1. Stars

2. Stars A star is a ball of different elements in the form of gases
The elements and gases give off electromagnetic radiation (from nuclear fusion) in the form of light
Scientists study the light coming from a star to determine:
The composition of the star
It’s temperature
It’s lifespan

3. Composition of Stars Starlight is analyzed through spectrographs
Spectrographs separate light (electromagnetic radiation) into different wavelengths or colors
The display of colors of light coming from a star is called a spectrum

4. White Board What is electromagnetic radiation?
Light
What process produces electromagnetic radiation in the sun?
Nuclear fusion

5. Composition of Stars Every element has its own pattern or spectra
Scientists can determine the elements that make up a star by studying its spectrum
So…how do we know what stars are made of?
Check it out…

6. Composition of Stars The most common element in stars in hydrogen
The second most abundant is helium
Elements like carbon, oxygen and nitrogen make up the rest of the stars

7. Temperature of stars
The temperature of a star is indicated by the star’s color
Colors range from blue, which is the hottest, to red, which is the coolest

8. Stellar Motion Stars have an apparent motion you can see at night
Apparent motion is caused by the movement of the Earth (rotation).

9. Stellar Motion Stars can have up to three types of actual motion
Rotate on an axis
Revolve around another star
Move away or toward our solar system
A stars spectrum tell astronomers how a star is moving

10. Stellar Motion
First scientists determine what the star is made of and how hot it is
Then they determine what color of light the star should be giving off
Last they determine whether the star is moving towards us or further away

11. White Board How do astronomers determine the temperature of a star?
Its color
What kind/color of star is our sun?
yellow

12. Doppler Effect
If a star is moving closer to us, the light it gives off gets squeezed together, which makes it appear bluer than it actually is
Blue shift
The colors of the spectrum are shifted toward blue
If the star is moving away from us, the light gets stretched out, which makes it appear more red than it really is
Red shift
The colors of the spectrum are shifted toward red
This shift in light is known as the doppler effect

13. Distance to stars
Scientists determine a star’s distance by measuring parallax
The shift in a star’s position when viewed from different locations
Distance = 1/p

14. White Board
What happens to the light of a star as it moves towards us?
The light gets compressed and appears blue

15. Stellar Brightness
Apparent brightness of a star as seen from Earth depends on how much light it emits and how far away it is
Absolute brightness is how bright they would be if all stars were equal distance from the Earth

16. White Board
What is the difference between apparent motion and absolute brightness of a star?
Apparent how much light it emits
Absolute – if all stars were equal distance from Earth

17. Organizing the Stars: The HR Diagram
Stars have different temperatures, brightnesses, sizes, chemical compositions, colors, etc.
It’s a lot to try to put together, let alone compare star to star.
Two astronomers, Hertzsprung and Russell (in 1908 and 1913 respectively), managed to find a way to put all the information about stars onto one graph.
This graph is now know as the Hertzsprung-Russell (HR) Diagram.
It is a graph of brightness vs. temperature.
Brightness also takes into account size.
Temperature takes into account color and chemical composition and thus spectral class.

18. Where different stars are located

19. There are 3 groups of stars:
Giants: Very large, very bright, cool stars.
Found in the top right.
Dwarves: Very small, very dim, hot stars.
Found in the bottom left.
Main Sequence: wide-ranging stars.
Found in the middle, running from top left to bottom right.
Most stars can be found here on the diagram.

21. Star Formation
A star begins in a nebula-an outside force compresses the cloud, particles condense from gravity
The central concentration of matter is a protostar
After millions of years electrons are stripped from parent atoms and gas turns into plasma
The birth of a star occurs when the temperature reaches about 10,000,000°C and nuclear fusion occurs

22. Life cycle of a Star
The main-sequence stage is the second and longest stage of a star’s life
A star reaches equilibrium when the inward force of gravity is balanced by the outward pressure from fusion and radiation
This is the stage that energy is produced by nuclear fusion
When fusion stops a star leaves the main sequence

23. Life cycle of a Star
A star enters the third stage when almost all of the hydrogen within the core fuses into helium
As stars expand because of cooling gas, they grow massive
Stars as big as our sun become giants
Stars that are more massive than the sun will become super giants

24. Life cycle of a Star
When fusion no longer produces energy a star enters its final stage
Planetary nebulas form when the outer gases of dying sun-like stars drifts outwards

25. Life cycle of a Star
Finally a dying star will collapse on itself and become a white dwarf
Once a white dwarf no longer gives off light it becomes a black dwarf – in theory
When a white dwarf is near a red giant the star can turn into a nova or a super nova
Nova – large explosion due to the accumulation of gases

26. Life cycle of a Star
Massive stars always become supernovas at the end of their life cycles
Supernovas can emit energy equal to the output of 400 million suns
After the supernova a very small but dense ball or neutron star may be formed

27. Life cycle of a Star
Pulsars are neutron stars that emit beams of radio waves
Some extremely massive stars will turn into black holes after the supernova explodes

28. Black Hole
Massive stars produce ‘leftovers’ too massive to become neutron stars
The star may contract under gravity crushing the core leaving a black hole
Gravity is so great nothing escapes it, not even light
Black Hole

29. Warm Up Take out a piece of paper Watch and listen
Describe in in your own words what the Doppler effect is and give me an example of it.