FOR a while there, Adam Frank and his fellow astronomers were worried. Their work had been moving along so nicely, but then suddenly everything changed. 鈥淲e realised we didn鈥檛 have a clue what was going on,鈥� says Frank, who is based at the University of Rochester in New York state.
Happily, they are back on their feet now. The ethereal, hovering clouds of glowing gas known as planetary nebulae are beginning to shed their mystique. And their unveiling could answer an intriguing question: what kind of spectacle will mark the last gasps of our sun, 5 billion years from now?
A planetary nebula is the spectacular swansong of a sun-like star. 鈥淲hat you are seeing is a star coming unhinged,鈥� says Frank. We know of some 1500 of these in our galaxy. A classic example is the Ring nebula, which lies about 2000 light years away in the constellation Lyra. Once the star at its centre had exhausted its helium core, it began to burn hydrogen and helium outside the core, swelling as it entered the final giant phase of its life. Radiation from the star blew a wind of gas and dust into space at a speed of about 10 kilometres per second.
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When the star ran out of fuel altogether, the stellar core started to shrink into a white dwarf star with a temperature of more than 100,000 掳C. The atmosphere started to stream outwards faster, at 1000 kilometres per second, ramming the earlier, slower wind into a thin shell. The molecules in the shell, excited by radiation from the central star, started to glow: from Earth the phenomenon can be seen as a giant shimmering ring a light year across.
But the simple structure of the Ring nebula is unusual. Most planetary nebulae are either elliptical or 鈥渂ipolar鈥�, with two lobes that stick out like butterfly wings or rocket jets. Others are reminiscent of jellyfish tendrils or skinny-legged insects, all with their own curious symmetry. 鈥淭here鈥檚 every possible shape you can imagine,鈥� Frank says. And these shapes are harder to explain.
In the early 1990s, astronomers thought they had the answer. An old giant star might belch out most gas and dust around its equator where the surface rotates fastest, they reasoned. Perhaps dark patches similar to sunspots around the star鈥檚 equator could also make the wind denser around the equator. Later, as the star turned into a white dwarf and ejected its atmosphere in a faster wind, the dusty doughnut at the equator would inhibit the outflow of gas, which would instead escape at the poles, creating the double lobes. 鈥淲e thought this model did a pretty good job of explaining the entire range,鈥� Frank says.
But then the Hubble Space Telescope came along. From 1994, Hubble beamed back image after image of planetary nebulae far too complex for any existing theory to explain. The Cat鈥檚 Eye, for example, which had appeared as a blobby bubble from ground-based telescopes, turns out to have extended gas shells with pointy caps, spiky jets at both ends, and curious little knots. Hubble images of other planetary nebulae showed with devastating clarity an array of strange and intricate symmetries, with multiple jets, radial wisps and even honeycomb-like structures. While the world marvelled at the pictures, the astronomers, who had thought the book on planetary nebulae was closed, discovered they had more thinking to do.
There was more puzzling evidence to come. Observations of many intricately shaped planetary nebulae have revealed starlight reflecting off spherical dusty shells around them. That suggests that the wind streaming out from an old, giant star is spherically symmetrical, so it would not create a dusty doughnut around the equator. The nail in the old model鈥檚 coffin came in 2001 when Javier Alcolea and his colleagues at Spain鈥檚 National Astronomical Observatory measured the momentum carried by the outflows from fledgling planetary nebulae: the winds carried up to 1000 times more momentum than the star鈥檚 radiation could supply. Where could the energy possibly come from?
One potential source, favoured by Frank and others, is magnetic fields. 鈥淭he star鈥檚 magnetic field could act as a drive belt that taps the star鈥檚 rotational energy and flings material away,鈥� Frank says. With his colleague Eric Blackman, Frank has modelled the way in which material from the star could be pulled into a disc around a companion star. Magnetic field lines threading through the disc channel hot ionised gas into outflows at right angles to the disc. If the disc wobbles slightly, or 鈥減recesses鈥� like a spinning top, the two jets would naturally swing around like a garden sprinkler, which could explain the odd shapes of some planetary nebulae.
One problem with this model is that while the central stars of at least 16 bipolar nebulae harbour a close companion, many others show no signs of one. That could be because the companion is too faint to see through the glare of the white dwarf, or because it has already fallen into the central star and vanished without trace. But it could also be that there is no companion, and never was.
