1. Chapter 19: Between the Stars: Gas and Dust in Space

2. February 14, 2006Astronomy 20102 Gas and Dust in Space To understand how stars are born, we begin by learning about the raw material from which they are made.  Interstellar matter: gas and dust that lies in the regions between stars.  Interstellar medium: the entire collection of interstellar matter.  The interstellar medium accounts for a large fraction of the atoms in the universe (>50%).  Provides the raw material for new stars.  Nebulae: latin for "clouds”, aggregates of interstellar matter that emits radio waves or light. Can produce colorful displays when lit by the light of nearby stars.

3. February 14, 2006Astronomy 20103 19.1: The Interstellar Medium  75% hydrogen, 25% helium, and 1% interstellar dust (by mass).  The matter comes together in clouds.  Density is low:  10 3 atoms per cubic centimeter (cc).  Air has 10 19 atoms per cc.  Best vacuum created on Earth has 10 7 atoms per cc.  But very large in size  Tens of LY’s.

4. February 14, 2006Astronomy 20104 19.2 Interstellar Gas  The color of a cloud tells us about its temperature and composition.  The common red color comes from hydrogen (H II).  H II regions contain ionized hydrogen.  H I indicates neutral hydrogen, H II is singly ionized hydrogen, and Fe III is doubly ionized iron. type of region temperature (K) HI: cold clouds 100 HI: warm clouds 5000 hot gas 500,000 HII regions 10,000 giant molecular clouds 10

5. February 14, 2006Astronomy 20105 H II Regions  temperature near 10 4 K – heated by nearby stars  ultraviolet light from hot O and B stars  ionizes the surrounding hydrogen gas  free electrons recombine – form excited H atoms  excited states emit light  red glow characteristic of hydrogen (red Balmer line) of hydrogen (red Balmer line)

6. HII regions: Orion Nebula closest to us, 1,500 LY 29x26 LY in size large star formation factory

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8. trapesizium cluster: stars that provide much of the energy which makes the brilliant Orion Nebula visible other stars obscured by nebula

9. February 14, 2006Astronomy 20109 Absorption Lines  Most of the interstellar medium is not ionized.  Mostly hydrogen, and helium  Other atoms and molecules seen: Ca, Na, CN, CH, H 2, CO  Cool gas between stars and Earth will cause an absorption spectrum.

10. February 14, 2006Astronomy 201010 19.2.2 Neutral Hydrogen Clouds  Vast clouds of neutral H I gas  Don't emit strong (visible) radiation  Spectroscopic binaries betray H I regions  binaries: doppler shift moves spectral lines  some lines don't move  reason: absorption lines in gas between binary pair and Earth XX interstellar gas

11. February 14, 2006Astronomy 201011 The Hydrogen 21 cm Line  Hydrogen: proton (p) plus electron (e)  Both p and e have spin – "up" or "down"  Ground state: p up, e down  Excited state: p up, e up  Can move between states by emitting/absorbing a photon.  The photon has a wavelength of 21 cm, a radio wave.

12. February 14, 2006Astronomy 201012 21cm Line From Cold H I Regions  This “spin flip” in hydrogen produces the 21cm radio waves.  Hydrogen clouds must be cold, about 100K.  First detected in 1951.  Seen by a radio telescope. top side

13. February 14, 2006Astronomy 201013 19.2.3 Ultra-Hot Interstellar Gas  Astronomers were surprised to discover hot interstellar gas.  Hot means about 1 million degrees K!  We now understand that the gas is heated by supernovae explosions.  This topic will be discussed in Ch. 22.

14. February 14, 2006Astronomy 201014 19.4 Cosmic Dust

15. February 14, 2006Astronomy 201015 Cosmic Dust  Dark regions seemingly empty of stars  Not voids – dark clouds  Dark nebulae block light from stars behind them  Composed of dust grains coated by ice.  Visible only in infrared  IR satellites IRAS, Hubble  Thermal spectrum gives the temperature, typically 10K to 100K.

16. February 14, 2006Astronomy 201016 Dust Phenomena  Extinction: light absorbed and scattered by dust  black regions Reflection nebula: scattered light illuminates nearby gas Reflection nebula: scattered light illuminates nearby gas scattered light tends to be more blue scattered light tends to be more blue the sky is blue the sky is blue Reddening: incomplete absorption Reddening: incomplete absorption direct light more red – blue light scattered away direct light more red – blue light scattered away the sunset is red the sunset is red

18. February 14, 2006Astronomy 201018 blue sky, red sunset blue light scattered more easily than red  blue light scattered out of white rays from Sun  lights atmosphere in blue  reflection nebula work the same way red sunset – direct light from the sun depleted of blue – reddening

19. reflection nebula in Orion

22. February 14, 2006Astronomy 201022 Trifid Nebula 3000 LY distant 50 LY across H II region (red) high-energy UV hits interstellar gas dark dust filaments – extinction due to debris from supernovae blue reflection nebula

23. February 14, 2006Astronomy 201023 dust filaments

24. February 14, 2006Astronomy 201024 Dust Glows in the Infrared infraredvisible

25. February 14, 2006Astronomy 201025 Dust Pillar very bright star blowing dust off of a star near the pillar's tip

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31. February 14, 2006Astronomy 201031Summary  The amount of gas and dust that exists between stars was an important discovery.  The gas and dust accounts for a large fraction of the mass of the Milky Way Galaxy.  The clouds are observed by a variety of means, including the 21cm radio waves.  Cool clouds of gas and dust (giant molecular clouds) are regions where new stars can be created.