Stars

The Suns of Other Worlds

What are Stars?

• Giant• Luminous• Plasma.

– Energized Gas

• Powered by fusion of hydrogen or heavier elements• Stars are NOT burning

– No oxygen

How do we learn about Stars?

• Studying the electromagnetic spectrum.• We can only see a tiny bit of the light stars

produce.• Visible light• Stars emit lots of different kinds of ‘light’

How did we learn about the EMS?

• Sir Isaac Newton

• If you pass sunlight through a prism it separated out into a spectrum of all the colors

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Infrared Light

• William Herschel discovered IR by accident.• Do colors have temperatures?• He found that an area just beyond the red part of

the spectrum was hotter.• He named that invisible light infra-red meaning

beyond red

Ultraviolet (UV) Light

• Johann Ritter

• Found chemicals that darkened when exposed to sunlight had a greater reaction just above the violet end of the spectrum

• Ultra-violet, meaning above violet.

What can we learn about Stars?

• Mass

• Temperature

• Stage of Development

• Chemical Composition

Star Light, Star Bright…

• Absolute magnitude – How much light a star produces

• Apparent magnitude– How much light actually makes to Earth

• Absolute magnitude requires we know the distance to the star.

How Far?

• Parallax– The apparent movement of an object

seen at different viewpoints. – The more the object moves (the

greater the parallax), the closer it is.– The Hipparcos satellite

Color

• Color• Different Temperature

– Mass – Temperature

• Missing Color– Shows presence of other chemicals

• Spectra– A view of the color produced by a star

Hertzsprung-Russell Diagram

• If you make a graph of stars brightness and color stars fall into certain areas

• Stars move through these areas as they age

Types of Stars

• Two different methods

– Temperature

• Color alone

– Age

• Color and brightness

• HR Diagram

Temperature

– O - 33,000K+ -bluest– B - 10,500 - 30,000K -bluish– A - 7,500 - 10,000K -blue-white– F - 6,000 - 7,200K -white– G - 5,500 - 6,000K -yellow white– K - 4,000 - 5,250 -orange– M - 2,600 - 3,850K -red

HR Diagram• Four major groups

– Super Giants– Giants– Main Sequence– White Dwarfs

• Some ‘stars’ didn’t get graphed– Neutron Stars– Black Holes – Black Dwarfs– Brown Dwarfs

Main Sequence Stars

• Dwarf stars

• Beginning of stars life

• Bigger, hotter stars spend less time here

• Fusing hydrogen as their main fuel.

Giants

• Larger(10x) and brighter than main sequence stars

• Average main sequence stars that have run out of hydrogen for fusion.

• Helium fusion is more explosive– outward pressure >inward

Super Giants

• Largest and brightest of all stars– 10-70 times size of the sun– 30,000+ times as bright as the sun

• Large MSS that ran out of hydrogen

• Short life spans

White Dwarfs

• The smallest and faintest of all stars– Stopped fusion process– Glow only due to stored energy

• Mass of sun, size of earth

• The final stage of most stars lives

• Eventually cool to form Black Dwarfs

Star Life Cycles

• Nebula – birth place of stars• Protostars – baby stars

– Gravity and fusion pressure not balanced yet

– Brown Dwarf – failed star • MSS – first stage of a stars life• What happens next depends on mass of star

Sun-like Stars

• Run out of hydrogen• Outer layers expand• Red giant• Outer layers eventually drift off into space

– Planetary nebula

• Hot core remains as a White Dwarf• Hypothetically if heat is gone becomes a Black

Dwarf

Big Stars

• Stars 1.5-3 times mass of sun• Become Red Super Giants• Fuse heavier elements until core is iron

– Collapses, causes explosion

• Super Nova – Create all other elements– Planets, people

Neutron Stars

• Neutrons.• 2x times mass of sun.• 20-40km

– Extremely dense– Teaspoon - 5,000,000,000,000kg

• Gravity makes surface perfectly smooth

Giant Stars

• Also become red super giants.• Same onion structure• Same Super Nova• If remaining core is >3 times the mass of the sun it

forms a Black Hole• All the matter is squeezed into a space smaller

than an atom.• Light can’t escape

Black Hole

• Any matter falling into a black hole produces deadly gamma rays.

• Event Horizon– Distance from the center of the black hole beyond

which nothing can escape

• Spaghettification– Youtube video

Star Structure

• Stars are made of several layers that each have differing properties.– Core– Radiation Zone– Convection Zone– Photosphere– Chromosphere– Corona

Core

• Site of Nuclear Fusion

• Extreme amounts of pressure

• 10,000,000 K

Radiation Zone

• Pressure from the core balance pressure from above layers, particles don’t move.

• Energy bounces around inside this layer for an average of 170,000 years.

• 7-2,000,000 K

Convection Zone

• Energy moves with motion of plasma.

• In main sequence stars, CZ is near the surface.

• In giant stars it is right next to the core.

• ~1,000,000 K

Photosphere

• Coolest Layer (6,000K)• Sun spots

– Disruptions in magnetic field

• Solar Flares– Violent explosions that send sub atomic

particles into space.– Affect satellites, astronauts, power grids.

Chromosphere

• Atmosphere of sun

• Hotter than photosphere due to magnetic activity.– 6-20,000 K

Corona

• 1,000,000 K

• Extends out several million km.

• Visible during solar eclipse