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Bloopernovas: troublesome exploding stars
BLOOPER
Much of what we know about the history of the universe is based on exploding stars. Problem is, we don’t really understand what makes them explode. So where does that leave the universe? Stuart Clark investigates
J.WARREN & J.HUGHES ET AL /RUTGERS/CXC/NASA
●
NOVA
IN NOVEMBER 1572, a dazzling new star appeared in the night sky. It became so bright so quickly that it soon outshone everything except the sun and the moon and could even be seen in daylight. Danish astronomer Tycho Brahe tracked the star for 16 months. As it slowly faded, the star changed colour from white to yellow then orange and finally faint red. We now know that what Brahe saw was probably a type Ia supernova, a species of exploding star that, over the past 30 years, has become increasingly important in astrophysics. Because they are all thought to explode with the same brightness, type Ia supernovae are used as “standard candles” to gauge distances across the universe. But type Ia supernovae are beset with problems. It has become clear that they do not all explode with the same brightness. What’s more, though astronomers were once sure they knew how they formed, they are no longer so confident. It is more than mere details that are at stake. The uncertainty threatens to undermine our ability to study dark energy, the mysterious force that is thought to pervade space and will ultimately determine the fate of the universe. Every year, astronomers report around 600 supernovae from galaxies across the universe. About half are type Ia. According to the classical explanation, type Ia supernovae are the death throes of a class of old, dead stars called white dwarfs. Most white dwarfs just sit in space doing nothing, but if one happens to be in orbit around a companion star things get more interesting. The dwarf’s gravity drags gas from its companion in a process known as accretion (see Diagram, right). When the white dwarf exceeds 1.38 times the mass of the sun, the socalled Chandrasekhar limit, it can no longer The probable remains of a type Ia supernova observed by Tycho Brahe in 1572 are still visible 52 | NewScientist | 27 October 2007
www.newscientist.com
(New Scientist, 5 February 2005, p 31). If dark energy does change, the best candidates to explain it are a previously unknown force of nature called quintessence or a modification of gravity over extremely large distances. On the other hand, if dark energy remains constant, the leading explanation is that empty space has an inherent energy that is driving the universe apart – a variant of Einstein’s cosmological constant (for more detail on these competing ideas see New Scientist, 17 February, p 28). No wonder astronomers are searching for
Hence the supernova problem. If astronomers do not know how they form, can they really be sure that the dark energy result holds up? “We have given physicists a huge problem to explain dark energy,” Kirshner says. “But we don’t actually understand the physics of the supernovae we used to deduce its existence.” For now, astronomers are confident that dark energy isn’t threatened. Despite the uncertainties, type Ia supernovae all still seem to go bang with more or less the same energy. There are some differences – up to a factor of
“We all believe white dwarfs explode, we just haven’t seen any on the path to destruction”
www.newscientist.com
about 2.5, according Ralf Napiwotzki of the University of Hertfordshire in Hatfield, UK – but astronomers feel they can spot these variations and average them away. “I don’t think dark energy is a house of cards waiting to tumble,” says Kirshner. At any rate, there is other evidence that dark energy exists. That doesn’t mean there’s nothing to worry about. Last month, astronomers reported that very distant supernovae – more than 8 billion light years away – were about 12 per cent brighter than less distant ones, for reasons that are not clear (New Scientist, 13 October, p 14). Until we fully understand what causes type Ia supernovae and why they vary, Kirshner says we will never be able to measure the rate of expansion precisely enough to answer a crucial question: has the rate, and hence the strength of dark energy, changed over time? This in turn will provide clues about the ultimate fate of the universe H;7:OJEHKC8B; ?dj^[ijWdZWhZ[nfbWdWj_ed\ehWjof[?Wikf[hdelW"Wm^_j[ ZmWh\WYYh[j[icWjj[h\hecWYecfWd_edijWhkdj_b_j][jiie cWii_l[_jYebbWfi[iWdZj^[d[nfbeZ[i
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support itself against gravity and it collapses, triggering a wave of nuclear fusion that sweeps through the star and blows it to pieces (The Astrophysical Journal, vol 186, p 1007). That’s the theory, anyway. In practice, astronomers struggle to find evidence that this scenario ever happens. The problem is that, while accretion from a companion is necessary to trigger a supernova, it is not sufficient. The rate of accretion is critical too. “In a computer model, you really have to finetune the mass flow rate to get a white dwarf to explode completely,” says Stephen Smartt of Queen’s University Belfast in the UK. If the mass flows too slowly, the white dwarf will take many billions of years to reach the limit. Too quickly and the pile-up of matter ignites prematurely, producing a weaker explosion. So while astronomers know of around 700 white dwarfs pulling matter off a companion, only a tiny number appear to be flowing at the right rate. “We all believe that white dwarfs accrete and explode,” says Robert Kirshner of Harvard University. “We just haven’t seen any stars on this path to destruction.” This puts astronomers in a tight spot, because type Ia supernovae are more than a curiosity – they are a crucial piece of evidence for the existence of dark energy. Because type Ia supernovae supposedly all explode with the same energy, differences in their brightness can be attributed to distance, making them good standard candles. In 1998, astronomers used type Ia supernovae to calculate the rate at which the universe had expanded throughout its history. They expected to see a deceleration due to gravity but found the opposite: the expansion was accelerating. The accepted explanation is that the universe must be filled with a mysterious “dark energy” pushing in the opposite direction to gravity (New Scientist, 3 April 1999, p 29).
