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Betelgeuse is one of the brightest and most easily recognisable stars in the night sky. It sits in the constellation of Orion and is visibly red. That’s because Betelgeuse is a luminous, red supergiant about 650 light years away.

And it is huge. If Betelgeuse sat at the centre of the Solar System, it would engulf, Mercury, Venus, Earth, Mars and the asteroid belt. In fact, this star is so big, that it is one of just a few stars that Earth-bound telescopes can see as a disc.

But there is one basic fact about Betelgeuse that astronomers have never been able to gauge accurately. Nobody knows its mass.

The main way of working out an astronomical object’s mass is to look at the objects that orbit it, since astronomers can use the orbital period to work out the mass. That has allowed them to work out the mass of all kinds of objects such as binary star systems, exoplanets and even entire galaxies.

But Betelgeuse doesn’t have a companion that astronomers can see. So they’ve had to rely on other ways to infer the mass.

One is to create a model of the way stars form and evolve and then work out how heavy a star like Betelgeuse ought to be. The answer according to this method turns out to be about 20 solar masses.

Another way is to measure the light the star produces which is the result of conditions at the star surface. This reveals the strength of gravity there. Combining this with measurements of the star’s radius gives a mass. The best answer for Betelgeuse turns out to be about 10 solar masses.

Clearly, these two methods do not agree, which is troubling for astrophysicists.

So Hilding Neilson at the University of Bonn in Germany and a few pals have come up with a new method. They’ve looked at Betelgeuse as a disc and measured how it appears to darken towards is edges, a well known effect that can be described in mathematical models.

In fact, so-called limb darkening models produce a relation between the amount of limb darkening and the ratio of the radius and mass.

Given that these guys know Betelgeuse’s radius and also its the limb-darkening, they’ve plugged the numbers in and come up with a value for the mass. The answer (drum roll) is about 12 solar masses, give or take a few.

So Betelgeuse really is about ten times bigger than the Sun.

Well, maybe. To be frank there’s no reason to think this number is much more reliable than the others. That’s because of uncertainty over Betelgeuse’s exact distance (and hence its measured radius) and also uncertainty over the accuracy of the mathematical model.

But if other astronomers can make a better fist of these, who knows how accurately they can measure Betelgeuse’s mass?

Ref: arxiv.org/abs/1109.4562: Weighing Betelgeuse: Measuring The Mass Of α Orionis From Stellar Limb-Darkening

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