1. Classifying Stars – the Hertzsprung-Russell Diagram

2. Classifying Stars All stars are huge spheres of glowing gas
They are made mostly of hydrogen, and they make energy by nuclear fusion
Stars are classified by their physical characteristics:
Size
Temperature
Brightness

3. Size of Stars
Size of Stars
Many stars are the same size of the sun, which is medium sized
Very large stars are called giant or supergiant stars
If Betelgeuse were located where our sun is, it would be large enough to fill the solar system as far out as Jupiter
White dwarf stars are about the size of the Earth
Neutron stars are even smaller, only about 20km in diameter

4. Size Comparison

5. Color and Temperature of Stars
A star’s color reveals its temperature
The coolest stars appear reddish in color
The hottest stars in the sky appear slightly bluer than the sun
The Color of a Star Song - kidsknowit

6. Brightness of a Star
Stars differ in brightness – the amount of light they give off
The brightness of a star depends upon its size and temperature
How bright a star looks from Earth depends on how far the star is from Earth and how bright the star actually is
A star’s brightness is described by its
Apparent magnitude
Absolute magnitude

7. Brightness of a Star

8. Apparent Magnitude
A star’s apparent magnitude is its brightness as seen from Earth
Just like a flashlight looks brighter the closer it is to you, a star looks brighter the closer it is to Earth

9. Absolute Magnitude
A star’s absolute magnitude is the brightness the star would have if it were at a standard distance from Earth
Stars in a globular cluster are all at about the same distance from Earth, so scientists can study clusters to determine absolute magnitude

10. The Hertzsprung-Russell Diagram
Two of the most important characteristics of stars are temperature and absolute magnitude
Ejnar Hertzsprung and Henry Norris Russell created graphs to show a relationship between the temperature of stars and brightness
Most stars form a diagonal band called the main sequence
In the main sequence, surface temperature increases as brightness increases

11. H-R Diagram
The sun is in the middle of this diagram. It is used for comparison
The vertical axis shows the star’s luminosity or absolute magnitude
The horizontal axis shows the star’s surface temperature, the axis is reversed from the way you normally see it, it decreases from left to right

12. Animated HR Diagram - sunshine
The Life and Death of Stars
Animated HR Diagram - sunshine

13. Hidden Violent Universe (5:20)