A Type II-P supernova, SN 2004dj, blazes at the top right of this Hubble Space Telescope image, taken in 2004. Credit: NASA , ESA , A.V. Filippenko (University of California, Berkeley), P. Challis (Harvard-Smithsonian Center for Astrophysics).

Astronomers have long relied on stellar explosions called Type Ia supernovae to measure the scale of the cosmos. A second class of supernovae may now be put to the same use, providing an independent check on measurements that were first used more than a decade ago to discover the accelerating expansion of the Universe.

A growing number of researchers are working on the idea that some Type II supernovae — which are caused by the gravitational collapse of giant stars with iron cores — may have a role as gauges of cosmic distance. The method could be put to use with next-generation sky surveys — including the Dark Energy Survey due to start at Cerro Tololo in Chile in late 2011, and the Large Synoptic Survey Telescope, still in the development phase, at Cerro Pachón, also in Chile. Both are expected to find tens of thousands of supernovae a year.

"We're at the stage where it would be stupid to ignore alternative methods to Type Ia," says Dovi Poznanski, an astrophysicist at the University of California, Berkeley, who has re-analysed results that he says show the promise of the new cosmic measuring sticks. His most recent findings were published on 1 October in the Astrophysical Journal1.

Standard candles

The key feature of Type Ia supernovae — which result from the explosion of white dwarf stars that are sucking up material from companion stars — is that they only go off as the white dwarf approaches a critical mass, the Chandrasekhar limit (1.4 times the mass of our Sun). This means that such supernovae are remarkably consistent in their behaviour. Their intrinsic brightness can be predicted by observing how their apparent brightness from Earth rises and falls, and used to calculate the distance away that they must be. By providing a measure of the distance from Earth to remote galaxies, such Type Ia 'standard candles' underpinned the discovery of the mysterious repulsive dark energy that is driving the Universe's accelerating expansion.

Galaxy-distance estimates using Type Ia supernovae are precise to within an error of 7%. But such measurements are also thought to suffer from systematic errors, such as the possibility that the type of galaxy hosting the supernova makes a difference to its brightness, a factor that could vary with distance from Earth and so skew results.

"It is always good to have more than one way to measure an important physical effect like the cosmic expansion," says Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics, who co-discovered the Universe's accelerating expansion in 1998, using Type Ia standard candle measurements2.

Working out the intrinsic brightness of a second class of supernovae, Type II-P, so-called because their brightness stays on a roughly constant plateau (P) for around 90 days before falling off, is considerably more complex. But it is feasible, thanks to a technique pioneered in 2002 by astronomers Mario Hamuy, now at the University of Chile in Santiago, and Philip Pinto, at the University of Arizona, Tucson3. The method was further developed by Poznanski and his colleagues4.

"It is unlikely that this technique will be able to compete with Ia, but it can contribute complementary cosmic information. It is coming into its own," says Adam Burrows, an astrophysicist at Princeton University in New Jersey who has worked on both types of supernovae.

For the Type II-P method, astronomers take a spectrum of the supernova around 50 days after it explodes, and use the shift relative to a standard of a spectral line caused by the absorption of light by iron to determine the speed at which the star is ejecting some of its material. That speed and the 50 day timescale are used to calculate the size of the explosion. Taken together with temperature measurements, that reveals the explosion's intrinsic brightness. Finally, a comparison with the apparent brightness as seen from Earth shows how far away it must be.

In 2009, Poznanski demonstrated that a sample of 17 Type II supernovae could be used to predict the distances from Earth of their host galaxies to within an error of 10%4 — not much more than the error of estimates using Type Ia explosions.

Ironing out the errors

But astronomers are still some way from being able to check the cosmic expansion using Type II measurements. All the supernovae in Poznanski's sample were near to Earth, and when astrophysicist Christopher D'Andrea of the University of Pennsylvania in Philadelphia and his colleagues tried to apply the method to supernovae further away — about 1.7 billion light years or 530 megaparsecs from Earth — their results gave around 15% error5. But D'Andrea and his colleagues used supernova spectra collected by the Sloan Digital Sky Survey at Apache Point Observatory in New Mexico, which was primarily looking for Type Ia supernova, and some of the spectra were taken only two weeks after the supernova's explosion. This doesn't deliver as accurate an estimate of speed of the explosion and hence brightness and distance.

Poznanski revisited D'Andrea's results in his October paper, and addressed the fact that the researchers had selected a biased sample of supernovae, favouring intrinsically brighter objects. By recalibrating spectral lines to calculate the speed of these supernovae explosions, he brought the error down to 11%1. He says that the Type II technique still works. D'Andrea agrees with Poznanski's diagnosis and says that he is very optimistic about the technique. "The lesson I learned is that you have to know what you're doing before you start the survey," he says.

Poznanski is now working with around 60 Type II supernovae, spotted by the Palomar Transient Factory survey at Palomar Observatory in California, to get the method to work at greater distances and let astronomers probe further back in the Universe's history. "[D'Andrea's results] show we need a better sample and that's what we're trying to do," he says.