I. Stars A.The Brightness of Stars -Star: A hot glowing sphere of gas that produces energy by fusion. -Fusion: The joining of separate nuclei. Common.

I. Stars

A. The Brightness of Stars

-Star: A hot glowing sphere of gas that produces energy by fusion.

-Fusion: The joining of separate nuclei. Common in nature, but not on Earth.

Actual vs. Apparent Brightness

• Variables which affect a star’s brightness:

1. Star size

2. Distance from Earth

3. Star temperature

• Apparent Brightness: The amount of light received on Earth from a star.

• Actual Brightness: How large and hot a star is in relation to other stars.

Star Brightness

• Example: (Fig. 20.1) Sirius has a greater apparent brightness then Rigel, even though Rigel is a much hotter and brighter star.

• Why?

B. The Origin of Stars

• Nebula: A large cloud of gas (helium and hydrogen) and dust which forms into a star.

• Dust and gas particles exert a gravitational force on each other which keeps pulling them closer together.

• Orion Nebula

More Nebulas

• As the particles pull closer together the temperature increases.

• At 10,000,000o C fusion takes place and energy radiates outward through the condensing ball of gas.

• Another view of Orion

C. Stellar Evolution

• Fusion uses up a star’s hydrogen supply rapidly casing the core to heat up and the outer temperature to fall. (Life cycle of the star)

• Star expands and becomes a red giant

• Red Giant

Stellar Evolution

• Core continues to heat and star expands to a super giant.

• As the core uses up its helium supply, the outer layers escape into space and the remaining core is white hot and called a white dwarf.

• White dwarfs

More Stellar Evolution

• When no more material is left in the core it explodes into a supernova.

• Smaller stars become neutron stars and most massive will collapse into a black hole.

• Neutron Star

Nothing (even light) can escape the gravity of a BLACK HOLE

Hertsprung-Russell Diagram

Determining a Star’s Temperature

• A star’s temperature can be determined by its color.

• All objects will glow a different color when heated differently

• Colors hottest to coolest: Blue/white yellow orange red.

E. Hydrogen Fusion: Energy of the Stars

• Stars have large amounts of hydrogen gas.

• Four hydrogen atoms fuse forming 1 atom of helium

• The mass of 4 hydrogen atoms is greater than the mass of 1 helium atom; the excess mass is converted to a tremendous amount of energy.

• This hydrogen helium fusion can power a star for billions of years

1. Determining a Star’s Composition

• Starlight is separated into a spectrum with a spectrometer

• A star’s light has dark bands along the spectrum, these bands are caused by the absorption of certain wavelengths of light by specific gases in the star.

• Different bands show what elements are in the star’s atmosphere.

A star’s spectrum

F. Light-Years

• Light-year: Distance light travels in one year. (Equal to about 9.5 trillion kilometers)

• Approximate distances:

-Sun to edge of solar system = 5.5 light hours

-Nearest star (Alpha Centauri) = 4.3 light years

-Center to edge of Milky Way = 50,000 light years

A. The Sun and You

• Our sun is a main sequence star according to the H-R Diagram.

• The actual brightness is average for a star of its average size.

2. Layers of the Sun (Fig. 20.9)

• Dense inner core which is the site of hydrogen fusion.

• Radiation zone: Energy bounces back and forth before escaping.

• Convections zone: Cooler layer of gas that is constantly rising and sinking.

Anatomy of Sun

• Photosphere: Bright source of much of the light we see.

• Chromosphere: Active layer which is home to many significant displays.

Anatomy of Sun

• Corona: Outer layer which is a gradual boundary between sun and space.

3. Sunspots

• Sunspots: Cool dark areas on the sun’s surface.

-First discovered by Galileo

-Not permanent features—Will appear and disappear

Cycle of Solar Activity

• Cycle of Solar Activity: 11 year cycle which see number of sunspots change.

• Sunspot Maximum: Time of many large sunspots.

• Sunspot Minimum: Time of few sunspots.

4. Prominences and Flares

• Prominence: A huge arching column of gas.

4. Prominences and Flares

• Solar Flares: Violent eruptions near a sunspot which suddenly brighten and shoot outward at high speed.

4. Prominences and Flares

• The interaction of solar flares with Earth’s magnetic field causes the aurora borealis/ aurora australis (Northern/Southern Lights)

A. Earth’s Galaxy—and Others

• Galaxy: A large group of stars, gas, and dust held together by gravity.

• Milky Way: Our galaxy which contains about 200 billion stars and many nebulas

• Spiral Galaxies

A. Earth’s Galaxy—and Others

• Galaxies are grouped together in clusters.

• The cluster the Milky Way belongs to is called the Local Group.

• Three types of galaxies:

• Cluster of galaxies

Elliptical Galaxies

• Elliptical Galaxies: Most common type of galaxy; large three-dimensional football shaped galaxies.

-Contain mostly older and dimmer stars.

Spiral Galaxies

• Spiral Galaxies: Circular galaxies that have arms curve outward from a central hub.– Arms are made up

of stars and dust

• Two spiral galaxies!!

More Spiral Galaxies

• Barred spiral galaxies: Have two spiral arms extending out.

Irregular Galaxies

• Irregular Galaxies: Come in many different shapes and are smaller and less common than elliptical or spiral galaxies.

B. The Milky Way Galaxy

• 100,000 light years in diameter

• Our sun orbits the center of the galaxy once every 240 million years

• Probably a barred spiral galaxy

• Contains over 200 billion stars

• Its where I live!!!!!

C. The Doppler Shift

• As waves (sound, light, etc.) move away, the lengths of the waves increases.

• Doppler Effect

C. The Doppler Shift

• If a star is approaching the dark lines of its spectrum will move toward the blue part of the spectrum.

• If a star is traveling away (as most are) the lines will move toward the red part of the spectrum.

C. The Red Shift

• The conclusion that the universe is expanding because of the red shift in light from galaxies.

• Quasars: Star like objects at the outskirts of the universe which give off tremendous energy.