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Artist’s ide of the developed stars known as red giants, at different distances in this illustration. Photo via NASA Goddard room Flight Center/ kris Smith (KBRwyle).

You are watching: What inevitably forces a star like the sun to evolve away from being a main sequence star?

What space red giants?

A red giant is a star in its death stages. It has slowly swollen increase to plenty of times its original size. Once at the red huge stage, a star could stay that means for as much as a billion years. Then the star will gradually contract and also cool to come to be a white dwarf: Earth-sized, ultra-dense star corpses radiating a tiny fraction of their original energy. Eventually, after billions of years, these stars will end up being totally cold and also radiate no energy. They’ll finish their resides as a so-called black color dwarf: a tiny, burned-out, virtually-invisible cinder.

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To become a red giant, a specific star must have actually between half our sun’s mass, and also eight times our time our sun’s mass. Astronomers call such stars low- or intermediate-mass stars. Therefore you can see that our sun is among the stars that will inevitably, someday, come to be a red giant.

So it’s our sun’s destiny to become a red gigantic star (and after that a white dwarf, and then a black dwarf). Yet what processes will journey the sun’s evolution to the red giant stage? and also what will take place exactly, inside the star, together it changes? Let’s study the fate the low- or intermediate-mass stars such as our sun, together they evolve to the red huge phase.

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One that the best known red giant stars is the old red star Betelgeuse. It’s also the nearest red supergiant star to Earth. This 2013 far-infrared image, native the Herschel space observatory, reflects the star and its enigmatic surroundings. The multiple arcs around the star are due to winds indigenous Betelgeuse crashing against the neighboring interstellar medium. They produce a bow shock together the star moves v space. Picture via ESA.

Hydrogen: A star’s 1st fuel

Stars radiate energy by converting hydrogen to helium via atom fusion. It’s this process that causes our sun to radiate light, heat and also other develops of energy as a byproduct. Yet nuclear blend in stars at first requires hydrogen. And also stars don’t have actually an unlimited supply that hydrogen. Our sunlight converts approximately 600 million loads of hydrogen right into helium every second. If that sounds together if the supply should because of this soon exhaust itself, just remember that the sunlight is a star practically a million mile across. And if you have trouble visualizing that, imagine boarding a jet airliner for a flight that is walk to last 226 days. The is just how long it would certainly take you to fly around our local star.

In truth, our sun, together an mean star of its type, was born with sufficient hydrogen to critical for about 10 billion years. Astronomers calculation our star is now roughly 4.5 billion years old. So the sunlight is leaving the halcyon job of the youth behind it. It’s entering into middle age. And also like people, it won’t be too long until its procedures start to readjust and falter.

Hydrogen burning and also the key sequence

We contact the current stage of ours star’s life the hydrogen-burning phase, due to the fact that its energy resource is the blend of hydrogen atoms. But burning is a little of a misnomer. It’s atom fusion, no chemical burning. Stars execute not burn in the conventional sense of the word. Still, astronomers perform use the ax burning to define the kind of fusion going on inside a star. Hence, you might hear that carbon burning or helium burning. Both space stages of atom fusion, consuming various elements, as soon as a star nears the finish of its life.

Stars that largely burn hydrogen space in what’s well-known as the main-sequence phase. As a main sequence star, our sun is in what’s dubbed stellar equilibrium. That way the outside radiation pressure from the sun’s internal combination reactions specifically balances the inward press of the sun’s very own gravity.

It’s essential to establish that, when the sun’s ~ above the main sequence, even the intake of thousands of millions of loads of hydrogen per 2nd does not instantly deplete the sun’s hydrogen. Just 0.7% of our sun’s hydrogen consumed in the fusion process will ultimately be radiated together energy. The remainder is provided up convert the hydrogen atom nuclei right into helium atoms. The tiny percent of power has been giving us all the light and heat we gain from the sunlight for the critical 4.5 exchange rate years!

Read an ext about this video: one all-sky red giant star symphony

The star starts to die

Eventually, as its nuclear fires falter, a star starts come contract under its very own gravity. At the same time the star is shrinking, the temperature is increasing. So the star i do not care brighter.

