Becoming a red giant | Stars, black holes and galaxies | Cosmology & Astronomy | Khan Academy

Summary

This video explores the life cycle of a star, starting from the formation of hydrogen atoms that fuse into helium, releasing energy to prevent gravitational collapse. As fusion continues, the core becomes denser and helium builds up, eventually causing hydrogen fusion to occur in a shell around the core. This process leads to the expansion of the star into a red giant. As temperatures rise, heavier elements like carbon and oxygen begin to form in the core. The video also hints at what happens to stars that don’t have enough mass to fuse carbon, setting the stage for future discussions on stellar evolution.

Takeaways

• 😀 Hydrogen atoms condense under pressure to form a mass of hydrogen nuclei, initiating fusion in the core of a star.
• 😀 When the pressure and temperature are high enough, hydrogen nuclei fuse, releasing energy and preventing the star from collapsing under gravity.
• 😀 The core of the star, primarily made of hydrogen, fuses into helium, creating immense energy that counteracts gravitational forces.
• 😀 As hydrogen continues to fuse into helium in the core, more helium accumulates, causing the core to become denser and shrink.
• 😀 With a denser core, the gravitational force becomes stronger, and hydrogen fusion begins in a shell around the core, leading to faster fusion reactions.
• 😀 As the fusion rate increases, the energy produced pushes the outer layers of the star outward, expanding its radius and causing it to become a red giant.
• 😀 A red giant has a larger surface area, which leads to cooler surface temperatures, causing it to emit light at a redder wavelength.
• 😀 The core of the star continues to contract and heat up, reaching temperatures of 100 million Kelvin, which is sufficient to ignite the fusion of helium into heavier elements like carbon and oxygen.
• 😀 When the temperature and pressure are high enough, helium in the core fuses into carbon and oxygen, starting the process of forming heavier elements in the star's core.
• 😀 In supermassive stars, the temperature and pressure can rise even further, allowing for the fusion of carbon and oxygen into even heavier elements, eventually leading to the formation of elements in the periodic table.

Q & A

What physical process marks the beginning of a star’s main sequence phase?

The main sequence begins when hydrogen nuclei in the core get close enough for nuclear fusion to occur, releasing energy that counteracts gravitational collapse.

Why does hydrogen fusion prevent a star from collapsing under gravity?

The energy released by hydrogen fusion creates an outward pressure that balances the inward pull of gravity, stabilizing the star.

Why does helium accumulate in the core during the main sequence?

As hydrogen nuclei fuse together, they form helium, which gradually builds up in the star’s core as a fusion byproduct.

How does the formation of helium affect the size of the star’s core?

Helium is denser than hydrogen, so as helium accumulates, the core shrinks while maintaining the same mass, becoming more compact and dense.

Why does hydrogen fusion eventually move into a shell around the core?

Once the core becomes mostly helium, hydrogen fusion can no longer occur there, so it shifts to a surrounding shell where hydrogen is still present under high pressure.

Why does fusion in the hydrogen shell occur faster than in the original core?

The denser helium core creates stronger gravitational pressure on the surrounding hydrogen, causing fusion to happen faster and over a larger region.

How does shell fusion cause the star to expand into a red giant?

The increased energy from faster fusion over a larger radius pushes the outer layers outward, dramatically increasing the star’s size.

Why is a red giant cooler on the surface despite intense fusion inside?

Although fusion is more intense, the energy is spread over a much larger surface area, reducing the surface temperature and making the star appear redder.

At what temperature does helium fusion begin in a sun-sized star?

Helium fusion begins when the core reaches approximately 100 million Kelvin.

What elements are primarily formed from helium fusion?

Helium fusion mainly produces carbon and oxygen, with some additional heavier elements forming in smaller quantities.

Why can some stars fuse carbon and oxygen while others cannot?

Only very massive stars can generate enough pressure and temperature—around 600 million Kelvin—to fuse carbon and oxygen into heavier elements.

What ultimately limits the sun’s ability to create heavier elements?

The sun does not have enough mass to raise core temperatures high enough to fuse carbon and oxygen, so its fusion process stops at that stage.