1. 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.

2. 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.

3. 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?

4. B. The Birth of a Star Stars form in dark, cool clouds called nebula.
Something happens to make these clouds contract and spin.
Orion Nebula

5. More Nebulas
Gravity pulls all of the pieces close to the center and squeezes them creating immense pressure.
This contraction could last over a million years.
Another view of Orion

6. Protostar Stage
The temperature of this gaseous body continues to rise until it starts emitting red light, like the burner of a stove.
Stars that are red have the lowest surface temperature and the longest wavelength light.
The inside of the protostar continues to increase in temperature. When it reaches about 20,000,000 degrees Fahrenheit, nuclear fusion of hydrogen begins and a star is born.

7. Main-Sequence Stage
It is this balance between pressure and gravity that forms a main sequence star.
Stars have an enormous amount of pressure inside of them that pushes them out.
Gravity holds this pressure back and keeps the star together.

8. Main Sequence Cont. Different stars age at different rates.
Large hot stars produce a large amount of short wavelength, high energy blue light. These stars burn up their fuel very quickly and burn out within a few million years.
A yellow star like our Sun, burns its’ fuel much slower and lasts about 10 billion years.
Small red stars that burn slowly and produce long wavelength, low energy red light can last for hundreds of billions of years.

9. Red Giant Stage
As a star gets older, the hydrogen fusion slows in the core and moves out toward the outer part of the star. When this happens, the Helium core contracts, becomes very hot and increases the hydrogen fusion on the outer part of the star.
This makes the star grow 100’s or 1,000’s of times its normal size.
Eventually the giant uses all of its’ Hydrogen and Helium fuel and shrinks under the force of gravity.

10. Burnout and Death
There are many ways that a star can die. The way in which this happens depends on the stars’ size.

11. A) Low Mass Stars
A low mass star never burns the helium in its’ core, so it never becomes a red giant.
Low mass stars collapse under the force of gravity and form white dwarfs.

12. B) Medium Mass Stars
Medium mass stars become red giants and fuse helium and hydrogen at a fast rate.
Medium mass stars shrink to white dwarfs too, but before they do, they eject their outer layer of gas. This is called a planetary nebula
Planetary Nebula

13. C) Massive Stars
Massive stars become red supergiants. They are very large when they finish consuming their hydrogen and helium. They collapse with such force that they explode, producing a supernova.
The intensity of a massive star collapsing produces a super-dense star called a neutron star. Imagine a star 1,000,000 times bigger than our sun becoming a star that is 20 km across. This would be like taking the Earth (8,000 miles across) and squeezing it down to 100 yards across. One pea sized sample of a neutron star weighs over 100 million tons.
In some rare cases, the red supergiant is so massive that when it collapses it produces a black hole.

14. Black Hole
Neutron Star

15. Black Holes
Black holes should be very hot. Usually hot things glow brightly, like a star. However, the gravity pulling the surface of a black hole toward the center is so strong that not even light can escape from its’ surface, so they completely disappear from sight.
Anything that gets too near to a black hole is swept in by its’ gravity and lost forever.

16. Study of Stars
­Scientist study stars by using the Hertsprung Russell Diagram.
The HR Diagram compares a stars Surface Temperature with its’ Absolute Magnitude.

17. Hertsprung-Russell Diagram

18. 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.

19. 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.

20. A star’s spectrum

21. 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

22. 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.

23. Layers of the Sun
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.

24. Anatomy of Sun Photosphere: Bright source of much of the light we see.
Chromosphere: Active layer which is home to many significant displays.

25. Anatomy of Sun
Corona: Outer layer which is a gradual boundary between sun and space.

26. Sunspots Sunspots: Cool dark areas on the sun’s surface.
-First discovered by Galileo
-Not permanent features—Will appear and disappear

27. 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.

28. Prominences and Flares
Prominence: A huge arching column of gas.

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

30. Prominences and Flares
The interaction of solar flares with Earth’s magnetic field causes the aurora borealis/ aurora australis (Northern/Southern Lights)