Will our Sun explode? Does every star end in a supernova? How are black holes formed? Everyday Einstein explores the science behind the stars.
Going Out with a Bang
After the burning of helium into carbon, the rest of a star’s life depends on how much mass the star had to start. More massive stars will continue the process of burning through an element in the core, leaving the core to contract until fusion of the next heaviest element begins.
Massive stars will move through the periodic table until they start making iron, the heaviest element that can be fused without requiring the input of additional energy. Iron cores are ultimately unstable and result in huge, fast explosions called supernovae.
One supernova can shine brighter than the light of 100 billion stars combined. Based on the number of massive stars in our galaxy, as well as the length of a star’s life before it reaches the iron core phase, we expect a supernova to happen once every 100 years. Have you seen one yet?
Small, low mass stars, like our Sun, will never get hot enough in their cores to burn any elements past helium. Their outer layers, the stuff that got puffed up during the red giant phase, get blown off due to strong winds. This ionized gas then forms beautiful rings around the star, which are called planetary nebula, even though they are unrelated to planets.
Once its helium fuel is exhausted, our Sun will become what is called a white dwarf star, no longer capable of generating energy. These stars are so dense that one teaspoon of white dwarf weighs 5.5 tons—that’s as much as an elephant here on Earth!
Even though they aren’t creating energy through fusion, white dwarfs can still emit light. All those photons produced in the core continue to fight their way to the surface, a process that can take hundreds of billions of years.
Eventually, the white dwarf will run out of energy, leaving behind a dense, dark core called a black dwarf. However, the universe is not more than 14 billion years old, so there are no stars that have been around long enough to have evolved into black dwarfs...yet.
White dwarfs still have a chance to go supernova if they are in a binary system—or in other words, if they rotate around a shared center of mass with another star. If a very dense white dwarf pulls material off of a nearby neighbor, eventually the weight of that added gravity will be too much for the star to support.
The heaviest mass a white dwarf star can support is 1.4 times the mass of our Sun—a limit astronomers call the Chandrasekhar limit. This is the difference between going out with a bang or a whimper. Once a white dwarf exceeds this limit, a sort of runaway reaction occurs as all of the star’s material collapses in a matter of seconds resulting in a violent supernova explosion.
Supernovae are extremely important to life as we know it for two reasons.
- All of the elements fused in the cores of stars would remain locked there forever, and of no use to us, if not for supernova explosions sending them out into space.
- We mentioned that elements on the periodic table beyond iron cannot be fused in stars, since such reactions require energy rather than produce it. Elements heavier than iron are formed thanks to the high speed collisions of other atoms, a byproduct of high energy supernova explosions. As the astronomer Carl Sagan once famously said, “We are all made of star stuff.”
How Are Black Holes Formed?
If a star is massive enough at the beginning of its life, more than 20 times more massive than our Sun, the core left behind after a supernova will be massive enough to form a black hole. Black holes are stellar cores so dense that not even light can escape. Anything entering the surface of the black hole, known as the event horizon, will be trapped there forever.>
Have any questions or comments about our Sun or stars in general? Ask them in the comments below or on Facebook.com/QDTEinstein.
Once again, I'm Sabrina Stierwalt, the Everyday Einstein, helping you make sense of science!