Stars and the Sun

Chapter 18.2

Objectives

• Describe the basic structure and properties of stars

• Explain how the composition and surface temperatures of stars are measured

• Recognize that all normal stars are powered by fusion reactions that form elements

• Discuss the evolution of stars

Vocabulary

• Constellation• Red star• White dwarf• Supergiant• Supernova• Neutron star• Black hole

Our sun is aTypical Star

Objective 1: The Structure of the Sun

• Stars are huge balls of mostly hydrogen and helium

• Held together by gravity• Inside the core there is enormous pressure

• Stars are driven by Fusion

Fusion

• Hydrogen is fused into helium, which releases energy

The sun’s external temperature

• 5500 Celcius• Gives off WHITE light• It appears yellow in our sky because our

atmosphere scatters the violet and blue light, leaving yellow

• The scattered light gives us our blue sky• When the sun is low, at sunset, more blue is

scattered and the sun can look orange

Heavy Metals

• Our solar system contains many heavy metals• Since these form when stars explode – our

solar system must have been formed from the star dust of exploding stars

1 Core2 Radiative zone3 Convective

zone4 Photosphere5 Chromosphere6 Corona7 Sunspot8 Granules9 Prominence

The Core

• The sun is Plasma not gas• Where most Fusion takes place• All of the sun’s energy comes from the core

Radiation and Convection Zones• Remember how heat moves?• Radiation the energy is absorbed and re radiated

out from the next particles• In the Radiation zone – Energy moves slowly• Between the Radiation zone and the Convection

zone is the Tachocline – a shear zone, the convection zone has fast moving

currents– may be responsible for the formation of the magnetic

field

Convection Zone• In Convection there are currents that help the

energy flow outward• Each current flows heat up, cools and sinks

back down• Forms magnetic regions all over the sun• The Granules are visible turbulence from

these flows• It takes 10,000 to 170,000 years for energy to

travel from the core to the surface (scientific estimates)

Photosphere

• The visible surface of the sun• Temperature at the surface is 6000 Celcius• It is not as dense as air on earth

Everything above the photosphere is the atmosphere of the

sun

There are 5 zones• Temperature minimum• Chromosphere• Transition• Corona• heliosphere

Temperature Minimum

• Coolest place in the sun is between the Photosphere and the Chromosphere

• 4000 Celcius• It is unknown why it is cooler

Chromosphere• Named from the

visible colors• Appears as a bright

flash before an eclipse• Temperature increases

to20,000 C

Transition

• Below transition gravity forms sun• Above transition gravity is less dominant• Temperature rises – Helium is ionized and

holds heat

Corona

• During a solar eclipse, the corona can be seen with the naked eye

Corona

• Can extend past earth• Temperatures can reach 1 to 3 million Celcius• Scientists don’t know why• One reason may be magnetic reconnection – a

process that releases energy stored in magnetic fields

Heliosphere

• Extends thru the solar system• Includes the solar wind and the suns magnetic

field• In 2004 the Voyager space probe encountered

a shock wave believed to be part of the heliosphere,

it was 50 AU away from the sun

Sunspots• Areas of the sun that appear darker because

they are cooler• They are cooler because they have strong

magnetic fields – magnetic fields decrease surrounding pressure and don’t let hot gases flow

• Most sunspots are the size of the earth• Can be seen with filters

Effect of sun spots on earth

• More sun spots – correlate to higher earth temperatures

• Fewer sunspots – cooler temperatures• In 1645-1715 sunspots disappeared and there

was a mini ice age• Sunspots increase activity of Northern Lights

Solar Cycle

• Sunspots increase and decrease every 11 years

• Last peak May 2000• We are heading back into a peak of sunspot

activity in 2011 – It’s time to take a trip to Alaska!

Granules

• It’s kind of like looking at the surface of a boiling pot of spaghetti sauce – the bubbling

• Top of convection columns in the sun, they surface in the photosphere

Prominence and

solar flares

What’s the difference

• A prominence is something sticking out – they are often shaped like loops

• They follow the magnetic field lines

• Can be stable for days or months

• Solar Flares are explosions under the surface of the sun – the blow clouds of gas out into space

• Much larger events

• Prominance

• Sun quake caused by solar flare

Looking at the sun• Brief periods don’t cause damage – more

likely to for young people or at high elevation• UV Exposure believed to contribute to

Cataracts (not from looking directly at sun)• Looking thru binoculars can damage eyes

(concentrated)• Looking at Partial eclipses without filters is

dangerous – the eye’s pupil doesn’t contract as much as it should

Objective 2: Explain how composition and surface temperatures of stars are

measured• Otherwise known as how do we know all this?• Cameras, telescopes, filters to detect… • visible light, radio waves, electromagnetic

radiation…• Spectroscopy• Can determine temperature, age, rotation,

magnetic field and movement towards or away from us

•

Spectroscopy

• Study light• Split light into wavelengths• Each element has a spectral pattern

it’s like a fingerprint for Hydrogen and Helium

• The red shift that shows us the Universe is expanding is seen because the fingerprint isn’t exactly where it is supposed to be

Young Stars contain more elements

• Stars are mostly hydrogen and helium• Each element contains a spectral pattern• Our Sun is middle aged

Temperature seen with color

• Again – we look at the spectral patterns• Blue suns are hotter – 220,000 C• Red suns are cooler – 17,600 C• Our Yellow sun – 55,000 C

Rotation

• Rotation is seen in smudges in the spectral pattern

Redshift

The boxes show redshifted Galaxies

Where are they in time?

Objective 3: Fusion

• The core of stars is extremely hot and dense – gravity squishes it together

• In these conditions - Stars convert hydrogen to helium

• As stars age, the run out of hydrogen and begin making carbon, oxygen, nitrogen…

• Older stars can produce elements as heavy as lead

Objective 4: Evolution of Stars

• Stars are born from massive gas clouds that coalesce from gravity

• When the pressure and temperature get high enough – fusion starts and the star starts to shine

How long will our

sun live?

What happens when the hydrogen runs out?

• When stars run out of hydrogen, the core collapses while the outer edge expands (it cools)

• The collapse lets the sun start fusing helium into heavier elements

• When our sun becomes a red giant it will swallow up earth – it’s diameter will reach mars

Planetary Nebula

• When the red giant has nothing left to fuse it releases the outer layers into space

• What’s left becomes a White Dwarf• White Dwarfs are as small as a planet but very

dense• They eventually Burn out

What happens to bigger stars?Up to 40 times our sun• When the star burns

it’s hydrogen it collapses very quickly

• Produces a shock wave and very bright light

• A Supernova• The final collapse of a

supernova creates a Neutron Star

Even Bigger• You get the same

supernova• The final collapse

creates a Black Hole

Supernova Explosion

What path will our sun take?What

path will other stars take?

Interaction of White Dwarf and Black Hole

• This is a computer simulation – scientists would like to find this happening out there

What happens after stars die?

• New stars form from Stardust• We are stardust• (better described as NUCLEAR WASTE)