News in Science

Supernova lifts the veil on star dust

Cosmic clue The creation of star dust in the shockwave of a supernova has been observed by astronomers for the first time.

The discovery, reported today in the journal Nature, helps explain the production of large quantities of star dust, which forms stars, planets and people.

Scientists have long known that grains of star dust are produced by the condensation of gas ejected in the stellar winds of old stars such as red giants, or blasted out in supernovae, the explosive deaths of stars at the end of their lives.

It's been unclear, however, exactly how and where these dust grains condense and grow, or how they avoid destruction due to the intense heat and radiation in the harsh environment of star-forming galaxies.

"We've achieved real time observations of how dust gets produced, and how the dust mass builds up," says lead author of the new study, Dr Christa Gall, of the Aarhus University.

New observations by Gall and colleagues show how star dust forms behind the supernova shockwave.

"We examined a very massive star that shed large quantities of gas prior to going supernova," says Gall.

Once the star exploded, the shockwave collided with this wall of gas, compressing the gas and cooling it down.

According to Gall, increasing gas density allows it to radiate away heat more efficiently.

"Once the material has cooled below 2000 Kelvin (1727°C), dust starts to condense out of the gas," says Gall.

Fast growing

Using the European Southern Observatory's Very Large Telescope in Chile, Gall and colleagues studied light from an unusually bright supernova, called SN2010jl, which exploded in the galaxy UGC 5189A, 160 million light years away.

Gall and colleagues found that dust formation starts soon after the explosion and continues over a long period of time.

The authors observed the supernova in visible and near-infrared wavelengths, nine times in the months following the explosion, detecting the rapid formation of large dust grains between 40 and 240 days after the initial blast.

The researchers found that dust grains, larger than one thousandth of a millimetre in diameter formed rapidly in the dense material surrounding the star.

These dust grains are big enough to resist destruction, suggesting a rapid and efficient mechanism for dust formation and growth.

"We were somewhat surprised because we expected to see small grains forming in supernovae, because that's what we see in the Milky Way," says Gall.

"But these were large grains, a micrometre in size, which for us is very large. Usually in the Milky Way large dust grains are just a quarter of that size.

"We suspect it's possible to grow larger sized grains because of the high densities [of gas]."

Gall and colleagues observed the supernova remnant again around 500 to 900 days after the initial explosion, detecting accelerated growth in the amount of star dust being produced.

If the dust production in SN2010jl continues to follow this trend, the supernova will produce about half the mass of the Sun in star dust within 25 years, which is similar to what's been observed in other supernovae.