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Seeing a stellar explosion in 3-D

Because of its relative closeness, Supernova 1987A has made it possible for astronomers to study the explosion of a massive star and its aftermath in more detail.
Provided by ESO, Garching, Germany
SN 1987A
Artist's impression of the material around SN 1987A.
ESO
August 5, 2010
Astronomers using the European Southern Observatory's (ESO) Very Large Telescope (VLT) have for the first time obtained a 3-D view of the distribution of the innermost material expelled by a recently exploded star. The original blast was not only powerful, according to the new results, but it was also more concentrated in one particular direction. This is a strong indication that the supernova must have been very turbulent, supporting the most recent computer models.

Unlike the Sun, which will die rather quietly, massive stars arriving at the end of their brief life explode as supernovae, hurling out a vast quantity of material. In this class, Supernova 1987A (SN 1987A) in the nearby Large Magellanic Cloud occupies a special place. Seen in 1987, it was the first naked-eye supernova to be observed for 383 years, and because of its relative closeness, it has made it possible for astronomers to study the explosion of a massive star and its aftermath in more detail. It is no surprise that scientists have met few events in modern astronomy with such an enthusiastic response.

SN 1987A has been a bonanza for astrophysicists. It provided several notable observational "firsts": the detection of neutrinos from the collapsing inner stellar core triggering the explosion; the localization on archival photographic plates of the star before it exploded; the signs of an asymmetric explosion; the direct observation of the radioactive elements produced during the blast; observation of the formation of dust in the supernova; and the detection of circumstellar and interstellar material.

New observations making use of a unique instrument, Spectrograph for Integral Field Observations in the Near Infrared (SINFONI) on ESO's VLT, have provided deeper knowledge of this amazing event because astronomers now have been able to obtain the first 3-D reconstruction of the central parts of the exploding material.

This view shows that the explosion was stronger and faster in some directions than others, leading to an irregular shape with some parts stretching out farther into space.

The first material to be ejected from the explosion traveled at an incredible 62 million mph (100 million km/h), which is about a tenth of the speed of light, or around 100,000 times faster than a passenger jet. Even at this speed, it has taken 10 years to reach a previously existing ring of gas and dust puffed out from the dying star. The images also demonstrate that another wave of material is traveling 10 times more slowly and is being heated by radioactive elements created in the explosion.

"We have established the velocity distribution of the inner ejecta of Supernova 1987A," said Karina Kjær. "Just how a supernova explodes is not very well understood, but the way the star exploded is imprinted on this inner material. We can see that this material was not ejected symmetrically in all directions, but rather seems to have had a preferred direction. Besides, this direction is different to what was expected from the position of the ring."

Such asymmetric behavior was predicted by some of the most recent computer models of supernovae, which found that large-scale instabilities take place during the explosion. The new observations are the first direct confirmation of such models.

SINFONI is the leading instrument of its kind, and only the level of detail it affords allowed the team to draw their conclusions. Advanced adaptive optics systems counteracted the blurring effects of Earth's atmosphere, while a technique called integral field spectroscopy allowed the astronomers to study several parts of the supernova's chaotic core simultaneously, leading to the build-up of the 3-D image.

"Integral field spectroscopy is a special technique where for each pixel we get information about the nature and velocity of the gas," said Kjær. "This means that besides the normal picture we also have the velocity along the line of sight. Because we know the time that has passed since the explosion, and the material is moving outwards freely, we can convert this velocity into a distance. This gives us a picture of the inner ejecta as seen straight on and from the side."
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