29 March 2005 AST 2010: Chapter 19 1

Between the Between the Stars:Stars:

Gas & Dust in Gas & Dust in SpaceSpace

29 March 2005 AST 2010: Chapter 19 2

Gas and Dust in SpaceGas and Dust in SpaceTo understand how stars form, we need to know the raw material from which they are madeAll the gas and dust material that lies in the region between stars is referred to as interstellar matterThe entire collection of interstellar matter is called the interstellar mediumThe 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

29 March 2005 AST 2010: Chapter 19 3

Interstellar MediumInterstellar MediumAbout 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 grainsThe density of interstellar matter is very low

It has 103 atoms per cubic centimeter (cc)Air has 1019 atoms per ccThe best vacuum created on Earth has 107 atoms per cc

The volume of space occupied by interstellar matter is huge

Consequently, the total mass of interstellar matter is humongous

29 March 2005 AST 2010: Chapter 19 4

Interstellar GasInterstellar GasThe color of a gas gives us clues about its temperature and compositionThe 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 region temperature (K)

HI: cold clouds 100

HI: warm clouds 5000

hot gas 500,000

HII regions 10,000

giant molecular clouds 10

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H II RegionsH II RegionsThese regions have temperatures near 104 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)

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H II Regions: Dusty Nebulae in H II Regions: Dusty Nebulae in Sagittarius ConstellationSagittarius Constellation

The red glow that dominates this image is produced by the red Balmer line of hydrogenThis indicates that there are hot stars nearby that ionize these clouds of gas

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Absorption LinesAbsorption LinesMost of the interstellar medium is cold and hence not ionized

Mostly hydrogen and heliumOther atoms and molecules are also seen: Ca, Na, CN, CH, H2, CO

The cool gas between the Earth and the stars will cause an absorption spectrum

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Neutral-Hydrogen CloudsNeutral-Hydrogen CloudsVast clouds of neutral-hydrogen (H I) gas are cold and, therefore, do not emit strong (visible) radiationThe 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

X X

interstellar gas

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The Hydrogen 21-cm LineThe Hydrogen 21-cm LineHydrogen: proton (p) plus electron (e)Both p and e have “spin” – "up" or "down"Ground spin-state: p up, e downExcited spin-state: p up, e upThe electron can move between the spin states by emitting or absorbing a photonThe photon has a wavelength of 21 cm, a radio wave

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21-cm Line From Cold H-I 21-cm Line From Cold H-I RegionsRegions

The “spin flip” in hydrogen was predicted to produce 21-cm-long radio wavesThe prediction was confirmed by observation in 1951 using radio telescopesThis indicates that neutral-hydrogen clouds must be cold, having temperatures of about 100 KMost 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

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Ultra-Hot Interstellar GasUltra-Hot Interstellar GasAstronomers 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 starsThis topic will be discussed in Ch. 22

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Cosmic DustCosmic DustThere 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 byblocking the light from distant stars reflecting the light from nearby starsmaking 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

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Blue Sky & Red SunsetBlue Sky & Red SunsetBlue 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

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Scattering of Light by Scattering of Light by Cosmic DustCosmic Dust

Interstellar dust particles are very small, about the same size as the wavelength of visible lightThe 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

Reflection NebulaeReflection NebulaeSome dense clouds of dust are close to luminous stars and scatter enough starlight to become visibleSuch a cloud is called a reflection nebula because the light that we see from it is starlight reflected off grains of dustSince dust grains are tiny, they scatter light with blue wavelengths better than light with red wavelengthsAs 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

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Trifid Nebula in Sagittarius Trifid Nebula in Sagittarius ConstellationConstellation

It is about 3000 LY from the Sun, and about 50 LY in diameterThe reddish H-II region is surrounded by a blue reflection nebula

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The The Dust Dust

FilameFilaments in nts in the the

Trifid Trifid Nebula Nebula

are are due to due to debris debris from from

supernsupernovaeovae

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Dust Glows in the InfraredDust Glows in the Infrared

infrared visible

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Visible and Infrared Visible and Infrared Images of Images of Horsehead Nebula in Horsehead Nebula in

OrionOrion

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very bright star blowing dust off of a star near the pillar's tip

Dust Pillar Dust Pillar

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Cosmic RaysCosmic RaysThese are particles that travel through interstellar space at a typical speed of 90% the speed of lightThe 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