Wednesday, December 16, 2009

In the valley of the jolly (ho ho ho!) red giant


The changing size and shape of chi Cygni
Image Credit: Sylvestre Lacour, Observatoire de Paris


Looking and behaving like one right jolly old elf, red giant stars tend to shake like a bowlful of jelly.  In a press release yesterday from the Center for Astrophysics (and, more importantly, a paper published in last week's issue of the Astrophysical Journal), a team of astronomers announced that they had obtained images of this shaking in one star, chi Cygni.  The illustration above shows the images of the star derived from their work.

Stars like the sun spend most of their lives looking a lot like the sun -- fairly small, at least in astronomical terms, fairly hot, and creating energy by fusing hydrogen atoms into helium atoms.  After some amount of time (a measly 10 billion years in the case of the sun), the star will exhaust its supply of hydrogen.  When it does so, it swells up into a truly monstrous size, and its outer layers cool off, fading and cooling from the blinding yellowish white of a star like the sun to a ruddy red or reddish orange color.  During this "red giant" stage, the star can get as large as the orbit of the Earth, Mars, or even Jupiter, depending on how massive ("heavy") the original star was.

At this stage, the star also becomes a little less stabple, and its outer layers can slowly pulse inwards and outwards, taking a year or longer to complete one cycle.  The most famous example of this is the star Mira, in the constellation Cetus the whale, which due to the pulsing changes in brightness from fainter than your unaided eye can see to being a little brighter than most of the stars in the Little Dipper. 


Despite their tremendous girth, red giant stars still tend to look just like points of light in even the best professional telescopes, because they are so far away.  In this new paper, a team led by French astronomer Sylvestre Lacour used a techniques called optical interferometry to obtain images of the star at several points throughout its pulsation cycle.  Optical interferometry is a technique that takes advantage of the wave-like properties of light to make sharper images of a star than normal optics allow. Interferometry has been used in radio waves for decades, but it gets harder as you go to shorter wavelengths of light.  Radio waves have wavelengths of centimeters to several meters (inches to several yards), while visible light has wavelengths of less than 1 micrometer (40 millionths of an inch, or roughly the size of a typical bacterium).  So, doing this in visible light is technically challenging, is also hampered by earth's atmosphere, and so it is just really starting to make big progress.

 The images of the Mira-like star chi Cygni clearly show the star changing in size.  You can also see that the star is kind of lumpy and not perfectly round.  In their paper, the authors state that this apparent lumpiness is due to some hot spots on the star, and the star may not actually have huge bumps on its surface.  One of the problems is that the outer parts of red giant stars are really fuzzy and low-density, and slowly trail off into the vacuum of space, so defining an edge of the star is really not possible. 

The authors use their observations of the star to determine how massive the star is.  This is really hard to do for red giant stars, because normal techniques we use for measuring masses don't work for red giants.  They found that chi Cygni is about twice the mass of the sun, so it gets a little bigger than the red giant sun will.

Measurements like this are important to help us figure out what the ultimate fate of the Earth will be.  Will the Earth be swallowed by the red giant sun, or will it survive?  Our best theoretical models of the sun place the Earth right on the hairy edge, and if you ask astronomers on different days of the week, their answers will change.  If we can actually measure the sizes of red giant stars as a function of their mass, we can test our models and, eventually, learn our own fate. 

Red giant stars don't last very long.  They eventually lose their outer layers, becoming a short-lived but picturesque planetary nebula.  Gravity squeezes the ashes at the center of the star into a white dwarf.  Any planets still around the white dwarf star are now safe, and can continue to orbit their parent star.  And, if we're lucky, my colleagues will then be able to find those survivors

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