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Chapter 19: Between the Stars: Gas and Dust in Space. 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.
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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. Astronomy 2010
19.1: The Interstellar Medium • 75% hydrogen, 25% helium, and 1% interstellar dust (by mass). • The matter comes together in clouds. • 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. • But very large in size • Tens of LY’s. Astronomy 2010
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. Astronomy 2010
H II Regions • temperature near 104 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) Astronomy 2010
HII regions: Orion Nebula • closest to us, 1,500 LY • 29x26 LY in size • large star formation factory
trapesizium cluster: • stars that provide much of the energy which makes the brilliant Orion Nebula visible • other stars obscured by nebula
Absorption Lines • Most of the interstellar medium is not ionized. • Mostly hydrogen, and helium • Other atoms and molecules seen: Ca, Na, CN, CH, H2, CO • Cool gas between stars and Earth will cause an absorption spectrum. Astronomy 2010
19.2.2 Neutral Hydrogen Clouds interstellar gas X X • 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 Astronomy 2010
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. Astronomy 2010
21cm Line From Cold H I Regions top side • 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. Astronomy 2010
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. Astronomy 2010
19.4 Cosmic Dust Astronomy 2010
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. Astronomy 2010
Dust Phenomena • 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 Astronomy 2010
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 Astronomy 2010
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 Astronomy 2010
dust filaments Astronomy 2010
Dust Glows in the Infrared infrared visible Astronomy 2010
Dust Pillar very bright star blowing dust off of a star near the pillar's tip Astronomy 2010
Summary • 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. Astronomy 2010
