Chapter 2 Stars and Galaxies. Where are you? The Earth circles the sun The sun is one of billions of billions of stars. To measure distances between stars.

Chapter 2

Stars and Galaxies

Where are you?• The Earth circles the sun

• The sun is one of billions of billions of stars.

• To measure distances between stars we a distance measurement called the Light- year

• 1 light-year is the distance light travels in one year.

Light-Year• Light moves at 300,000 km/sec

That’s 186,000 mile/sec

• It would reach the sun in about 5 minutes

• How far would it go in a year?

• Nearest star is 4.3 light years away

Binary Stars• Most stars are found in pairs

• These stars revolve around each other

• If a dim star passes in front of a bright star, it will block its light.

• Called an eclipsing binary

• Algol dims every 69 hours

• The closest star -Alpha Centari is actually a triple star system

Binary System

Top Side

Constellations• Groups of stars that appear to stay

together

• Zodiac

• Named after gods, animals, and heroes

• Stars are not necessarily near each other

Nova• A star getting suddenly brighter

• Occurs in a binary star system

• Gases from one star are pulled into the other.

• Causing a nuclear explosion.

Clusters• Smaller groups of stars within a galaxy

• Globular Clusters- Spherical shaped with may (up to 100,000 stars)

• Open clusters- less organized- with fewer stars ( hundreds )

Nebula• Gas and dust clouds in space.

• Most can’t be seen

• If they reflect light from nearby stars they can be seen

• Probably the birthplace of new stars

Ring Nebula

Galaxies• Huge collections of stars

• each may contain hundreds of billions of stars

• The major feature of the universe

• Maybe as many as 100 billion galaxies

Types of Galaxies• Elliptical - round, from flat disks to

spheres - contain older stars

• Spiral- Flattened arms that spin around a center

• Irregular- no definite shape -less common

Andromeda

Large Magellanic Cloud

M51

M83

Milky Way• Our galaxy• almost all the stars

you can see in the sky

• 100,000 light-years across

• 15,000 top to bottom• 100 to 200 billion

stars

Spectrum

Prism• White light is made

up of all the colors of the visible spectrum.

• Passing it through a prism separate it.

If the light is not white• Stars give off different

colors of light

• Passing this light through a prism does something different.

• How we know what stars are made of.

• Spectra from stars will have lines missing

Doppler Effect• Change in wavelength caused by the

apparent motion of the source.

• Cars moving by you

• Same things happen to light

• Light from objects coming toward you is compressed looks more blue

• Light from objects away looks more red

Red Shift• Light from galaxies moving away

Blue Shift• Light from galaxies moving toward us

A big surprise• No Galaxies showed blue shift

• All galaxies showed red shift.

• Which means All galaxies were moving away The universe is expanding

The Big Bang Theory• The universe started with a

concentrated area of matter and energy.

• 15-20 billion years ago

• Then it exploded and has been expanding ever since

• Faster moving stuff traveled farther

• Explained red shift

Big Bang Theory

• Predicts energy should be evenly distributed

• Astronomers did find it

• Called background radiation

• Evenly spread throughout the universe.

Gravity• Force of attraction

• All objects attract each other.

• Pulled matter into clumps

• These clumps became bigger

• became galaxies

Open or closed?• Two possible results of big bang.

• Open universe will continue expanding

• Stars will eventually lose all energy

• end of universe is emptiness.

• In a few hundred billion years

Closed Universe• Gravity will eventually pull all the

galaxies back together.

• Eventually all matter will come back together at the center of the galaxy

• Blue shift

• Packed into a area as small as a period.

• Then another big bang

• Every 80 to 100 million years.

Quasars• Quasi - stellar radio source

• Quasi- means “something like”

• stellar means “star”

• Most distant objects in the universe -12 billion light years

• Give off tremendous energy as x-rays and radio waves

• as much as 100 galaxies

Quasars• 1 sec, enough for 1 billion years

electricity for Earth

• At the edge of the universe

• At the very beginning of the universe

Another Tool• Spectroscope

• Breaks the light of a star up into its colors

• Called a spectrum

• Kind of spectrum tells scientists what the star is made of which way and how fast it is moving

