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“The sun is the only star we know its age,” said astronomer David Soderblom of the Space Telescope Science Institute in Baltimore, United States (US). “The others were pulled from there.”
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Even well-studied stars sometimes surprise scientists. For example, when the red giant star Betelgeuse dimmed in 2019, astronomers weren’t sure if it was just going through a phase or a supernova explosion was imminent. This suggests astronomers may not fully understand the midlife timeline.
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Because of this, it takes physical calculations and indirect measurements of the star’s age to give astronomers a rough estimate. Here are three methods or ways scientists calculate the age of stars in the galaxy, as reported by: Science News.
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Diagram Hertzsprung-Russell
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At the turn of the 20th century, astronomers Ejnar Hertzsprung and Henry Norris Russell, independently came up with the idea of reading a star’s temperature against its brightness. The patterns on the Hertzsprung-Russell (HR) diagram relate to the locations of the different stars in the life cycle.
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Currently, scientists are using these patterns to determine the age of star clusters whose stars are thought to have formed at the same time.
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But there is a note. If you do a lot of math and modeling, this method can only be used for stars in groups, or by comparing the color and brightness of a single star with a theoretical HR diagram. “This is not very precise,” said astronomer Travis Metcalfe of the Institute for Space Science in Boulder, Colorado, USA. “Nevertheless, this is the best thing we have.”
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In the 1970s, astrophysicists had noticed a trend: stars in younger clusters were spinning faster than stars in older clusters. And in 1972, astronomer Andrew Skumanich used the rotational rate and surface activity of stars to propose a simple equation for estimating the age of stars.
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This method worked for individual stars for decades, but was discontinued because it turned out that some stars didn’t slow down when they reached a certain age. Instead, they maintain the same rotational speed for the rest of their lives.
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“Rotation is the best thing to use for stars that are younger than the sun,” says Metcalfe. “For stars that are older than the sun, have to use another method.”
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With the Kepler space telescope, not only is it a boon for exoplanet research, but it also pushes stellar seismology to the forefront of simply staring at the same star for very long periods of time to provide clues to its age.
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Scientists view changes in the brightness of stars as an indicator of what is happening beneath the surface. And through modeling, roughly the age of the star can be calculated. To do this requires a very large dataset on star brightness — which the Kepler space telescope can provide.
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This approach has previously provided some clues about the evolution of the Milky Way. About 10 billion years ago, our galaxy collided with a dwarf galaxy. The scientists found that the stars left behind by the dwarf galaxy are younger or roughly the same age as the original stars of the Milky Way. Thus, the Milky Way may have evolved more rapidly than previously thought.
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In addition to curiosity about our own stars, stellar age also has implications beyond the solar system, from planet formation to galaxy evolution. Even the search for extraterrestrial life.
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“One day — maybe a long time ago — someone will claim that they saw signs of life on a planet around another star. The first question people will ask is, ‘How old is that star?'” Soderblom said. “That will be a difficult question to answer,” he concluded.
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