Shakespeare in Space #9: Death Will Come

5,000 light years away from Earth is a dying star surrounded by a rapidly expanding cloud of gas and dust. It's known as the Calabash Nebula

Calabash Nebula
Image Credit: NASA, ESA, Hubble, MAST, Judy Schmidt
Quote source: William Shakespeare, "Julius Caesar"

Eventually, all stars run out of fuel. For the largest stars, that point can become explosive—as they use up all of their hydrogen and begin fusing heavier elements together, the energy released can override its deep gravitational well and rip the entire star apart.

But in the case of stars like that in the center of the Calabash Nebula, death is more of a whimper rather than a bang. And even a whimpers can look cool sometimes. Rather than exploding and spewing its guts out into the cosmos, the star above is actually shrinking to become a white dwarf. One day, our Sun will do the same.

A brief explainer about the lives of stars:

When a star similar to our Sun (as the Calabash Nebula was) burns through all of its hydrogen fuel, its radius changes. A star's radius is a delicate balance between the fusion energy being released in its core and the gravitational field that all of its mass creates. It's really just a perpetual explosion in space that's constantly falling back in on itself—just like Sisyphus rolling his boulder up a hill for all eternity, only to have it roll down again.

As a star's mass increases, its rate of fusion increases. This means that it explodes with more force, which slightly increases its radius—but at the same time, having a higher mass means higher gravity, and this higher gravity pulls the explosion back in on itself with ever greater force.

The result of this balancing act is that a star twice as massive as our Sun won't actually be twice the size; it would only be a little bit bigger (though its actual size depends on the rate of fusion in its core, which itself depends on its elemental composition). 

The more hydrogen a star has, the slower its burn. Because hydrogen fuses at lower pressures than other elements, a star will always burn through its hydrogen fuel before heavier elements (such as helium). But once it runs out of hydrogen, the magic begins.

When its hydrogen fuel has been used up, the star enters a new phase. Stars like our Sun (and Calabash) become red giants—they begin burning helium fuel, releasing significantly more energy, which increases the radius of the star exponentially. While the star's mass remains the same as before, it can actually increase in radius by several orders of magnitude:

What a red giant will look like
This is what our Sun will look like in a few billion years...
Image Credit: Wikimedia Commons

When this happens to our Sun, the inner planets (Mercury and Venus) will be swallowed up, and Earth will be scorched into a molten oblivion. As the Sun swells in size, it'll also form distinct layers of gasses. And as the Sun finally finishes fusing all of its helium fuel, it will shed those outer layers, as its residual mass will no longer be large enough to hold it together.

At this point, the core of the Sun will shrink down into a white dwarf—an extremely dense mass of leftover elements that aren't under enough gravitational pressure to undergo fusion. What's behind left is a low-energy mass about the size of Earth but with the approximate mass of a star—a teaspoon of white dwarf material would weigh as much as 15 tons

This is what's happening in the Calabash Nebula right now, and we get to witness it from a distance. This star had previously been like our Sun, but now it's burned through all of its fuel, completed its red giant phase, and lost its outer gaseous layers, thus forming the nebula imaged here by the Hubble Space Telescope:

Calabash Nebula
At least when stars die, they do it in style.
Image Credit: NASA, ESA, Hubble, MAST, Judy Schmidt

Death is inevitable, even for stars that can live for billions of years. They lived spectacularly, and they die spectacularly as well—the material being ejected into the cosmos will one day be used to form new stars and new planets, some of which may even enable life to form. 

Death is a necessary end, but with every death comes new opportunities for life to emerge. Without it, we wouldn't exist at all. 

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