After forming out of a cloud of gas and dust, a mid-sized star like our Sun sits nicely on the main sequence and burns hydrogen in its core for some 5 billion years. Then, the end begins. (This one’s a bit longer, but stay with it… you’ll know much more about how stars work in just a couple of minutes…)
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* What you’ve learned already: stars on the main sequence fuse hydrogen into helium in their cores, releasing heat and light for tens of millions to billions of years. Massive stars burn fast, hot, and blue; less massive stars burn slow, cool, and white or yellow or red. Eventually, all but the smallest stars run out of hydrogen, and that’s when they start burning heavier elements in the core and quickly evolve off the main sequence.
* After 5-10 billion years, when a mid-sized star like our Sun runs low on hydrogen, nuclear fusion in the core slows. With less light to push back against gravity, the star contracts and heats the helium-rich core, re-igniting a thin shell of hydrogen, which pushes out the star’s atmosphere.
* The star cools and swells by 50-100 times, becoming a red giant. It moves to the right and upwards on the HR diagram. Planets closest to the star may get swallowed up. This fate awaits Mercury and Venus. Earth may be spared, but the Sun will expand to fill much of our sky.
* In some cases, depending on the star’s mass, the helium core will be squeezed enough to suddenly ignite in what’s called the “helium flash“. This expels as much energy as 100,000,000 Suns. But the flash is brief, and the released energy does not disturb the outer layers of the star.
A Deeper Look
* After helium starts burning, the hydrogen shell around the core expands and cools, which means the outer layers of the star contract again. The star shrinks and moves to the left and downwards on the HR diagram, but does not return to the main sequence.
* Helium burning is notoriously unstable, so the star begins to pulsate irregularly. Mira is an example of a star in this late phase of life. The outer layers are expelled as a planetary nebula. Once the helium is finished burning, the core becomes rich in carbon and oxygen. The core collapses again, but this time it doesn’t get hot enough to continue burning. What’s left of the core become a white dwarf, which we’ll cover in the next issue.
* The sun has enough hydrogen fuel to burn for 5 billion more years before it enters the red giant stage. But here’s the kicker… as the hydrogen burns, the Sun slowly contracts and becomes too hot to comfortably sustain life on Earth. This will happen not in 5 billion years, but just 500 million years. So plan accordingly.
Good To Know
A word about classification. On the main sequence, in the prime of life, a star has a luminosity class of “V” (five). So the sun is a G2V star, and Sirius is a B3V star. When mid-sized stars become red giants, they have luminosity class III (three) or IV (four). Aldebaran in the constellation Taurus, for example, is a K5III star, which means it’s become a red giant.
A little knowledge of star types will help you better understand what you see in the sky. For example, go out on a fine spring night and find Arcturus in the constellation Bootes. It’s a red giant star, type K2III, and it’s at this very moment burning a shell of hydrogen near its core; our Sun will look very much like this in some 6 -7 billion years. Now you know why.