Dr. van Belle has been the instrument scientist for ESO's Phase Referenced Imaging and Microarcsecond Astrometry instrument for the Very Large Telescope Interferometer for the past two years. He has been enjoying life in Germany as an American ex-pat. He is joined by his wife, three sons, two dogs, and a cat, all of whom find the German climate a rather brisk change from southern California.

400 years ago, many of the remarkable discoveries of Galileo were built upon the ability of the newly invented telescope to magnify planets into disks - worlds in their own right. The rings of Saturn, the phases of Venus, and Jupiter's major moons were all a result from the application of new technology to the night sky. In much the same way, astronomers are advancing the knowledge of stars through use of new technology in the study of these thermonuclear furnaces.

Seeing Stars

Throughout all of recorded human history, the night sky full of stars has captured the imagination and interest of people worldwide. However, aside from our own star, the Sun, the stars were nothing more than "pin-pricks in the curtain of night". This is due to their extreme distance — they are physically large bodies, but significantly further away from the Earth than the Sun. Alpha Centauri, the star system nearest to our Solar System, is more than 250,000 times further than the Sun.

Why (most) stars are round

The Sun, like all stars, is an incredible energy-releasing machine. It is, simply put, a big ball of gas: about 75% hydrogen, 25% helium. It's physically large — about 109 times the diameter of the Earth — but more importantly, it's massive, roughly 333,000 times the mass of the Earth. The self-gravity of that much stuff — the propensity for each atom in the Sun to be gravitationally attracted to every other atom in the Sun — is a tremendous reservoir of potential energy. On average, the tendency is for all of these atoms to be drawn to the centre of the Sun. However, these atoms don't all immediately fall to the centre of the Sun and *poof* form a black hole. Rather, they tend to bump into each other.

Near the core of the Sun, there's a lot of bumping going around. The crushing weight of the entire star is pushing down at that point, so the temperature is very high. So high, in fact, that some of these atoms cannot just bump into each other but get stuck to each other. Nuclear fusion is taking place, which releases a net positive amount of energy into the surroundings. This lets the Sun keep the "bumping" going on. More correctly stated, the pressure of the gas that makes up the star is large enough to counteract the tendency of gravity to contract the star towards its central point. This is true even though there is a net energy loss of the Sun since it is shining, bleeding away energy into deep space in the form of electromagnetic radiation.

Summing it up: stars are machines that convert gravitational potential energy into light, using nuclear fusion to power a near-balance between gas pressure and the force of gravity.

If you think of a star that is static in space — not moving, and more importantly, not spinning — the force of gravity and the pressure of gas are the same everywhere across the star's surface, so it tends to be round.

Getting in a spin

Once you've built your star, one thing you can consider doing is making it spin. The Sun, in fact, rotates roughly once a month, which is a positively leisurely rate. However, some stars rotate with substantially more alacrity.

Let's consider the case of a star that is spinning very fast. The two competing, but previously balanced effects of gravity versus gas pressure are now starting to get out of whack. The reason is that the outward-pushing gas pressure is getting an assist from a similarly outward-pushing centripetal force. Just try pulling a hand mixer out of a bowl of cake batter while the mixer is still spinning and you'll have a dramatic (and messy) demonstration of centripetal force.

However, this centripetal force that is competing for gravity's attention, along with the gas pressure, is greatest at the equatorial regions of the star, not present at all at the poles of the star, and scaling along the way in the in-between latitudes. As a child, you may have felt this sort of effect, and its scaling, while riding a merry-go-round or carousel — at the edge, farthest from the rotational axis, you felt the most pull to the outside, while at the centre, you may still have gotten dizzy, but felt no pull towards the outside.

The latitude dependence of the centripetal force versus the non-latitude dependence of the other two factors, gravity and gas pressure, is what makes the star deform into a non-round, "oblate" shape.

Taking pictures of rapidly rotating stars

In 2001, after many failed attempts by astronomers worldwide, my team and I were successful in measuring the effect of rotation on a rapidly rotating star. We looked at the star Altair, a nearby hot, rapidly spinning star, with the Palomar Testbed Interferometer, an ultra-high resolution technology prototype telescope designed to test techniques for finding planets about other stars. We found that a simple size measurement of the star was greater in one direction than another, and deduced that Altair's rapid rotation was finally revealing itself directly.

More recently, a newer array of telescopes operated by Georgia State University's Centre for High Angular Resolution Astronomy (CHARA) has managed to fully image Altair. By combining the light from six small telescopes spread over the top of Mount Wilson in southern California, the CHARA Array effectively creates a telescope that has the resolution of a 330 metre telescope — roughly 130x more resolution than the Hubble Space Telescope. Investigators from the University of Michigan were able to utilise a special new instrument that combined the light from those individual telescopes and take a picture of Altair.

Pretty pictures are nice, but extracting interesting astrophysics is even more compelling. One intriguing aspect of imaging these sorts of stars is that they appear brighter at the poles than at their equators. Indeed, the star Altair shows a temperature difference of nearly 2000K between its hot pole and cool equators; this effect tells astronomers a great deal about how the star is put together on the inside, in the presence of such extreme rotation. This detection is a powerful demonstration of the ability of a simple image to contain a great deal of meaningful astrophysics. It has taken many decades of telescope advancements to get to this point where stellar surfaces can be directly imaged!

There is a promising future for this particular kind of work. Currently there are more than five dozen such targets that will be observed with the CHARA Array and the European Southern Observatory's own Very Large Telescope Interferometer. Seeing the stars in this new way will tell us even more about how these markers in the nighttime sky are put together, and how they live and ultimately die.

This feature article was written as part of the Cosmic Diary Cornerstone project for the International Year of Astronomy 2009. To find out more, check out www.cosmicdiary.org and www.astronomy2009.org.