How Astronomers Used Asteroids to Measure Stars

Astronomers have used a unique method and an extraordinary telescope array to reveal the diameters of distant stars.

It's amazing how much information you can coax out of a few photons. For decades, astronomers have watched asteroids as they blocked out stars behind them — these occultations revealed the asteroids' shape and size as their "shadows" swept across Earth. Now, astronomers are using asteroid occultations to reveal the sizes of the stars themselves.

Astronomers used the Very Energetic Radiation Imaging Telescope Array System (VERITAS), located at the Fred Lawrence Whipple Observatory in Arizona, to do just that, publishing the results in Nature Astronomy. VERITAS is a group of four 12-meter telescopes, each composed of 350 hexagonal mirror segments. They're set up to watch for the faint blue flashes of Cherenkov radiation, produced when energetic charged particles crash into Earth's atmosphere. But Wystan Benbow (Center for Astrophysics, Harvard & Smithsonian) and colleagues turned the telescopes toward asteroid shadows.

First, the asteroid 1165 Imprinetta passed in front of a star designated TYC 5517-227-1. In this proof-of-principle observation, the astronomers used VERITAS to snap 300 images a second, pegging the star at 11 times the Sun's width. Then a few months later, 201 Penelope passed in front of another star, TYC 278-748-1. This time VERITAS captured 2,500 images every second as the shadow passed over Earth, allowing it to measure an even smaller star only twice the Sun's diameter.

Although the mirrors of the VERITAS array are crude by astronomical standards, it was the telescopes' incredible time resolution that made it possible to measure the stars' diameters. “VERITAS telescopes are very large, and that is very important for taking accurate measurements with very fast sampling frequencies,” says Tarek Hassan (DESY, Germany). Large detectors like the VERITAS telescopes also don't see as much scintillation noise, the "twinkling" caused by turbulent motions in Earth's atmosphere.

To measure an occulted star's size, the astronomers first need to capture the delicate "fringes" of the diffraction pattern along the edge of the shadow it casts. These fringes are where light waves merge to alternately boost or cancel the signal. (In fact, every shadow has diffraction fringes at its edges, but for the objects we interact with on a daily basis at visible wavelengths, these fringes are imperceptible.)

When an asteroid passes in front of a star, the asteroid itself is invisible, but an observer will see the star it's blocking briefly wink out. Thanks to diffraction fringes along the asteroid shadow's edge, the star's brightness will vary in a predictable way right before and after shadow sweeps across Earth. By comparing the fringes around an occulted star to those from a true point source, the astronomers infer the diameter of the star.

The Asteroids & The Stars

The 60-km-wide asteroid Imprinetta passed in front of the first star, the 10.2-magnitude red giant TYC 5517-227-1 in the constellation Crater, the Cup, on February 22, 2018. Observations revealed the star to be have an apparent size of 0.125 milliarcseconds. (For reference, that's more than 1,000 times better than what the Hubble Space Telescope can resolve.) Given the star's distance of 2,674 light-years, its tiny angular size equates to a girth 11 times that of the Sun. It's the most distant star to date with an accurate measurement of its angular size.

A second chance for the team came on May 22, 2018, when the 88-km asteroid Penelope swept in front of the 9.9-magnitude star TYC 278-748-1 in the constellation Virgo. Occultation measurements revealed that the star, at 700 light-years away, had a diameter just over twice the Sun's.

Catching Occultations

Occultations are not easy to predict or observe. The first attempted observation of an asteroid occultation was on February 19, 1958, when 3 Juno was predicted to pass in front of a star. Observations of this event, however, later proved to be a false positive. The first successful capture of a stellar asteroid occultation was by asteroid 2 Pallas on October 2nd, 1961. Only a handful of such events were observed up until the 1980s. Today, Steve Preston's site lists dozens of such events occurring worldwide, every month.

“The [occultation] technique itself is not really new,” Hassan notes. “What's new is that now we have better knowledge of the location and speed of many solar system objects so far away... and thanks to Gaia and other missions we have unprecedented knowledge of the stars' exact location on the sky.”

Astronomers have also used a diffraction method during lunar occultations, but it only works for stellar diameters down to about 1 milliarcsecond. Only 17 stars have a resolved angular diameter greater than this, and of those, only Antares lies along the current path of the Moon. Occultations of Antares are quick, as we witnessed from our backyard in Florida in 2009:

Because asteroids are much farther from Earth than the Moon, astronomers can use them to measure much smaller stars, explains Daniel. “This is why our asteroid occultation measurements were able to measure the smallest angular size stars thus far measured.”

Even so, using occultations to measure star sizes remains a chancy business because of the uncertainty in asteroid orbits, and if the predicted path changes, tough luck, because large telescopes aren't terribly portable. The shadow of asteroid Imprinetta, for example, had a 50% chance of crossing over the VERITAS array, and Penelope's shadow had an even lower chance of 29%. Nevertheless, the researchers figure, any telescope capable of observing a 10th-magnitude star would see about five occultations per year.

This method could also be applied using the next generation of megatelescopes: The Giant Magellan Telescope, for example, could reach to far fainter stars, and the Large Synoptic Survey Telescope would capture fast images. Both telescopes are set to see first light early in the coming decade.