Astronomy: New Dimensions for the Stars

Ever since astronomers started staring through their telescopes, they have been trying to measure the size of the stars. It is an almost impossible job.

Stars are so far away that even in the biggest telescopes they never show up as anything more than mere points of light. The only direct means of measuring them is with an interferometer: an instrument that collects the light from a star in two different mirrors a short distance apart, then feeds the two beams into the same telescope. Despite the star’s enormous distance, light from each of its edges must travel a slightly different distance to reach one mirror than to reach the other. This tiny variation causes the light waves in the two beams to interfere with each other, and the pattern of that interference can be interpreted to give the star’s apparent diameter.

The method is straightforward enough, but interferometers attached to conventional telescopes are extremely difficult to use; they can measure only the nearest and biggest stars — red giants such as Betelgeuse, which has 290 times the diameter of the sun. To measure smaller or more distant stars, the beam-collecting mirrors must be much farther apart than is possible with a single optical telescope.

With the help of modern electronics, Australian and British astronomers at the University of Sydney have managed to extend their star measurements by using a pair of 22-ft. parabolic mirrors. There is no need for the mirrors to be ground carefully enough to bring star light to the precise optical focus that is required by a standard telescope. All that is necessary is that they feed the light into photomultiplier tubes that turn it into electric current. Each mirror is made of 250 small hexagonal mirrors, good enough for the purpose and a great deal cheaper than the single dishes of big telescopes. When the mirrors are turned toward the dim light from a distant star, two fluctuating currents come from their photomultiplier tubes. Combined, those currents form the electronic equivalent of interferometer light patterns; they, too, can tell the apparent diameter of a far-distant star.

The Australians reported in Nature that Vega, the first star measured by the new instrument, turns out to have an apparent diameter of 0.0037 seconds of arc, about one five-hundred-thousandth of the breadth of the full moon. Since the distance of Vega is known, its absolute diameter can be calculated as 2,800,000 miles—about 3.2 times the size of the sun.