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ASTRONOMY. It may be the venerable grandaddy of science, but over the last century it has been reduced to the poor relation of every other branch of modern science.

That’s because it is missing one of the foundation stones on which to build its house of knowledge. It has observation and theory in spades but, unlike most other scientific disciplines, it lacks experiment. No one can bench-test the formation of the gas giant Jupiter or nip out the back and explode a star to see if their ideas are correct. No, we have simply had to make do with what the universe throws at us. Until now, that is.

We have at last found a way to bring the universe inside the laboratory. Home-made stars and planets are allowing our various ideas and assumptions about celestial objects to be put to the test in unprecedented ways. Not only do the measurements have the power to resolve arguments in astrophysics that have raged for decades, they could also finally reveal how solar systems form and the innermost secrets of stars.

So just how do you recreate stars and planets in the lab? The answer is lasers. Really, really big ones. Scattered around the world are about a dozen megalasers, each one housed in its own warehouse-sized building.

Using the energy created by these megalasers, we can compress gases to recreate the conditions inside a giant planet or blow things apart just as inside an exploding star. We can mimic the behaviour of a young star expelling snake-like jets of plasma and recreate the environments around a feeding black hole. In the not-too-distant future, we might even be able to reproduce miniature versions of the titanic fireballs that engulf those mysterious cosmic explosions, gamma-ray bursts.

Laser astrophysics began just over 15 years ago, born out of ongoing efforts to develop nuclear fusion reactors. Ironically, we have never been able to manufacture a fusion reactor that gives out more energy than it takes to run, even though there are countless natural ones throughout the universe. You can see about 3000 of them every time you look up on a clear night. They are, of course, stars, nuclear furnaces which mostly fuse hydrogen into helium, releasing vast amounts of energy in the process.

Lights, camera, action

In their attempts to understand how to construct artificial fusion reactors, physicists have built powerful lasers that can produce terawatt and petawatt pulses. With these, they could compress fuel pellets to densities similar to those found inside stars and see what would happen. That’s when the penny dropped: they realised that by attempting to reproduce the conditions inside a star, they were glimpsing realms that astrophysicists had only dreamed of seeing.

For Bruce Remington at in California, the wake-up call came in 1995 when he saw two different pictures, in two different journals, and was astonished at how similar they were. One showed the simulation of a fusion fuel-pellet imploding after being hit by powerful laser beams. The other showed a simulation of an exploding star, a supernova, whose core implodes just before the outer layers are blown off. Although the scales of time and distance were vastly different, the patterns of turbulence were almost identical.

“Tens of nanoseconds in a laser experiment accurately scales up to a few thousand seconds in a star,” says Paul Drake at the University of Michigan in Ann Arbor, one of the first researchers to use lasers to mimic supernovae.

The depth of our ignorance about stellar explosions was literally thrown in our faces in 1987. That was when we saw go boom. Astronomers watched in surprise as events 160,000 light years away unfolded in unforeseen ways.

That was not what they expected to see at all. The interior of a giant star on the verge of exploding had been thought to resemble an onion, with layers of different chemicals surrounding an inert heart of iron and nickel. According to the best calculations at the time, those layers are supposed to explode outwards in order, so the last elements we would observe would be the iron and nickel from the core. Yet in supernova 1987A, iron and nickel were some of the first to become visible. “The iron was ejected at higher speeds than the outer layers,” says Drake. “To some degree, the star turned itself inside out.”

As computer simulations had failed to predict this, Drake and his colleagues turned to lasers in an attempt to . First, they spent a year and a half convincing themselves that laser experiments really could say something about stars. Since then they have been running experiments at the University of Rochester’s in New York.

At the facility, the team make mini supernovae from a sheet of dense plastic topped with carbon foam one-tenth as dense. Then they blast it with 10 laser beams at once, instantly superheating the plastic layer. This creates a blast wave that forces its way through the less dense carbon foam at 50 million atmospheres of pressure (see diagram).

Almost simultaneously, the team fires X-rays through the exploding cloud of plastic onto a photographic plate, allowing the team to take a single snapshot of it. By repeating the same experiment many times, they can build up a complete picture of the mini supernova.

“These experiments are easy to describe but devilishly difficult to perform,” says Remington. Everything happens so quickly, the experiment must be timed to perfection.

With perseverance, Drake and his colleagues have gained insight into what might be wrong with the computer simulations of stellar explosions. Their experiments reveal that computer models have been underestimating the viscosity of stellar material, whose value affects how much turbulence is produced. “It’s the difference between air and molasses,” says Drake. Correct for this and the computer simulations start to mimic supernovae more accurately.

