5.4 The Death of Low Mass Stars - GRCC Astronomy with Dr. Woolsey

Summary

This video explores the life cycle of low-mass stars (less than 10 solar masses), detailing their evolution from hydrogen fusion to the creation of red giants, followed by the formation of white dwarfs. After the core runs out of fusion fuel, the outer layers are expelled, forming a planetary nebula, leaving behind a dense core. If a white dwarf exceeds the Chandrasekhar limit (1.4 solar masses), it can undergo a catastrophic supernova explosion. The video also discusses novae, where white dwarfs in binary systems accumulate material, leading to a brief fusion flash on their surfaces.

Takeaways

• 😀 Low mass stars are stars with less than 10 times the mass of the Sun (less than 10 solar masses).
• 😀 Mass determines everything about a star: its existence, behavior, and lifespan.
• 😀 Stars with insufficient mass become brown dwarfs, while those with too much mass form multiple star systems.
• 😀 All stars start by fusing hydrogen into helium in their cores, but eventually, they run out of fuel.
• 😀 When fusion ceases, the core of the star contracts and heats up, while the outer layers expand, forming a red giant.
• 😀 In low mass stars, the fusion of helium into carbon occurs through the triple-alpha process, creating a new balance.
• 😀 The outer layers of low mass stars puff out and form a planetary nebula, leaving behind an exposed core.
• 😀 The exposed core is called a white dwarf, which is small (about the size of Earth) and very hot, but dim.
• 😀 White dwarfs are supported by electron degeneracy pressure, which resists further compression.
• 😀 A white dwarf cannot exceed 1.4 solar masses (the Chandrasekhar limit); otherwise, it will collapse into a supernova.
• 😀 Accretion of material from a companion star can trigger a nova, a brief flash of fusion at the surface of a white dwarf.
• 😀 If a white dwarf exceeds the Chandrasekhar limit, it will collapse and explode as a Type 1a supernova, leaving nothing behind.

Q & A

What defines a low-mass star in this context?

A low-mass star is typically defined as a star with a mass less than 10 times that of the Sun, or less than 10 solar masses.

How does mass affect the characteristics of a star?

Mass determines everything about a star, including whether it can even become a star at all, its overall lifetime, its fusion processes, and whether it can remain a stable single star or form a binary or multiple star system.

What happens when a star runs out of fusion in its core?

When fusion in a star's core stops, the core contracts and heats up, while the outer layers expand due to the increased pressure from the hotter core, leading to the star becoming a red giant.

What is the triple-alpha process?

The triple-alpha process is a type of fusion that occurs in the core of a star when helium nuclei (alpha particles) fuse to form carbon. This process only happens at high temperatures and densities.

Why do stars become red giants during their later stages?

Stars become red giants because their cores contract and heat up, but the outer layers expand due to increased pressure from the hotter core. This expansion causes the star to become much larger in size.

What is a planetary nebula?

A planetary nebula is formed when the outer layers of a dying low-mass star are ejected, leaving behind the exposed, hot core. The nebula consists of the star's outer layers, which have detached from the star itself.

What is a white dwarf?

A white dwarf is the remnant core of a low-mass star after it sheds its outer layers. It is extremely dense, hot, and small, about the size of Earth, but can have a mass similar to the Sun.

How does electron degeneracy pressure support a white dwarf?

Electron degeneracy pressure is the quantum mechanical force that prevents the electrons in a white dwarf from being squeezed closer together. This pressure counteracts gravity and supports the star from further collapse.

What happens if a white dwarf exceeds the Chandrasekhar limit?

If a white dwarf exceeds the Chandrasekhar limit (about 1.4 solar masses), it can no longer be supported by electron degeneracy pressure and will collapse, leading to a catastrophic explosion known as a supernova.

What is the difference between a nova and a supernova?

A nova occurs when a white dwarf in a binary system accretes material from a companion star, causing a brief flash of fusion. A supernova, on the other hand, happens when a white dwarf exceeds the Chandrasekhar limit and collapses, releasing an enormous amount of energy.