Hubble Revisits Supernova 1987A

Supernova 1987A's ring, about a light-year across,
was probably shed by the star about 20,000 years before it exploded.
The dozens of bright spots around the ring mark where a shock wave
unleashed by the stellar blast is slamming into the ring's material.

It's been more than two decades since we Earthlings got word that a star in the Large Magellanic Cloud had blown itself to smithereens.

Supernova 1987A peaked at 3rd magnitude, making it a snap to spot by eye. But, with its declination of -69°, the blast was invisible to virtually everyone north of the equator. I'm jealous of my southern astro-friends because I never got to see it. (In fact, I wonder how the popular perception of and appreciation for astronomy might be different had this event been in view from northern skies — a topic for another day!)

Fortunately, the Hubble Space Telescope started observing SN 1987A within months of its launch in 1990. Those first views revealed a ring of matter thrown out by the star about 20,000 years before its demise. The supernova's expanding shock wave eventually crashed into that ring, creating a necklace of bright knots first spotted in 1995 HST images. Observers have kept tabs on this ring ever since — primarily with an instrument called the Space Telescope Imaging Spectrograph, or STIS, which astronauts installed on Hubble in 1997.

Unfortunately, an electronic failure in 2004 rendered STIS inoperable, and astronomers had to make do without its services until last year, when spacewalking astronauts replaced a faulty circuit board and brought STIS back to life.

In the September 2nd edition of Science Express, a star-studded international team of astronomers describe observations of the supernova made with STIS earlier this year, the first in six years. The ejected ring is still there, now studded with about 30 hotspots. Over time, as the supernova's shock wave continues to barrel outward, these should merge into a single bright band.

More interesting is the insight being gleaned from a second shock wave, this one triggered by the ring itself and propagating back toward what's left of the progenitor star and through the supernova's expanding debris. The team reports that spectra of this inner shock reveal lots of hydrogen, as you'd expect, but they also see some other emissions that are probably from nitrogen and perhaps from carbon.

Essentially, the now-gone star has laid bare whatever was inside when it exploded, and over time careful observations by HST and other telescopes will, in Humpty Dumpty fashion, attempt to put the progenitor star back together again.

"I think a great thing here is the resurrection of STIS," notes coauthor Robert Kirshner (Harvard-Smithsonian Center for Astrophysics). "Astronauts zipping out 114 screws while wearing boxing gloves were not just doing it for the challenge! This paper shows that the instrument is back working, and that we're finding out new things about an object that is about the same age as HST."