1. Lecture 19 The Interstellar Medium The Stuff Between The Stars

2. Announcements Read Units 49 and 61 for Wednesday. Homework 10 Due Now Homework 11 – Due Monday, April 23 Unit 49: TY2 Unit 60: RQ2, 4, P1, TY1, 2 Unit 61: RQ2, TY2, 3 Unit 72: P3, TY1, 2

3. We’re Going To Talk About Clouds In Space Today…

4. But Why Should You Care? They’re pretty! And neat to look at! But there is a more important reason…

5. The Nebula-Star Connection… The stars don’t last forever. New stars have to be born to replace the ones that die. But where do new stars come from? How do they form?

6. The Nebula-Star Connection… We know stars form from clouds of gas out in space, but… Space is full of many different types of gas clouds. Can stars form from any gas cloud, or only certain types? If only certain types, which types?

7. The Star-Nebula Connection… So understanding the basics of the stuff between the stars will help us understand where new stars come from. We’ll talk about… How we know there are clouds of dust and gas out in space. What the different types of clouds are. Which types stars can form from.

8. The Stuff Between The Stars MYTH: Space is a vacuum. REALITY: Nearly 10% the mass of the galaxy is in the form of gas and dust between the stars.

9. The Evidence? How do we know there is stuff out there? Nebulae Some small portions of interstellar gas and dust clouds glow brightly. Extinction and Reddening Even invisible dust clouds make stars beyond look dimmer and redder.

10. Nebulae There are three kinds of nebulae. Emission Nebulae Occur near hot, bright stars (O and B). Ultraviolet light from the stars strips electrons from hydrogen atoms in the gas cloud. As the electrons recombine with the hydrogen, they give off pink light. Called HII regions: HI = Neutral Hydrogen HII = Ionized Hydrogen

11. Nebulae Reflection Nebulae Dust in space scatters light. Scattered means a photon of light moving straight away from the star “bounces off” something in some random direction. Short wavelength (blue) light is scattered much more often than long wavelength (red) light. Many more photons of blue light than red are scattered toward us. Make the cloud seem to glow blue. “us”

12. SAME REASON THE SKY IS BLUE

13. Nebulae Dark Nebulae Thick cloud of dust and gas Either: No stars nearby Stars deep inside cloud. All light from stars inside and behind cloud are blocked. Make cloud look black. Related to Extinction and Reddening.

14. Extinction and Reddening Extinction Dust sometimes absorbs photons. More dust means more photons absorbed. Dust clouds make stars inside or beyond look fainter. Called extinction. Thicker dust = more extinction Long distance = more dust = more extinction.

15. Extinction and Reddening Reddening Sometimes dust scatters blue light coming directly from the star away from the Earth. Makes the star look redder – called reddening. Thicker dust = more reddening Long distance = more reddening SO: extinction and reddening happen at the same time! The fact that both are observed suggest there is stuff between the stars.

16. The Stuff Between The Stars The Interstellar Medium (ISM) is what astronomers call the material between the stars. The ISM is mostly gas with some dust mixed in.

17. The Interstellar Medium The gas is mostly hydrogen and helium (about 99%). Other major components include: Oxygen Nitrogen Carbon The gasses are the raw materials that stars form from.

18. The Interstellar Medium The dust is similar to very fine sand. Tiny silicate particles. Major effect on temperature. Affects how fast a hot gas cools off. Molecules form on the surface of dust grains. Include organic molecules – the stuff living things are made of!

19. Interstellar Dust Probably formed in the atmospheres of cool stars. Mostly observable through infrared emission. IRAS (infrared) image of infrared cirrus of interstellar dust. Infrared and radio emissions from molecules and dust are efficiently cooling gas in molecular clouds

20. The Components of the ISM The ISM is not just evenly spread between the stars. Divided into four components based on: Temperature How dense the gasses are

21. The Four Components Of The ISM NEUTRAL HYDROGEN (HI) CLOUDS INTERCLOUD MEDIUM (HII) CORONAL GAS MOLECULAR CLOUDS

22. Neutral Hydrogen Clouds Galaxy filled with clouds of neutral hydrogen. Cool: 100 K “Relatively” Dense (for ISM) 1-1000 atoms of hydrogen per cm 3 Still better than any vacuum created on Earth. About 300 LY in diameter. About 6 to 10 per 3,000 LY in the galaxy.

23. The Intercloud Medium Inside the galaxy. Fills the space between the denser clouds (HI and molecular) Hot: 5,000 K Very Thin – over 10,000 times thinner than the HI clouds.

24. The Coronal Gas Outside the galaxy – forms a halo that surrounds the galaxy. Absurdly Hot: 500,000 K Extremely Thin - Hundreds of times thinner than the HII gas. Probably originates in supernova explosions and winds from hot stars

25. Molecular (Hydrogen) Clouds Very Cold: 20-50 K Very Dense: 10,000 atoms of hydrogen per cm 3 Large range in sizes: Smallest are a few tens of LY across. Largest are thousands of LY across. Typically dark – light can’t make it through.

26. Hydrogen, Hydrogen, Everywhere What exactly IS the difference between neutral hydrogen (HI), ionized hydrogen (HII), and molecular hydrogen (H 2 )? Neutral hydrogen is one electron orbiting one proton. Ionized hydrogen is when the electron is torn away from the proton. Molecular hydrogen is when two neutral hydrogen atoms share their electrons to form a hydrogen molecule.

27. What Does All Of This Have To Do With Stars? Stars form when nebulae collapse under their own gravity. A nebulae is MORE likely to collapse when… It is VERY COLD Causes the “air pressure” inside the nebula to be very low. It is VERY DENSE Very dense means more mass. More mass means more gravity. More gravity means more likely to collapse.

28. Where Stars Form… Only one part of the ISM is a good place for stars to form. MOLECULAR CLOUDS! Very cold Very dense

29. Where Stars Form… So now we know where the stars form They only form from certain types of clouds in space: Molecular clouds. But we still need to know: HOW they form? AND how are newborn stars related to emission and reflection nebulae? We’ll find that out next time!