Astronomy Cast Ep. 108: The Life of the Sun

The sun formed 4.6 billion years ago. A giant cloud of gas and dust (nebula) started to contract and formed many young stars. The nebula was cold, which allowed it to collapse. If it was hot, it would not have been able to condense. The sun started out 10 times brighter than it is now. As the sun continued to contract it got fainter and fainter. At first, the sun was just a glowing ball of hydrogen and helium and eventually reached a time when it could start fusion. This is the protostar stage. In the life of the sun, it represents a very small chunk. The sun then settled into a main-sequence star, which it is now. When the Sun first became main-sequence, it was a little hotter than it is now, but it has been mostly constant. Slowly, the sun is starting to get warmer and warmer (over millions of years). Currently, the sun appears to be in a slight cooling stage. The sun will most likely stay main-sequence for another 5 billion years. The Earth is about half way through its life right now. When the sun nears the end of its life, it will get hotter and will evaporate the oceans. In about 5 billion years, the sun will start to run out of hydrogen for fusion and will start to contract. Once the sun collapses, a shell of hydrogen surrounding a helium core will ignite. This phase is the red giant phase. As the sun continues to collapse, pressure from the weight of the material causes the core temperature to continue to rise. Once the core temperature reaches 10^8 degrees, the helium core ignites, otherwise known as a helium flash. The Sun is now a Horizontal Branch Star or the sub-giant phase. The sun now begins to bloat and drop in luminosity. Eventually, all the helium runs out. The star collapses and forms a carbon-cored star surrounded by a shell of burning hydrogen and helium. The Sun is now AGAIN a red giant. The sun could also become a variable star, where the sun pulsates and has a variable luminosity. If the sun does become a variable star, it would be a Mira variable. The sun's atmosphere now reaches to the orbit of Earth. Some of the outer layers are so far away that they can actually be released into space. The sun is now only 4,000 K and the star is red, but the star is so much larger that it has a larger luminosity. The sun spends almost all of its life as main-sequence, and the other stages occur over a combined couple hundred millions of years. When the Sun starts to expel its atmosphere, a planetary nebula will form. The sun starts to run out of energy. All hydrogen and helium burning stop and the atmosphere drift away. The Sun enters the Degenerate Gas Phase, where the helium and hydrogen are packed into a sphere, forming a white dwarf. Although no fusion is occurring, the Sun is still hot. It is releasing heat as it cools to oblivion. The planetary nebula blends in with the surrounding interstellar medium and the dwarf cools to the temperature of space. The sun is now a black dwarf or cold white dwarf. As the sun loses mass, the planets' orbits will expand and the Earth will actually survive the changing phases.

I was amazed by the life our Sun will endure in the future. The constant changing that will occur over such a short period of time after a long life of little variability is truly amazing. It would truly be interesting to see the sun change and see its impact on the surrounding planets. I've learned a lot from this podcast that I previously did not have a clue about and the evolution of stars is incredible.