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How to weigh a star using a moon Friday, October 29, 2010
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How do astronomers weigh a star that is billions of miles away and is too large for a bathroom scale? In most cases, can not, although they may achieve a better estimation using computer models of stellar structure. A new study conducted by astrophysicist David Kipping said that in special cases, we can directly Despite a star. If the star has a planet, and the planet has a moon, and they cross in front its star, then we can measure the sizes and orbits to learn more about the star.
"I often wonder how astronomers weigh the stars. We've added a new technique to our toolbox for this purpose, "says Kipping, a PhD student at the Harvard-Smithsonian Center for Astrophysics.
Astronomers have found more than 90 planets crossing in front - or passing - its star. By measuring the amount of starlight blocked, they can calculate how big is the planet on its star. But they can not know exactly how big is the planet unless they know the actual size of the star. Computer models give a good estimate, but in science, the actual measurements are the best. Kipping
realized that if a transiting planet had a moon large enough so that we saw (also blocking the starlight), then the planet-moon-star could be measured in a way that would allow us to calculate exactly how big and massive were the three bodies.
"Basically, we measure the orbits of the planets around the star and the moon around the planet. Then, through Kepler's Laws of Motion, it is possible to calculate the mass of the star, "explains Kipping.
The process is not easy and requires several steps. Measuring how the star's light dims when transiting planet and moon, astronomers achieve three key issues: 1) The orbital periods of the moon and the planet, 2) the size of their orbits in relation to the star, and 3) the size the planet and moon in relation to the star.
Introducing these numbers in Kepler's Third Law is achieved density of the star and planet. Since density is mass divided by volume, the densities and relative sizes give the relative mass. Finally, scientists measure the wobble of the star due to the gravitational pull of planet, known as radial velocity. Combining velocity measurements with the relative masses can calculate the mass of the star directly.
"If there is no moon, the whole exercise would be impossible," says Kipping. "I do not have moons means that no one can calculate the density of the planet, so that the whole process stops."
Kipping has not even this method in practice because there is no star known to have both a planet and a moon in transit. However, NASA's Kepler mission should find several such systems.
"When they are, we are ready to weigh" Kipping said.
The research appears in the issue of the journal
Monthly Notices of the Royal Astronomical Society .
Original Date: October 15, 2010 Original Link
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