Shock Wave Sheds New Light on Fading Supernova
The Hubble telescope is giving astronomers a ringside seat to a never-before-seen titanic collision of an onrushing stellar shock wave with an eerie glowing gas ring encircling a nearby stellar explosion, called supernova 1987A.
Though the star's self-destruction was first seen nearly 11 years ago on Feb. 23, 1987, astronomers are just now beginning to witness its tidal wave of energy reaching the "shoreline" of the immense light-year-wide ring. Shocked by the 40-million-mile-per-hour sledgehammer blow, a 100-billion-mile-wide knot of gas in a piece of the ring has already begun to "light up," as its temperature surges from a few thousand degrees to a million degrees Fahrenheit. For comparison, the Hubble image on the left was taken before the collision. The picture on the right shows a glowing ball of gas [denoted by arrow].
NASA's Hubble Space Telescope is giving astronomers a ringside seat to a never before seen titanic collision of an onrushing stellar shock wave with an eerie glowing gas ring encircling a nearby stellar explosion, called supernova 1987a.
Though the star's self-destruction was first seen nearly 11 years ago on Feb. 23, 1987, astronomers are just now beginning to witness its tidal wave of energy reaching the "shoreline" of the immense light-year wide ring.
Shocked by the 40-million mile per hour sledgehammer blow, a 100-billion mile diameter knot of gas in a piece of the ring has already begun to "light up", as its temperature surges from a few thousand degrees to a million degrees Fahrenheit.
"We are beginning to see the signature of the collision, the hammer hitting the bell. This event will allow us to validate ideas we have built up over the past ten years of observation," says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, MA. "By lighting up the ring, the supernova is exposing its own past."
Astronomers predict it's only a matter of years before the complete ring becomes ablaze with light as it absorbs the full force of the crash.
Illuminating the surrounding space like a flashlight in a smoky room, the glowing ring is expected to literally shed a brilliant new light on many unanswered mysteries of the supernova: What was the progenitor star? Was it a single star or binary system? Are a pair of bizarre outer rings attached to an invisible envelope of gas connecting the entire system?
"We have a unique opportunity to probe structure around the supernova and uncover new clues to the final years of the progenitor star before it exploded," adds Richard McCray of the University of Colorado in Boulder, CO. "The initial supernova flash only lit up a small part of the gas that surrounds the supernova. Most of it is still invisible. But the light from the crash will give us a chance to see this invisible matter for the first time, and then perhaps we can unravel the mystery of the outer rings."
Though scientists will never solve the paradox of what happens when an irresistible force meets an immovable object, the supernova collision is the closest real-world example yet. "This supernova gives us an unprecedented opportunity to directly witness new physics of shock interactions," says McCray. "Though astronomers have measured shock effects from the expanding debris of many supernovae which are centuries-old, their impact velocities are at least ten times slower than the ones we see today in supernova 1987a."
The ring was formed 20,000 years before the star exploded. One theory is that it resulted from stellar material flung off into space as the progenitor star devoured a stellar companion. The ring's presence was given away when it was heated by the intense burst of light from the 1987 explosion. The ring has been slowly fading ever since then as the gas cools.
Several years ago radio waves and X-rays were detected as the fastest moving explosion debris slammed into cooler invisible gas inside the ring. In spring of 1997 the newly installed Space Telescope Imaging Spectrograph (STIS) first measured the speed of the supernova debris pushing along the shock wave. "The STIS lets you see the invisible stuff," says George Sonneborn of Goddard Space Flight Center in Greenbelt, MD. "We see the shock happening everywhere around the ring." In July, Hubble Wide Field and Planetary Camera-2 images taken by Robert Kirshner and co-investigators showed that a compact region on the ring lit up like a sparkling diamond set in an engagement ring.
Supernova 1987a is the brightest stellar explosion seen since Johannes Kepler observed a supernova in the year 1604. It is located about 167,000 light-years from Earth in the Large Magellanic Cloud.
