Star Formation

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

10. The Trifid Nebula – copyright Jason Ware

11. Haemission nebulae Copyright - Jason Ware 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.

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, gas inside is very, very cold. • Inside molecules 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. COOLER HOTTER Star Formation • Remember lecture 4: • Cold interstellar clouds: No heat = no velocity = no outward pressure. Gravity wins. • Gas begins to contract.

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 are 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: • 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 #9 • For Feb 17: • Read B17.3 – 17.5, B18.1 – 18.3 • Do: • Ch17 : Problem 1, 4, 25 • Ch18: Problem 1, 4, 11 • (Math folks replace Ch18 Problem 1 with Ch 18 Problem 19)