The Sciences

Wonder twins telescope sees star's dying gasps

Bad AstronomyBy Phil PlaitFeb 18, 2009 8:56 PM


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500 light years away, the star T Leporis is dying. It used to be much like the Sun, but the store of nuclear fuel in its core is running out. Due to the nuclear processes going on deep inside it, its energy production has vastly increased, blasting out thousands of times the energy it did when it was a stable star. The outer layers of the star absorb this energy, and, like a hot air balloon, expand hugely. Even though it is now far, far brighter than it used to be, the expansion actually cools the star's surface. It has become a bloated, swollen red giant. Because the surface is cooler, more complex molecules can form there. What astronomers call dust, they are blasted by the intense brightness of the light coming up from below. And because the star has expanded so much, its gravity at the surface is much lower, too. The huge force upwards from the light cannot be held back by the feeble gravity, and the dust is launched into space, creating a spherical shell around the star. And now, for the first time, astronomers have taken the sharpest infrared pictures ever of that shell as it is hurled into space.

The image above is not an illustration, it's an actual image of a red giant star undergoing dying paroxysms and blasting a dense shell of molecules into space. As you can see by the comparison drawing of the Earth's orbit around the Sun, T Lep has expanded to a diameter of nearly 300 million kilometers (180 million miles). At first, I thought the ring around the star was not real, and was instead what's called an imaging artifact; a mirage due to optical effects inside a telescope or camera. But it's actually the dense shell of ejected material from the star. This incredible picture was not taken by a single telescope. It was produced by combining the light of four different 1.8 meter telescopes in a process called interferometry. It's a fiendishly complex process that virtually creates a telescope that has the same resolution (ability to see small objects) as a single telescope spanning the separation between the smaller ones. In other words, separate the smaller 'scopes by 100 meters, and you can create a virtual telescope 100 meters across. The longer the wavelength, the easier this process is (though it's never easy); in this case the image was taken in the infrared. It took multiple observing sessions over several nights, but in the end the astronomers were able to see objects as small as two milliarcseconds across-- much smaller than Hubble can resolve, and equivalent to an object just four meters across sitting on the Moon's surface! The ESO created a cool zoom-in of T Lep, starting just south of Orion and ending with the super-high-resolution image: This superior resolution can allow astronomers to determine the size, shape, density, and structure of the stellar wind blowing off the surface of T Lep, which will help them understand how stars like this die. And by "stars like this", I mean stars like the Sun. Some day, about 6 to 7 billion years from now, our Sun will run out of hydrogen in its core and swell up into a red giant just like T Lep has. Take another look at that image; see how the surface of the star is almost touching the size of the Earth's orbit? We don't know exactly how big the Sun will get, but that size right there is a pretty good guess. What happens to the Earth at that point is fairly clear, and the news won't be good. The Sun's clock is ticking. It has a lot of ticks left, but the number is finite. Studying stars like T Lep let us take a peek into our own distant future, and techniques like infrared interferometry make sure that the glimpse we get is ever-more sharply focused. If you want to learn more about how the Sun will die, then read Chapter 7 of my book,Death from the Skies!You'll get all the details, maybe more than you want. Photo credit: ESO/J.-B. Le Bouquin et al.

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