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A stellar dust factory

STRIKING NEW OBSERVATIONS with the Atacama Large Millimetre/submillimetre Array (ALMA) telescope capture, for the first time, the remains of a recent supernova brimming with freshly formed dust. If enough of this dust makes the perilous transition into interstellar space, it could explain how many galaxies acquired their dusty, dusky appearance.

Cosmic dust consists of silicate and graphite grains – minerals also abundant on Earth. The soot from a candle is very similar to cosmic graphite dust, although the size of the grains in the soot are ten or more times bigger than typical grain sizes of cosmic graphite grains.

This image shows the remnant of Supernova 1987A

This image shows the remnant of Supernova 1987A seen in light of very different wavelengths. ALMA data (in red) shows newly formed dust in the middle of the remnant. Hubble Space Telescope (in green) and Chandra Space Observatory (in blue) data show the expanding shock wave. Credit: ALMA (ESO/NAOJ/NRAO) / A. Angelich. Visible light image: the NASA/ESA Hubble Space Telescope. X-Ray image: The NASA Chandra X-Ray Observatory

Galaxies can be remarkably dusty places and supernovae – exploded stars – are thought to be a primary source of that dust, especially in the early universe. But direct evidence of a supernova’s dust-making capabilities has been slim up to now, and could not account for the copious amount of dust detected in young, distant galaxies. But now observations with ALMA are changing that.

An international team of astronomers used ALMA to observe the glowing remains of Supernova 1987A, which is in the Large Magellanic Cloud, a dwarf galaxy orbiting the Milky Way about 160,000 light-years from Earth. SN 1987A is the closest observed supernova explosion since Johannes Kepler’s observation of a supernova inside the Milky Way in 1604. Being far in the southern sky, it is clearly visible only from the Southern Hemisphere.

The Tarantula Nebula and its surroundings

This is an image of the Tarantula Nebula and its surroundings in the Large Magellanic Cloud galaxy, taken in 1987. Supernova 1987A is the bright star just to the right of centre. Credit: ESO

“This is the first time we’ve been able to really image where the dust has formed, which is important in understanding the evolution of galaxies,” said Remy Indebetouw, an astronomer at the National Radio Astronomy Observatory (NRAO) and the University of Virginia, both in Charlottesville, USA

Astronomers predicted that as the gas cooled after the explosion, large amounts of dust would form as atoms of oxygen, carbon, and silicon bonded together in the cold central regions of the remnant. However, earlier observations of SN 1987A with infrared telescopes, made during the first 500 days after the explosion, detected only a small amount of hot dust.

With ALMA’s resolution and sensitivity, the team was able to image the far more abundant cold dust, which glows brightly in millimetre and submillimetre light. The astronomers estimate that the remnant cloud now contains about 25 percent the mass of the Sun in newly formed dust. They also found that significant amounts of carbon monoxide and silicon monoxide have formed.

Aerial view of dishes of the Atacama Large Millimetre/submillimetre Array

Aerial view of dishes of the Atacama Large Millimetre/submillimetre Array (ALMA) telescope. Credit: ALMA

“SN 1987A is a special place since it hasn’t mixed with the surrounding environment, so what we see there was made there,” said Indebetouw. “The new ALMA results, which are the first of their kind, reveal a supernova remnant chock full of material that simply did not exist a few decades ago.”

There’s more information on Supernova 1987A, including an interview with Australian astronomers, on the ABC’s web site.

Adapted from information issued by NRAO.

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Supernovae make raw material for planets

SN 1987A

Observations with the Herschel Space Observatory show that supernova 1987A produced enough dust to make 200,000 new planets.

THE HERSCHEL SPACE OBSERVATORY has discovered that titanic stellar explosions can be excellent dust factories. In space, the dust mixes with gas to become the raw material for new stars, planets and, ultimately, life. This discovery may solve a mystery of the early Universe.

The discovery was made while Herschel was charting emission from cold dust in the Large Magellanic Cloud, a small galaxy near to the Milky Way. It is the perfect observatory for the job because cold dust radiates far-infrared light, the wavelengths Herschel is designed to detect.

Herschel saw a spot of light at the location of supernova 1987A, an exploding star first seen from Earth in February 1987, and the closest known supernova in the past 400 years.

Astronomers have been studying the remains of the explosion as its blast wave expands into its surroundings.

Planet factories

Herschel’s images are the first clear-cut far-infrared observations of SN1987A. They reveal cold dust grains at about -250 degrees C, which nevertheless emit more than 200 times the Sun’s energy.