Stars• Are formed by the same forces

• Have different Size Composition Temperature Color Mass Brightness

Size• 5 main categories

• Medium sized - like our sun from 1/10 size of sun to 10 times it’s

size

• Giant stars- 10 to 100 times bigger than the sun

• Supergiant stars- 100 to 1000 times bigger than the sun

Size• White dwarfs- smaller than 1/10 the size

of the sun

• Neutron stars - smallest stars - about 16 km in diameter

Composition• Determined with a spectroscope

• by the colors of light it gives off

• The lightest element Hydrogen makes up 60 - 80 % of a star

• Helium is second most

• 96-99 % is hydrogen and helium

• rest is other elements -

Temperature• Color also

indicates temperature

• hottest surface 50000 °C

• coolest -3000°C

Blue

White

Yellow

Red-orange

Red

35,000 °C

10,000 °C

6,000 °C

5,000 °C

3,000 °C

Brightness• Magnitude - measure of brightness• Apparent magnitude - how bright it

looks from earth• Absolute magnitude - how bright it really

is• Variable stars - brightness changes

from time to time• Cephid variables - pulsating variables-

change both brightness and size

Hertzsprung-Russell diagram• Found that as temperature increased,

so did absolute magnitude

• 90% of stars followed this pattern

• Called main sequence stars

• Other 10% were once main sequence stars but have changed over time

50000 20000 10000 6600 6000 5000 3000

Main sequence

Supergiants

Giants

White DwarfsAbs

olut

e M

agni

tude

Distance to stars• One method is parallax

• Apparent change in position as the earth goes around the sun

Measure the angle to the star

Wait half a year

Measure the angle to the star

Triangle tells distance

Distance to stars• Parallax works only to 100 light-years

• More than 100 light-years they use a complicated formula based on apparent and absolute magnitude.

• More than 7 million light-years they use the red shift

Why Stars Shine• Stars are powered by nuclear fusion• Hydrogen atoms join to form helium• Happens because gravity pulls the atoms in

the core so close together• The sun turns 600 billion kilograms of

hydrogen to 595.8 kilograms of helium every second

• The 4.2 billion kilograms of mass are turned to energy -light, heat, UV, x-rays

• E= mc2

The Sun• An average star

• Over 1 million earth’s would fit inside

• 1/4 the density of the Earth

• made of 4 layers

Corona- Outermost layer•Temp-1,700,000ºC•Few particlesChromosphere-middle of atmosphere•Temp-27,800ºC•1000’s of km thick

Corona

Chromosphere

Photosphere-•Temp-6000ºC•550 km thick•Surface of the sun

Core-1,000,000ºC

15,000,000ºC

Activity on the Sun• Storms on the sun• Prominences- Loops or arches of gas that

rise from the chromosphere• Solar Flares- Bright bursts of light, huge

amounts of energy released• Sunspots- Dark areas on the suns surface• in the lower atmosphere• Motion shows the rotation of the sun• Interferes with radio

Solar Prominence

Solar Wind• Continuous stream of high energy

particles.

• Can also interfere with radio and TV

Star Life Cycles• Stars change over time

• New stars form from nebulae

• Gravity pulls the dust and gas together

• Mostly hydrogen

• Forms a spinning cloud

• Hydrogen atoms hit each other and heat up

Star Life Cycle• When the temperature reaches

15,000,000 °C fusion begins

• Makes a protostar - a new star

• What determines the life cycle of the star is how much mass it starts with.

Medium-Sized stars• Shine for a few billion years as

hydrogen turns to helium.

• When hydrogen is used up, the core is almost all helium.

• Helium core shrinks and heats up

• Makes outside expand and cool

• Gives off red light

• Becomes red giant

Medium-Sized Stars• Helium in core turns to carbon

• Last of hydrogen gas drifts away to become a ring nebula or a planetary nebula.

• When last of helium is used up the core collapses and becomes a white dwarf

• Incredibly dense- a teaspoon will weigh tons

How long• It depends on the mass.

• The smaller a star starts out, the longer it takes

• From a few to 100 billion years for medium sized stars

• The sun will take about 10 billion years

Massive Stars• Start with at least 6 times the mass of

the sun.

• Like medium stars up until they become red giants.

• The helium in the core becomes carbon, but it keeps getting hotter.

• Carbon atoms for heavier elements like oxygen and nitrogen and even iron

Massive stars• Can’t go further than iron.

• Iron absorbs energy until it explodes in a supernova

• Temperatures up to 100,000,000,000°C

• Then heavier atoms can form

• Explosion results in a new nebula,but with the new elements

Neutron Stars• If the star started out 6 to 30 times the

mass of the sun, the core of the exploding star becomes a neutron star.

• As massive as the sun, but only 16 km across.

• Neutron stars spin rapidly and give off pulses of radio waves

• If these radio waves come in pulses it is called a pulsar

Black holes• If the star was bigger than 30 times the

mass of the sun

• The left over core becomes so dense that light can’t escape its gravity.

• Becomes a black hole.

• Grab any nearby matter and get bigger

• As matter falls in, it gives off x-rays.

• That’s how they find them