A Star’s True Dimension | by Brian Koberlein



26 July 2020

The VERITAS array, an air Cherenkov telescope designed to detect low-energy cosmic rays.
The VERITAS array, an air Cherenkov telescope designed to detect low-energy cosmic rays.

In astronomy, the sharpness of your picture relies upon upon the dimensions of your telescope. When Galileo and others started to view the heavens with telescopes centuries in the past, it modified our understanding of the cosmos. Objects comparable to planets, seen as factors of sunshine with the bare eye, might now be seen as orbs with floor options. However even below these early telescopes, stars nonetheless appeared as a degree of sunshine. Whereas Galileo might see Jupiter or Saturn’s measurement, he had no technique to know the dimensions of a star.

Image showing the red supergiant Betelgeuse.
Alex Lobel, Andrea Dupree, Ronald Gilliland, CfA, STScI, NASA, ESA
Picture exhibiting the pink supergiant Betelgeuse.

That didn’t change till 1995 when the Hubble Area Telescope made a picture of Betelgeuse not as a degree however as a blurry disk. It was the primary time astronomers might decide the dimensions of a star instantly. Astronomers might lastly evaluate the obvious measurement of a star to theoretical calculations primarily based on mass, colour, and brightness. Since then, each floor and space-based optical telescopes have imaged stars and even planets instantly. However astronomy at different wavelengths posed new challenges.

In radio astronomy, the problem was with the wavelength of radio gentle. Optical telescopes use gentle with wavelengths on the order of some hundred nanometers, the wavelengths utilized by radio telescopes are usually millimeters or centimeters. For the reason that decision of telescope scales with the wavelength of sunshine, a radio telescope would should be practically 1,000,000 occasions bigger to create a pointy picture. It isn’t possible to create such a big radio antenna dish. So as an alternative, radio astronomers use a method referred to as interferometry.

A high-resolution image of Betelgeuse taken by the ALMA radio telescope.
ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella
A high-resolution picture of Betelgeuse taken by the ALMA radio telescope.

With radio interferometry, an array of antenna dishes view the identical object from broadly separated positions. Waves of sunshine from the article at barely completely different occasions, relying on their location. By correlating the antenna indicators, astronomers can create a digital telescope the dimensions of the array. That is what makes observatories such because the VLA and ALMA so highly effective. With radio interferometry, astronomers may even create a digital telescope the dimensions of Earth, which they used to instantly picture a black gap.

However you don’t must make a high-resolution picture of a star to measure its measurement instantly. Just lately a workforce measured the dimensions of two stars, β Canis Majoris and ϵ Orionis, and so they did it an array of gamma-ray telescopes referred to as VERITAS.

Artist concept of using stellar intensity interferometry to increase resolution.
CfA, M. Weiss
Artist idea of utilizing stellar depth interferometry to extend decision.

Whereas radio wavelengths are for much longer than seen gentle, gamma rays have a lot shorter wavelengths. So brief that gamma rays act nearly like particles. When gamma rays strike Earth’s environment, they will create flashes of optical gentle known as Cherenkov gentle. VERITAS observes Cherenkov gentle to check gamma ray astronomy, which isn’t fitted to the kind of interferometry utilized by radio telescopes. So the workforce repurposed the detectors to make use of one other sort of interferometry referred to as depth interferometry. With this technique, a number of antennas solely measure the depth or brightness of a supply, so it doesn’t want to fret in regards to the wave property of optical gentle.

Each β Canis Majoris and ϵ Orionis are blue large stars. The previous is about 500 light-years away, whereas the latter is 2,000 light-years away. Their obvious sizes are lower than a milliarcsecond, which is smaller than the Hubble Area Telescope’s decision. Utilizing this technique, the workforce measured the obvious measurement of those stars with an uncertainty of lower than 5%.

The VERITAS array solely comprises 4 antennas, so that is only a first step. With extra antennas, this technique may very well be used to create extraordinarily exact observations of distant stars.

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