16.3: A star is born

Because the Sun is so massive, it is able to achieve tremendous pressures in its interior. These pressures are so high, they can actually force two atoms into the same space, overcoming their immense repulsion for one another, and causing their two nuclei to merge. As two atoms combine to make one more massive atom, energy is released. This process is thermonuclear fusion. Once it begins, stars begin to give off light.

Figure \(\PageIndex{1}\): A Hertzprung-Russell diagram, plotting the temperatures of stars (how hot they are) are against their luminosities (how much light they emit). The position of a star in the diagram provides information about its present stage and its mass. Stars that burn hydrogen into helium lie on the diagonal branch, the so-called main sequence. Our Sun is an example of a main sequence star, about halfway through its “life” expectancy. Red dwarfs lie in the cool and faint lower right corner. When a star exhausts all the hydrogen, it leaves the main sequence and becomes a red giant or a supergiant, depending on its mass (upper right corner). Stars with the mass of the Sun which have burnt all their fuel finally develop into a white dwarf (lower left corner). Modified from an original by the European Southern Observatory.

The ability of stars to make big atoms from small ones is key to understanding the history of our solar system and our planet. Planet Earth is made of a wide variety of chemical elements, both lightweight and heavy. All of these elements must have been present in the nebula, in order for them to be included in Earth’s “starting mixture.” Elements formed in the Sun today stay in the Sun, fusing low-weight atoms into heavier atoms. So all the elements on Earth today came from a pre-Sun star. We can go outside on a spring day and enjoy the Sun’s warmth, but the carbon that makes up the skin that basks in that warmth was forged in the heart of another star, a star that’s gone now, a star that blew up.

This exploding star was the source of the nebula where we began this case study: it’s the backstory that occurred before the opening scene. Our solar system is like a “haunted house,” where billions of years ago, there was a vibrant, healthy main-sequence star right here, in this part of the galaxy. Perhaps it had planets orbiting it. Perhaps some of those planets harbored life. We’ll never know: the explosion wiped the slate clean, and “reset” the solar system for the iteration in which we live. The ghostly remnants of this time before our own still linger, in the very stuff we’re made from. This long-dead star fused hydrogen to build the carbon in our bodies, the iron in our blood, the oxygen we breathe, and the silicon in the rocks of our planet.

This is an incredible realization to embrace: everything you know, everything you trust, everything you are, is stardust.

Did I Get It? - Quiz

Exercise \(\PageIndex{1}\)

What process do stars undergo?

a. Thermonuclear fusion

b. Accretion

c. Galactic hydrogenation

d. Fission

Answer

a. Thermonuclear fusion

Exercise \(\PageIndex{2}\)

The fusion of smaller atoms (like hydrogen and helium) in stars results in ______________ and ___________.

a. emission of light; nebulae

b. emission of light; bigger atoms

c. nebulae; bigger atoms

Answer

b. emission of light; bigger atoms

Exercise \(\PageIndex{3}\)

Everything you've ever touched is made of __________.

a. chondrules

b. radiation

c. helium

d. stardust

Answer

d. stardust