the correct explanation of type Ia supernovae. So what do we know? The evidence for white dwarfs being involved is compelling. For a start, type Ia supernovae explode in all celestial environments, often far away from areas of recent star formation. This indicates that the progenitors must be old. Secondly, the way their brightness decreases suggests that the exploding object is not surrounded by an envelope of gas, which rules out most types of star. Taken together, these make a convincing case for white dwarfs. The problem, as Smartt puts it, is “how to get them to explode”. One possibility is that the classical model of accretion is wrong. Astronomers have devised another possible scenario in which a pair of white dwarfs in orbit around one another spiral inwards and collide. Again, though, observational evidence is sparse. In a survey of 1014 white dwarfs, Napiwotzki found 139 pairs, but none was convincing as a supernova candidate. Another option on the table is that there are numerous ways to make a white dwarf go bang. “It would not be shocking to discover several different ways to trigger a type Ia supernova,” says Saul Perlmutter of Lawrence Berkeley National Laboratory in California, who was part of the team that deduced the existence of dark energy. Perlmutter now heads the Supernova Acceleration Probe (SNAP), one of three projects being considered by NASA for a major mission to probe dark energy. One part of the project will be to observe thousands of supernovae as they happen. The hope is that the reams of supernova data the mission will gather could help crack the riddle. Only then will we know whether our ideas about them stand up, or are about to be blown away in a giant cosmic explosion. ● Stuart Clark is a science journalist based in London 27 October 2007 | NewScientist | 53
Much of what we know about the history of the universe is based on exploding stars. Problem is, we don’t really understand what makes them explode. So where does that leave the universe? Stuart Clark investigates
J.WARREN & J.HUGHES ET AL /RUTGERS/CXC/NASA
●
NOVA
IN NOVEMBER 1572, a dazzling new star appeared in the night sky. It became so bright so quickly that it soon outshone everything except the sun and the moon and could even be seen in daylight. Danish astronomer Tycho Brahe tracked the star for 16 months. As it slowly faded, the star changed colour from white to yellow then orange and finally faint red. We now know that what Brahe saw was probably a type Ia supernova, a species of exploding star that, over the past 30 years, has become increasingly important in astrophysics. Because they are all thought to explode with the same brightness, type Ia supernovae are used as “standard candles” to gauge distances across the universe. But type Ia supernovae are beset with problems. It has become clear that they do not all explode with the same brightness. What’s more, though astronomers were once sure they knew how they formed, they are no longer so confident. It is more than mere details that are at stake. The uncertainty threatens to undermine our ability to study dark energy, the mysterious force that is thought to pervade space and will ultimately determine the fate of the universe. Every year, astronomers report around 600 supernovae from galaxies across the universe. About half are type Ia. According to the classical explanation, type Ia supernovae are the death throes of a class of old, dead stars called white dwarfs. Most white dwarfs just sit in space doing nothing, but if one happens to be in orbit around a companion star things get more interesting. The dwarf’s gravity drags gas from its companion in a process known as accretion (see Diagram, right). When the white dwarf exceeds 1.38 times the mass of the sun, the socalled Chandrasekhar limit, it can no longer The probable remains of a type Ia supernova observed by Tycho Brahe in 1572 are still visible 52 | NewScientist | 27 October 2007
www.newscientist.com
(New Scientist, 5 February 2005, p 31). If dark energy does change, the best candidates to explain it are a previously unknown force of nature called quintessence or a modification of gravity over extremely large distances. On the other hand, if dark energy remains constant, the leading explanation is that empty space has an inherent energy that is driving the universe apart – a variant of Einstein’s cosmological constant (for more detail on these competing ideas see New Scientist, 17 February, p 28). No wonder astronomers are searching for
Hence the supernova problem. If astronomers do not know how they form, can they really be sure that the dark energy result holds up? “We have given physicists a huge problem to explain dark energy,” Kirshner says. “But we don’t actually understand the physics of the supernovae we used to deduce its existence.” For now, astronomers are confident that dark energy isn’t threatened. Despite the uncertainties, type Ia supernovae all still seem to go bang with more or less the same energy. There are some differences – up to a factor of
