1. Star Formation
A Star is Born

2. Goals What is there between the stars? What are dust clouds?
What are nebulae?
Where do stars come from?

3. The Stuff Between Stars
Space isn’t empty.
Interstellar Medium – The gas and dust between the stars.
All the interstellar gas and dust in a volume the size of the Earth only yields enough matter to make a pair of dice.

4. The Distribution Picture the dust under your bed.
Fairly uniform thin layer
Some small clumps
Occasional big complexes
Interstellar dust and gas is the same.

5. Dust Space is dirty. Dust blocks or scatters some light.
Result: black clouds and patterns against the background sky.

6. Absorption and Scattering
Q: Why are sunsets red?
Light is absorbed or scattered by objects the same size or smaller than its wavelength.
Dust grains = wavelength of blue light
Dust clouds:
Opaque to blue light, UV, X-rays
Transparent to red light, IR, radio
A: Whenever there is a lot of dust between you and the Sun, the blue light is absorbed or scattered leaving the only the red light.

7. The IR Universe
Orion – by IRAS
Orion - visible

8. The Trifid Nebula – copyright Jason Ware

9. Interstellar Gas
In Lab 2 we talked about spectral lines and how they apply to hot and cool gases.
Ha emission nebulae
Copyright - Jason Ware

10. Dust obscuring Ha emission nebula
Horsehead Nebula – copyright Arne Henden
Dust obscuring Ha emission nebula

11. HII Regions For light: atoms must be excited.
Orion Nebula – copyright Robert Gendler
HII Regions
For light: atoms must be excited.
Energy comes from very hot stars.

12. Concept Test An HII region appear red because
it is hot and things that are hot glow red.
it is ionized hydrogen which appears red because the brightest emission line is red.
it is cold and things that are cold appear red.
it is full of red stars.
dust between the observer and the region blocks the blue light, but lets the red light through.

13. Cold Dark Clouds
If dust clouds block light, then inside thick dust clouds there should be no light at all.
Without light, there is little energy.
With little energy, gas inside is very, very cold.
Inside molecules form.

14. Gravity vs. Pressure
Stars and other interstellar material are in a perpetual battle between forces pulling in (gravity) and forces pushing out (pressure).
Gravity comes from the mass of the cloud or star.
Pressure comes from the motion of the atoms or molecules.
Think of hot air balloons.
The hotter the air, the bigger the balloon.

15. Star Formation Remember: Cold interstellar clouds:
No heat = no velocity = no outward pressure.
Gravity wins.
Gas begins to contract.
HOTTER
COOLER

16. How to Make a Star 1 2 3

17. 1. The Interstellar Cloud
Cold clouds can be tens of parsecs across.
Thousands of times the mass of the Sun.
Temperatures 10 – 100 K.
In such a cloud:
Something makes a region denser than normal.
Force of gravity draws material to denser region.
Gravitational collapse begins.

18. Orion Nebula – copyright Robert Gendler

19. Eagle Nebula – copyright J. Hester

20. Visible and IR image of protostars in the Orion Nebula.

22. Concept Test
A new star reaches the main sequence when inward gravitational collapse is:
Halted by degeneracy pressure in the core.
Halted when the atoms are pushed up against one another and contraction stops.
Finally balanced by outward thermal pressure from nuclear reactions.
Finally balanced by radiation emitted in the photosphere.
none of the above.

23. …and the Nebula? Cloud around the protostar spins faster.
Flattens to a disk.
Pizza dough.

24. Planetesimals Dust and gas condense onto dust grains.
Small clumps grow bigger.
Bigger clumps have more mass and attract more matter.
Planetesimals are the building blocks of the planets.
Orion Nebula – Copyright O’Dell and Wong

25. An H-R Life-Track

26. The Main Sequence For the Sun: Bigger Stars: Smaller Stars:
While it took 40 – 50 million years to get here, the new star will spend the next 10 billion years as a main sequence star.
Bigger Stars:
Everything goes quicker.
Smaller Stars:
Everything longer.

27. Now what?
The mass of the star that is formed will determine the rest of its life!
Recall: the more massive the star, the more pressure in the core.
The more pressure, the more fusion.
More fusion:
More energy produced
Hotter
Shorter life span

28. Concept Test
Which Cluster is the oldest? a. b. c. d.

29. Open Clusters These are the new stars. Small groups of young stars.
Slowly drifting apart.
Jewel Box – copyright MichaelBessell

30. Homework #11 For Oct 15: Read B17.3 – 17.5
Do:
Ch17 : Problem 4, 25
Ch18: Problem 11