1. The Life and Death of Stars
The Universe
The Life and Death of Stars

2. What is a Star?

3. WHAT IS A STAR?
STAR:
sphere of hot gas & plasma
emits light & energy
the sun is a main sequence star & nearest to Earth
LIGHT-YEAR: distance that light travels in one year - about 9.5 trillion kilometers
Earth is 8 light-minutes from the Sun

4. POWERING STARS WITH STAR FUEL
NUCLEAR FUSION REACTIONS: the nuclei of hydrogen (H) atoms combine into helium (He).
When two particles fuse, energy is released.
Takes place in the core of a star
Core = extremely hot, extremely dense, and under extreme pressure (perfect for fusion)

5. At 15,000,000 ºC in the core of the star, fusion ignites!
NUCLEAR FUSION BASICS
At 15,000,000 ºC in the core of the star, fusion ignites!
4 hydrogen atoms smash into each other --> 1 helium atom + particles + ENERGY
Where does the energy come from?
Mass of four 1H atoms > Mass of one 4He atom
Extra mass in converted to energy
E = mc2
(Energy) = (mass) x (speed of light)2

6. OUR STAR – THE SUN
Converts 700 million tons of hydrogen to helium every second
Converts 4.7 million tons of matter to energy every second
We receive two billionths of that energy here on Earth - each second
Enough to power 100 light bulbs for 5 million years
The sun formed about 5 billion years ago.
1 Solar mass = size of the Sun
Example: 3 solar masses = size of 3 Suns

7. WE STUDY STARS BY STUDYING LIGHT Brightness of a Star
Depends on the star’s temperature, size, and distance from Earth.
The brightest star in the night sky = Sirius (Because relatively close to Earth, 9 light years)
Stars produce not only visible light but all types of electromagnetic radiation waves
Gamma Rays, X-rays, Ultraviolet, Infrared, Microwaves, and Radio waves.
Scientists use special telescopes and satellites to see the different types of radiation coming from the stars

8. STUDYING STARS TEMPERATURE
A star’s color is related to its temperature.
Hotter objects glow with blue light that has shorter wavelengths.
Cooler objects glow with red light that has longer wavelengths

9. Types of Stars

10. The Hertzsprung- Russell Diagram (H-R Diagram)
1911: Danish astronomer Ejnar Hertzsprung compared and plotted magnitude of stars with their color (temperature)
1913: American astronomer Henry Norris Russell plots stars’ spectral class against magnitude

11. Together, the plots show a relationship between temperature and luminosity
Shows that stars fall into distinct groups

16. Life and Death Depends on the Mass of Star
Main Sequence Stars (Sun) Massive Stars

17. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

18. STAR LIFE CYCLE: STAGE 1 – BIRTH MAIN SEQUENCE AND MASSIVE STARS
All stars begin their life the same
They form from a Stellar Nebula = Cloud of gas and dust at beginning of star life cycle
Steps that lead to the birth of a star
Gravitational attraction causes the gas and dust to accumulate (this is called accretion)
A protostar forms (not yet a star)
Temperature and pressure increase until nuclear fusion begins (now a star)
Nuclear fusion beginning means that a star has been born

19. Omega Nebula - Emission Nebula

20. Orion Nebula - Diffuse Nebula

21. “Pillars of Heaven” from the Eagle Nebula

22. Horsehead Nebula - Dark Nebula

23. The Pleiades - Reflection Nebula

24. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

25. STAR LIFE CYCLE: STAGE 2 – MIDDLE AGE MAIN SEQUENCE AND MASSIVE STARS
Nuclear Fusion has begun and continues…
Hydrogen fuses into helium and releases lots of energy
The fusion reactions in the core of the star produce an outward force that balances the inward force due to gravity.
Massive Stars = higher fusion rates
More gravity = hotter cores = uses hydrogen faster
Over time, the percentage of the star’s core that is helium becomes larger.
After millions of years, core will run out of hydrogen and star will begin to die.

26. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

27. STAR LIFE CYCLES: STAGE 3 - DYING
MAIN SEQUENCE STAR
LIFE CYCLE
As fusion slows, the outer layers of the star will cool and expand
Becomes a red giant
RED GIANT:
10 to 100 times larger
Converts helium to carbon and oxygen
When the star runs out of helium, fusion stops, outer layers will expand, and leave the star’s orbit.
MASSIVE STAR
LIFE CYCLE
As fusion slows, the outer layers of the star will cool and expand
Becomes a supergiant
SUPERGIANT:
200 to 1500 times larger
Converts helium to carbon, oxygen, and…
Has enough mass (gravity) so heavier elements can fuse into iron

30. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

31. STAR LIFE CYCLES: STAGE 4 – DEATH
MAIN SEQUENCE STAR
LIFE CYCLE
Fusion stops at helium
Outer gas layers from red giant stage are released to form planetary nebula
PLANETARY NEBULA:
Cloud of gas and dust at end of stars life cycle
Sends enriched elements into space
When planetary nebula clears, a white dwarf is left (part of Stage 5 for a Main Sequence Star)
MASSIVE STAR
LIFE CYCLE
Supergiants can fuse heavier elements beyond helium
Fusion stops at iron
Develop layers of different elements with iron at the core
Iron will not fuse
Force of gravity becomes greater than force of fusion
Eventually the core collapses and then explodes in a Type II supernova.
SUPERNOVA: a gigantic explosion in which a massive star collapses and throws its outer layers into space

32. Cat’s Eye Nebula - Planetary Nebula

33. Ring Nebula - Planetary Nebula

34. Crab Nebula from a supernova

35. Layers of heavy elements in a supergiant

36. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

37. STAR LIFE CYCLES: STAGE 5 – AFTER DEATH
MAIN SEQUENCE STAR
LIFE CYCLE
When planetary nebula clears, a white dwarf is left
WHITE DWARF:
a small, hot dim star that is the leftover carbon center of a main sequence star.
Very dense - 1 ton in 1cm3
Will cool until it becomes a black dwarf (no more light, big lump of coal)
MASSIVE STAR
LIFE CYCLE
After a Type II supernova, either A. neutron star or B. black hole forms.
If the remaining core has a mass of 1.4 to 3 solar masses = neutron star (densest star).
If the core has a mass that is >3 solar masses = black hole.
BLACK HOLE: an object so massive and dense that not even light can escape its gravity
NEUTRON STAR: composed of neutrons, 1 teaspoon would weigh 5 x 1012 kg

40. Crab Nebula from a supernova

43. Crab Pulsar

44. STAGE 1
STAGE 2
STAGE 3
STAGE 4
STAGE 5
Main Sequence Star

45. WHY DO WE CARE ABOUT STARS?
We get all elements lighter than Iron from fusion in stars
We get all elements heavier than Iron from supernovae
Without stars, we wouldn’t have:
Elements which make up our body
Energy needed to sustain life