Chapter 19: Between the Stars: Gas and Dust in Space

Chapter 19: Between the Stars:Chapter 19: Between the Stars:Gas and Dust in SpaceGas and Dust in Space

February 14, 2006February 14, 2006 Astronomy 2010Astronomy 2010 22

Gas and Dust in SpaceGas and Dust in SpaceTo understand how stars are born, we begin by To understand how stars are born, we begin by learning about the raw material from which they learning about the raw material from which they are made.are made.

Interstellar matterInterstellar matter: gas and dust that lies in : gas and dust that lies in the regions between stars.the regions between stars.

Interstellar mediumInterstellar medium: the entire collection of : the entire collection of interstellar matter.interstellar matter.

The interstellar medium accounts for a large The interstellar medium accounts for a large fraction of the atoms in the universe (>50%).fraction of the atoms in the universe (>50%).

Provides the raw material for new stars.Provides the raw material for new stars. NebulaeNebulae: latin for "clouds”, aggregates of : latin for "clouds”, aggregates of interstellar matter that emits radio waves or interstellar matter that emits radio waves or light. Can produce colorful displays when lit light. Can produce colorful displays when lit by the light of nearby stars.by the light of nearby stars.


19.1: The Interstellar Medium19.1: The Interstellar Medium75% hydrogen, 25% helium, and 1% 75% hydrogen, 25% helium, and 1% interstellar dustinterstellar dust (by mass). (by mass).

The matter comes together in clouds.The matter comes together in clouds.Density is low:Density is low:

101033 atoms per cubic centimeter (cc). atoms per cubic centimeter (cc). Air has 10Air has 101919 atoms per cc. atoms per cc. Best vacuum created on Earth has 10Best vacuum created on Earth has 1077 atoms atoms per cc.per cc.

But very large in sizeBut very large in size Tens of LY’s.Tens of LY’s.


19.2 Interstellar Gas19.2 Interstellar Gas The color of a cloud tells us about its The color of a cloud tells us about its temperature and composition.temperature and composition.

The common red color comes from hydrogen (H II).The common red color comes from hydrogen (H II). H IIH II regions contain ionized hydrogen. regions contain ionized hydrogen. H I indicates neutral hydrogen, H II is singly H I indicates neutral hydrogen, H II is singly ionized hydrogen, and Fe III is doubly ionized ionized hydrogen, and Fe III is doubly ionized iron.iron.

type of regiontype of region temperature (K)temperature (K)

HI: cold cloudsHI: cold clouds 100100

HI: warm cloudsHI: warm clouds 50005000

hot gashot gas 500,000 500,000

HII regionsHII regions 10,00010,000

giant molecular giant molecular cloudsclouds

10 10


H II RegionsH II Regionstemperature near 10temperature near 1044 K – heated by K – heated by nearby starsnearby stars ultraviolet light from hot O and B stars ultraviolet light from hot O and B stars ionizes the surrounding hydrogen gasionizes the surrounding hydrogen gas

free electrons recombine – form excited free electrons recombine – form excited H atoms H atoms excited states emit light excited states emit light red glow characteristic red glow characteristic

of hydrogen of hydrogen (red Balmer line)(red Balmer line)

HII regions: Orion NebulaHII 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


AbsorptionAbsorption Lines LinesMost of the interstellar medium is not Most of the interstellar medium is not ionized.ionized. Mostly hydrogen, and heliumMostly hydrogen, and helium Other atoms and molecules seen: Ca, Na, CN, Other atoms and molecules seen: Ca, Na, CN, CH, HCH, H22, CO , CO

Cool gas between stars and Earth will Cool gas between stars and Earth will cause an absorption spectrum.cause an absorption spectrum.


19.2.2 Neutral Hydrogen Clouds19.2.2 Neutral Hydrogen CloudsVast clouds of neutral Vast clouds of neutral H I gas H I gas

Don't emit strong Don't emit strong (visible) radiation(visible) radiation

Spectroscopic binaries Spectroscopic binaries betray H I regionsbetray H I regions binaries: doppler shift binaries: doppler shift moves spectral lines moves spectral lines

some lines don't move some lines don't move reason: absorption reason: absorption lines in gas between lines in gas between binary pair and Earthbinary pair and Earth

X X

interstellar gas


The Hydrogen 21 cm LineThe Hydrogen 21 cm Line Hydrogen: proton (p) plus electron (e)Hydrogen: proton (p) plus electron (e) Both p and e have spin – "up" or "down"Both p and e have spin – "up" or "down" Ground state: p up, e downGround state: p up, e down Excited state: p up, e upExcited state: p up, e up Can move between states by emitting/absorbing a Can move between states by emitting/absorbing a photon.photon.

