How Can We Know How Old A Star Is?

We have a significant amount of information about stars. After aiming telescopes toward the night sky for decades, professional and amateur astronomers can determine essential details about every star, such as its mass or composition. They can simply look at the star's orbital period and use some maths to get the star's mass. Examine the star's light spectrum to learn what it is comprised of. But time and age are factors that researchers have not completely figured out how to deal with.

Sometimes even stars that have been extensively examined, confound astronomers. Astronomers were not sure if the red supergiant star Betelgeuse's dimming in 2019 was merely a phase or if a supernova explosion was about to occur. The sun also caused a stir when researchers noted that it was not acting like other middle-aged stars (turns out it was simply a phase.) In comparison to other stars of the same age and mass, it has less magnetic activity. The timeframe of middle age may not be fully understood by astronomers as a result of this.

Astronomers can obtain rough estimations of a star's age using physics calculations and extrapolations. And certain techniques are more effective with particular kinds of stars. Here are three methods used by astronomers to determine a star's age.

Hertzsprung-Russell diagrams

Star birth, life, and death are all topics that scientists are rather well-versed in. For instance, stars exhaust their hydrogen fuel, inflate, and then release their gases into space either with a bang or a whimper. But the tricky part is figuring out precisely when a star goes through each step of its life cycle. Various stars reach such places after varying lengths of time depending on their masses. Less massive stars can last for billions of years whereas more massive stars tend to die earlier.

Ejnar Hertzsprung and Henry Norris Russell, two astronomers, independently developed the concept of plotting stars' temperatures against their luminosities around the start of the 20th century. These Hertzsprung-Russell, or H-R, diagrams showed patterns corresponding to the stages of the life cycles of several stars. These patterns are now used by scientists to estimate the age of star clusters, whose stars are considered to have originated simultaneously.

The catch is that this approach can only be used for stars in clusters or by comparing a single star's coloring and luminance with theoretical H-R diagrams unless you embark on a lot of arithmetic and modeling. It is not very accurate, but it is the best we have, says astronomer Travis Metcalfe of the Boulder, Colorado-based Space Science Institute.

Rotation rate

Astrophysicists began to observe a pattern in the 1970s: Stars in younger clusters spin more quickly than those in older clusters. Astronomer Andrew Skumanich proposed a straightforward equation in 1972 to calculate a star's age using its rotation rate and surface activity: Rotation rate = (Age) - 12.

For decades, this was the standard technique for studying individual stars, but recent results have called into question its applicability. It seems that some stars do not slow down as they become older. Rather, they continue to rotate at the same pace for the remainder of their lives.

For stars that are younger than the Sun, Metcalfe asserts that rotation is the optimum method. Other approaches work better for stars older than the Sun.

Stellar seismology

The Kepler space telescope, which searches for exoplanets, provided the fresh data that proved rotation rate was not the greatest method to determine a star's age. Kepler was not merely a gift for exoplanet research; by simply spending a lot of time gazing at the same stars, it brought stellar seismology to the forefront.

A star's age may be inferred by observing how it flickers. Scientists use fluctuations in a star's brightness as a sign of what is going on deep within the star and use modeling to roughly determine the star's age. To do this, a very large dataset on the star's brightness is required, which the Kepler telescope may offer.

According to Soderblom, "Everyone believes it was all about discovering planets, which was accurate." But the Kepler expedition was a covert stellar physics mission, as he likes to joke.

This method revealed the Sun's magnetic midlife crisis and lately offered some hints regarding the Milky Way's development. Our galaxy and a dwarf galaxy collided some 10 billion years ago. According to research, the stars that dwarf galaxy leaves behind are either younger or around the same age as the stars that were born in the Milky Way. Consequently, it is possible that the Milky Way developed more swiftly than previously imagined.

Astrophysicists will be able to develop fresh estimates for additional stars when more parts of the sky are surveyed by satellite telescopes like NASA's TESS and the European Space Agency's CHEOPS.

Star ages have ramifications for processes like planet formation, galaxy evolution, and even the hunt for alien life, outside of our own solar system.

Even with all the new technologies spreading throughout the world; the stars, lighting up our whole world is still a hidden concept amongst us.

Works Cited:

21, O., 1999. How do scientists determine the ages of stars? is the technique really accurate enough to use it to verify the age of the universe? Scientific American. Available at:

Anon, 2022. How do scientists determine the age of a star? Science ABC. Available at:

Grossman, L. & Thompson, H., 2022. How do scientists calculate the age of a star? Science News. Available at: the 1970s%2C astrophysicists had,%3D (Age) -½.

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