Stellar Evolution

‘The life-cycle of stars’

What is a star?

• LARGE GLOWING BALL OF GAS

• Composed of H and He

• Fixed points of light in space

• Undergo fusion in their cores

• Generate heat and light

• Give off tremendous radiation

Star Energy

• Nuclear Fusion – a nuclear reaction in which to atoms are fused together…

• New elements are created and energy is released.

• This process is responsible for creating ALL elements found in the universe…

in other words, we are all made from star dust.

Star Energy (con’t)• Hydrogen fusion

H + H He +

• Helium fusionHe + He Be +

• As the mass of elements increases, energy production increases

Energy

More Energy

Properties of Stars

• Color & Temperature

• Brightness– Apparent vs. Absolute

• Size & Mass

• Composition

Hertzsprung-Russel Diagram

• A graph showing the surface temperature and absolute brightness of a group of stars

• Used to compare several properties of stars and estimate their sizes and distances

• Shows star color, size, temperature and brightness

H-R Diagram

Protostar

Nebula

• A cloud of gas and dust

• Gravity causes the cloud to collapse and condense

• Temperatures begin to increase = Glows

• Fusion begins at VERY high temps.

(Some of the extra gas and dust may form planets)

Protostar

• Gravity pulls a nebula’s dust and gas into a denser cloud

• As a nebula heats up, it contracts

• A contracting cloud of dust with enough mass to form a star

Main Sequence Stars

• Core reaches a temp of ~15 million K.

• Hydrogen begins to fuse into Helium in the core.

• ~90% of stars lifetime is spent in the main sequence stage.

•Classified based on temperature and luminosity

Giant Stars

• Core decreases in size as all (or most) H is consumed

• He fusion is occurring – producing more energy

• Diameter increases x10• Surface temp decreases

as star expands

Super Giants

• Form from massive stars• A chain of reactions take

place in the core producing He, C, O, Ne, Mg, Si, S, Ar, Ca, Ti, Cr, Fe

• Highest temperature = Blue Super Giants

• Usually explode in a tremendous event called a supernova

Planetary Nebula

• Energy from star becomes to decrease; no more elements to fuse

• Star begins to collapse

• Dying star surrounded by gases

Nova/Super Nova

• The core of a giant star produces too much energy in it’s core and causes the outside of the star to expand.

• The outer layer of gasses are blown out to space (nova, or supernova), leaving behind a small, hot core…

White Dwarf

• Stars decrease in size ~ same diameter as earth, mass stays the same

• Mass of sun, size of Earth DENSITY INCREASES TREMENDOUSLY

• Solid, but still hot, so it is glowing

• Luminosity decreases

Neutron stars

• Form from the remains of extremely massive stars after a supernova.

• Very small ~ 30 km across

• Density of 2 x 1014

• Would be comparative to 1 sugar cube = mass of humanity

Black hole• If the core’s mass is even greater/denser

than a neutron star, it collapses.

• Surface gravity is so great that no matter can escape it…not even electromagnetic waves!

The fate of our Sun