Copyright © Karl Dahlke, 2022
Once in a while there is a star in the sky that is red, and very large. (This is in contrast to the red dwarfs, described in the next chapter.) These stars are called red giants. They are bright, due to their large size, so if you see a red star in the sky, it is probably a red giant, and not a red dwarf, even though red dwarfs are common in our galaxy.
Our sun, and most of the medium size stars, become a red giant at the end of their lives. This phase lasts a billion years or less, a fraction of the stars overall lifetime, thus there are comparatively few red stars in the sky. By analogy, if you select a thousand people at random, very few of them will be over 90 years old.
Our sun is middle aged, but some day it too will become a red giant. Fortunately we have a couple billion years to prepare for this cataclysmic event.
Recall that a star burns hydrogen in its core, in a slow and steady process that can last for billions of years, but eventually, the hydrogen in the core is spent. The thick soup is mostly helium, and the remaining hydrogen atoms don't collide with each other very often - not often enough to keep the home fires burning. With the nuclear furnace winding down, the core begins to collapse under its own weight. The shell around the core moves inward, and experiences higher pressure and temperature, sufficient to fuse hydrogen. This hydrogen shell begins to burn, as it floats atop a superheated core of helium. The heat of fusion will not allow this shell to contract any further, nor the outer layers of the star. In fact, this heat, plus the gravitational heat from the shrinking core, causes the outer layers to expand. The star's diameter can increase by a factor of 100, or more.
Our sun's radius is 430 thousand miles, but this will expand to more than 80 million miles, engulfing Mercury and Venus, and approaching earth. It may not touch the earth, but it will turn our planet into a scorched molten ball of lava, with all the air and water boiled away. Not a single bacteria will survive. Let's hope we can find a new home before then. Somewhat obsessed by this catastrophe, I wrote a science fiction novel, describing our attempt to move out into the solar system and survive.
So why are these giant stars red? suppose the diameter increases by a factor of one hundred - thus the surface area increases by a factor of ten thousand. If the star remained white hot, it would radiate ten thousand times as much energy out into space. yes, it burns hydrogen in its shell, but not that much hydrogen. The surface temperature must decrease, to compensate for the increase in surface area. The surface temperature drops by half, or more. This causes the star to glow red, instead of white - thus it is a red giant.
After its red giant phase, a star can die in several ways, depending on its size and chemical composition. Our sun will quietly cool, and collapse into a white dwarf, an earth sized ball of carbon with a thin helium atmosphere. This is the cinder that remains, after 10 billion years of energy and heat and light made life possible on our little blue planet. Larger stars collapse with a bang, an explosion that we see as a nova, or even a supernova. For a few days, a supernova can outshine all the other stars in the galaxy. This happened in 1054 A.D., when a supernova in the night sky was brighter than the full moon. Astronomers around the world documented this event, describing the same bright star in the sky. Yet even a supernova cools eventually, its fuel spent, and becomes a quiet, compact remnant of its former self.
Located in the constellation Orion, Betelgeuse is the tenth brightest star in the night sky. Along with its luminosity, its distinct red color makes it easy to spot. If we could see in infrared, it would be the brightest star in the night sky.
Even by the standards of red giants, Betelgeuse is huge! If it were placed at the center of our solar system, it would swallow Mercury, Venus, Earth, Mars, and Jupiter, and might even approach Saturn. It's hard to believe anything could be that large, a ball of gas, red hot, and well over a billion miles in diameter. It is probably the largest contiguous object in the universe. A probe, designed to map the surface of Betelgeuse, would take over 20 years to orbit the bloated star, assuming it could hug the surface, which it can't. If you want to orbit Betelgeuse at a safe distance, you probably have to wait a century to go all the way around.
Betelgeuse has 16 times the mass of our sun. It was a large white star, probably blue-white as it burned hot and fast. The core exhausted its fuel, and collapsed, then the shells started burning furiously, and puffed out the star to a gargantuan size. It is classified as a red supergiant. This star will explode as a supernova some time in the next hundred thousand years. That's a blink of an eye in cosmic time. You shouldn't tap your foot waiting for this wondrous event, but it could happen tomorrow. You could look up at Orion, and discover that your familiar red star has been replaced with a brilliant white beacon, brighter than the full moon, bright enough to see during the day. It could outshine the other stars for weeks, until it fades, leaving a neutron star or black hole in its wake. That would be an amazing sight to behold. It's not likely to happen in my lifetime, but it will happen, and our descendants will step outside every night to witness the magnificent new star in the sky. I hope they still possess technology, ground and space based telescopes to observe the star as it evolves. I hope we have not reverted back to the stone age by then.