The Sun is about 4.6 billion years old – measured by the age of other bodies in the Solar System that formed around the same time. Based on observations of other stars, astronomers expect that they will reach the end of their life in about another 10 billion years.
There are other things that will happen along the way, of course. In about 5 billion years, the Sun is set to turn into a red giant. The star’s core will shrink, but its outer layers will extend into the orbit of Mars, engulfing our planet in the process.
One thing is for sure: by then, we will almost certainly not be around. In fact, humanity has only a billion years left unless we find a way to get away from this rock. This is because the sun’s brightness increases by about 10% every billion years.
It doesn’t sound like much, but an increase in brightness would end life on Earth. Our oceans will evaporate, and the surface will become too hot for water to form. Several previous studies have found that in order to form a bright planetary nebula, the protostar must be twice the mass of the Sun.
However, the 2018 study used computer modeling to determine that, like 90% of other stars, our Sun is likely to shrink from a red giant to a white dwarf and then end as a planetary nebula.
When a star dies, it ejects a mass of gas and dust – known as its atmosphere – into space. The envelope can be up to half the mass of the star. This reveals the star’s core, which is turned on at this point in the star’s life. Astrophysicist Albert Zelstra of the University of Manchester in the UK, one of the authors of the paper, said: ‘The hot core makes the ejected mantle shine brightly for about 10,000 years – a brief period in astronomy. This is what makes the planetary nebula visible. Some are so bright that they can be seen. From extremely large distances, measured in the tens of millions of light years.”
The data model the team created actually predicts the life cycle of different types of stars, figuring out the brightness of planetary nebulae associated with different star masses.
Planetary nebulae are relatively common throughout the observable universe, with the most famous being the Helix Nebula, the Annular Nebula, and the Bubble Nebula.
They are called planetary nebulae not because they are actually related to the planets, but because when William Herschel first discovered them in the late 18th century, they were similar in appearance to the planets through telescopes at the time.
Nearly 30 years ago, astronomers noticed something strange: the brightest planetary nebulae in other galaxies all had the same brightness. This means that, in theory at least, by looking at planetary nebulae in other galaxies, astronomers can calculate their distance.
The data showed that this was true, but models contradicted it, which has infuriated scientists since the discovery.
“Old, low-mass stars should make planetary nebulae much fainter than younger, more massive stars,” Zilstra said. “This has become a source of conflict over the past 25 years. The data suggests that bright planetary nebulae can be obtained from low-mass stars like the Sun.” Models show that this is not possible, as anything less than twice the mass of the Sun would give a planetary nebula too faint to be seen.”
The 2018 models solve this problem by showing that the Sun is approaching the minimum mass of a star that can produce a visible nebula.
Even a star less than 1.1 times the mass of the Sun would not produce a visible nebula. On the other hand, larger stars up to 3 times the mass of the Sun will produce the brighter nebulae.
For all the other stars in between, the expected brightness is very close to what was observed.
“This is a good result,” Zilstra said. “Not only do we now have a way to measure the presence of stars over several billion years in distant galaxies, a range that is remarkably difficult to measure, but we have also discovered what the Sun will do when it dies!”
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