May 7, 2015
Jet Propulsion Laboratory, Pasadena, Calif.
NASA Headquarters, Washington
Supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope more than 400 years ago, is no stranger to the NASA/ESA Hubble Space Telescope. The observatory has been on the frontline of studies into this brilliant dying star since its launch in 1990, three years after the supernova exploded on 23 February 1987. This image of Hubble’s old friend, retreived from the telescope’s data archive, may be the best ever of this object, and reminds us of the many mysteries still surrounding it. Dominating this picture are two glowing loops of stellar material and a very bright ring surrounding the dying star at the centre of the frame. Although Hubble has provided important clues on the nature of these structures, their origin is still largely unknown. Another mystery is that of the missing neutron star. The violent death of a high-mass star, such as SN 1987A, leaves behind a stellar remnant — a neutron star or a black hole. Astronomers expect to find a neutron star in the remnants of this supernova, but they have not yet been able to peer through the dense dust to confirm it is there. The supernova belongs to the Large Magellanic Cloud, a nearby galaxy about 168 000 light-years away.
Large Magellanic Cloud
Even though the stellar explosion took place around 166 000 BC, its light arrived here less than 25 years ago. This picture is based on observations done with the High Resolution Channel of Hubble’s Advanced Camera for Surveys [ACS].
The field of view is approximately 25 by 25 arcseconds. Credit: NASA/ESA Hubble
The plot of data from NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR (right), amounts to a “smoking gun” of evidence in the mystery of how massive stars explode. The observations indicate that supernovae belonging to a class called Type II or core-collapse blast apart in a lopsided fashion, with the core of the star hurtling in one direction, and the ejected material mostly expanding the other way (see diagram in Figure 1). NuSTAR made the most precise measurements yet of a radioactive element, called titanium-44, in the supernova remnant called 1987A. NuSTAR sees high-energy X-rays, as shown here in the plot ranging from 60 to more than 80 kiloelectron volts. The spectral signature of titanium-44 is apparent as the two tall peaks. The white line shows where one would expect to see these spectral signatures if the titanium were not moving. The fact that the spectral peaks have shifted to lower energies indicates that the titanium has “redshifted,” and is moving way from us. This is similar to what happens to a train’s whistle as the train leaves the station. The whistle’s sound shifts to lower frequencies. NuSTAR’s detection of redshifted titanium reveals that the bulk of material ejected in the 1987A supernova is flying way from us at a velocity of 1.6 million miles per hour (2.6 million kilometers per hour). Had the explosion been spherical in nature, the titanium would have been seen flying uniformly in all directions. This is proof that this explosion occurred in an asymmetrical fashion.
The bright ring consists of material ejected from the dying star before it detonated. The ring is being lit up by the explosion’s shock wave.
NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, has found evidence that a massive star exploded in a lopsided fashion, sending ejected material flying in one direction and the core of the star in the other.
The findings offer the best proof yet that star explosions of this type, called Type II or core-collapse supernovae, are inherently asymmetrical, a phenomenon that had been difficult to prove before now.
“Stars are spherical objects, but apparently the process by which they die causes their cores to be turbulent, boiling and sloshing around in the seconds before their demise,” said Steve Boggs of the University of California, Berkeley, lead author of a new study on the findings, appearing in the May 8 issue of Science. “We are learning that this sloshing leads to asymmetrical explosions.”
The supernova remnant in the study, called 1987A, is 166,000 light-years away. Light from the blast that created the remnant lit up skies above Earth in 1987. While other telescopes had found hints that this explosion was not spherical, NuSTAR found the “smoking gun” in the form of a radioisotope called titanium-44.
“Titanium is produced in the very heart of the explosion, so it traces the shape of the engine driving the disassembly of the star,” said Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology in Pasadena. “By looking at the shift of the energy of the X-rays coming from titanium, the NuSTAR data revealed that, surprisingly, most of the material is moving away from us.”
Last year, NuSTAR created detailed titanium-44 maps of another supernova remnant, called Cassiopeia A, also finding evidence of an asymmetrical explosion, though not to as great an extent as in 1987A.
Together, these results suggest that lopsidedness is at the very root of core-collapse supernova.
When supernova 1987A first lit up our skies decades ago, telescopes around the world had a unique opportunity to watch the event unfold and evolve. Outer, ejected materials lit up first, followed by the innermost materials powered by radioactive isotopes, such as cobalt-56, which decayed into iron-56. In 2012, the European Space Agency’s Integral satellite detected titanium-44 in 1987A. Titanium-44 continues to blaze in the supernova remnant due to its long lifetime of 85 years.
“In some ways, it is as if 1987A is still exploding in front of our eyes,” said Boggs.
NuSTAR brought a new tool to the study of 1987A. Thanks to the observatory’s sharp high-energy X-ray vision, it has made the most precise measurements of titanium-44 yet. This radioactive material is produced at the core of a supernova, so it provides astronomers with a direct probe into the mechanisms of a detonating star.
The NuSTAR spectral data reveal that titanium-44 is moving away from us with a velocity of 1.6 million mph (2.6 million kilometers per hour). That indicates ejected material flung outward in one direction, while the compact core of the supernova, called a neutron star, seems to have kicked off in the opposite direction.
“These explosions are driven by the formation of a compact object, the remaining core of the star, and this seems to be connected to the core blasting one direction, and the ejected material, the other,” said Boggs.
Previous observations have hinted at the lopsided nature of supernova blasts, but it was impossible to confirm. Telescopes like NASA’s Chandra X-ray Observatory, which sees lower-energy X-rays than NuSTAR, had spotted iron that had been heated in the 1987A blast, but it was not clear if the iron was generated in the explosion or just happened to have been in the vicinity.
“Radioactive titanium-44 glows in the X-rays no matter what and is only produced in the explosion,” said Brian Grefenstette, a co-author of the study at Caltech. “This means that we don’t have to worry about how the environment influenced the observations. We are able to directly observe the material ejected in the explosion.”
Future studies by NuSTAR and other telescopes should further illuminate the warped nature of supernovae. Is 1987A particularly askew, or in line with other objects in its class? A decades-old mystery continues to unravel before our eyes.
NuSTAR is a Small Explorer mission led by the California Institute of Technology in Pasadena and managed by NASA’s Jet Propulsion Laboratory, also in Pasadena, for NASA’s Science Mission Directorate in Washington.
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Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. Although the facility has a Pasadena postal address, it is actually headquartered in the city of La Cañada Flintridge , on the northwest border of Pasadena. JPL is managed by the nearby California Institute of Technology (Caltech) for the National Aeronautics and Space Administration. The Laboratory’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network.