91av

FOR the first time, astronomers have directly measured the girth of a normal
star. They found that Altair, the twelfth brightest star in the sky, spins so
fast that it is 14 per cent wider at its equator than from pole to pole.

The researchers made the discovery by chance while using an instrument on
Mount Palomar in California to observe another star. “We were using Altair as a
check star for a study of Vega,” says Gerard van Belle of the Jet Propulsion
Laboratory in Pasadena. Measurements of their diameters showed that Vega was
circular but Altair was not.

Astronomers have measured stellar diameters before, most notably when the
Hubble Space Telescope gauged the size of the supergiant star Betelgeuse. But
Altair is a much tougher target. Although it is only 16 light years away, Altair
is a normal star just 1.8 times the Sun’s diameter and looks only 1/25th the
size of Betelgeuse in the sky.

Van Belle’s team got the measure of Altair using the Palomar Testbed
Interferometer, an instrument that combines light collected by pairs of
50-centimetre telescopes 110 metres apart. Such an approach can give a much
finer resolution than a single large telescope.

Measuring the size of a star’s equatorial bulge gives a good indication of
its rotation speed. The size of the bulge depends on how strongly the star’s
rotational velocity counteracts its gravity. Altair’s bulge suggests the star
takes less than 10.4 hours to rotate once. And if one of its rotational poles is
tilted toward us, Altair could actually be spinning faster. Its surface velocity
is 210 kilometres per second.