The Interstellar Medium

1. The Interstellar Medium No, it’s not a space psychic

2. Goals • What is the interstellar medium? • 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. And allows us to see dust where we wouldn’t otherwise expect it.

10. Scattered light • Q: But what happens to the blue light that is scattered? • A: Reflection nebulae. The Witchhead Nebula

11. The Trifid Nebula – copyright Jason Ware

12. Haemission nebulae Copyright - Jason Ware Interstellar Gas • In lecture 2B we talked about Kirchhoff’s laws and how they apply to hot and cool gases. • Let’s look at some hot and cool gases in space.

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

14. 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.

15. 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.

16. Gravity vs. Pressure • Stars and other interstellar material are in a perpetual battle between forces pushing 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.

17. COOLER HOTTER Star Formation • Remember lecture 2A: • Cold interstellar clouds: No heat = no velocity = no outward pressure. Gravity wins. • Gas begins to contract.

18. How to Make a Star

19. 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: • If a star’s worth of matter should clump together in a denser region than the rest of the cloud: • Gravitational attraction will win out over their combined pressure. • The clump will begin to collapse. • The cold cloud will fragment. • Time: A few million years.

20. Orion Nebula – copyright Robert Gendler

21. 2. Contracting Fragments • A fragment about the same mass as the Sun slowly contracts due to its gravity. • Now a few hundredths of a parsec across or 100 times the size of the Solar System. • Temperature still about the same. • In the center some heat begins to be retained. • Time: several tens of thousands of years.

22. Eagle Nebula – copyright J. Hester

23. 3. Continued Contraction • Fragment now a gaseous sphere. • Size of the Solar System • (10,000 x the size of the Sun) • Inner region is dense enough to be opaque to light (remember the convection zone of Sun). • Result: inner region heats up – 10,000 K. • Outer edge still cool. • The central opaque part is called a protostar. • Mass increases as material rains down on it. • Time: 100,000 years

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

25. 4. Protostellar Evolution • Gravity is still winning. • The protostar is still shrinking. • Size: Mercury’s orbit. • The protostar is also still warming. • Core = 1,000,000 K (recall core of Sun 15,000,000 K) • Still not hot enough for nuclear fusion • Surface temp = 2000 – 3000 K (recall Sun = 5800 K)

26. The Evolutionary Track • Given the low temperature, the protostar has a low flux (very little energy is emitted per area of its surface). • The surface area is huge, however! • Result: Very high total luminosity (or absolute magnitude).

27. 5. T Tauri Phase • Protostar still shrinks: 10 x the Sun. • Smaller size  smaller surface area  smaller luminosity. • Luminosity = 10 x 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.

29. 6. A Star is Born • Time: 6 million years since the protostar formed (Stage 3). • 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 almost counter the contraction from gravity.

30. 7. The Main Sequence • Time: 30 million years since Stage 6. • Central temp is finally 15,000,000 K • Pressure from energy of fusion balances gravity • Contraction ends • Surface temp is now 6000 K • 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.

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