Will the Sun Explode?

Will our Sun explode? Does every star end in a supernova? How are black holes formed? Everyday Einstein explores the science behind the stars.

Sabrina Stierwalt, PhD,
July 12, 2016
Episode #139

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Hi, I’m Dr. Sabrina Stierwalt, your Everyday Einstein, bringing you Quick and Dirty Tips to help you make sense of science.

We rely on our Sun to keep us warm and we trust that every night when we go to bed, the Sun will rise again the next morning to light up our day. However, the Sun won’t do this forever. What will happen to our Sun as it ages? Will it explode? How soon?.

A Day in the Life of Our Sun

Our Sun is one of 300 billion stars in our galaxy, the Milky Way. These stars are born out of collapsing clouds of gas. As a gas cloud contracts, or collapses under the weight of its own gravity, the atoms in the cloud have less and less room to move around. This causes the pressure and temperature inside the cloud increase.

Once the pressure is high enough to ignite hydrogen burning, the newborn stars will “turn on,” or start converting hydrogen atoms into helium. The leftover energy generated by this process is released in the form of photons, or light.

As these photons fight their way out of the core, they exert their own pressure outward against gravity (called radiation pressure), which acts to prevent the cloud’s collapse. Once these photons make it to the star’s surface and are released, the star begins to shine. Most of the stars in the universe are currently in this hydrogen burning phase.

Photons leaving our Sun do not arrive instantaneously on Earth, but instead, travel at the finite speed of light. That travel time is roughly 8 minutes. So if a super villain threw an enormous, heat-resistant blanket over the Sun, we would live in blissful unawareness for long enough to make ourselves one last sandwich (or listen to an Everyday Einstein podcast!).

Eventually, after 10 billion years or so, depending on how big the star was initially, the star will run out of hydrogen in its core to burn. Our Sun is about half way through its hydrogen burning phase. This means it has enough fuel for about another 4.5-5 billion years.

Red Giant Stars

Once a star like our Sun exhausts the hydrogen in its core, the core will start to contract again.  Except this time, there won’t be any radiation pressure to balance the gravity. During this core contraction stage, the temperature and pressure in the core increase again.

The star also burns the hydrogen in the layers surrounding the core, a process that heats the star’s outermost layers. These heated outer layers expand, causing the overall size of the star to increase.

The now larger, puffier star has a lower surface temperature (since the same amount of heat is now spread out over a larger surface), and thus appears reddened. Stars in this phase of their evolution are dubbed “red giant stars,” and their puffy, poorly-defined outer edges give them a fuzzy appearance.

Red giant stars are roughly 20-100 times the size of our Sun. When the Sun reaches the red giant phase, it will become so large that it will swallow Mercury and Venus! Astronomers disagree on whether our future red giant Sun will also engulf the Earth or just become dangerously close. Either way, we won’t be able to live here anymore.

Meanwhile, back in the red giant star’s core, which is still contracting, the temperature and pressure eventually get so high that the star begins to burn the next heaviest element—helium. Sometimes this happens in a burst of energy called a helium flash. The red giant phase only lasts for about a billion years or so before helium sparks—a relatively short time for stellar evolution.


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