- 3570 - STARS - how to measure their size and weight? When passages of foreground stars in front of background stars astronomers, using ultra-precise measurements from the Gaia satellite. Each event will produce shifts in the background star's position due to the deflection of light by gravity, and will allow the measurement of the mass of the foreground star, which is extremely difficult to determine by other means.
--------------------- 3570 - STARS - how to measure their size and weight?
- Every star in the Milky Way is in motion. But because of the distances their changes in position, called “proper motions“, are very small and can only be measured using large telescopes over long time periods.
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- In very rare cases, a foreground star passes a star in the background, at close proximity as seen from Earth. Light from this background star must cross the gravitational field of the foreground star where, instead of following straight paths, the light rays are bent.
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- This is like a lens, except here the deviation is caused by the space and time distortion around any massive body. This effect was one predictions of Einstein's general theory of relativity and has been verified in solar system tests for decades. This distortion of the light by the foreground star is called “gravitational lensing”
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- Gravitional lensing is when the light of the background star is deviated or focused into a smaller angle, and the star appears brighter. The main effect is the change in the star's apparent position on the sky because the deviation shifts the center of light relative to other more distant stars.
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- Both of these effects depend on the mass of the lensing body, in this case that of the foreground star. Thus, gravitational lensing is a method for weighing stars. Actually, measuring the mass of stars that are not part of a binary star is otherwise extremely difficult to do.
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- Previously, the difficulty in this method was being able to predict the motions of the stars with high enough precision. The spectacular data set of literally billions of stellar positions and proper motions recently published as the Gaia Data Release 2 by the ESA Gaia consortium has made this research possible.
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- These data were used to search for such close passages of stars. Of the many close encounters which will happen in the next 50 years, two passages are going on right now (May 2022) : the closest angular separations will be reached in the next few weeks with measurable effects on the positions of the background stars.
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- The names of these two foreground stars are “Luyten 143-23” and “Ross 322“; they move across the sky with apparent velocities of about 1,600 and 1,400 milliarcseconds per year. The closest angular separations between foreground and background stars will occur in July and August 2018, when the apparent positions of the background stars will be shifted, due to the astrometric microlensing effect, by 1.7 and 0.8 milliarcseconds.
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- One milliarcsecond corresponds to the angle under which a human being lying on the surface of the moon would be seen. It is a challenging task, but with the best telescopes on Earth, these displacements of stellar positions are measurable.
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- The closest star to Earth is more than four light years away, at a distance of 25 trillion miles. There are more than 100 billion stars in our Milky Way Galaxy, and, while we have learned much about them, there are relatively few whose size has been directly measured because they are so far away.
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- A star's size is a key piece of information that unlocks many other mysteries about it. Several methods have been used to measure star sizes, yet each has its limitations. There is a new way to determine the size of stars. This method draws on the unique capabilities of the “Very Energetic Radiation Imaging Telescope Array System” (VERITAS) at the Fred Lawrence Whipple Observatory in Arizona—and asteroids that pass by at just the right place and time.
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- Using the technique revealed the diameters of a giant star 2,674 light-years away, and a sun-like star at a distance of 700 light-years which is the smallest star measured in the night sky to date.
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- How big and how hot a star is tells you how it was born, how long it will shine, and how it will eventually die. Yet almost any star in the sky is too far away to be measured accurately by even the best optical telescopes.
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- Scientists use an optical phenomenon called “diffraction” to measure a star's diameter. When an object passes in front of a star, an event called an "occultation," the shadow and surrounding pattern of light waves can be used to calculate the star's size.
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- The object passing in front of the star was an asteroid, a bit of space rubble likely leftover from when the planets were formed about 4.6 billion years ago.
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- Asteroids travel at an average speed of 15 miles per second. Normally, the VERITAS telescopes watch for the faint bluish blip that high-energy cosmic particles and gamma rays produce when they race through Earth's atmosphere.
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- While the telescopes do not produce the best optical images, they are extremely sensitive to fast variations of light, including starlight, thanks to their huge mirrored surface, segmented in hexagons like a fly's eye.
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- Using the four large VERITAS telescopes on February 22, 2018, the team could clearly detect the diffraction pattern of the star “TYC 5517-227-1” as the 37 mile diameter asteroid “Imprinetta” passed by.
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- The occultation itself takes only a few seconds, but we point the telescope at the star for about 15 minutes or so to get an estimate of what it looks like before and after the event. If you want to detect a shadow, you need to know what the object looks like without anything blocking it.
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- The VERITAS telescopes allowed the team to take 300 snapshots every second. From these data, the brightness profile of the diffraction pattern could be reconstructed with high accuracy, resulting in an angular, or apparent, diameter of the star of 0.125 milliarcseconds. Together with its distance of 2,674 light-years, the scientists determined that the star's true diameter is 11 times that of our sun, categorizing it as a red giant star.
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- This star is about 200 million times farther away from us than the sun, but it's still well within our Milky Way Galaxy, which is 100,000 light years across.
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- The researchers repeated the feat three months later on May 22, 2018, when asteroid Penelope with a diameter of 88 kilometers occulted the star TYC 278-748-1. The measurements resulted in an angular size of 0.094 milliarcseconds and a true diameter of 2.17 times that of our sun, the smallest star ever measured directly.
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- This star is a G dwarf, twice as big as our sun and about 700 times farther away from us than our closest star. While the new technique delivers a ten times better resolution than the standard method astronomers have been using, based on lunar occultation, and is twice as sharp as size measurements using interferometric techniques.
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- Asteroids pass by Earth every day. "VERITAS is gearing up to increase its observations and extend its observation range, building data on a whole new population of stars.
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May 5, 2022 STARS - how to measure their size and weight? 3570
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