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 Side Top

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

Supergiants Giants Main sequence Absolute Magnitude White Dwarfs 50000 20000 10000 6600 6000 5000 3000

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 particles Chromosphere- 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 Then heavier atoms can form 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