Notes: 24.3 Evolution of Stars

The H-R Diagram The Hertzsprung- Russell Diagram is one of the most important tools in the study of stellar evolution. Developed in early 1900’s by astronomers Ejnar Hertzsprung & Henry Russell.

What does it do? H-R Diagram compares the temperature (spectral type) of stars against their brightness (absolute magnitude). Brightness Temperature

Spectral Classification Found on bottom of H-R Diagram along with temperature. Letters represent a star’s temperature classification from hot to cool: O B A F G K M Hot Cool

Star Groups on H-R Diagrams 6 5 4 1 2 Red & Brown Dwarfs 3

Star Types on H-R Diagram Main Sequence - diagonal region, upper left to lower right. (90% of all stars) White Dwarf – lower left. (small & hot) Red Dwarf – lower right. (small & cool) Giants – upper right. (large & cool) Supergiants – uppermost & right. (largest & cooler) Blue Giants – upper left. (large & hot)

Blue Giants Supergiants Giants Main Sequence Red Dwarfs White Dwarfs

Where Stars are Born Stars start their lives as clouds of gas & dust called Nebulas. Gravity causes nebula to contract into star. All stars form in clusters.

Newborn Stars Newborn stars that form in nebulas are called Protostars. Surrounded by rotating disk of matter. Increasing heat from contraction.

Stellar Equilibrium When the core of the star reaches 10 million K (20 million F0 Nuclear Fusion begins. Star expands until gravity stops it. Equilibrium is reached when outward forces equal inward forces.

MASS The main factor that shapes the life and death of a star. Low mass stars live for billions of years. High mass stars live for millions of years. (burn fuel quicker!)

Brown Dwarfs Smaller bodies (0.08 mass of sun) that can’t reach temp. for fusion. “failed star” Hard to see… faint glow & small.

Main Sequence Stars Fuse hydrogen atoms to form helium atoms in their cores. Most stars are Main Sequence (90%). – including our Sun! Where stars spend most of their lives.

Different Size Stars Sizes vary from one tenth the mass of our sun to 200 times as massive. Red Dwarf – smallest. Blue Giants – largest.

Red Giants Late stage of life for low mass Star. Has run out of Hydrogen fuel, burning Helium. Grows to large size, very bright, low surface temp. Makes atoms He – Fe.

Planetary Nebula Expanding shell of gas surrounding a dying low mass star. Ejected from the star’s outer layers. Wrongly named in 1700’s, they have nothing to do with planets.

White Dwarfs Small, very dense star typically the size of a planet. Collapsed core of low mass stars (Sun). Has run out of fuel, no Fusion is taking place!

White Dwarf

Black Dwarf White dwarf turns into this when it cools off. Gives off almost no radiation.

Red Supergiants Late stage of life for high mass stars. Cool temps., Brightest because of it’s huge size. Largest size of any star. Ex: Betelguese

Supernova Death of massive stars. Huge explosion from star collapsing in on itself. Creates all elements heavier than Iron. One million times brighter than the sun.

Neutron Star Star made entirely of neutrons. Smallest & densest stars known! Size of small city. (11km dia.) Teaspoon of material would weigh 1000’s of tons!

Pulsar Rapidly spinning neutron star. Some rotate 600x/sec Produce lighthouse-like beam of radiation.

Black Hole Made from supernova of super massive stars. Matter has been squeezed into small space where strong gravity will not let light escape. Sometimes called a “singularity”.

3 Types of Black Holes Stellar Black Holes – formed when a massive star collapses. Supermassive Black Holes – exists in center of most galaxies. Miniature Black Holes – could have formed after the Big Bang. (have not been discovered yet!)

Detecting Black Holes You can see the effects on their surroundings. Matter near B.H. heats up, spins very fast around it, then falls in. Accretion Disk of Gas & dust forms around it.

Particle Jets Some matter is shot out as jets of High Energy Particles moving close to the speed of light!

Actual Pictures of Black Hole Jets

Actual Pictures of Black Hole Jets

Why do Stars Twinkle? because dust in the atmosphere reflects the stars light.

Starlight is Old! You see the stars as they looked when the light left the star’s surface. Distant stars – light left many 1000’s of years ago!

Life Cycles of Stars Stars are born, pass through middle life, and die. Examples of each stage of life are found in the night sky.

Stellar Life Cycle

A Star begins as a Nebula Large gas and dust cloud. Gas – mostly Hydrogen. Made from dead stars.

Gravity The force responsible for turning a nebula into a star.

Birth Stage 1 As a nebula collapses it starts to spin. Temperature increases. 1

Protostar 2 A newborn star where gravity is the source of heat. Also called a “Protosun”. 2

Main Sequence Star When Nuclear Fusion begins. (starts when core gets to 15,000,000 0C) Star spends most of it’s life in this stage.

Old Age Stage Red Giants SuperGiants – ave. sized Stars – massive Stars Hydrogen fuel is changed to Helium. Core shrinks, outer shell expands. Outer shell cools and reddens.

Death of Ave. Mass Star (like our Sun) RED GIANT– when hydrogen fuel runs out, star cools and expands.

Death of Ave. Mass Star (like our Sun) Planetary Nebula – when outer shell of Hydrogen gas drifts away. (has nothing to do with planets!!)

Planetary Nebulas Ring Nebula Hourglass Nebula

Planetary Nebulas Spirograph Nebula Eskimo Nebula

Death of Ave. Mass Star (like our Sun) White Dwarf – hot, dense core of matter left over after an average star collapses. Nuclear fusion stops, star cools and fades. About size of Earth.

Death of Ave. Mass Star (like our Sun) Nova – a white dwarf star that increases it’s brightness by 1000’s of times. Common in binary systems.

Large Mass Stars Burn brighter and die sooner.

Death of large Mass Star Supergiant Stage after Main Sequence. Very large & bright. Forms heavy elements like Carbon, Nitrogen & Iron.

Death of large Mass Star supernova Sudden violent explosion of a massive star. Millions of times brighter than original star, lasts for weeks. Crushes the core. Creates a new nebula.

Fate of Supernova Core High mass = Neutron Star Depends on the Starting Mass of the Star. High mass = Neutron Star Highest mass = Black Hole

Fate of Supernova Core Neutron Star Atoms are turned to neutrons. Shrinks to only 12km in size. Extremely dense – teaspoon worth weighs 100 million tons!

Fate of Supernova Core Pulsar A Neutron star that pulses and gives off radio waves.

Fate of Supernova Core Black Hole Occurs only from the most massive stars. This object’s gravity is so great that light can’t escape.