There is another problem with supernovae that laser astrophysics hopes to solve. As the dense inner material is flung through the less dense outer layers of a star, it creates turbulence and mixes everything up. Traditional computer simulations do not model turbulence well.

“Our theoretical understanding of turbulence is incomplete,” says astrophysicist of the University of Chicago. In other words, you cannot write down a set of equations describing the state of a turbulent system at any given time and then use them to predict what it will look like next. Instead, you have to employ a brute-force approach, using sheer computer muscle.

To see the scale of this problem, take your morning cup of coffee and stir in some milk. You are using turbulence to mix two fluids. To determine how they mix, physicists mentally split the cup into boxes and assign numbers to represent the properties inside each box, such as the temperature and density of the fluid. A computer then calculates how each box interacts with its neighbours during one brief instant of time and then re-evaluates those numbers. Once it has done this for every box, it starts again for the next slice of time and so on.

To do this massive computation perfectly, each box should be tiny and contain just one fluid particle, but before you can get anywhere near this sort of precision, the numbers become mind-bogglingly large. Scientists talk of degrees of freedom as a measure of both the number of particles in a system and the number of ways each particle can interact with those around it. A single cup of coffee possesses a staggering 10⁴⁰ degrees of freedom – far more than you can model in today’s computers. “Maybe in 10 years we will be able to fully model a cup of coffee,” says Khokhlov.

“We don’t understand turbulence. Maybe in 10 years we will be able to fully model a cup of coffee”

Until then the computation will always be approximate, and thus prone to errors, because small-scale physical interactions are not being taken into account. Scale that up to celestial proportions and those “small scale” interactions might be over the size of the solar system, or larger. If it is going to take 10 years to fully model a cup of coffee, how long until we can model an entire star?

“Never,” Khokhlov says. “Not until someone comes up with a clever theory that does not depend on what is happening on the small scale.” The only hope is to continue to investigate turbulence in order to learn how to better approximate its behaviour. So teams around the world continue to create mini supernovae, hoping to understand just that little bit more about turbulent mixing with every plastic star they blast to smithereens. “Many people are very pleased by what we are doing,” says Drake.

One argument laser astrophysics hopes to resolve soon concerns the formation of the gas-giant planets. Different scenarios not only produce different interiors for these planets but also have implications for the way the rest of the solar system formed. Yet current observations and computer models cannot decide between the two front-runners. Did Jupiter and Saturn form as the result of a sudden collapse of gaseous matter? Or did they form as the terrestrial planets did, through the gradual collision of rocky building blocks?

Inside Jupiter

If they formed early and fast, as in the gas-instability scenario, then they would have had enough gravity to influence the formation of the other planets. If rocky collisions were involved, however, then Jupiter and Saturn simply grew large enough to finish their leisurely formation by pulling gas from the surrounding nebula, by which time other planets would have formed independently. “At the moment we can’t even say whether Jupiter and Saturn formed in the same way as each other,” says Remington. And that’s where lasers come in handy.

We know that Jupiter and Saturn are made mostly from hydrogen, with a dash of helium. What we do not know is how hydrogen and helium mixtures behave under great pressure.

The behaviour of a substance is usually formulated in its equation of state, which links its volume, pressure and temperature. For the interior of a gas giant like Jupiter, however, there are five competing equations of state for hydrogen alone, all of which are consistent with astronomical observations.

Remington is one of a group of researchers using high-powered lasers to compress hydrogen and then investigate its behaviour. Their aim is to determine which of the equations of state is correct. But the laser experiments take time, and once Remington and colleagues have worked through hydrogen, they will still have helium to contend with. Beyond that, they will need to repeat the experiment with a variety of mixtures of the two gases. When these elements’ equations of state are worked out, we will finally be able to be confident in our knowledge of the interiors of the gas giants.

It is not a quick fix, but it is a permanent one. And once we know how matter behaves in these extreme situations we will be able to apply that knowledge more widely, not only within our own solar system, but also to the discovered outside our solar system.

Supernovae and planets are not the only celestial objects appearing in the lab. Attempts are under way to recreate the energetic jets that spew from young stars and black holes as they devour the gas that surrounds them (see “Electric dreams”).

And this is just the start of what might be achieved within a decade. Remington is already eyeing emerging technology that can shorten a megalaser’s energy pulse to less than a billionth of a second. This boosts its heating effect, allowing particles of matter and antimatter to be created.

Stars are thought to be engulfed in a maelstrom of particles and antiparticles from gamma-ray bursts exploding into space. So lasers might be able to recreate a mini gamma-ray burst too. Any insights would be a boon to astronomers who continue to puzzle over these almighty, far-off explosions despite a growing body of observational evidence.

Bringing one into the lab to study might sound like an impossible task, but megalasers are living up to their mega reputation.