Frank is unperturbed, however. He thinks a 鈥渕agnetic explosion鈥� on the star鈥檚 surface could create some of the strange shapes without the need for a companion star. This idea is supported by some of Alcolea鈥檚 results. His team has shown that gas flowing from young planetary nebulae follows an intriguing pattern in which the outflowing gas furthest from the core moves much faster than the inner layers. In fact, the speed of the outflow is directly proportional to the distance from the central star, and that velocity profile is the hallmark of a sudden explosion. If a bomb exploded in space, for instance, fast shrapnel would go farther than slow shrapnel, and the distance of any fragment from the explosion site would always be proportional to its speed.
This explosion could occur if the newborn white dwarf has a strong magnetic field and rotates very fast, while the envelope of gas around it expands and rotates very slowly, Frank says. A powerful dynamo would be created as the magnetic field lines fold tightly at the white dwarf鈥檚 surface. Eventually, there would be so much stored energy in the twisted field that the field would reconfigure in a giant explosion, accelerating the atmosphere outward over just a few decades.
Magnetic key
鈥淚t鈥檚 like winding up a big coiled spring 鈥� eventually, you have a big magnetic explosion,鈥� Frank says. Simulations of this outburst by Frank, Blackman and Sean Matt of McMaster University in Hamilton, Ontario, in Canada show that factors including the mass of the stellar core, its rotation rate and its magnetic field strength will affect the shape of the planetary nebula. By changing these inputs, it is possible to recreate many of the typical multi-lobed shapes Hubble observed.
Last year, a team led by Stefan Jordan of the Astronomy Research Institute in Heidelberg, Germany, provided further confirmation that magnetism is a key force in creating bipolar planetary nebulae. The team reasoned that if magnetism is important, bipolar nebulae should only form around highly magnetised stars. To test this idea, they used the knowledge that a strong magnetic field splits the energy levels of atoms in a characteristic way that polarises the light that atoms emit. Using the Very Large Telescope (VLT) at the European Southern Observatory in Chile, they measured the polarisation of starlight from four white dwarfs inside bipolar nebulae. Their results suggested that all four have strong magnetic fields, roughly 1000 times that of the sun (Astronomy & Astrophysics, vol 432, p 273). Next month, Jordan plans to use the VLT to find out if the same principle holds for spherical planetary nebulae. If the magnetic field theory is correct, then the central stars of these nebulae should have weak magnetic fields, or none at all.
Not everyone is convinced that magnetic fields are necessary to explain the nebulae. Noam Stoker at the Technion-Israel Institute of Technology in Haifa, for example, still thinks that a companion star, or even a giant planet like Jupiter, could give rise to bipolar nebulae. The fact that some don鈥檛 have a visible companion is not reason enough to dismiss the idea, he says. The issue may be resolved by astronomers who are monitoring the central stars of planetary nebulae to see if the gravitational pull of an unseen companion is giving them a slight wobble.
So there is still no definitive explanation. But astronomers now have some more observations to help them make sense of the strange zoo of planetary nebulae. NASA鈥檚 Chandra X-ray Observatory and Europe鈥檚 XMM-Newton satellite have revealed intense X-rays coming from several planetary nebulae. They signal the existence of outflows of gas at temperatures of several million degrees. These might result from shock waves as gas outflows collide or from flares powered by intense magnetic fields, but no one knows for certain.
Frank thinks enlightenment might come from using top-line telescopes to capture images of nebulae over many years to make 鈥渕ovies鈥�. These could be matched against a theoretical model to show how planetary nebulae evolve. He points out that new computer techniques are slashing the time needed to develop the theories. 鈥淭o make those movies would be very important,鈥� he says. 鈥淲e could see how all the stuff is moving and get away from this snapshot mentality.鈥�
鈥淚t might be possible to predict what the last gasp of the sun will look like 5 billion years from now鈥�
He is confident that astronomers are on the right track with a mix of magnetic fields, rotation and possibly companion stars. 鈥淚n the mid-1990s, we were flailing around trying to figure out which direction to go,鈥� he says. 鈥淚 believe we have a good direction now, and in the next few years we鈥檙e going to make a lot of progress.鈥� This could have benefits beyond understanding planetary nebulae, as similar processes involving magnetism and rotation may well be a driving force in other astronomical objects, from newborn stars to energetic gamma-ray bursts.
The results could also give us a glimpse of our sun鈥檚 likely future. The sun is roughly halfway through its life, and in about 5 billion years鈥� time will swell into a red giant. The swelling sun is expected to swallow the innermost planets Mercury and Venus, and possibly the Earth too, before entering its planetary nebula phase. If magnetism is a major player in shaping planetary nebulae, and if we can predict the sun鈥檚 future magnetic field, it might be possible to take a stab at what the sun鈥檚 nebula will look like. Perhaps our home will play host to the most beautiful nebula of all. It鈥檚 a shame we won鈥檛 be around to see it.
