Adam Frank for McGraw-Hill

People have been staring at the stars for a long time and for most of our history we have been pretty clueless. In the absence of high-powered telescopes, advanced mathematical physics and digital computers it was difficult to see the stars as much more than mysterious points of light. That is why, for most of humanity's time on Earth, we have mapped our imagination's most heartfelt wanderings onto the stars. Throughout history (or even before it) we have seen the stars as Gods and Goddesses, the illuminated souls of heroes or the scattering of angel dust. Only in the last few centuries have we begun to understand the true nature of these ever-present companions in the night sky. But the star's real truth, the details of their internal workings, has only been revealed to us in the last seventy years. Now most of the mystery is gone. We know what the stars are and how they work. The loss of that mystery, however, is not to be mourned. The truth about stars is more amazing than fiction, even if that truth starts in as unlikely a place as your kitchen.

"We understand stars." This is a statement you might hear in any astronomy 101 class. It is a remarkable claim for astronomers to make given the vast stretches of time people have spent not knowing what stars were. Our ability to make such a seemly outrageous claim and make it with confidence stems from our understanding of the laws of physics. Over the last five centuries humanity has come to understand there is a fundamental order built into the world. From the falling of a leaf to the flow of heat from a cup of hot chocolate, the physical world appears to be based on principles that can be expressed in the language of mathematical physics. These principles of physics were first discovered by watching how things behaved here on Earth. Through a continual tension between experiments and mathematical theorizing physicists slowly build up a pretty complete picture of the forces shaping the world we experience. Astronomers then took those laws and applied them to the worlds we don't experience like the inside of stars.

Stars are pretty far away. We can't send probes directly to a distant sun to tell us what is happening inside, or even directly outside, of its fiery surface. What astronomers have found however is that the same laws of physics that operate in Earth-bound laboratories also apply in distant stars. The most mundane example of this is the simple physics of boiling.

Stars produce energy in their cores through nuclear fusion. The energy released in the fusion reactions has to go somewhere and so it streams outward through the layers surrounding the core. As the energy moves outward making its way towards the star's surface, the lower layers of the star are heated in much the same way as the bottom of a pot of water on a stove gets heated by a gas flame. What happens in the star is exactly what happens in the pot. The hot gas in the bottom layers becomes buoyant and rises upward. When the gas reaches the surface it's exposed to space and cools. The cooled material becomes heavy and sinks back down to the lower depths where the process starts all over again.

Look down on a pot of super hot water and you can see the tumbling motions as water from the bottom of the pot pops to the surface and sinks back down. Look down on the surface of the Sun and you will see much the same thing. In the pot the process is called boiling and physicists have a pretty good handle on how it works. In a star the process is called convection and over the years astronomers have also gained a good understanding of how it works. Of course the star is ten billion times larger than the kitchen pot so astronomers have to use techniques like supercomputer simulations to study convection (a link to a NASA program to simulate stellar convection is provided below).

Regardless of the tools or the names boiling is boiling. Our ability to understand the world we experience in terms of basics physics and then apply that understanding to more distant parts of the world is the basis of our claim to understand stars. Boiling and Convection are particularly prosaic examples of this link between heaven and Earth. But whether we are talking about boiling fluid or inverse beta decay (a sub-atomic process described by quantum mechanics) the foundations of physics built in our labs allow us to gain extraordinary new insight into the sky. There is an ancient saying ``As above so below''. Could the sages who dreamed that idea up more than a thousand years ago have imagined how right they would turn out to be?

NASA Project to Simulate Stellar Convection
http://astro.uchicago.edu/Computing/HPCC/Presentations/Preshtml/sld001.htm

Some Movies of Convection
http://astro.uchicago.edu/Computing/

Simulating a Giant Star
http://www.lcse.umn.edu/research/RedGiant/