1. The Birth of Stars Stellar Evolution Stellar Remnants
The Lives of Stars
The Birth of Stars
Stellar Evolution
Stellar Remnants
We are going to talk about:
How stars are created - where does it all begin?
Stellar evolution - how do stars change over time?
Remnants- What is left when they burn out?

2. The Life Cycles of Stars
Depending on their size, stars evolve differently .
This version was uploaded to the Imagine site on 8/28/03 as life_cycles_v2.ppt
Feb 26, 2004, life_cycles_v2.1.ppt - changed 3rd mnemonic on slide 12.
Imagine Life Cycle Poster Image

3. Twinkle, Twinkle, Little Star ...
Hubble Heritage image of Sagittarius Star field. Note that along the horizontal axis, the image is 13.3 light-years across.
What do you notice by looking at this image. Hopefully they will notice the different colors. You can then ask them what the different colors mean [different temperatures]
Image from

4. How I Wonder What You Are ...
Stars have
Different colors
Which indicate different temperatures
The hotter a star is, the faster it burns its life away.
By looking at previous slide, audience should determine that stars have different colors, and deduce that this means different temperatures..
They can deduce that hotter stars burn faster by analogy with a regular fire.

5. Stellar Nursery Space is filled with the stuff to make stars.
M16 - Eagle Nebula Pillars
(from Hubble,

6. Star Birth - Nebula
**Stars form from clouds of gas and dust called nebulas**

7. Star Birth: Nebula
*The gas in the nebula is mostly H, He, and traces of heavier elements*
Add this information to your notes.

8. Star Birth: Gravitational Collapse
*Gravitational collapse may be triggered by the explosion of a nearby star or other process. *
Stars usually form in groups with in a cloud: e.g. Trapezium in Orion and the Pleiades in Taurus.

9. Some Famous Clusters of Young Stars
Pleiades & Trapezium in Orion

10. Protostars *Star begins with slow accumulation of gas and dust
*Protostar continues to collapse.
Gravity increased
Temperature increases
Density increases

11. From Infant to Adult .
*Fusion occurs in Main Sequence stars at 10,000,000 oC. (about 18,000,000 F)
* Hydrogen fuses to Helium
*Fusion balances the inward force of gravity.
*This only takes about 1 million years for an average size star.
Finally, when the proto star gets hot enough it is able to begin fusion. Fusion starts using the star’s fuel. Stars first use Hydrogen. This is a main sequence star.

12. Stars: Life on the Main Sequence
Stars spend 90% of their lives
fusing H into He in their cores.
These stars are in equilibrium - the inward force of gravity is just balanced by the outward force of gas pressure.

13. The Life Cycle Sun-like Stars Massive Stars
Stars are either low mass or high mass. Their mass determines their fate.
Sun-like Stars
Massive Stars

14. Lives of Small Mass Stars
Watch video

15. The Beginning of the End: Red Giants
*After Hydrogen is exhausted in core ...
Core collapses
Outer layer expand
Temp. and pressure increase
*Star begins He fusion
Stars on main sequence have finite lifetime. Why ? -limited amount of fusion fuel.
What happens when fuel exhausted ? Fusion ceases, and the “Balancing Act” comes into play.
As core collapses, the kinetic energy of the collapse is converted into heat. This heat expands the outer layers.

16. Planetary Nebula- Sun like stars
*After Helium exhausted, outer layers of star expelled
Planetary Nebulae
Planetary nebula - after Helium has been consumed, the core collapses again. Outer atmosphere expelled, and then ionized (I.e. glows) by the hot remaining core
From Left to Right:
Ring Nebula - true colors, representing different elements. helium (blue), oxygen (green), and nitrogen (red).
[Images from Hubble Heritage:

17. White Dwarfs *At center of Planetary Nebula lies a White Dwarf.
*Size of the Earth with Mass of the Sun “A ton per teaspoon”
Inward force of gravity balanced by repulsive force of electrons.
Basic characteristics of white dwarfs: about the size of the earth, with a mass of about the sun. 1 million g/cm3 = “1 ton/teaspoon”
White Dwarfs are stable because inward force of gravity is balanced by the repulsive force of the electrons.

18. High Mass Stars High mass stars also go through red giant phases.
*High mass stars can burn nuclear fuels up to Fe.
This requires higher and higher temperatures

19. Periodic Table Light Elements Heavy Elements 4 (1H) 4He + energy
C-N-O Cycle
28Si + 7(4He) Ni + energy Fe
3(4He) C + energy
4He + 16O Ne + energy
4He + 12C O + energy
16O + 16O S + energy
12C + 12C Mg + energy
Periodic table is from
We’ve seen (click #1) 1H -> 4He and (2) 4He -> 12C.
These are further representative reactions that occur in massive stars:
(3) Carbon to Magnesium (12C -> 24Mg)
(4) Helium and Carbon to Oxygen (4He + 12C -> 16O)
(5) Oxygen to Silicon (16O -> 32Si) or Oxygen to Sulfer and He
(6) Helium and Oxygen to Neon (4He + 16O -> 20Ne)
(7) Also note the CNO cycle which uses C,N,O, as catalysts for H-> He in hotter stars. These are noteworthy as the building blocks of life.
(8) Helium and Silicon to Nickel (which decays to Cobalt and then to Iron via successive positive beta decays) (28Si + 7(4He) -> 56Ni -> 56Co + e+ -> 56Fe + e+
Iron and neutrons to isotopes of Iron (not shown)
Through fusion, nearly all the elements up to Iron are created

20. The End of the Line Supernova.
*Once the high mass star has reached an Fe core, nuclear fusion stops!

21. Supernova Explosions
In a few seconds, the explosion releases more energy than the Sun produces in its lifetime. For a brief time, the star outshines its host galaxy.

22. The Stellar Graveyard Low mass stars end as white dwarfs
High mass stars end as either neutron stars or black holes.

23. Neutron Stars
*One possible end to a high mass star - roughly 10 km in diameter (6 miles)

24. Black Holes *End point of most massive stars
The escape speed is greater than that of light - not even light can escape!

25. Which Brings us Back to ...
This brings us back to our life cycle !

26. Stellar Evolution: Two Basic Facts
Gravity drives stellar evolution. From the moment of its birth to its final death.
*The ultimate fate of a star depends on its mass!
FOR MORE SEE:

27. End