“The supernova remnant was much brighter at infrared wavelengths than we were expecting,” says Mikako Matsuura, University College London, who is the lead author on the scientific paper detailing these results.

The remnant’s brightness was used to estimate the amount of dust. Surprisingly, there turned out to be about a thousand times more dust than astronomers had thought a supernova was capable of producing—enough to make 200,000 planets the size of Earth.

Artist's impression of the Herschel Space Observatory.

Artist's impression of the Herschel Space Observatory.

The origin of dust in the Universe is of great interest. The dust’s heavy atoms like carbon, silicon, oxygen and iron were not produced in the Big Bang and must have formed later.

Although they are only a minor part of the Universe and our Solar System, they are the main constituents of rocky planets like Earth and thus of life itself—many of the atoms we are made of were once part of the dust in the Universe.

Dust factories

However, it is not fully understood where this dust comes from, and especially where it came from in the young Universe. But scientists now have a clue.

The many old red giant stars in today’s Universe are thought to be the major dust producers, with the grains condensing like soot in a chimney as warm gases flow away from the star.

However, there were no such stars in the early Universe—yet we know there was already dust.

Now Herschel has shown that supernovae can produce enormous amounts of dust. The astronomers speculate that the dust condenses from the gaseous debris as it expands from the explosion and cools.

Since there were plenty of supernovae in the young Universe, this could help to explain the origin of dust seen at those times.

Adapted from information issued by ESA. Images courtesy ESA / C. Carreau / P. Challis (CfA).

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Blast from the past glows anew

Hubble image of SN 1987A

This Hubble Space Telescope image of SN 1987A shows an odd-shaped central blob of debris from the exploded star, which has now begun to brighten. The brightening is due to illumination by X-rays coming from the surrounding ring of hot gas.

IN 1987, LIGHT FROM AN EXPLODING STAR in a neighbouring galaxy, the Large Magellanic Cloud, reached Earth. Named Supernova 1987A, it was the closest supernova explosion witnessed in almost 400 years, and its proximity has enabled astronomers to study it in unprecedented detail as it evolves.

A team of astronomers has now announced that the supernova debris, which had been fading over the years, is now brightening. This shows that a different “power source” has begun to light up the debris, and marks its transition from a supernova to a supernova remnant.

“Supernova 1987A has become the youngest supernova remnant visible to us,” said Robert Kirshner of the Harvard-Smithsonian Centre for Astrophysics (CfA).

Kirshner leads a long-term study of SN 1987A with NASA’s Hubble Space Telescope. Since its launch in 1990, Hubble has provided a continuous record of the changes in SN 1987A.

A new power source

SN 1987A is surrounded by a ring of gas that blew off the progenitor star thousands of years before it exploded. The ring is about one light-year (10 trillion kilometres) across. Inside that ring, the “guts” of the star are rushing outward in an expanding debris cloud.

Most of a supernova’s light comes from radioactive decay of elements created in the explosion. As a result, it fades over time. However, the debris from SN 1987A has begun to brighten, suggesting that a new power source is lighting it.

Supernova 1987A

Supernova 1987A was the closest exploding star seen in almost 400 years. Astronomers are continuing with long-term studies of it.

“It’s only possible to see this brightening because SN 1987A is so close and Hubble has such sharp vision,” Kirshner said.

A supernova remnant consists of material ejected from an exploding star, as well as the pre-existing material the blast wave sweeps up.

The outflowing debris from SN 1987A is beginning to crash into the surrounding gas ring, creating powerful shock waves that generate X-rays, which have been detected by NASA’s Chandra X-ray Observatory. Those X-rays are illuminating the debris, and shock heating is making it glow.

The same process powers other well-known supernova remnants in our galaxy, such as Cassiopeia A.

Change you can see

Because it’s so young, the remnant of SN 1987A still shows the history of the last few thousand years of the star’s life recorded in the knots and whorls of gas. By studying it further, astronomers may decode that history.

“Young supernova remnants have personality,” Kirshner agreed.

Eventually, that history will be lost when the bulk of the expanding stellar debris hits the surrounding ring and shreds it. Until then, SN 1987A continues to offer an unprecedented opportunity to watch a cosmic object change over the course of a human lifetime. Few other objects in the sky evolve on such short time-scales.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics. Images courtesy NASA / P. Challis (CfA) / David Malin (AAO).

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