A Star’s True Size
26 July 2020
In astronomy, the sharpness of your image depends upon the size of your telescope. When Galileo and others began to view the heavens with telescopes centuries ago, it changed our understanding of the cosmos. Objects such as planets, seen as points of light with the naked eye, could now be seen as orbs with surface features. But even under these early telescopes, stars still appeared as a point of light. While Galileo could see Jupiter or Saturn’s size, he had no way to know the size of a star.
That didn’t change until 1995 when the Hubble Space Telescope made an image of Betelgeuse not as a point but as a blurry disk. It was the first time astronomers could determine the size of a star directly. Astronomers could finally compare the apparent size of a star to theoretical calculations based on mass, color, and brightness. Since then, both ground and space-based optical telescopes have imaged stars and even planets directly. But astronomy at other wavelengths posed new challenges.
In radio astronomy, the issue was with the wavelength of radio light. Optical telescopes use light with wavelengths on the order of a few hundred nanometers, the wavelengths used by radio telescopes are typically millimeters or centimeters. Since the resolution of telescope scales with the wavelength of light, a radio telescope would need to be nearly a million times larger to create a sharp image. It isn’t feasible to create such a large radio antenna dish. So instead, radio astronomers use a technique known as interferometry.
With radio interferometry, an array of antenna dishes view the same object from widely separated positions. Waves of light from the object at slightly different times, depending on their location. By correlating the antenna signals, astronomers can create a virtual telescope the size of the array. This is what makes observatories such as the VLA and ALMA so powerful. With radio interferometry, astronomers can even create a virtual telescope the size of Earth, which they used to directly image a black hole.
But you don’t need to make a high-resolution image of a star to measure its size directly. Recently a team measured the size of two stars, β Canis Majoris and ϵ Orionis, and they did it an array of gamma-ray telescopes known as VERITAS.1
While radio wavelengths are much longer than visible light, gamma rays have much shorter wavelengths. So short that gamma rays act almost like particles. When gamma rays strike Earth’s atmosphere, they can create flashes of optical light called Cherenkov light. VERITAS observes Cherenkov light to study gamma ray astronomy, which is not suited for the type of interferometry used by radio telescopes. So the team repurposed the detectors to use another type of interferometry known as intensity interferometry. With this method, multiple antennas only measure the intensity or brightness of a source, so it doesn’t need to worry about the wave property of optical light.
Both β Canis Majoris and ϵ Orionis are blue giant stars. The former is about 500 light-years away, while the latter is 2,000 light-years away. Their apparent sizes are less than a milliarcsecond, which is smaller than the Hubble Space Telescope’s resolution. Using this method, the team measured the apparent size of these stars with an uncertainty of less than 5%.
The VERITAS array only contains four antennas, so this is just a first step. With more antennas, this method could be used to create extremely precise observations of distant stars.
Abeysekara, A. U., et al. “Demonstration of stellar intensity interferometry with the four VERITAS telescopes.” Nature Astronomy (2020): 1-6. ↩︎