), it is the youngest supernova remnant (SNR) we know of, and a virtually unique laboratory for everything from high energy physics to the astrophysics of nuclear reactions. Three years after the light from 1987A arrived at the Earth, the Hubble Space Telescope was launched and it later began monitoring the changing effects of the debris shell as it plowed into the interstellar space around where the star once shone. Nearly 20 years of data show an amazing view, never before seen, as a dying star literally reconfigures the contents of the space around it.
New observations from Chandra, an X-ray-sensitive cousin of Hubble, show that after having faded for a while after initially encountering a ring of material probably blown off in successive bubbles from the dying progenitor star, the light from the explosion faded. But it is brightening again. A new paper, to appear in the Astrophysical Journal Letters by Park et al. suggests that the shock front moving outward from the explosion, after having encountered an increasing density of interstellar material for the first 20+ years after the explosion, is now propagating into a region of decreasing density, so less of its mechanical energy is going into compressing and heating up the interstellar material. They surmise that the material through which the shock wave is plowing might have been blown out into space by the progenitor star itself near the end of its life when it went through what is called a Luminous Blue Variable phase. (These objects are also called Wolf-Rayet stars.) Sometimes their intense ultraviolet energy lights up the material, causing it to glow like in the nebula NGC 6888:
Note that the star in the middle is significantly bluer than the ones around it – that’s the Wolf-Rayet star. The pinkish light comes from excited hydrogen atoms that were once part of its outer atmosphere. But the progenitor of SN 1987A didn’t show a nebula like this; we know because the star before the explosion was identified on old photographic plates, giving vital information about it’s pre-supernova properties. So the mystery is certainly far from solved.
Astronomy is very much now moving into the “time domain”, in which observations in which time is the variable provide information to which we never had access previously. Expect exciting discoveries in the coming decades as major new facilities come online designed to tap into this underutilized resource.