1. EQ: What factors determine a star’s lifecycle?

2. General Information Constellations are: imaginary patterns of stars.
We use constellations to locate objects in the night sky.
Stars are huge spheres of glowing gas made up mostly of hydrogen and they produce energy through the process of nuclear fusion.

3. Astronomers classify stars according to:
COLOR
TEMPERATURE
SIZE
CHEMICAL COMPOSITION
BRIGHTNESS

4. Color A stars color reveals its surface temperature. Example:
When you watch a toaster heat up you see the wires turn red as they get hotter
The coolest stars (3200°C) are also reddish
The hottest stars in the
sky are approximately
20,000° Celsius and appear
bluish.

5. Size
Many stars are actually about the size of the sun. (the sun is closest to us which is why it looks so large)
Very large stars are called giant stars or super giant stars.
Most stars are much smaller than the sun.
Example: stars can be 20 km (neutron star) in diameter to 420 million kilometers (Betelgeuse)
Star Size Comparison HD

6. Chemical Composition The gases in a star: (most stars) Hydrogen: 73%
Helium: 25%
Other elements: 2%

7. A spectrograph is a device that breaks light into colors and produces an image of the results.
By comparing a stars spectrum with the spectrums of elements, scientists are able to figure out how much of each element is found in the star.

8. Brightness/ magnitude
The brightness of a star depends upon size and temperature.
Think about this: The sun looks very bright. This does not mean the sun gives off more light than all the other stars. The sun looks so bright simply because it is so close. In reality, the sun is a star of only average brightness.
Apparent brightness (magnitude): a star’s brightness as seen from Earth.
The lower the number, the brighter the object.
Negative numbers indicate extreme brightness.

9. Absolute brightness (magnitude): the brightness the star has if it were a standard distance from the Earth.
More complex to figure out
Need both the apparent brightness and its distance from the Earth
Can vary tremendously

10. Hertsprung-Russell Diagram

11. Hertsprung-Russell Diagram
Plots brightness versus surface temperature
Also called H-R Diagram
Used to classify stars and to understand how stars change over time
90% of stars are called main sequence stars
Dimmest stars are located at the top of the diagram

12. The Lives of Stars Stars do not last forever
Each star is born, goes through its life cycle, and eventually dies
See star worksheets:
Life Cycles
H-R Diagram
Star Death
Magnitude
Star Heat
A Star is Born
Life and Death (Discovery Channel)
Nebula pictures
Stellar Evolution