How Astronomers Used Asteroids to Measure Stars

April 24, 2019 - Comment

Astronomers have used a singular technique and a unprecedented telescope array to disclose the diameters of distant stars. A closeup view of one of many telescopes within the VERITAS array. NSF / VERITAS It is wonderful how a lot data you may coax out of some photons. For many years, astronomers have watched asteroids as

Astronomers have used a singular technique and a unprecedented telescope array to disclose the diameters of distant stars.


A closeup view of one of many telescopes within the VERITAS array.

It is wonderful how a lot data you may coax out of some photons. For many years, astronomers have watched asteroids as they blocked out stars behind them — these occultations revealed the asteroids’ form and measurement as their “shadows” swept throughout Earth. Now, astronomers are utilizing asteroid occultations to disclose the sizes of the celebs themselves.

Astronomers used the Very Energetic Radiation Imaging Telescope Array System (VERITAS), positioned on the Fred Lawrence Whipple Observatory in Arizona, to just do that, publishing the ends in Nature Astronomy. VERITAS is a gaggle of 4 12-meter telescopes, every composed of 350 hexagonal mirror segments. They’re set as much as look ahead to the faint blue flashes of Cherenkov radiation, produced when gamma rays crash into Earth’s environment. However Wystan Benbow (Heart for Astrophysics, Harvard & Smithsonian) and colleagues turned the telescopes towards asteroid shadows.

First, the asteroid 1165 Imprinetta handed in entrance of a star designated TYC 5517-227-1. On this proof-of-principle commentary, the astronomers used VERITAS to snap 300 photos a second, pegging the star at 11 instances the Solar’s width. Then a number of months later, 201 Penelope handed in entrance of one other star, TYC 278-748-1. This time VERITAS captured 2,500 photos each second because the shadow handed over Earth, permitting it to measure a fair smaller star solely twice the Solar’s diameter.


The 4 telescopes within the VERITAS array.

Though the mirrors of the VERITAS array are crude by astronomical requirements, it was the telescopes’ unimaginable time decision that made it attainable to measure the celebs’ diamters. “VERITAS telescopes are very giant, and that is essential for taking correct measurements with very quick sampling frequencies,” says Tarek Hassan (DESY, Germany). Massive detectors just like the VERITAS telescopes additionally do not see as a lot scintillation noise, the “twinkling” brought on by turbulent motions in Earth’s environment.

DESY occultation

An artist’s conception of the occultation of a star by an asteroid.
DESY/Lucid Berlin

To measure an occulted star’s measurement, the astronomers first must seize the fragile “fringes” of the diffraction sample alongside the sting of the shadow it casts. These fringes are the place gentle waves merge to altenately enhance or cancel the sign. (In actual fact, each shadow has diffraction fringes at its edges, however for the objects we work together with every day at seen wavelengths, these fringes are imperceptible.)

When an asteroid passes in entrance of a star, the asteroid itself is invisible, however an observer will see the star it is blocking briefly wink out. Due to diffraction fringes alongside the asteroid shadow’s edge, the star’s brightness will differ in a predictable means proper earlier than and after shadow sweeps throughout Earth. By evaluating the fringes round an occulted star to these from a real level supply, the astronomers infer the diameter of the star.

The Asteroids & The Stars

The 60-km-wide asteroid Imprinetta handed in entrance of the primary star, the 10.2-magnitude purple big TYC 5517-227-1 within the constellation Crater, the Cup, on February 22, 2018. Observations revealed the star to be have an obvious measurement of zero.125 milliarcseconds. (For reference, that is greater than 1,000 instances higher than what the Hubble Area Telescope can resolve.) Given the star’s distance of two,674 light-years, its tiny angular measurement equates to a girth 11 instances that of the Solar. It is probably the most distant star thus far with an correct measurement of its angular measurement.

occultation graph

Graphs of the 1165 Imprinetta occasion with depth versus time, displaying information collected throughout ingress (left) and egress (proper).
Benbow et al

A second probability for the crew got here on Might 22, 2018, when the 88-km asteroid Penelope swept in entrance of the 9.9-magnitude star TYC 278-748-1 within the constellation Virgo. Occultation measurements revealed that the star, at 700 light-years away, had a diameter simply over twice the Solar’s.

Catching Occultations

Occultations usually are not straightforward to foretell or observe. Newbie astronomers captured their first profitable commentary of an asteroid occultation on February 19, 1958, when three Juno handed in entrance of a star. However solely a handful of such occasions had been noticed up till the 1980s. At the moment, Steve Preston’s web site lists dozens of such occasions occurring worldwide, each month.

“The [occultation] method itself shouldn’t be actually new,” Hassan notes. “What’s new is that now we have now higher data of the situation and velocity of many photo voltaic system objects so distant… and because of Gaia and different missions we have now unprecedented data of the celebs’ precise location on the sky.”

Astronomers have additionally used a diffraction technique throughout lunar occultations, nevertheless it solely works for stellar diameters right down to about 1 milliarcsecond. Solely 17 stars have a measured angular diameter larger than this, and of these, solely Antares lies alongside the present path of the Moon. Occultations of Antares are fast, as we witnessed from our yard in Florida in 2009:

As a result of asteroids are a lot farther from Earth than the Moon, astronomers can use them to measure a lot smaller stars, explains Daniel. “That is why our asteroid occultation measurements had been in a position to measure the smallest angular measurement stars to this point measured.”

Even so, utilizing occultations to measure star sizes stays a chancy enterprise due to the uncertainty in asteroid orbits, and if the anticipated path modifications, robust luck, as a result of giant telescopes aren’t terribly moveable. The shadow of asteroid Imprinetta, for instance, had a 50% probability of crossing over the VERITAS array, and Penelope’s shadow had a fair decrease probability of 29%. Nonetheless, the researchers determine, any telescope able to observing a 10th-magnitude star would see about 5 occultations per 12 months.

This technique is also utilized utilizing the following era of megatelescopes: The Large Magellan Telescope, for instance, might attain to far fainter stars, and the Massive Synoptic Survey Telescope would seize quick photos. Each telescopes are set to see first gentle early within the coming decade.

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