Chapter 19 Between the Stars Gas and Dust

Chapter 19: Between the Stars: Gas and Dust in Space February 14, 2006 Astronomy 2010 1

Gas and Dust in Space To understand how stars are born, we begin by learning about the raw material from which they are made. u Interstellar matter: gas and dust that lies in the regions between stars. u Interstellar medium: the entire collection of interstellar matter. u The interstellar medium accounts for a large fraction of the atoms in the universe (>50%). u Provides the raw material for new stars. u 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. February 14, 2006 Astronomy 2010 2

19. 1: The Interstellar Medium u 75% hydrogen, 25% helium, and 1% interstellar dust (by mass). u The matter comes together in clouds. u Density is low: § 103 atoms per cubic centimeter (cc). § Air has 1019 atoms per cc. § Best vacuum created on Earth has 107 atoms per cc. u But very large in size § Tens of LY’s. February 14, 2006 Astronomy 2010 3

19. 2 Interstellar Gas u The color of a cloud tells us about its temperature and composition. u The common red color comes from hydrogen (H II). u H II regions contain ionized hydrogen. u H I indicates neutral hydrogen, H II is singly ionized hydrogen, and Fe III is doubly ionized iron. February 14, 2006 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 Astronomy 2010 4

H II Regions u temperature near 104 K – heated by nearby stars § ultraviolet light from hot O and B stars § ionizes the surrounding hydrogen gas u free electrons recombine – form excited H atoms § excited states emit light § red glow characteristic of hydrogen (red Balmer line) February 14, 2006 Astronomy 2010 5

HII regions: Orion Nebula • closest to us, 1, 500 LY • 29 x 26 LY in size • large star formation factory February 14, 2006 Astronomy 2010 6

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trapesizium cluster: • stars that provide much of the energy which makes the brilliant Orion Nebula visible • other stars obscured by nebula February 14, 2006 Astronomy 2010 8

Absorption Lines u Most of the interstellar medium is not ionized. § Mostly hydrogen, and helium § Other atoms and molecules seen: Ca, Na, CN, CH, H 2, CO u Cool gas between stars and Earth will cause an absorption spectrum. February 14, 2006 Astronomy 2010 9

19. 2. 2 Neutral Hydrogen Clouds u. Vast clouds of neutral H I gas u. Don't emit strong (visible) radiation u. Spectroscopic binaries betray H I regions interstellar gas § binaries: doppler shift moves spectral lines § some lines don't move § reason: absorption lines in gas between binary pair and Earth February 14, 2006 Astronomy 2010 X X 10

The Hydrogen 21 cm Line u Hydrogen: proton (p) plus electron (e) u Both p and e have spin – "up" or "down" u Ground state: p up, e down u Excited state: p up, e up u Can move between states by emitting/absorbing a photon. u The photon has a wavelength of 21 cm, a radio wave. February 14, 2006 Astronomy 2010 11

21 cm Line From Cold H I Regions top u This “spin flip” in hydrogen side produces the 21 cm radio waves. u Hydrogen clouds must be cold, about 100 K. u First detected in 1951. u Seen by a radio telescope. February 14, 2006 Astronomy 2010 12

19. 2. 3 Ultra-Hot Interstellar Gas u. Astronomers were surprised to discover hot interstellar gas. u. Hot means about 1 million degrees K! u. We now understand that the gas is heated by supernovae explosions. u. This topic will be discussed in Ch. 22. February 14, 2006 Astronomy 2010 13

19. 4 Cosmic Dust February 14, 2006 Astronomy 2010 14

Cosmic Dust u Dark regions seemingly empty of stars u Not voids – dark clouds u Dark nebulae block light from stars behind them u Composed of dust grains coated by ice. u Visible only in infrared § IR satellites IRAS, Hubble u Thermal spectrum gives the temperature, typically 10 K to 100 K. February 14, 2006 Astronomy 2010 15

Dust Phenomena u Extinction: light absorbed and scattered by dust black regions • Reflection nebula: scattered light illuminates nearby gas • • scattered light tends to be more blue the sky is blue • Reddening: incomplete absorption • • direct light more red – blue light scattered away the sunset is red February 14, 2006 Astronomy 2010 16

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blue sky, red sunset blue light scattered more easily than red u blue light scattered out of white rays from Sun u lights atmosphere in blue u reflection nebula work the same way red sunset – direct light from the sun depleted of blue – reddening February 14, 2006 Astronomy 2010 18

reflection nebula in Orion February 14, 2006 Astronomy 2010 19

reflection nebula in Orion February 14, 2006 Astronomy 2010 20

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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 February 14, 2006 Astronomy 2010 22

dust filaments February 14, 2006 Astronomy 2010 23

Dust Glows in the Infrared infrared February 14, 2006 visible Astronomy 2010 24

Dust Pillar very bright star blowing dust off of a star near the pillar's tip February 14, 2006 Astronomy 2010 25

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Summary u. The amount of gas and dust that exists between stars was an important discovery. u. The gas and dust accounts for a large fraction of the mass of the Milky Way Galaxy. u. The clouds are observed by a variety of means, including the 21 cm radio waves. u. Cool clouds of gas and dust (giant molecular clouds) are regions where new stars can be created. February 14, 2006 Astronomy 2010 31