Astronomers Watch a Supernova and See Reruns
It’s “Groundhog Day” in the cosmos.
In the 1993 Bill Murray movie, a weatherman finds himself reliving the same day over and over again. Now astronomers using the Hubble Space Telescope say they have been watching the same star blow itself to smithereens in a supernova explosion over and over again, thanks to a trick of Einsteinian optics.
The star exploded more than nine billion years ago on the other side of the universe, too far for even the Hubble to see without special help from the cosmos. In this case, however, light rays from the star have been bent and magnified by the gravity of an intervening cluster of galaxies so that multiple images of it appear.
Four of them are arranged in a tight formation known as an Einstein Cross surrounding one of the galaxies in the cluster. Since each light ray follows a different path from the star to here, each image in the cross represents a slightly different moment in the supernova explosion.
This is the first time astronomers have been able to see the same explosion over and over again, and its unique properties may help them better understand not only the nature of these spectacular phenomena but also cosmological mysteries like dark matter and how fast the universe is expanding.
“I was sort of astounded,” said Patrick Kelly of the University of California, Berkeley, who discovered the supernova images in data recorded by the space telescope in November. “I was not expecting anything like that at all.”
Dr. Kelly is lead author of a report describing the supernova published on Thursday in the journal Science.
Robert Kirshner, a supernova expert at the Harvard-Smithsonian Center for Astrophysics who was not involved in the work, said: “We’ve seen gravitational lenses before, and we’ve seen supernovae before. We’ve even seen lensed supernovae before. But this multiple image is what we have all been hoping to see.”
Supernovas are among the most violent and rare events in the universe, occurring perhaps once per century in a typical galaxy. They outshine entire galaxies, spewing elemental particles like oxygen and gold out into space to form the foundations of new worlds, and leaving behind crushed remnants called neutron stars or black holes.
Because of the galaxy cluster standing between this star and the Hubble, “basically, we got to see the supernova four times,” Dr. Kelly said. And the explosion is expected to appear again in another part of the sky in the next 10 years. Timing the delays between its appearances, he explained, will allow astronomers to refine measurements of how fast the universe is expanding and to map the mysterious dark matter that supplies the bulk of the mass and gravitational oomph of the universe.
The heavens continue to light candles for Albert Einstein. On March 14 he would have been 136, and this year marks a century since his greatest achievement, the general theory of relativity that transformed our understanding of space, time and gravity. Dr. Kelly’s paper appears in a special issue of Science devoted to the anniversary of that theory.
Einstein proposed that matter and energy warp the geometry of space the way a heavy body sags a mattress, producing the effect we call gravity. One consequence of this was that even light rays would be bent by gravity and follow a curved path around massive objects like the sun, as dramatically confirmed during a solar eclipse in 1919.
In effect, space itself could become a telescope.
How this cosmic telescope works depends on how the stars are aligned. If a star and its intervening lens are slightly out of line, the distant light can appear as arcs. If they are exactly lined up, the more distant star can appear as a halo known as an Einstein ring, or as evenly separated images — the Einstein Cross.
Astronomers have learned how to use entire galaxies and galaxy clusters as telescopes to see fainter objects beyond them that would otherwise be lost in the fog of time.
Hubble scientists have recently been using this trick in a program known as Glass, or Grism Lens-Amplified Survey from Space, to explore around clusters of galaxies, the most massive and thus most powerful gravitational lenses in the universe. This has enabled them to extend Hubble’s already powerful vision deeper into the past, in one case to a galaxy that existed when the universe was only half a billion years old.
Dr. Kelly’s job was to inspect the images for distant supernovas. He was not expecting to see four versions of the same explosion at once.
They appeared in images recorded in November of a spiral galaxy roughly nine billion light-years from here. The light from this spiral has been bent and magnified both by the gravity of the intervening cluster, which is five billion light-years distant, and by one very massive galaxy in the cluster.
As a result, ghost images of the spiral appear throughout the cluster and in particular in an Einstein Cross around that one galaxy. Because the lensing effect gathers light that would not otherwise be sent to our eyes or a telescope, the image of the host galaxy is not split so much as multiplied, explained Adi Zitrin, a team member from the California Institute of Technology.
“We simply see more appearances than we would if the lens were not present,” he said.
So far the supernova, named after a Norwegian astrophysicist, Sjur Refsdal, has been detected in only the four images in the Einstein cross. Based on computer modeling of the cluster, Dr. Kelly and his colleagues suspect that Supernova Refsdal has appeared before, around 1964 and 1995, in other lensed images of the spiral galaxy.
It should appear again elsewhere in the same cluster within the next few years, Dr. Kelly’s team predicts. The exact timing of Supernova Refsdal’s reappearance depends on how the dark matter in the galaxy cluster is distributed, which will tell astronomers much about a part of the universe they cannot see any other way. The longer the path length or the stronger the gravitational field the light ray goes through, the longer the delay.
Because of the expansion of the universe, the star and its galaxy are receding from us so fast that, according to relativity, clocks there appear to run markedly more slowly than clocks here. As a result, two months from the point of view of the supernova corresponds to nearly six months on Earth.
From our point of view, Dr. Kelly said, “it’s going on in slow motion.”
A star might die only once, but with Einstein’s telescope, if you know where to look, you can watch it scream forever.