How to measure a star

Stars in the night sky appear as tiny points of light because they are too far away for your eyes to resolve. But even through powerful telescopes, stars still appear as mere points because they are too small to see their true physical size at vast distances. Now, a group of astronomers from over 20 different institutions has found a way to combine a unique telescope array with passing asteroids to measure the diameter of two distant stars, including the smallest star directly measured to date at just over twice the size of our Sun.Their work appeared April 15 in Nature Astronomy . And using this new information, astronomers can better refine their picture of the properties and life cycles of stars, which are the building blocks that make up our galaxy and every other galaxy in the universe.To measure the size of a star, the collaboration used events called occultations . An occultation occurs when one object — such as an asteroid or planet in our own solar system — passes in front of a star. Occultations can reveal a wealth of information about either the object passing in front of the star, or about the star itself. An occultation in 1977 led astronomers to discover the rings of Uranus , while occultations of Pluto allowed researchers to probe the dwarf planet’s tenuous atmosphere.In this pilot study, the team went after even more difficult-to-see occultations — those caused by asteroids passing in front of background stars. To catch the events, they used the Very Energetic Radiation Imaging Telescope Array System, or VERITAS , which contains four telescopes designed to detect gamma rays. Each 12-meter telescope is made up of 350 individual mirrors, which allow the array to detect the extremely faint, fast flashes of light caused when gamma rays interact with our atmosphere. The array can also spot asteroids in our solar system occulting distant stars; as an asteroid zips in front of a star at about 15 miles (24 kilometers) per second, the dip in starlight is extremely short-lived. But the way the star’s light behaves as it diffracts around the asteroid during that time reveals the star’s size.Occultations occur often, but the shadow cast by the event is tiny compared to the size of Earth. Shadows only fall over a minuscule portion of the planet, so many astronomers who study occultations take to the road (or sea, or sky) with mobile telescopes. VERITAS can’t move, however, so to study occulted stars, the team had to wait for an occultation to occur that would be visible from the Fred Whipple Observatory in Arizona, where the array is located.