1. 2 August 2005AST 2010: Chapter 191 Between the Stars: Gas & Dust in Space

2. 2 August 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 Some interstellar material is concentrated into giant clouds, called nebulae (the Latin for “clouds”) Interstellar gas and dust can produce colorful displays when lit by the light of nearby stars AnimationAnimation: flight thru nebula

3. 2 August 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 and helium 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 about one atom 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 exceedingly large Consequently, its total mass is humongous The total mass of interstellar matter in our Milky Way Galaxy has been estimated to be about 20% of the total mass of its stars

4. 2 August 2005AST 2010: Chapter 194 Interstellar Gas Depending on where it is located, interstellar gas has temperatures ranging from a few kelvin (only a few degrees above absolute zero) to more than a million kelvin Since hydrogen (H) is the main constituent of interstellar gas, astronomers often characterize a region of space according to whether its hydrogen is neutral or ionized A cloud of ionized hydrogen is usually called an H II region In contrast, the hydrogen atoms in an H I region are not ionized and hence are neutral In an H II region, the hydrogen is ionized by ultraviolet radiation from nearby stars The proton, however, will not remain alone for long, but will quickly recombine with one of the electrons near it The resulting neutral atom can then absorb UV radiation again, and the process is repeated H II regions are not very common because they require very hot stars, which are rare

5. 2 August 2005AST 2010: Chapter 195 H II Regions These regions have temperatures ~10,000 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 hydrogen atoms Excited states emit light The reddish glow is characteristic of H, corresponding to the red Balmer line in its emission-line spectrum One way to excite an atom

6. 2 August 2005AST 2010: Chapter 196 Example of H II Regions Dusty Nebulae in the 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. 2 August 2005AST 2010: Chapter 197 Absorption Lines Most of the interstellar gas is cold and hence not ionized It consists mostly of hydrogen and helium Other atoms and molecules are also seen: Ca (calcium), Na (sodium), CN, CH, etc. The cool gas located between the Earth and the stars can yield absorption-line spectra Although cold hydrogen does not produce spectral lines in the visible range, some of the other elements do produce strong spectral lines when they are cold

8. 2 August 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 Strong visible radiation is produced by the some of the other elements in the gas The first evidence for absorption by interstellar clouds in H I regions came from the analysis of spectroscopic binary stars The Doppler effect was expected to cause the spectral lines to move But some of the lines did not move Explanation: the stationary lines were absorption lines produced by cold gas located between the binary stars and the Earth XX interstellar gas

9. 2 August 2005AST 2010: Chapter 199 The Hydrogen 21-cm Line A hydrogen atom consists of a proton (p) & an electron (e) Both p and e have “spin”, which could be “up” or “down” In the ground (lowest energy) state, p is up and e down In the slightly excited state, both p and e are up The electron can move between the spin states by emitting or absorbing a photon Such a photon has a wavelength of 21 cm, a radio wave

10. 2 August 2005AST 2010: Chapter 1910 21-cm Line From Cold H-I Regions The “spin flip” in neutral hydrogen was predicted to produce 21-cm-long radio waves The prediction was confirmed by observation in 1951 using sensitive radio telescopes This indicates that neutral-hydrogen clouds must be cold, having temperatures of about 100 K Most of the 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. Stellar Question What’s a supernova?

12. 2 August 2005AST 2010: Chapter 1912 Ultra-Hot Interstellar Gas Astronomers were surprised to discover very-hot interstellar gas in some regions of space, even though there was no visible source of heat nearby The hot temperatures are about 1 million K! Theoretical calculations have now shown the source of energy that can yield such extreme temperatures is a supernova, the explosion of a massive star Some stars, nearing the end of their lives, become unstable and literally explode (to be discussed in more detail in Ch. 22) Supernova remnant Cassiopeia A

13. 2 August 2005AST 2010: Chapter 1913 Cosmic Dust There are dark regions on the sky that are seemingly empty of stars They actually are not voids, but are clouds of dark dust The dust betrays its presence by blocking the light from distant stars making distant stars look redder and fainter than they really are reflecting the light from nearby stars Each dust particle has a rocky core that is either sootlike (carbon-rich) or sandlike (containing silicates) and a mantle made of icy material

14. 2 August 2005AST 2010: Chapter 1914 Interstellar Extinction Interstellar dust particles are very tiny, just slightly smaller than the wavelength of visible light Consequently, they readily interact with visible light Since interstellar dust grains both absorb and scatter the starlight that they intercept, they reduce the amount of light from distant stars that can reach us, making the stars look dimmer This dimming effect is called interstellar extinction The situation is similar to that of the dimming of light by fog or smoke

15. 2 August 2005AST 2010: Chapter 1915 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

16. 2 August 2005AST 2010: Chapter 1916 Like air particles in the Earth’s atmospheric, the grains of interstellar dust interact with the different colors of visible light differently Consequently, interstellar dust particles also make the distant stars look redder This is called interstellar reddening Strictly speaking, this process should more properly be called “de-blueing” because the blue and related colors have been removed (scattered) by the dust Interstellar reddening can even make some stars that are extremely hot (and hence should look bluish) appear reddish Interstellar Reddening

17. 2 August 2005AST 2010: Chapter 1917 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

18. 2 August 2005AST 2010: Chapter 1918 Trifid Nebula in Sagittarius Constellation It is about 3000 LY from the Sun and about 30 LY in diameter The reddish H-II region is surrounded by a blue reflection nebula

19. 2 August 2005AST 2010: Chapter 1919 Detecting Interstellar Dust in the Infrared While interstellar dust clouds are too cold to radiate measurable amount of visible light, they emit heat radiation and hence glow brightly in the infrared InfraredVisible Horsehead Nebula in Orion

20. 2 August 2005AST 2010: Chapter 1920 Cosmic Rays These are particles that travel through interstellar space at a typical speed of 90% the speed of light They have nearly the same composition as ordinary interstellar gas But they behave very differently from the gas Most cosmic rays are hydrogen nuclei (protons) About 9% of cosmic rays are nuclei of helium and heavier elements Positrons (anti-electrons) are also found Many cosmic rays are probably produced in supernova explosions

21. 2 August 2005AST 2010: Chapter 1921 Recycling of Cosmic Material Much of interstellar matter may have been ejected by old and dying stars The most massive stars end their lives with the giant explosions called supernovae The ejected gas and dust will likely become part of the raw material for the formation of future stars

22. 2 August 2005AST 2010: Chapter 1922 The dust filaments in the Trifid Nebula are supernova debris