Life Cycle of StarsPumbaa: Timon?

Timon: Yeah?

Pumbaa: Ever wonder what those sparkly dots are up there?

Timon: Pumbaa. I don't wonder; I know.

Pumbaa: Oh. What are they?

Timon: They're fireflies. Fireflies that uh... got stuck up on that big... bluish-black... thing.

Pumbaa: Oh. Gee. I always thought that they were balls of gas burning billions of miles away.

Timon: Pumbaa, wit' you, everything's gas.

A Star is Born• Stars are born in nebulae. Huge clouds of

dust and gas collapse under gravitational forces.

• As the cloud collapses the temperature increases and a protostar is formed.

• These young stars undergo further collapse, becoming hot enough to fuse hydrogen into helium as main sequence stars.

Looking at the Birthplace of Stars

• Horsehead Nebula

• Rosette Nebula

Everything Depends on Mass

• The more mass a star starts out with, the brighter and hotter it will be.

• The color of the star depends on the surface temperature of the star.

• Its temperature depends on its mass, or how much gas and dust were accumulated during formation.

The Constant Battle Against Gravity

• Gravity is always present, compressing stars toward their center of mass.

• Nuclear Fusion is the force that acts against gravity, releasing energy outwards

Adolescent Stars

• The force of gravity is pulling in on the star while the radiation from fusion is pushing out.

• The star is burning its most abundant fuel, HYDROGEN!

Nuclear Fusion

• Conservation of Mass and Energy E = mc2

• Nucleus 1 + Nucleus 2 Nucleus 3 + Energy

• Proton - Proton Chain Reaction

• 1H + 1H 2D + Energy

• 2D + 1H 3He + Energy

• 3He + 3He 4He + 1H + 1H + Energy

• After a while all the Hydrogen gets turned into He and lots of energy has been released.

Red Giants• After millions to billions of years, depending on their initial

masses, stars run out of their main fuel - hydrogen.

• Without the outward pressure generated from these reactions to counteract the force of gravity, the outer layers of the star begin to collapse inward toward the core.

• Just as during formation, when the material contracts, the temperature and pressure increase.

• This newly generated heat temporarily counteracts the force of gravity, and the outer layers of the star are now pushed outward.

• The star expands to larger than it ever was during its lifetime --

a few to about a hundred times bigger.

Planetary Nebula

• After expanding and reaching the enormous red giant phase, the outer layers of the star continue to expand.

• The core contracts; the helium atoms in the core fuse together, forming carbon atoms and releasing energy.

• The core is now stable since the carbon atoms are not further compressible.

• Now the outer layers of the star start to drift off into space, forming a planetary nebula (a planetary nebula has nothing to do with planets).

• The star loses most of its mass to the nebula.

Egg Nebula formed only a few hundred years ago.

It takes light about 3000 years to reach us from the Egg Nebula.

White Dwarf

• The star is now a white dwarf, a stable star with no nuclear fuel. It radiates its left-over heat for billions of years.

• When its heat is all dispersed, it will be a cold, dark black dwarf - essentially a dead star (perhaps full of diamonds, highly compressed carbon).

A white dwarf in the M4 Globular Cluster

SUPERNOVA: A star that dramatically increases in brightness because of an explosion on its surface.

• Means “new” in Latin but is actually the death of a star.

• Here’s a before and after picture of a nova in 1992.

A Closer Look

H-R Diagram

Our Star the Sun.

• Diameter 1.4 x 106 km

• Mass 2.0 x 1030 kg (300,000 times bigger than the Earth).

• On the H-R Diagram our sun is a yellow star in the main sequence.

• In about 5 billion years our Sun will start to become a Red Giant

Chemistry and the Sun

Element Abundance Abundance

(% of total # of atoms) (% of total mass)

Hydrogen 91.2 71.0

Helium 8.7 27.1

Oxygen 0.078 0.97

Carbon 0.043 0.40

Nitrogen 0.0088 0.096

Silicon 0.0045 0.099

Magnesium 0.0038 0.076

Neon 0.0035 0.058

Iron 0.0030 0.140

Sulfur 0.0015 0.040