In one aging star, this phase of shrinking and also brightening can last for numerous million years. The shrinking core, which is heater up together it shrinks under gravity, brings much more hydrogen towards the facility of the star, right into the place previously occupied by the now-shrunken core. Eventually, temperatures and also pressures are sufficient to ignite this shell of hydrogen approximately the core: radiation indigenous this new hydrogen-shell burning pushes outward through the star, resulting in its outer layers to expand. Over there are complicated physical processes at work here, but the laws of the conservation of energy, in conjunction with the means gravity behaves, mean that if the main point of the star shrinks, the rest of the star must expand. The star has actually started evolving into what is well-known as a subgiant star, representing an intermediate phase between the main sequence and the red huge stage.

It becomes a red giant

The hydrogen-shell burning occurs through blend processes that room far much more intense 보다 they were once the star to be on the key sequence. The an outcome is that the star brightens by a modest amount. However the external layers the the expanding star, now being additional away native the hydrogen shell around the core, cool at the exact same time, dropping from a best of between 6,000 and 30,000 degrees down come 5,000. This also method that the star’s light reddens, in the same means that a cooling poker eliminated from a fire will cool native white v yellow come red end time.

The hydrogen-burning phase have the right to last for in between a few hundred million come a billion years, relying on the initial massive of the star. For stars in between 0.8 and also 2 time the fixed of our sun, this outcomes in a subgiant i beg your pardon is 10 times the diameter of our sun. Stars of mass external this variety may climate follow various evolutionary paths, yet for a star like the sun the next phase will be a substantial increase in size, a large rise in brightness and an ext cooling. The driving power for this will arise indigenous the helium core, collapsing, getting denser until, at the end of the subgiant phase, it i do not care hot enough to burn that is helium. This reasons a huge increase of power output which pressures the growth of the star. Eventually, after ~ perhaps numerous millions that years, the star will certainly be a hundred time the diameter of the sun and distinctly red in color.

And so a red huge is born.

A star will be in the red gigantic phase for typically approximately a exchange rate years. What happens following will count on the star’s mass. High-mass stars will certainly explode together supernovae. Low- to intermediate-mass stars like our sun will progressively shrink and cool right into white dwarf stars.

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Comparison of the size of our sunlight to the smallest and largest red giant for i beg your pardon oscillations (starquakes) have been detected with the Kepler room Telescope. Picture via Daniel Huber/ college of Sydney.

In the night sky

We have the right to look up and see several red giants with our unaided eyes. Aldebaran and also Antares are just two examples. But perhaps the most well known is Betelgeuse, in the constellation the Orion. That famous due to the fact that it hit the headlines a small while ago when it all of sudden started getting dimmer, end a duration which it is long for number of months in 2019. That brightness dropped by much more than 60%, meaning it to be noticeably dimmer in the night sky. Read much more about Betelgeuse’s especially dimming.

So what of ours sun? end the next couple of hundred million years, it will gradually increase in brightness and also start to radiate more energy throughout the electromagnetic spectrum, together it heads towards its subgiant phase. That’s bad news because that the Earth. In around a billion years, boosting radiation from our star will have sterilized our planet, extinguishing every life.

Eventually, as our sunlight completes its alters from a usual G-type star right into a red giant, that will expand to gulp down Mercury, Venus and perhaps planet too. And that will certainly be the end of our world.

The study of red giants is complex, together there are many variables and exceptions. It can throw out the unexpected, like the dimming that Betelgeuse. But these huge stars are just going through a natural phase of life, obtaining old and also dying. By the moment our sun, for example, ends its life as a white dwarf, that will have actually lived for ten exchange rate years. And also perhaps, once our star swells up to substantial size, it will certainly be studied by alien civilizations looking from afar, as we examine Betelgeuse and the other giants in our skies. They will certainly have tiny idea that, once, a tiny blue period orbited that star, whose occupants looked come the stars and wondered, too.

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Bottom line: What room red giants? Most main sequence stars, favor our sun, will end up being one. Red giants puffy to together a huge size they have the right to swallow their inner planets.