Astronomers measure smallest star size to date using clever asteroid trick

15 Apr 2019

An illustration of an asteroid passing in front of a star’s diffraction pattern. Image: DESY/Lucid Berlin

A team of international astronomers, including Irish researchers, has measured a tiny, distant star by tracking the shadows of nearby asteroids.

Given the almost incomprehensible vastness of the universe, being able to measure the most distant objects can often be quite difficult for astronomers here on Earth. For this reason, astronomers sometimes turn to physics to come up with some ‘tricks’ that help them do things that would normally not be possible.

One such new trick – using a distant asteroid to estimate a star’s size – has been revealed by an international team of 80 astronomers, including scientists from University College Dublin (UCD), NUI Galway and Cork IT. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) collaboration published its findings to Nature Astronomy, revealing the smallest star size in the night sky to date using a technique called diffraction.

Explaining diffraction, Dr Tarek Hassan of DESY, the German high-energy physics institute, said it illustrates the wave nature of light and occurs when an object, such as an asteroid, passes in front of a star.

“The incredibly faint shadows of asteroids pass over us every day,” he said, “but the rim of their shadow isn’t perfectly sharp. Instead, wrinkles of light surround the central shadow, like water ripples.”

Measurements taken by VERITAS’s four 12-metre gamma-ray telescopes showed that the first star (TYC 5517-227-1), located 2,674 light years away, is 11 times the diameter of our sun. This was achieved by taking 300 images per second to detect the diffraction pattern in the shadow sweeping past the telescopes as the star was occulted by the 60km asteroid Imprinetta.

The researchers repeated the feat three months later, when asteroid Penelope, with a diameter of 88km, occulted the star TYC 278-748-1, located 700 light years from Earth. The star was shown to be 2.17 times the diameter of our sun with an angular size of 0.094 milliarcseconds. This is the smallest angular size of a star ever measured directly, the team said.

VERITAS spokesperson and one of those behind the research, associate professor John Quinn of the UCD School of Physics, said: “The large light-collecting ability of the VERITAS telescopes, coupled with their fast light detectors and electronics, allows measurement of the rapidly varying wrinkles in the shadow.

“This study opens up the opportunity to use VERITAS to study a whole variety of rapidly varying optical phenomena and it is fantastic to see this innovative use of the telescopes for scientific applications beyond which they were originally designed.”

Colm Gorey was a senior journalist with Silicon Republic