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