The Death of our Star

"What will happen to the Earth when the Sun explodes?" This question, or variations on the theme, is asked by young visitors to the Dome Planetarium with some frequency. They are always relieved to learn that our Sun will not explode, it is not big (massive) enough. However, nothing lasts forever, not even stars. So what will the eventual end, or death, of our star be like? Read on to learn more, and join us for a free lecture on the topic by the esteemed astronomer, Dr. James Kaler, on October 17!

Our Sun, captured by Solar Dynamics Observatory.

Our Sun is a yellow star, meaning it is of medium size and hotness. It probably formed about 4.5 billion years ago, and all the planets in the Solar System formed out of the same disc of materials. The Sun has not changed much since it formed, and is expected to stay stable for another 4 billion years or so. 

The Moon eclipsing the Sun in 2014. Image Credit: SDO

How will our star "die"?

A star is a ball of hot gas, held together by its own gravity. It is powered by nuclear reactions taking place in its core. Nuclear power plants on Earth generate energy by fission, the process in which an atom's nucleus is split into fragments. The nuclear reaction taking place in stars is fusion, a process in which nuclei combine. In our Sun, two hydrogen nuclei (a single proton) are combining into helium nuclei, which contain two protons and one neutron. The fusion of these nuclei releases energy in the form of heat and light. 

Image Credit: Wikipedia Commons

In 5 billion years or so, the Sun will have converted so much hydrogen into helium that the core of the star will be filled with helium nuclei. The helium nuclei will not be able to fuse into heavier elements in our star because the temperature is too low (hotter stars can fuse heavier elements). Eventually, the core will become an inert ball of helium, causing the energy production in the core to fall. As the Sun produces less energy, the weight of the outer layers of the star will force the core to contract. 

Image Credit: Wikipedia Commons

As the core contracts, it will get hotter, which will cause the outer layers of the Sun to puff up and expand. The Sun will become a red giant, eventually puffing up so large that it will swallow up Mercury and Venus, and possibly even Earth. After about a billion years as a red giant, the Sun's outer layers will drift away and the core will shrink down to a small cool star, a white dwarf. It would have a similar mass to our Sun today, but be about the size of the Earth, and shine very very faintly. 

One of the few imaged white dwarfs is Sirius B, the small dot of light near the very bight Sirius A. Image Credit: Wikipedia Commons.

The process I have described above is not true for every star. There are so many complexities of the stellar cycle that it is hard to describe them adequately in a blog post. If you are interested and would like to learn more, join us for a FREE lecture by Dr. James Kaler, celebrated astronomer and author of over 20 books, many on the stellar cycle. He will present in our Dome Planetarium on October 17 at 6:00 pm. This lecture is put on in partnership with the Great Burr Oak Science Club, an organization dedicated to increasing public interest in the sciences. Call to reserve your free ticket at 309-686-7000, or just show up the night of the lecture (knowing there is a possibility the seats will fill!). Click here for more information. We hope to see you here!