Much as new parents excitedly share photographs of their baby's first smile and first steps, on June 4 an international team of astronomers presented images of their first-ever look at the early development of an infant supernova remnant. Presented to the 200th meeting of the American Astronomical Society (AAS) in Albuquerque, N.M., these images document for the first time the emergence and movement of "hot spots" at the points of impact between debris from an exploded star and surrounding gas clouds.
In this case the "baby" is the remains of Supernova 1987A, a stellar explosion discovered in February of 1987. The images confirm general theoretical expectations and provide several surprises as a cosmic "smoke ring" puffed off by a dying star is obliterated by a tremendous shock wave traveling at millions of miles per hour.
A supernova occurs when a massive star collapses in on itself, sending its outer layers, called ejecta, off in a titanic explosion that can briefly outshine an entire galaxy filled with billions of stars. The ejecta, moving at millions of miles per hour, collide with and shock surrounding interstellar gas clouds, superheating them to millions of degrees. As this gas cools, it produces spectacular displays of X-rays, radio waves and visible light in what is known as a supernova remnant. Supernovae are quite rare and all nearby supernova remnants are many centuries old. Much of their energy and debris have already been absorbed and diluted by swept-up interstellar gas. The details of the transition from supernova to remnant have never been seen before.
The closest and brightest supernova in nearly four centuries, Supernova 1987 is the only such object ever to be resolved in any significant detail. Supernova 1987A took place in the Large Magellanic Cloud, a small galaxy orbiting our home Milky Way galaxy, estimated to lie 160,000 light-years distant in the southern constellation of Dorado.
False color HST image of SN1987A in 1994-1995
Surrounding the supernova are three glowing rings of gas, which the doomed star expelled some 20,000 years prior to exploding. Observations indicate that the rings are approximately flat and circular, and appear elliptical due to the 45-degree tilt of our viewing angle. The rings were initially lit up by the ultraviolet radiation released during the explosion and have been gradually fading ever since.
Although "close" by cosmic standards, the supernova remnant is by no means easy to study. The apparent size of the innermost ring as viewed from Earth is the same as an American quarter seen at a distance of 2 miles. Only the Hubble Space Telescope (HST) and the most advanced ground-based telescopes are able to monitor the system's detailed evolution.
When the star exploded, it launched its ejecta outward at speeds over 50 million miles per hour. Astronomers expected this ejecta would collide with the innermost ring first, crushing it with tremendous shock waves and raising the gas temperature to millions of degrees. Evidence for this impact came in 1997, when a single brightening "hot spot" was detected on the inner ring in visible-light HST images. Using sensitive data-processing techniques, astronomers determined this first spot appeared very faintly two years earlier, making March 1995 the "birthday" of Supernova Remnant 1987A at visible wavelengths.
Initially thought to mark the beginning of a full-scale collision, astronomers expected this first hot spot to spread out and rapidly light up the entire inner ring. Instead, over the next two years, the first spot remained a single point and no new spots were detected, leading astronomers to speculate that the large-scale collision was still to come.
Ben Sugerman, a doctoral candidate from Columbia University, representing an international collaboration of astronomers known as the Supernova Intensive Survey, or SinS, reported at the AAS meeting that this prediction has occurred. Sugerman and several members of the SInS team also present their work in the June 10, 2002 issue of The Astrophysical Journal.
Beginning in early 1999, new hot spots began appearing at various sites around the ring with each new HST observation: six in 1999, six more in 2000, and four more as of late 2001. The continuing trend of individual spots rather than extended bright arcs suggests that the hot spots mark the impact of the ejecta with narrow, inward-pointing protrusions distributed around the visibly clumpy ring. "These protrusions are being struck first, much as the piers of a coastal city are the first things destroyed by a tidal wave, just before it reaches the shore," notes Stephen Lawrence of Hofstra University.
"The hot spots have popped up one-by-one, much like a baby cutting her first teeth," describes Arlin Crotts of Columbia. "What's surprising is that the spots haven't appeared in any regular or predictable order, and they still haven't spread out into resolved arcs."
HST images of SN1987A in 1997 (left) and 2002 (right) showing the dramatic increase in hot spot activity.
At this point there are hot spots scattered around almost the full circumference of the ring. "The tidal wave of ejecta is close to colliding with the edge of the ring itself, and then the real fireworks will begin, as the total amount of shocked, superheated gas suddenly skyrockets," Lawrence adds. (Click to view the appearance of hot spots in Figure 2 and more dramatically, an animation.)
As with any cosmic phenomenon observed for the first time, unexpected surprises tell astronomers the most. Roughly twice as many hot spots have appeared on the eastern side of the ring as compared to the west, and the eastern spots have tended to appear earlier. This indicates that the ejecta have reached this side of the ring first, suggesting the ring was not exactly centered on the former star. While the oldest spots are typically brightest, the ninth spot to appear (located at the 12 o'clock position in the images) has brightened so quickly in two years that it is now second brightest. This indicates there are significant differences in the shapes or densities of the shocked protrusions, details too small to resolve with even Hubble's exquisite vision.
Sugerman has also discovered that many of the hot spots appear to move outward in time. "The precise center of the first hot spot is traveling away from the site of the supernova at over five million miles per hour," he notes. "At this speed it would take less than two seconds to travel from New York City to Los Angeles."
But the spot motion is an optical illusion of sorts. Because of the tremendous distance of this system, astronomers cannot directly observe any detail in the shocked protrusions, even with HST. Rather, what they see as a single hot spot is actually the unresolved emission from all of the shocked gas within a large, clumpy protrusion. "This is similar to your nighttime view of a distant town from the window of an airliner," notes Crotts. "The individual lights can be blurred by the distance into a single, unresolved glowing patch."
The illusion of motion is created as the shock sweeps past more of the protrusion, making the average position of all the light-emitting material move outward as time passes. "This is the smoking gun of the ejecta's invisible front edge," says Sugerman, "and the velocity of the spot's apparent motion gives us a minimum speed for the shock wave itself."
Astronomers hope that the combination of pre-hot spot images with the locations, birth dates, motions and rates of brightening of the hot spots will allow them to infer details about the shapes and properties of ring structures too small to see even with HST.
"Being witness to the birth of a supernova remnant is certainly a once-in-a-lifetime opportunity," says Sugerman. And, he notes, one that could have significant results in many fields of astronomy. With the exception of hydrogen and helium, all other elements in the Universe were created in the interiors of massive stars and expelled into space in supernova explosions. These newly created elements are then mixed into the general interstellar medium during the evolution of a supernova remnant.
"All the oxygen we breath, the calcium in our bones, the iron in our blood, the carbon in our DNA -- all of these atoms came from massive stars that lived and died long ago,'' describes Lawrence. Some astronomers have proposed that the shock wave from a supernova remnant triggered the collapse of the interstellar cloud that formed our Solar System five billion years ago. "In watching Supernova 1987A give birth to a supernova remnant," he adds, "we are literally looking at the fires of creation."
Sugerman's doctoral research has been funded in part by grants from the National Science Foundation and the National Aeronautics and Space Administration.