1. 29 March 2005AST 2010: Chapter 191 Between the Stars: Gas & Dust in Space

2. 29 March 2005AST 2010: Chapter 192 Gas and Dust in Space To understand how stars form, we need to know the raw material from which they are made All the gas and dust material that lies in the region between stars is referred to as interstellar matter The entire collection of interstellar matter is called the interstellar medium The interstellar medium accounts for a large fraction of the atoms in the universe (>50%) and provides the raw material for new stars Clouds of interstellar gas or dust that are seen to glow with visible light or infrared radiation are usually called nebulae (the Latin for "clouds) Interstellar gas and dust can produce colorful displays when lit by the light of nearby stars

3. 29 March 2005AST 2010: Chapter 193 Interstellar Medium About 99% of the interstellar matter is in the form of gas (individual atoms or molecules) The most abundant elements in the interstellar gas are hydrogen (75%) and helium (25%) The remaining 1% of interstellar matter is in the form of solid interstellar dust grains The density of interstellar matter is very low It has 10 3 atoms per cubic centimeter (cc) Air has 10 19 atoms per cc The best vacuum created on Earth has 10 7 atoms per cc The volume of space occupied by interstellar matter is huge Consequently, the total mass of interstellar matter is humongous

4. 29 March 2005AST 2010: Chapter 194 Interstellar Gas The color of a gas gives us clues about its temperature and composition The red color commonly seen in interstellar gas comes from ionized hydrogen, or H II The proton recombines with an electron which then moves down to the lowest-energy orbit by emitting a red-wavelength photon H I refers to a neutral hydrogen, and Fe III a doubly ionized iron type of regiontemperature (K) HI: cold clouds100 HI: warm clouds5000 hot gas500,000 HII regions10,000 giant molecular clouds10

5. 29 March 2005AST 2010: Chapter 195 H II Regions These regions have temperatures near 10 4 K, heated by nearby stars The ultraviolet light from hot O and B stars ionizes the surrounding hydrogen gas The free electrons recombine with protons, forming excited H atoms Excited states emit light The red glow is characteristic of hydrogen (the red Balmer line)

6. 29 March 2005AST 2010: Chapter 196 H II Regions: Dusty Nebulae in Sagittarius Constellation The red glow that dominates this image is produced by the red Balmer line of hydrogen This indicates that there are hot stars nearby that ionize these clouds of gas

7. 29 March 2005AST 2010: Chapter 197 Absorption Lines Most of the interstellar medium is cold and hence not ionized Mostly hydrogen and helium Other atoms and molecules are also seen: Ca, Na, CN, CH, H 2, CO The cool gas between the Earth and the stars will cause an absorption spectrum

8. 29 March 2005AST 2010: Chapter 198 Neutral-Hydrogen Clouds Vast clouds of neutral-hydrogen (H I) gas are cold and, therefore, do not emit strong (visible) radiation The first evidence for absorption by interstellar clouds in H I regions came from the analysis of spectroscopic binary stars binaries: doppler shift moves spectral lines some lines don't move reason: absorption lines in gas between binary pair and Earth XX interstellar gas

9. 29 March 2005AST 2010: Chapter 199 The Hydrogen 21-cm Line Hydrogen: proton (p) plus electron (e) Both p and e have spin – "up" or "down" Ground spin-state: p up, e down Excited spin-state: p up, e up The electron can move between the spin states by emitting or absorbing a photon The photon has a wavelength of 21 cm, a radio wave

10. 29 March 2005AST 2010: Chapter 1910 21-cm Line From Cold H-I Regions The spin flip in hydrogen was predicted to produce 21-cm-long radio waves The prediction was confirmed by observation in 1951 using radio telescopes This indicates that neutral-hydrogen clouds must be cold, having temperatures of about 100 K Most of cold hydrogen is confined to a very flat layer (less than 300-LY thick) that extends throughout the disk of the Milky Way Galaxy top side

11. 29 March 2005AST 2010: Chapter 1911 Ultra-Hot Interstellar Gas Astronomers were surprised to discover hot interstellar gas, even though there was no visible source of heat nearby The hot temperatures are about 1 million degrees K! We now understand that the gas is heated by supernovae, the explosions of massive stars This topic will be discussed in Ch. 22

12. 29 March 2005AST 2010: Chapter 1912 Cosmic Dust There are dark regions on the sky that are seemingly empty of stars But they are not voids, but clouds of dark dust The dust betrays its presence by blocking the light from distant stars reflecting the light from nearby stars making distant stars look redder and fainter than they really are Each dust particle has a rocky core that is either sootlike (carbon-rich) or sandlike (containing silicates) and a mantle made of icy material

13. 29 March 2005AST 2010: Chapter 1913 Blue Sky & Red Sunset Blue light is scattered more easily than red because red wavelengths are longer than blue The blue colors in sunlight are scattered repeatedly by molecules in the air, and this makes our sky look blue Seen directly, the Sun looks yellowish, as the light from it is missing some of its blue At sunrise or sunset, the Sun appears redder than at noon because the light from it travels a longer path through the air than at noon and hence is missing more of its blue

14. 29 March 2005AST 2010: Chapter 1914 Scattering of Light by Cosmic Dust Interstellar dust particles are very small, about the same size as the wavelength of visible light The particles scatter blue light more efficiently than red light, thereby making distant stars appear redder and giving clouds of dust near stars a bluish hue

15. Reflection Nebulae Some dense clouds of dust are close to luminous stars and scatter enough starlight to become visible Such a cloud is called a reflection nebula because the light that we see from it is starlight reflected off grains of dust Since dust grains are tiny, they scatter light with blue wavelengths better than light with red wavelengths As a result, a reflection nebula usually appears bluer than its illuminating star A reflection nebula (NGC 1999), illuminated by a star, which is visible just to the left of center

16. 29 March 2005AST 2010: Chapter 1916 Trifid Nebula in Sagittarius Constellation It is about 3000 LY from the Sun, and about 50 LY in diameter The reddish H-II region is surrounded by a blue reflection nebula

17. 29 March 2005AST 2010: Chapter 1917 The Dust Filaments in the Trifid Nebula are due to debris from supernovae

18. 29 March 2005AST 2010: Chapter 1918 Dust Glows in the Infrared infraredvisible

19. 29 March 2005AST 2010: Chapter 1919 Visible and Infrared Images of Horsehead Nebula in Orion

20. 29 March 2005AST 2010: Chapter 1920 very bright star blowing dust off of a star near the pillar's tip Dust Pillar

22. 29 March 2005AST 2010: Chapter 1922 Cosmic Rays These are particles that travel through interstellar space at a typical speed of 90% the speed of light The most abundant elements in cosmic rays are the nuclei of hydrogen and helium Positrons (anti-electrons) are also found Many cosmic rays are probably produced in supernova explosions