“We all believe white dwarfs explode, we just haven’t seen any on the path to destruction”
www.newscientist.com
about 2.5, according Ralf Napiwotzki of the University of Hertfordshire in Hatfield, UK – but astronomers feel they can spot these variations and average them away. “I don’t think dark energy is a house of cards waiting to tumble,” says Kirshner. At any rate, there is other evidence that dark energy exists. That doesn’t mean there’s nothing to worry about. Last month, astronomers reported that very distant supernovae – more than 8 billion light years away – were about 12 per cent brighter than less distant ones, for reasons that are not clear (New Scientist, 13 October, p 14). Until we fully understand what causes type Ia supernovae and why they vary, Kirshner says we will never be able to measure the rate of expansion precisely enough to answer a crucial question: has the rate, and hence the strength of dark energy, changed over time? This in turn will provide clues about the ultimate fate of the universe H;7:OJEHKC8B; ?dj^[ijWdZWhZ[nfbWdWj_ed\ehWjof[?Wikf[hdelW"Wm^_j[ ZmWh\WYYh[j[icWjj[h\hecWYecfWd_edijWhkdj_b_j][jiie cWii_l[_jYebbWfi[iWdZj^[d[nfbeZ[i
9ecfWd_edijWh
M^_j[ZmWh\ 7YYh[j_edZ_iY CWjj[hfkbb[Ze\\ YecfWd_edijWh
IEKH9;0;IE
support itself against gravity and it collapses, triggering a wave of nuclear fusion that sweeps through the star and blows it to pieces (The Astrophysical Journal, vol 186, p 1007). That’s the theory, anyway. In practice, astronomers struggle to find evidence that this scenario ever happens. The problem is that, while accretion from a companion is necessary to trigger a supernova, it is not sufficient. The rate of accretion is critical too. “In a computer model, you really have to finetune the mass flow rate to get a white dwarf to explode completely,” says Stephen Smartt of Queen’s University Belfast in the UK. If the mass flows too slowly, the white dwarf will take many billions of years to reach the limit. Too quickly and the pile-up of matter ignites prematurely, producing a weaker explosion. So while astronomers know of around 700 white dwarfs pulling matter off a companion, only a tiny number appear to be flowing at the right rate. “We all believe that white dwarfs accrete and explode,” says Robert Kirshner of Harvard University. “We just haven’t seen any stars on this path to destruction.” This puts astronomers in a tight spot, because type Ia supernovae are more than a curiosity – they are a crucial piece of evidence for the existence of dark energy. Because type Ia supernovae supposedly all explode with the same energy, differences in their brightness can be attributed to distance, making them good standard candles. In 1998, astronomers used type Ia supernovae to calculate the rate at which the universe had expanded throughout its history. They expected to see a deceleration due to gravity but found the opposite: the expansion was accelerating. The accepted explanation is that the universe must be filled with a mysterious “dark energy” pushing in the opposite direction to gravity (New Scientist, 3 April 1999, p 29).
the correct explanation of type Ia supernovae. So what do we know? The evidence for white dwarfs being involved is compelling. For a start, type Ia supernovae explode in all celestial environments, often far away from areas of recent star formation. This indicates that the progenitors must be old. Secondly, the way their brightness decreases suggests that the exploding object is not surrounded by an envelope of gas, which rules out most types of star. Taken together, these make a convincing case for white dwarfs. The problem, as Smartt puts it, is “how to get them to explode”. One possibility is that the classical model of accretion is wrong. Astronomers have devised another possible scenario in which a pair of white dwarfs in orbit around one another spiral inwards and collide. Again, though, observational evidence is sparse. In a survey of 1014 white dwarfs, Napiwotzki found 139 pairs, but none was convincing as a supernova candidate. Another option on the table is that there are numerous ways to make a white dwarf go bang. “It would not be shocking to discover several different ways to trigger a type Ia supernova,” says Saul Perlmutter of Lawrence Berkeley National Laboratory in California, who was part of the team that deduced the existence of dark energy. Perlmutter now heads the Supernova Acceleration Probe (SNAP), one of three projects being considered by NASA for a major mission to probe dark energy. One part of the project will be to observe thousands of supernovae as they happen. The hope is that the reams of supernova data the mission will gather could help crack the riddle. Only then will we know whether our ideas about them stand up, or are about to be blown away in a giant cosmic explosion. ● Stuart Clark is a science journalist based in London 27 October 2007 | NewScientist | 53