1. Warm Up 6/6/08
If star A is farther from Earth than star B, but both stars have the same absolute magnitude, what is true about their apparent magnitude?
a. Star B has the greater apparent magnitude.
b. Both stars have the same apparent magnitude.
c. Star A has the greater apparent magnitude.
d. Apparent magnitude is not related to distance.
A Hertzsprung-Russell (H-R) diagram shows the relationship between ____.
a. temperature and absolute magnitude
b. apparent magnitude and parallax
c. absolute magnitude and apparent magnitude
d. parallax and temperature
Which of the following refers to the change in wavelength that occurs when an object moves toward or away from a source?
a. spectroscopy c. wave theory of light
b. Doppler effect d. chromatic aberration
Answers: 1) a. 2) a. 3) b.

2. Stellar Evolution
Chapter 25, Section 2

3. The Birth of a Star
The birthplaces are dark, cool interstellar clouds (nebulae)
The initial contraction of the nebula can be triggered by the shock wave from an explosion of a nearby star
Once this begins, gravity squeezes the particles, pulling every particle toward the center

4. The Birth of a Star
The Orion Nebula in normal color and infrared

5. Protostar Stage The initial contraction can span millions of years
The temperature of the nebula slowly rises until it is hot enough to radiate energy from its surface
Protostar – a developing star not yet hot enough to engage in nuclear fusion
When the core of a protostar has reached about 10 million K, pressure within is so great that nuclear fusion of hydrogen begins, and a star is born
Heat from hydrogen fusion causes the gases to increase their motion

6. Protostars in the Horsehead Nebula are circled

7. Concept Check What is a protostar?
A protostar is a developing star not yet hot enough to engage in nuclear fusion.

8. Main-Sequence Stage
Main-Sequence Stage – From the moment of birth until the star’s death
The internal gas pressure struggles to offset the unyielding force of gravity
Hydrogen fusion will last for a few billion years and provides the outward pressure to keep the star from collapsing
The more massive a main-sequence star, the shorter its life span
A yellow star, like our sun, can remain in the main-sequence for approximately 10 billion years
Once the hydrogen fuel of the star’s core is depleted, it evolves rapidly and dies

9. Main-Sequence Stage

10. Red-Giant Stage
Occurs because the zone of hydrogen fusion continually moves outward
When all the hydrogen is consumed, the core no longer has outward pressure supporting it, and will contract
The core will grow hotter, the heat is radiated outward and expands the surface
As the surface expands, it cools down, producing its reddish color
The core will eventually reach a temperature which allows Helium to Carbon fusion
Eventually all the fusion fuel will be consumed
The Sun will spend less than 1 billion years as a Red-Giant

11. Red-Giant Stage
Globular Star Cluster, some of the oldest stars in the universe

12. Life Cycle of a Sun-like Star

13. Concept Check What causes a star to die?
A star runs out of fuel and collapses due to gravity.

14. Burnout and Death
We do not know that all stars, regardless of their size, eventually run out of fuel and collapse due to gravity
Low Mass Stars – consume fuel at a slow rate, may remain on main-sequence for up to 100 billion years, end up collapsing into white dwarfs
Medium Mass Stars – go into red-giant stage, followed by collapse to white dwarf by blowing out their outer layer, and eventually light up planetary nebulae
Massive Stars – these have relatively short lifetimes, end with a large supernova (brighter than the sun if near Earth), this huge explosion blasts apart the star
Supernova – an exploding star that increases in brightness many thousands of times

15. Possible Function of a Binary Pair

16. Planetary Nebula

17. Concept Check What is a supernova?
A supernova is the brilliant explosion that marks the end of a massive star.

18. White Dwarfs
White Dwarf – remains of low and medium mass stars, extremely small stars with densities greater than anything on Earth
The sun begins as a nebula, spends much of its life as a main-sequence star, becomes a red-giant, planetary nebula, white dwarf, and finally, black dwarf

19. White Dwarfs

20. Neutron Stars
The smaller white dwarfs are actually a result of the more massive stars
Neutron Stars – remnants of supernova events, stars that are smaller and more massive than white dwarfs
Electrons are forced to combine with protons to form neutrons, because of how closely packed the matter is

21. Neutron Stars

22. Supernovae
The outer layer of a star is ejected, while the core condenses to form a very hot neutron star
As the star collapses, it rotates faster, and it generates very strong radio waves situated at its magnetic poles creating pulses
Pulsar – a variable radio source of small size that emits radio pulses in very regular periods

23. Supernovae

24. Black Holes
Black Hole – A massive star that has collapsed to such a small volume that its gravity prevents the escape of everything, including light
How does astronomer find a black hole?
They look for material that is being gravitationally swept up by a location that we cannot see
Material that is swept in should be very hot and emits large amounts of X-rays

25. Black Holes

27. Stellar Evolution

28. Assignment Read Chapter 25, Section 2 (pg. 707-714)
Do Chapter 25 Assessment #1-31 (pg )