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I. Stars. 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. Variables which affect a star’s brightness: Star size Distance from Earth Star temperature.

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i stars
I. Stars
  • 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.

1 actual vs apparent brightness
Variables which affect a star’s brightness:

Star size

Distance from Earth

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.

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


Star Brightness
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

B. The Origin of Stars
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

More Nebulas
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

C. Stellar Evolution
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

Stellar Evolution
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

More Stellar Evolution
determining a star s temperature
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
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.
1 determining a star s composition
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.
f light years

Pierre Public Schools:

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.

A. The Sun and You
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.

2. Layers of the Sun (Fig. 20.9)
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
3 sunspots
Sunspots: Cool dark areas on the sun’s surface.

-First discovered by Galileo

-Not permanent features—Will appear and disappear

3. Sunspots
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.

Cycle of Solar Activity
4 prominences and flares24
Solar Flares: Violent eruptions near a sunspot which suddenly brighten and shoot outward at high speed. 4. Prominences and Flares
4 prominences and flares25
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
a earth s galaxy and others27
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

A. Earth’s Galaxy—and Others
elliptical galaxies
Elliptical Galaxies: Most common type of galaxy; large three-dimensional football shaped galaxies.

-Contain mostly older and dimmer stars.

Elliptical Galaxies
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!!

Spiral Galaxies
irregular galaxies
Irregular Galaxies: Come in many different shapes and are smaller and less common than elliptical or spiral galaxies.Irregular Galaxies
b the milky way galaxy
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 shift34
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 Doppler Shift
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. 

C. The Red Shift