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A heart still beats in
a dying star

Posted: December 15, 2009

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A throbbing red giant star called chi Cygni, 550 light years away in the constellation Cygnus, has been imaged in unprecedented detail, giving a clear insight into the fate that awaits our own Sun in five billion years time.

The variations in brightness and size of chi Cygni over a period of 408 days. When the star has contracted to its smallest size, notable hotspots can be seen on its surface. Image: Sylvestre Lacour (Observatoire de Paris).

When a Sun-like star begins to run out of hydrogen gas to fuel its nuclear reactions that generate energy in its core, it swells into a red giant, engulfing its innermost planets. As the star’s central engine begins to falter the red giant pulses in and out. By combining the light from the now disassembled trio of telescopes at the Smithsonian Astrophysical Observatory’s infrared Optical Telescope Array (IOTA) at Whipple Observatory in the United States, astronomers were able to image these pulsations, creating a movie from the still images.

“IOTA offered unique capabilities”, says Marc Lacasse of the Harvard–Smithsonian Center for Astrophysics, who was co-author on a paper in The Astrophysical Journal; describing the results. “It allowed us to see details in the images that are about 15 times smaller than can be resolved in images from the Hubble Space Telescope.”

The images show that the pulsations are not symmetrical, but are lumpy, with hotspots and clouds ejecting in different directions, and the star contracts and expands every 408 days. We call these stars Mira-type variables, after their prototype star Mira in the constellation Cetus. Imaging chi Cygni was no simple task – infrared light, which can pass through the clouds of dust that surround the star, was the only way to do it.

An artist’s impression of a pulsating red giant star similar to chi Cygni. Image: ESO/L Calçada.

At its smallest, chi Cygni is 482.8 million kilometres across, with huge spots covering its surface (granules blown up large) and clouds of boiling plasma. As it expands, it reaches 772.5 million kilometres across. If placed in our Solar System, it would incinerate Mercury, Venus, Earth and Mars and extend well into the Asteroid Belt.

In time, our own Sun will run out of hydrogen and follow suit, engulfing the inner planets; in a sense chi Cygni warns us of our own Solar System’s fate. With each pulse, the red giant throws off some more of its outer layers; over time, these layers will accumulate, spreading out into a beautiful planetary nebula, the parting shot in the life of a star.

An animation combining the individual images of the pulsation of the star chi Cygni is available here.