The photon has a wavelength of 21 cm, a radio The photon has a wavelength of 21 cm, a radio wave.wave.


21cm Line From Cold H I Regions21cm Line From Cold H I RegionsThis “spin flip” in This “spin flip” in hydrogen produces the 21cm hydrogen produces the 21cm radio waves.radio waves.

Hydrogen clouds must be Hydrogen clouds must be cold, about 100K.cold, about 100K.

First detected in 1951.First detected in 1951.Seen by a Seen by a radio telescoperadio telescope..

top

side


19.2.3 Ultra-Hot Interstellar Gas19.2.3 Ultra-Hot Interstellar GasAstronomers were surprised to Astronomers were surprised to discover hot interstellar gas.discover hot interstellar gas.

Hot means about 1 million degrees Hot means about 1 million degrees K!K!

We now understand that the gas is We now understand that the gas is heated by supernovae explosions.heated by supernovae explosions.

This topic will be discussed in Ch. This topic will be discussed in Ch. 22.22.

February 14, 2006February 14, 2006 Astronomy 2010Astronomy 2010 141419.4 Cosmic Dust19.4 Cosmic Dust


Cosmic DustCosmic DustDark regions seemingly Dark regions seemingly empty of starsempty of stars

Not voids – dark cloudsNot voids – dark cloudsDark nebulae block light Dark nebulae block light from stars behind themfrom stars behind them

Composed of dust grains Composed of dust grains coated by ice.coated by ice.

Visible only in infrared Visible only in infrared IR satellites IRAS, HubbleIR satellites IRAS, Hubble

Thermal spectrum gives Thermal spectrum gives the temperature, the temperature, typically 10K to 100K.typically 10K to 100K.


Dust PhenomenaDust Phenomena Extinction: light absorbed and scattered by Extinction: light absorbed and scattered by dust dust black regions black regions

• Reflection nebulaReflection nebula: : scattered light scattered light illuminates nearby illuminates nearby gasgas• scattered light tends scattered light tends to be more blueto be more blue

• the sky is bluethe sky is blue

• ReddeningReddening: incomplete : incomplete absorptionabsorption• direct light more red direct light more red – blue light scattered – blue light scattered away away

• the sunset is redthe sunset is red


blue sky, red blue sky, red sunsetsunset

blue light scattered more blue light scattered more easilyeasily

than redthan red blue light scattered out blue light scattered out

of white rays from Sunof white rays from Sun lights atmosphere in blue lights atmosphere in blue reflection nebula work the reflection nebula work the

same waysame way

red sunset – direct lightred sunset – direct light

from the sun depleted offrom the sun depleted of

blue – blue – reddening reddening

reflection nebula in Orion

reflection nebula in Orion


Trifid Trifid NebulaNebula3000 LY

distant50 LY across •H II region (red) high-energy UV hits interstellar gas•dark dust filaments – extinction due to debris from supernovae•blue reflection nebula


dust dust filamentsfilaments


Dust Glows in the InfraredDust Glows in the Infrared

infrared

visible


Dust Dust PillarPillar

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



SummarySummaryThe amount of gas and dust that The amount of gas and dust that exists between stars was an exists between stars was an important discovery.important discovery.

The gas and dust accounts for a The gas and dust accounts for a large fraction of the mass of the large fraction of the mass of the Milky Way Galaxy.Milky Way Galaxy.

The clouds are observed by a The clouds are observed by a variety of means, including the variety of means, including the 21cm radio waves.21cm radio waves.

Cool clouds of gas and dust (giant Cool clouds of gas and dust (giant molecular clouds) are regions where molecular clouds) are regions where new stars can be created.new stars can be created.

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Chapter 19: Between the Stars: Gas and Dust in Space