1. Stars
Essential Questions:
1. What relationship exist between the temperature and brightness of most stars?
2. What is the difference between absolute brightness and apparent magnitude?
Objectives:
1. Describe and classify stars by examining their characteristics such as apparent magnitude, absolute brightness, temperature, color, and solar radius.

2. What is a Star?
Stars are large celestial bodies that are composed of gas and emit light.
They form inside a nebula (cloud of dust, hydrogen, helium, and other gases) due to gravity pulling matter into a dense mass that causes increased pressure and raises the temperature.
Hydrogen atoms fuse together producing helium and large amounts of energy.
Stars are made mostly of hydrogen and helium along with other elements in small amounts (oxygen, carbon, neon, iron).
Stars vary in brightness, temperature, and size.
Cooler stars are red, hotter stars are blue
Stars can range in size from 1/100th the size of the Sun to 1,000 times larger.

3. Measuring Brightness Apparent Magnitude
The measure of a star’s brightness as seen from Earth.
The brightness appears different in comparison to other’s because of a star’s distance from the Earth.
We use a magnitude system to measure brightness:
Faint stars have positive (larger) numbers (faintest star= +30)
Bright stars have negative (smaller) numbers (brightest star= -2)
Absolute Magnitude
Used to measure the luminosity or actual brightness of a star.
Absolute magnitude measure of how bright a star would be if the stars were all located at a standard distance.
We compare the brightness to a star whose distance from Earth we know.
Absolute magnitude also uses the magnitude scale.

4. These all have the same absolute brightness
But, they have different apparent
magnitudes
Comprehension Check: Compare and contrast absolute magnitude and apparent magnitude.

5. Measuring Temperature
Stars have different colors because they have different temperatures.
Stars with the coolest temperatures are red.
Stars with the highest surface temperatures are blue.
Color
Surface Temperature °C
Blue
Above 25,000
Blue-white
10,000 – 25,000
White
7,500 – 10,000
Yellow-white
6,000 – 7,500
Yellow
5,000 – 6,000
Orange
3,500 – 5,000
Red
Below 3,500

6. Measuring Size The size of stars is measured by using solar radii.
Our Sun is a medium-sized star. It is used as the measuring stick for measuring other stars.
The Sun= 1 solar radius
Large stars have diameters that are 10 to 100 times the diameter of the sun.
Small stars can have a diameter 100 times smaller.
The temperature and color of a star depends directly on its size.
Bigger=hotter, more gravity makes it burn faster raising its temperature
Larger stars have a shorter life than smaller stars due to the faster burning of hydrogen fuel

7. Hertzsprung-Russell Diagram (H-R)
Diagram named after two astronomers.
Used to help us understand stars.
The x-axis plots the star’s temperature.
The y-axis plots absolute magnitude.
Stars that are brighter should be hotter.
Click Diagram for Animation

8. Main Sequence Stars Most stars you see are main sequence stars.
Main sequence stars are in their main life cycle.
Two forces keep these stars balanced: gravity pulling the atoms toward the center and fusion’s heat pushing atoms outward.
When the hydrogen fuel runs out they get bigger and become super giants.

9. Fun facts about Stars
Most of the stars in the universe are red dwarfs.
They twinkle because of movement in the Earth's atmosphere.
Many stars come in pairs called binary stars. There are some groupings with up to 4 stars.
The smaller they are the longer they live. Giant stars are bright, but tend to burn out fast.
The nearest star to Earth is Proxima Centauri. It is 4.2 light-years away, meaning you would have to travel at the speed of light for 4.2 years to get there.
The Sun is around 4.5 billion years old.

10. HR Diagram Video

11. Star Size Comparison Video