Visiting asteroids help scientists measure the size of distant stars
The Very Energetic Radiation Imaging Telescope Array System must be be very energetic indeed to capture 300 images a second.
The VERITAS array searches for very high-energy gamma rays and cosmic rays from its location in the Santa Rita Mountains south of Tucson, Arizona. (Harvard/Smithsonian Center for Astrophysics photo)
On Feb. 22, 2018, as the asteroid Imprinetta passed between the Earth and a distant star known as TYC 5517-227-1, a team of astronomers who had predicted the event captured it with a camera shooting 300 images per second. Serving as the camera’s lens were the four 12-meter gamma ray optical reflectors of the Very Energetic Radiation Imaging Telescope Array System (VERITAS) telescope at Harvard- Smithsonian Fred Lawrence Whipple Observatory in Amado, Az.
Details preserved in the images as the 60-kilometer diameter asteroid passed in front of TYC 5517-227-1 and caused it to dim momentarily were then used to accurately calculate the size of the star for the first time.
“The brightness profile of the diffraction pattern was reconstructed with high accuracy,” says Michael Daniel, operations manager of VERITAS for the Smithsonian Astrophysical Observatory, and one of the co-authors of a paper on the study in April 16, 2019 issue of the journal Nature Astronomy. “This allowed us to determine the actual diameter of the star, and determine it to be a red giant.”
Because TYC 5517-227-1 is some 2,674 light years away from Earth, like most other stars in the night sky, it is too distant for its size to be measured reliably with even the most accurate optical telescopes. By recording the diffraction pattern caused as Imprinetta passed in front of TYC 5517-227-1—creating a shadow called an occultation—the astronomers were then able to use this information to calculate the star’s size. “The only chance to catch the diffraction pattern is to make very fast snapshots when the shadow of the occultation sweeps across the telescope,” Daniel says.
When an asteroid passes in front of a star, the resulting diffraction pattern (here greatly exaggerated) can reveal the star’s angular size. (Credit: DESY, Lucid Berlin)
“The incredibly faint shadows of asteroids pass over us every day,” says study co-author Tarek Hassan, an astronomer at the Deutsches Elektronen-Synchrotron. “But the rim of the shadow isn’t perfectly sharp. Instead, wrinkles of light surround the central shadow, like water ripples.”
On May 22, 2018, the team conducted a similar experiment when asteroid Penelope occulted star TYC 278-748-1, which is located 700 light years from Earth. Penelope is 88 kilometers in diameter.
“Using the same formula for data collection and calculations, we determined this star to be 2.17 times the diameter of Earth’s Sun,” Daniel said. “This direct measurement allowed us to correct an earlier estimation that placed the star’s diameter at 1.415 times that of our sun.”
With almost any star on the night sky too distant from Earth to be directly measured using even the best of optical telescopes, scientists overcame these limitations using diffraction, which occurs when an object, like an asteroid, passes in front of a star, making a shadow called an occultation.”
For VERITAS scientists, however, the task was not as easy as turning telescopes to the sky. “Asteroid occultations are difficult to predict,” Daniel says.
Astronomers have used a similar method to measure angular sizes of stars occulted by Earth’s moon.
“The trouble is that not many telescopes are large enough for the occultation method to measure the diffraction pattern with confirmed accuracy over the turbulence in the Earth’s atmosphere,” Daniel says.
VERITAS telescopes however are uniquely sensitive “as we use them primarily for observing faint light from very-high-energy gamma rays and cosmic rays. While they do not produce images as elegant as those from traditional optical telescopes, they see and capture fast variations of light, and we estimate that they can analyze stars up to ten times farther away with extreme accuracy than optical telescopes using the lunar occultation method can.”
Astronomers now look forward to using this new method of measuring star size of other distant stars visible on the night sky.
2. The VERITAS gamma-ray observatory is located at the Fred Lawrence Whipple Observatory in Amado, Arizona. (Harvard/Smithsonian Center for Astrophysics photo)
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Posted: 10 May 2019
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Categories:
Astrophysical Observatory , Feature Stories , Science and Nature
