Lecture 11 - Lifecycle of the Sun

What are molecular clouds made of?

Mostly hydrogen (¾), helium (¼), and traces of other elements.

Why do molecular clouds appear dark in visible light?

Dust blocks and absorbs visible light.

What kind of light do cold molecular clouds emit?

Very long wavelength “sub-millimeter” light (not visible to the eye).

What does sub-mm light tell astronomers?

Where cold, dense regions of star formation exist.

What force causes a molecular cloud to collapse into a protostar?

Gravity.

As a cloud collapses, what happens to its core?

It becomes denser and hotter.

Fusion begins when the core is:

Hot & dense

What must overcome for protons to fuse?

Their mutual electric repulsion.

What two conditions are needed for fusion?

Very high temperature and very high density.

What reaction powers the Sun?

Hydrogen nuclei (protons) fusing into helium.

What famous equation explains the energy release in fusion?

E = mc²

Define hydrostatic equilibrium.

The balance between gravity pulling inward and pressure from fusion pushing outward.

If core density rises above equilibrium, what happens first?

Fusion increases

What happens when fusion increases?

Core temperature and pressure rise, causing expansion and restoring balance.

What maintains stability in main-sequence stars?

Self-regulating hydrostatic equilibrium.

Name the Sun’s three internal zones (core outward).

Core → Radiative Zone → Convective Zone.

Which zone in the Sun transfers energy by moving hot plasma upward?

The convection zone.

What happens to hydrogen in the Sun’s core over time?

It’s converted into helium, increasing core helium concentration.

When hydrogen runs out in the core, what happens next?

Fusion stops, the core collapses, and hydrogen fusion continues in a surrounding shell.

What type of star does the Sun become after core hydrogen is gone?

A red giant.

Why does the envelope of a red giant expand?

Hydrogen shell fusion produces much more energy, inflating the outer layers.

What supports the collapsing helium core before helium fusion begins?

Electron degeneracy pressure.

Helium fusion begins when:

Temperature is high enough

What new elements form from helium fusion?

Carbon and oxygen.

What happens when helium is used up in the core?

Core collapses again, while helium and hydrogen fusion continue in shells.

What does the Sun eject when outer layers are blown away?

A planetary nebula.

What remains after the envelope is lost?

A white dwarf (inert carbon–oxygen core).

How dense is a white dwarf compared to Earth materials?

Extremely dense — roughly 1 ton per cubic centimeter.

What happens to a white dwarf over billions of years?

It cools and fades slowly.

Order the Sun’s lifecycle stages:

Molecular Cloud → Protostar → Main Sequence → Red Giant → Planetary Nebula → White Dwarf.