Life Cycle of Stars

• 1st Step:–Stars form from nebulas

• Regions of concentrated dust and gas left from the BIG BANG (or other stars’ demise)

–Gas and dust begin to collide, contract and heat up• All due to gravity, even the smallest particle has gravitational attraction!

–[What 2 things affect gravity?]

Orion Nebula

Eagle Nebula

Horsehead Nebula

Triangulum Nebula

• 2nd Step:

– As nebula contracts, a small star is formed

• Called a protostar

– Eventually, the protostar will begin nuclear FUSION

• Hydrogen protons attract to each other

– Strong nuclear force (Nucleus particles stick)

» Heat (15,000,000° C) + gravity and…..

FUSION begins

• Continued FUSION is necessary for stars to survive.

• Can take 30,000,000 years for protostar to form

• 3rd Step: The Hertzsprung-Russell Diagram

– Star joins the main sequence• 90% of stars spend life here• Nuclear fusion = Hydrogen into

Helium• Mass of star determines location on

main sequence• Ranges in size from ½ Sun to 20

times the Sun’s size• Color depends on the surface

temperature

• Beginning of the End:– Stars begin to die when they run out of

hydrogen• Gravity begins to take over

Really Low Mass Star

• Doesn’t burn Hydrogen (dud) • Begins to cool and forms a Brown Dwarf

Low Mass Stars

• From ½ all the way down to 0.075% of the Sun’s mass

• Burn cool, less than 3500 K. Dim light.

• Can live a REALLY long time. 10,000,000,000,000 years?

• Our nearest star neighbor is Proxima Centauri, a red dwarf.

• Most numerous stars in the entire Universe!

Intermediate mass Stars (like the Sun up to 5x Sun)

– Star core begins to shrink; outer core of hydrogen begins to fuse and expand

• Star gets bigger

Red Giant!

Eventually the outer shell is blown off (planetary nebula) and the remaining core is a white, hot and

small…White Dwarf!

High Mass Stars—form supergiants!

Red Giant

S

S

Supergiant

Supernova!

• Core is no longer fusing (iron is it!)

• The balance of pressure loses and the entire star collapses

• Massive explosion – Creates heavier elements

Neutron Star• Star with a core from 1.4 to 3 times the size of the

Sun becomes a neutron.

• Electrons and neutrons combine into neutrons.

• 10 km (6 mi) in diameter with a mass more than our Sun!

• A teaspoon of neutron star would be about 10 million tons

• Acts like a huge magnet with magnetic poles, can be a pulsar

Black Holes Info Sheet

What is a Black Hole???

– An object so massive and dense that not even light can escape its gravity

– The end result from a supernova that leaves a core of 3+ times the Sun.

– Collapses to a singularity

• Recap: Death of Stars: Low and Medium Mass

Red Giant

Main Sequence Star

Planetary Nebula

White DwarfBlack Dwarf

Red Dwarf

• Death of Stars: High Mass

Red Super Giant

Main Sequence Star

Supernova

Neutron Star Black Hole

• Recap: High Mass Stars:– Core size greater than 1.4x our sun

• Create high mass elements such as iron

– Neutron Star• Formed if remaining star is 1.4-3x sun’s mass.

– Black Holes• Formed if remaining star > 3x sun’s mass

Recap Video

• Death of Stars:– What stars end up as depend on mass

– Low and Medium mass stars• Planetary nebula --------- white dwarf

– High mass stars• Supernova --------- neutron star or black hole

• As fusion begins to slow down, the core of the sun will contract

• Temperature in the core will rise

• The outer layers of the sun will expand, consuming in the inner planets

• Sun will become a Red Giant

Video Link

– Core of the sun will begin to fuse helium into larger elements such as carbon and oxygen

– Continuing over the next 100 million years…• Core will become entirely carbon and

oxygen• Core will contract• Outer layers will expand

– Outer layers will form a planetary nebula• Core of sun will become a white dwarf

What about our solar

system?

How did it start?

Gravity, gas pressure, magnetic fields, and rotation cause disk to form around protostar.

Conservation of angular momentum!