1. Star Formation
A Star is Born

2. Goals What is there between the stars? What are dust clouds?
What are nebulae?
How do these lead to the formation of star?
Where do baby 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.
But what light gets through, and what light doesn’t?

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. Interstellar Reddening
Same thing with dust clouds in space.
Since space is full of dust, the farther away stars are, the redder they look.
Enough dust and eventually all visible light is scattered or absorbed.

8. Dust and IR In a dark dust cloud:
Even though all visible light may be gone, we can still use IR.
If dust is warm, IR will show its blackbody emission.

9. The IR Universe
Orion – by IRAS
Orion - visible
And allows us to see dust where we wouldn’t otherwise expect it.

10. The Trifid Nebula – copyright Jason Ware

11. Interstellar Gas
In Lab 2 we talked about spectral lines and how they apply to hot and cool gases.
Let’s look at some hot and cool gases in space.
Ha emission nebulae
Copyright - Jason Ware

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

13. Orion Nebula – copyright Robert Gendler
In order for the hydrogen to emit light, the atoms must be in the process of being excited.
The energy for the excitation comes from very hot stars (O and B stars) within the cloud.

14. 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, any gas inside is very, very cold.
Inside molecules can form.

15. 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.

16. Star Formation Remember lecture 4: Cold interstellar clouds:
No heat = no velocity = no outward pressure.
Gravity wins.
Gas begins to contract.
HOTTER
COOLER

17. How to Make a Star 1 2 3

18. 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.

19. Orion Nebula – copyright Robert Gendler

20. Contracting Fragments
Cloud about the size of solar system.
In the center:
Collapsing material continues to heat up.
Density causes heat to be retained.
Higher density makes center opaque.

21. Eagle Nebula – copyright J. Hester

22. 2. Protostar The central opaque part is called a protostar.
Mass increases as material rains down on it.

23. Visible and IR image of the hot protostars in the Orion Nebula.

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

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

26. 3. T Tauri Phase Protostar still shrinks: 10x the Sun.
Still heats up: surface = 4000 K
Core temp = 5,000,000 K
Violent surface activity creates strong winds that blow material away near the protostar’s surface.
Clear away the dust and gas between planets.

28. A Star is Born Time: 40- 50 million years since the collapse started.
Radius: 1,000,000 km (Recall the Sun = 700,000 km)
Core temp: 10,000,000 K (Sun = 15,000,000 K)
Surface temp = 4500 K
Fusion begins in core.
Energy released creates the pressure needed to counter the contraction from gravity.
Contraction ends!

29. An H-R Life-Track

30. 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.

31. 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

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

34. Homework #7 For 10/16: Read B16.3 – 16.5, B17.1 – 17.3 Do:
Ch16 : Review Questions 9, 18, Problem 1
Ch17: Review Question 1, Problem 4