Asteroids help scientists measure distant stars

Watch the sky on a clear night and you will see a lot of stars. Sometimes they seem almost at hand or at least a short rocket ride. But the closest star to the Earth – not counting our sun – is more than four light-years away, at a distance of 25,000 billion kilometers.

Our galaxy of the Milky Way contains more than 100 billion stars and, although we have learned a lot, there are relatively few whose size has been measured directly because they are so far apart. The size of a star is essential information that can solve many other mysteries about it. Several methods have been used to measure the size of stars, but each has its limits.

But now, an international team, including researchers from the University of Delaware, has discovered a new way to determine star size. Their method is based on the unique capabilities of the highly energetic radiation imaging telescope system (VERITAS) at the Fred Lawrence Whipple Observatory in Arizona and the asteroids that move to the right place at the right time.

Using this technique, a collaboration of 23 universities and research institutes, led by Tarek Hassan of the Deutsches Elektronen Synchrotron (DESY) and Michael Daniel of the Smithsonian Astrophysical Observatory, revealed the diameter of a giant star at 2,674 light-years and a sun-like star at a distance of 700 light-years – the smallest star measured in the night sky to date. The search was reported on Monday, April 15 in the newspaper Nature Astronomy.

Choose a star

"Knowing the size of a star is of general importance," said Jamie Holder, an associate professor in the UD's Department of Physics and Astronomy and co-author of the 39; study. "What is the size and warmth of a star that tells you how she was born, how long she will shine and how she will eventually die."

Yet almost all the stars in the sky are too far to be accurately measured, even by the best optical telescopes.

"You can not solve the image of a dot-shaped star," said Holder. "It will look blurry through your telescope."

To overcome this limitation, scientists use an optical phenomenon called diffraction to measure the diameter of a star. When an object passes in front of a star, an event called "blackout", the shadow and the surrounding light wave pattern can be used to calculate the size of the star.

In this pilot study, the object that passed in front of the star was an asteroid – a bit of rubble in the space probably due to the formation of the planets there are about 4.6 billion of them. ; years.

Asteroids travel at an average speed of 15 miles per second, which adds to the team's challenge. Normally, VERITAS telescopes watch for the bluish bluish that cosmic particles and high-energy gamma rays produce when they pass through the earth's atmosphere. Although telescopes do not produce the best optical images, they are extremely sensitive to rapid changes in light, including starlight, thanks to their immense reflective surface, segmented into hexagons like a fly's eye. Holder was involved in the construction and commissioning of the telescopes in 2006, and all light sensor modules for the four telescopes were assembled at UD.

A doctoral student from UD makes pioneering observations

Using the four large VERITAS telescopes on February 22, 2018, the team was able to clearly detect the TYC 5517-227-1 star diffraction pattern when passing the 60 km asteroid Imprinetta (37 miles) UD PhD student Tyler Williamson was present for the observation.

"It was the first time we carried out this type of measurement, so we made sure to give ourselves enough time to prepare ourselves and follow the procedure to the letter," said Williamson, one of the three scientists on duty tonight. "The occultation itself only takes a few seconds, but we point the telescope at the star for about 15 minutes to get an estimate of what it looks like before and after the event." If you want to detect a shadow, you must know what the object looks like without anything blocking it. "

Usually, when team members take data, a computer gives them a real-time view of what they are collecting as they arrive. But they had no way of seeing this occultation happen. They just had to point the telescope and wait.

"Nobody was sure that the occultation would even be visible from our location in the first place," he said. "According to our latest estimates, at night we had about a 50% chance that our shadow would be projected onto our observatory.The asteroid is small, and the size and trajectory of the camera were uncertain. shadow would fall. "

The crew took the data, e-mailed it to the project's senior investigators and called them overnight.

"I remember waking up the next afternoon to receive an IP email with a nice plot showing a clear detection of the shadow," Williamson said. "We were all very excited and, as observers, we were very happy to be part of the result."

The VERITAS telescopes allowed the team to take 300 snapshots per second. From these data, the luminance profile of the diffraction pattern could be reconstructed with great accuracy, which would give an angular or apparent star diameter of 0.125 milliarcseconds. With their distance of 2674 light-years, scientists have determined that the actual diameter of the star is 11 times that of our sun, classifying it as a giant red star.

According to Holder, this star is about 200 million times farther from us than the sun, but it is still good in our galaxy of the Milky Way, which is 100,000 light years away.

The researchers repeated the exploit three months later, on May 22, 2018, when a Penelope asteroid of 88 km in diameter obscured the TYC 278-748-1 star. The measurements resulted in an angular size of 0.094 milliarcseconds and a real diameter of 2.17 times that of our sun – the smallest star ever measured directly.

But "small" is relative. "This star is a G dwarf, twice as big as our sun and about 700 times farther from us than our nearest star," Holder said.

While the new technique offers a resolution ten times greater than the standard method used by astronomers, based on lunar occultation, and is twice as accurate as size measurements using interferometric techniques, Ms. Holder said that the team was working on perfecting it for greater accuracy.

"Asteroids go through Earth every day," said Holder. "VERITAS is preparing to increase its observations and extend its range of observation, by building data on a new population of stars."