How do astronomers weigh stars?    In most cases stars that are trillions of miles away can't be measured, though astronomers can get a best estimate using numerical models.

But in some instances a star could be measured using inference.    If the star has a transit planet, and that planet has a moon, and both of them cross in front of their star, the size and orbit can be measured to learn about the star.  Has it been done yet?   Well, no, since we have not found a star with both a planet and moon that transit but NASA's Kepler spacecraft should discover several such systems.

So far, astronomers have found more than 90 planets that cross in front of, or transit, their stars. By measuring the amount of starlight that's blocked, they can calculate how big the planet is relative to the star. But they can't know exactly how big the planet is unless they know the actual size of the star. Computer models give a very good estimate but science would like more than estimates.

Astrophysicist David Kipping says that if a transiting planet has a moon big enough for us to see (by also blocking starlight), then the planet-moon-star system could be measured in a way that lets us calculate exactly how large and massive all three bodies are.

This is an artist's concept of an exoplanet and its moon transiting a sun-like star. Such a system could be used to directly weigh the star.   Photo: David A. Aguilar (CfA)

"Basically, we measure the orbits of the planet around the star and the moon around the planet. Then through Kepler's Laws of Motion, it's possible to calculate the mass of the star," explained Kipping.

The process isn't easy and requires several steps. By measuring how the star's light dims when planet and moon transit, astronomers learn three key numbers: 1) the orbital periods of the moon and planet, 2) the size of their orbits relative to the star, and 3) the size of planet and moon relative to the star.

Plugging those numbers into Kepler's Third Law yields the density of the star and planet. Since density is mass divided by volume, the relative densities and relative sizes gives the relative masses. Finally, scientists measure the star's wobble due to the planet's gravitational tug, known as the radial velocity. Combining the measured velocity with the relative masses, they can calculate the mass of the star directly.

"If there was no moon, this whole exercise would be impossible," stated Kipping. "No moon means we can't work out the density of the planet, so the whole thing grinds to a halt."