1. Stars Characteristics
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2. What are the characteristics of a star?
Stars differ in…
Mass
Size
Temperature
Color
Luminosity

3. Mass and Composition
A star is a massive ball of glowing gases, mainly hydrogen and helium.
Bigger stars have more mass, greater gravity and are hotter than smaller stars.

4. Star Size Large Stars: Giants or Super Giants
Medium Stars: Main Sequence (the Sun)
Small Stars: White Dwarf or Neutron
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6. Star Energy
The enormous pressure and heat in a star’s core converts matter into energy.
This reaction is called nuclear fusion
In nuclear fusion, hydrogen atoms fuse to form helium atoms.
During each step of the process, mass is lost and energy is released.

7. Star Energy cont.
The energy released causes the star (Sun) to shine and gives the star its high temperature.
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8. Star Temperature
Stars in the sky show tinges of different colors which reveal the stars’ temperatures.
Blue: hottest
Yellow: medium temperature
Red: coolest
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9. Classification of Stars
Color
Surface temp (ºC)
Examples
Spectral Class
Blue
Above 30,000
10 Lacertae O
Blue-white
10,000 – 30,000
Rigel, Spica B
White
7,500 – 10,000
Vegas, Sirius A
Yellow-white
6,000 – 7,500
Canopus, Procyon F
Yellow
5,000 – 6,000
Sun, Capella G
Orange
3,500 – 5,000
Arcturus, Aldebaran K
Red
Less than 3,500
Betelgeuse, Antares M
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10. Star Luminosity/Brightness
The brightness of a star depends upon the distance and its luminosity.
Apparent Brightness/Magnitude: a star’s brightness as seen from Earth.
Absolute Brightness/Luminosity: the actual brightness of the star.
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11. Stop and Think
If person A has a pen flashlight and person B has a mega flashlight and both were standing at the front of the classroom, which light would appear brighter?
If person A (pen light) stood in the classroom and person B (mega light) stood at the baseball fields across the street, which light would appear brighter?
You cannot tell by looking in the sky how bright a star truly is. The farther away the star is, the less bright it will appear.
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12. The Hertzsprung-Russell Diagram
An H-R diagram plots stars according to their luminosity and temperature.
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13. Location of Stars on an H-R Diagram
The main sequence is a major grouping of stars in the H-R Diagram
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14. Star Mass Position on H-R depends on mass
The larger the amount of mass in a star, the more luminous it is.
Giants: more mass, more luminous
Dwarfs: less mass, less luminous
During its lifetime a star will evolve on the H-R diagram depending on its mass.
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