Jul 10, 2000

Comet Borrelly or Bust

Sizzling Comets Circle a Dying Star

Astronomers have detected a massive cloud of water vapor around an aging star. It could be the telltale sign of innumerable dying comets and a glimpse of things to come in our own solar system.


Marshall Space Flight Center

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July 11, 2001 -- Stargazers here on Earth get excited whenever a comet comes close to the Sun. Comets -- icy visitors from the outer solar system -- are easily vaporized. Even a little sunlight will cause one to bubble and froth, dispensing a tail that stretches millions of kilometers through space and far across the night sky. Bright comets are unforgettable sights.

Now imagine a solar system in which not one but billions of comets are vaporizing -- all at the same time!

That's what astronomers may have found around an aging star called IRC+10216. "We see substantial concentrations of water vapor around this star," says Harvard's Gary Melnick, Principal Investigator for the Submillimeter Wave Astronomy Satellite (SWAS), the orbiting observatory that made the discovery. "The most plausible explanation for this water vapor is that it is being vaporized from the surfaces of orbiting comets -- 'dirty snowballs' that are composed primarily of water ice."

Above: In this artist's rendering, the aging star IRC+10216 vaporizes a belt of comet-like objects in its vicinity. Image courtesy of the Smithsonian Astrophysical Observatory


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IRC+10216, also known as CW Leonis, was once a well-behaved main-sequence star as our own Sun is now. But the aging giant is substantially heavier than the Sun --as a young star it contained 1.5 to 4 solar masses-- so it evolved more quickly. About one billion years ago, say astronomers, IRC+10216 ran out of hydrogen fuel in its core. Since then hotter fusion reactions involving helium and carbon have replaced simple hydrogen burning -- swelling the star enormously. If IRC+10216 was located at the center of our solar system, it would engulf Mercury, Venus, Earth, Mars and Jupiter.

"We think we are witnessing the type of apocalypse that will ultimately befall our own planetary system," says SWAS team member David Neufeld, professor of physics and astronomy at Johns Hopkins. "Several billion years from now, the Sun will become a giant star and its power output will increase five thousand fold. As the luminosity of the Sun increases, a wave of water vaporization will spread outwards through the solar system, starting with Earth's oceans and extending well beyond the orbit of Neptune. Icy bodies as large as Pluto will be mostly vaporized, leaving a cinder of hot rock."

Below: Several billion years from now, our Sun will become a giant star. It will expand, swallowing Mercury -- then contract temporarily -- and then expand once again to engulf the inner planets. This

shows the entire process. It's slow to download, but worth the wait. [more]

From its vantage point in orbit above the absorbing haze of water in Earth's atmosphere, SWAS can detect distinctive radiation at sub-millimeter wavelengths emitted by water vapor in deep space. When astronomers turned the satellite toward IRC+10216 they discovered a substantial cloud of water vapor about 100 AU across. ("AU" --short for Astronomical Unit-- is a unit of length used by astronomers. One AU equals the mean distance between Earth and the Sun.)

"There must be about four Earth-masses of frozen water around IRC+10216 to produce the vapor cloud we see," says Melnick. The water vapor probably does not come from the vaporization of oceans on an Earth-like planet, because there wouldn't be enough water on such a planet to supply the cloud. Instead, the researchers speculate, the vapor cloud might come from a swarm of several hundred billion comets orbiting 75 AU to 300 AU from the star.

That might sound like a lot of comets, but we know that such swarms can exist. For example, there is a great reservoir of dormant comets right here in our own solar system -- a doughnut-shaped cloud called the Kuiper Belt. The Kuiper Belt extends far beyond the orbit of Neptune and harbors a vast population of small icy bodies. Scientists think they are primitive remnants from the earliest days of our solar system, leftover bits and pieces that were never consumed by growing planets.

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Left: The Kuiper Belt, located beyond the orbit of Neptune, contains many billions of icy objects, of which the planet Pluto is the largest currently known. Image courtesy Johns Hopkins University.

"In our own solar system, these [Kuiper Belt] comets orbit the Sun quietly for the most part," says Saavik Ford, a Johns Hopkins graduate student who contributed to the SWAS discovery. "Occasionally a comet comes in close to the Sun, starts to vaporize, and displays the characteristic coma and tail that we are familiar with. But IRC+10216 is so much more luminous than the Sun that comets start to vaporize even at the distance of the Kuiper Belt. So one has several hundred billion comets all vaporizing at once."

In recent years planet-hunters have found more than 50 stars with planets. Most of those systems, which harbor very large planets close to the central star, don't seem to be much like our own solar system. However, the observations of water vapor around IRC+10216 suggest that some stars might indeed have planetary systems similar to our own, with an alien Kuiper Belt and abundant water. Water, of course, is a key ingredient for life as we know it -- and that's the most tantalizing prospect of all!

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"This just goes to show that a star doesn't have to be bright to be interesting," says Neufeld. IRC+10216 is a dim 18th magnitude star located 500 light years from Earth in the constellation Leo. "The star seems so faint because it is shrouded in its own cocoon of obscuring dust and gas," he explains. "But at infrared and submillimeter wavelengths it's one of the brightest objects in the sky."

Right: The orbiting Submillimeter Wave Astronomy Telescope reveals what the human eye cannot see. [more information]

Indeed, if humans could see infrared light, IRC+10216 would be an eye-catching beacon in the night sky. But in real life it's practically invisible. By piercing the star's veil of mystery, SWAS has revealed what astronomers long-suspected: there's much more to IRC+10216 than meets the eye.

The discovery of water vapor around IRC+10216 is reported in the July 12th edition of the journal Nature. In addition to Melnick, Neufeld and Ford, the other co-authors are Dr. David Hollenbach of NASA's Ames Research Center and Dr. Matthew Ashby of the Harvard-Smithsonian Center for Astrophysics.


Sizzling Comets Circle a Dying Star
July 11, 2001
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 Thursday's Classroom

These lessons and activities for 6th to 12th grade classrooms are based on the Science@NASA news story "Sizzling Comets Circle a Dying Star"

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Web Links

The Main Sequence Blues - a different perspective on the Main Sequence -- which our own star is still on, but IRC+10216 long ago departed -- from Robert J. Nemiroff's Comedy of Science web page.

The Submillimeter Wave Astronomy Satellite - learn more about SWAS, a small radio observatory launched by NASA in 1998. SWAS was built and operated by NASA with support from the German government and the participation of the Harvard-Smithsonian Center for Astrophysics, the University of Massachusetts at Amherst, Cornell University, the Johns Hopkins University, the University of Cologne, Ball Aerospace, and Millitech (now Telaxis Communication Corp.).

The Story of IRC 10°216 -- learn more about this intriguing star. This readable essay is by Dr Ian Griffin.

More pictures -- from the SWAS web team at Harvard's Center for Astrophysics

What is a Red Giant? -- in plain language, from NASA Goddard's Imagine the Universe.

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