The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
February 23, 1987: Canadian astronomer Ian Shelton at Las Campanas Observatory in Chile takes a telescopic photo of a small galaxy 167,000 light-years from Earth called the Large Magellanic Cloud. For Shelton, it is just routine work – until he develops the photographic plate. On that plate, he notices an extremely bright star, an intruder that he had not seen in previous observations of the same area. He races outside and looks up at the sky. There it is: a star of about the fifth magnitude, glowing in the sky. He realizes this "new star" actually is an aging massive star that has blown itself apart in a supernova explosion. (The star actually blew up about 165,000 BC, but its light arrived here in 1987.)
Astronomers are excited with this discovery because it is the nearest supernova in 400 years, since Johannes Kepler observed one in our Milky Way Galaxy in 1604.
Data taken by a small telescope aboard the International Ultraviolet Explorer (IUE) satellite help astronomers identify the exploding star's location as Sanduleak -69 degrees 202, the former site of a blue supergiant about 20 times the mass of the Sun. Astronomers name the exploding star supernova 1987a.
Astronomers believe the star swelled up to become a red supergiant, puffed away some mass, then contracted and reheated to become a blue supergiant. Then, in less than a second, the star's core collapsed, and a wave of neutrinos heated the inner core to 10 billion degrees Fahrenheit. This process triggered a shock wave that ripped the star apart, propelling a burst of neutrinos – ghostly particles from the star's core – into space.
The neutrinos are picked up by deep underground detectors: the IMB detector in Ohio and Kamiokande II in Japan. These invisible particles are the first signal of the supernova explosion, arriving even before the bright light from the dying star.
May 1987: IUE discovers an abundance of chemical elements in the supernova debris, an indicator that the progenitor star had already passed through the red giant phase.
July 1987: The Japanese satellite GINGA and a West German X-ray telescope called HEXE, attached to the Soviet Mir space station, detect X-rays emanating from the supernova debris.
August to November 1987: Several research missions, including the Solar Maximum Satellite, detect high-energy gamma rays – released in the decay of radioactive elements formed in nuclear reactions at the core of the dying star. The data show that the explosion created from simple building blocks a multitude of chemical elements. Among them was radioactive nickel, which decays into cobalt, which rapidly transforms into stable iron. The discovery confirms a widely held theory that supernovas produce the heavy chemical elements that make up most things on Earth.
December 1989: Optical observations by the European Southern Observatory's New Technology Telescope in La Silla, Chile, show a bright doughnut or ring-like feature around the supernova.
August 1990: The Faint Object Camera, an instrument aboard the newly deployed Hubble Space Telescope, clearly shows a narrow ring around the supernova. The distance between the ring and the supernova is about three-quarters of a light-year. Astronomers believe this ring was formed before the supernova explosion, ejected by the blue supergiant star about 20,000 years before its violent demise.
1990: Rapidly brightening radio emissions are detected by the Australia Telescope National Facility. (Radio waves were detected for two weeks after the supernova was first spotted.) Astronomers determine that the radio waves are coming from an area that lies between the ring and the glowing debris of the supernova at the center of the ring. In that region, the most rapidly moving debris of the supernova is crashing into gas. Optical telescopes cannot detect the gas because its density is too low and its temperature is too high.
1992: The NASA-Germany ROSAT satellite detects rapidly brightening X-rays from the supernova. The X-rays evidently are coming from the same collision area as the radio waves.
May 1994: The Hubble telescope's Wide Field and Planetary Camera 2 (WFPC2) shows that two outer loops of glowing gas, first identified several years earlier in ground-based images, are surprisingly thin. Puzzled by Hubble's unexpected new details, astronomers are challenged to explain the processes which formed such unusual structures.
January 1997: WFPC2 shows a dumbbell-shaped structure one-tenth of a light-year long. The structure consists of two blobs of debris in the center of the supernova racing away from each other at nearly 6 million mph.
May 1997: The Hubble telescope's Space Telescope Imaging Spectrograph (STIS) produces a detailed ultraviolet image of the inner ring, identifying specific gases such as oxygen, nitrogen and hydrogen, and sulfur. By dismantling the ring into its component elements, astronomers hope to assemble a picture of how the ring was created.
June 1997: Astronomers measure the fast-moving gas ejected by the supernova explosion as it crashes into gas expelled by the progenitor star about 20,000 years before its demise. This gas was invisible until observed in ultraviolet light by STIS. The spectrograph detects the presence of glowing hydrogen expanding at a speed of 33 million mph inside the inner ring.