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MOST Measures the Pulse of a Star.

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Pulse of a Star.
Pulse of a Star: Image credit: Canadian Space Agency.

The Canadian-built MOST space telescope has shed new light on how stars like our own Sun can actually be quite turbulent, vibrating and flaring up unexpectedly. MOST tracked a star called eta Bootis for 28 straight days without interruption, and measured its brightness more than 250,000 times - 10 times more accurately than any previous instrument could reach. MOST should also assist planet hunters by watching how a star brightens and dims as planets pass in front of it, similar to Venus' recent transit across the face of the Sun.

MOST, Canada’s first space telescope, is shaking up the way Astronomers think about stars - and putting a new spin on the life story of our own Sun - by allowing Astronomers to see in unprecedented detail how stars shake and spin.

The first results from MOST, a Canadian Space Agency mission which was also the first scientific satellite to be launched by Canada in over 30 years, include the detection of a strong 'pulse’ in a young adult star called eta Bootis, and a bad case of stellar acne and hyperactivity in a 'pre-teen’ version of the Sun, kappa 1 Ceti. These data offer a unique perspective on what our own Sun may have been like in its youth.

"All this talk of stellar pulses and hyperactivity must sound like ER Meets star Trek," admitted MOST Mission Scientist Dr. Jaymie Matthews of the University of British Columbia, who presented the findings today in a keynote address to the annual meeting of the Canadian Astronomical Society in Winnipeg. "But we really are doing diagnostic check-ups of stars at different points in their lives, by placing them under intensive observation for weeks at a time."

Matthews made the presentation to a gathering of physicists, astrophysicists, and medical physicists at a unique conference of Canadian physics societies (CAP/CASCA/COMP/BSC CONGRESS 2004) hosted by the Department of physics and astronomy at the University of Manitoba in celebration of the Faculty of Science's 100th anniversary.

These are ambitious results from a Canadian-built and -operated orbiting observatory which is no bigger than a suitcase but can monitor the brightnesses of stars with unmatched precision and thoroughness. MOST, which stands for Microvariability and Oscillations of STars, was launched into orbit last summer and has been collecting data for the last few months.

"MOST is a major advance in the way Astronomers study stars, made possible by innovative Canadian technology," noted Canadian Space Agency President, Dr. Marc Garneau. "It is the world’s most precise light meter, capable of recording variations as small as one ten thousandth of a percent in the brightness of a star."

How small is that?

"If all the lights in all the offices of the Empire State Building were on at night," explains Dr. Garneau, "you could dim the total light by 1/10,000th of a percent if you pulled down just one window blind by only one centimetre."

From its vantage point in polar orbit, 820 km high, the tiny MOST space telescope can stare at stars without interruption for up to eight weeks. No other observatory or network of telescopes, including the Hubble, can do this. The unique combination of precision and time coverage enables MOST to look for subtle vibrations in stars that will reveal secrets hidden beneath their surfaces. It also gives MOST the best chance to detect light directly from planets outside our solar system and study their atmospheres and weather.

MOST is a Canadian Space Agency mission. Dynacon Inc. of Mississauga, Ontario, is the prime contractor for the satellite and its operation, with the University of Toronto Institute for Aerospace Studies (UTIAS) as a major subcontractor. The University of British Columbia (UBC) is the main contractor for the instrument and scientific operations of the MOST mission. MOST is tracked and operated through a global network of ground stations located at UTIAS, UBC and the University of Vienna.

The MOST Canadian space telescope was launched from northern Russia in June 2003 aboard a former Soviet ICBM (Intercontinental Ballistic Missile) converted to peaceful use. Weighing only 54 kg, this suitcase-sized microsatellite is packed with a small telescope and electronic camera to study stellar variability.

One of its early targets was the star eta Bootis, a slightly more massive and younger version of the Sun. Astronomers had picked out this star as one of the best candidates for the new technique of "asteroseismology" - using surface vibrations to probe the inside of a star, similar to how geophysicists use earthquake vibrations to probe the Earth’s core.

MOST monitored eta Bootis for 28 days without interruption, placing the star under a 24-hour scientific 'stake-out’ that revealed behaviour that was hidden from the limited view possible for Earth-bound telescopes. Accumulating almost a quarter of a million individual measurements of this star, MOST reached a level of light-measuring precision at least 10 times better than the best ever achieved before from Earth or space.

The data reveal the star is vibrating, but at a pitch well below the range of human hearing. The stellar melody should allow the MOST team of scientists, including Dr. David Guenther of the Canadian Institute for Computational Astrophysics at St. Mary’s University, Halifax, to determine the age and structure of eta Bootis. "We’re now in a position to explore new physics in stars, with observations like these," said Dr. Guenther.

Before observing eta Bootis, while still in the shakedown phase of its mission, MOST was aimed for testing purposes at a fainter star called kappa 1 Ceti. Astronomers already suspected this was a younger version of our Sun, with an age of about 750 million years. The Sun’s age is about 4.5 billion years, and it’s just entering middle age. In terms of a human life, the Sun would be about 45 years old while kappa 1 Ceti would be eight years old – barely a pre-teen.

Like many human kids, Kappa 1 Ceti is hyperactive, flaring up from time to time, and spinning with much more kinetic energy than sedate older stars like the Sun. It also has a severe case of acne - dark spots on its face which are much larger than those visible on the Sun's surface. The MOST data, following Kappa 1 Ceti for 29 days, show in exquisite detail how the spots move across the visible side of the star as it spins once every nine days or so. And because a star is not solid, different parts of its gaseous surface spin at different rates. MOST has been able to measure this effect directly in a star other than the Sun for the first time. These results are being prepared for submission to The Astrophysical Journal.

Future targets for MOST include other stars representing the Sun at various stages in its life, and stars known to have giant planets. MOST is designed to be able to register the tiny changes in brightness that will occur as a planet orbits its parent star. The way in which the light changes will tell Astronomers about the atmospheric composition of these mysterious worlds, and even if they have clouds.

"It’s like doing a weather report for a planet outside our Solar System," says Dr. Jaymie Matthews, MOST Mission Scientist, of the University of British Columbia.

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