1. Characteristics of Stars
Ch. 8 sect. 2

2. Galaxy A cluster of hundreds of billions of stars.
            
Galaxy
A cluster of hundreds of billions of stars.
Our galaxy is The Milky Way
250 million billion kilometers to the center of our galaxy
Traveling at the speed of light it would take 25,000 years to travel to the center of our galaxy.
The next closest galaxy to us is Andromeda, 2 million light years away.

3. Universe Space and everything in it.
There are billions of galaxies in the Universe

4. Distance to Stars
Instead of kilometers, astronomers use a unit called the light-year.
Light travels at the speed of 300,000 kilometers per second.
A light-year is the distance that light travel in one year, or about 9.5 million million kilometers.

5. Parallax
Astronomers have found a way to measure the distance to a nearby star.
A parallax is the apparent change in position of an object when you look at it from different places.
The less the star appears to move the farther away it is.

6. Parallax

7. Classifying Stars
Stars are classified by their size, temperature and brightness.
The sun is neither the largest nor the brightest star but it is the closest to us.
The next closest star is Proxima Centauri.

8. Size of Stars The Sun is a medium sized star.
Very large stars are called giants or even supergiants.
Betelgeuse is a supergiant.
Small stars are called White dwarf, about the size of Earth.
Neutron stars are even smaller, only about 20 kilometers in diameter.
Betelgeuse

9. Brightness of stars
The brightness of a star depends upon its size and temperature.
Betelgeuse is fairly cool but it is very large so it gives off a lot of light.
Rigel is much smaller but it is very hot so it appears to shine as brightly as Belelgeuse.

10. Apparent Magnitude
A star’s apparent magnitude is its brightness as seen from Earth.
How does it appear.
The sun appears to be the brightest

11. Absolute Magnitude
A star’s absolute magnitude is the brightness the star would have if it were at a standard distance from Earth.

12. The Hertzsprung-Russel Diagram
The H-R diagram is a graph of stars temperature and brightness.
Most stars fall within a diagonal band called the main sequence.
In the main sequence surface temperature increases as brightness increases.