24 interstellar medium

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C h a p t e r 1 C h a p t e r 1 1 1 The Interstellar Medium The Interstellar Medium Group 24 Group 24

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Page 1: 24 Interstellar Medium

C h a p t e r 1 1C h a p t e r 1 1

The Interstellar MediumThe Interstellar Medium

Group 24Group 24

Page 2: 24 Interstellar Medium

Gas and DustGas and Dust

• Interstellar medium is made up of 2 components—gas and dust

• The gas is made up mainly of individual atoms and small molecules

• The dust consists of clumps of atoms and molecules (like the ones that make up smoke or soot)

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Gas and Dust IIGas and Dust II

• Interstellar matter obscures the light from stars beyond

– Obscuration is caused by dust

– Light can be obscured (absorbed or scattered) only by particles having diameters comparable to or larger than the wavelength of radiation involved.

– Obscuration produced by particles of a given size increases with decreasing wavelength

Page 4: 24 Interstellar Medium

Gas and Dust IIIGas and Dust III

• The typical diameter of an interstellar dust particle, or dust grain, is about 10-7 m, comparable in size to the wavelength of visible light– The effect is that dusty regions of interstellar space

are transparent to long-wavelength radio and infrared radiation but opaque to shorter-wavelength optical, ultraviolet, and X-ray radiation

– Because of this, light from distant stars is robbed of its higher-frequency (blue) components. So in addition to being generally diminished in overall brightness, stars also appear redder than they really are (reddening)

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Density and Density and Composition of the Composition of the Interstellar MediumInterstellar Medium

• Gas and dust are found everywhere in interstellar space• No part of the galaxy is truly devoid of matter, although

the density of the interstellar medium is extremely low– The gas averages roughly 106 atoms per cubic meter—just one

atom per cubic centimeter– Interstellar dust is even rarer—about one dust particle for every

trillion or so atoms

• Space in the vicinity of the Sun contains about as much mass in the form of interstellar gas and dust as exists there in the form of stars

• Interstellar matter is distributed very unevenly

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Density and Composition Density and Composition of the Interstellar Medium of the Interstellar Medium

IIII• The composition of interstellar gas is reasonably well known

from spectroscopic studies of interstellar absorption lines•It mirrors the composition of the other astronomical objects, such as the Sun, the stars, and the jovian planets

•Most of the gas—about 90%--is atomic or molecular hydrogen, 9% is helium, and the remaining 1% consists of heavier elements

•The gas is deficient in some heavy elements, such as carbon, oxygen, silicon, magnesium, and iron, most likely because these elements have gone to form the interstellar dust

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Density and Composition of the Interstellar Medium

III• The composition of the dust is not well

known, although there is some infrared evidence for silicates, carbon, and iron, supporting the theory that interstellar dust forms out of interstellar gas

– The dust probably also contains some “dirty ice,” a frozen mixture of water ice contaminated with trace amounts of ammonia, methane, and other compounds, much like cometary nuclei in our own solar system

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Important Definitions #1Important Definitions #1

• Interstellar medium - the matter between the stars

• Dust grain - an interstellar dust particle

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Milky Way MosaicMilky Way Mosaic

Fig 11.3 shows mosaic of Milky Way Galaxy from horizon to horizon• Dust causes the patchy obscuration of background stars

Infrared observations indicate dust composed of silicates, carbon, and iron

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Emission Nebulae

Charles Messier, 18th century French astronomer saw sky much like Fig 11.4

He observed several fuzzy patches of light that he couldn’t classify as stars, planets, comets, or asteroids

Page 11: 24 Interstellar Medium

Emission Nebulae

These are now known as Emission Nebulae, as seen in Fig 11.5 and 11.6

Nebulae large enough to measure size with simple geometry

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Emission NebulaeEmission Nebulae

UV radiation from a forming O- or B-type star ionizes surrounding nebular gasesAs hydrogen electrons recombine with nuclei, they emit red colored visible radiationNebular gas spectrum indicates composition similar to our sun, stars, and other ISMSpectral line widths imply that gas atoms and ions have temperature around 8000KSize information coupled with estimates of matter in our line of sight (as revealed by nebulae’s total light emission) ascertain nebular density - @ 10^8 particles per cubic meter

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Emission NebulaeEmission Nebulae

Through examination of interaction between dust and nebular gas, we see that dust lanes are part of the nebulae and not just dust in our line-of-sight

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Important Definitions #2Important Definitions #2

Reddening - the way a star appears redder than it truly is

Emission nebulae - glowing clouds of hot intersteallar matter

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Dark Dust Dark Dust CloudsClouds

• More than 99% of space is free from emission nebulae and stars

• Dark dust clouds are typically bigger than our solar system

• Made primarily of dust (causes the absorption of starlight)

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21-Centimeter 21-Centimeter RadiationRadiation

• Much gas in interstellar space consists of atomic hydrogen

• Astronomers observe interstellar regions by using the process known as the 21-centimeter line

• This form of observation does not require light, only enough hydrogen to produce a detectablesignal

•Wavelength of 21-centimeter radiation is much larger than the typical size of interstellar dustparticles

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Molecular GasMolecular Gas

• Forms molecular Forms molecular cloudsclouds

• Only long wavelength Only long wavelength radio radiation can radio radiation can escape from these escape from these parts of parts of interstellar spaceinterstellar space

• Can not use 21-cm Can not use 21-cm observationsobservations

Astronomers use radio Astronomers use radio observations of other observations of other "tracer" molecules to "tracer" molecules to study study these regionsthese regions

•Molecular clouds do Molecular clouds do not exist as distinct not exist as distinct and separate objects and separate objects in spacein space

•Molecules are found Molecules are found only in the densest only in the densest and darkest of and darkest of interstellar cloudsinterstellar clouds

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Important Definitions Important Definitions #3#3

• Dark dust cloudsDark dust clouds - huge clouds made primarily - huge clouds made primarily of gas that absorb starlight and are cooler than of gas that absorb starlight and are cooler than their surroundingstheir surroundings

• Molecular cloudsMolecular clouds - gas particles (molecules) - gas particles (molecules) that are in the dense interstellar regionsthat are in the dense interstellar regions

• Molecular cloud complexesMolecular cloud complexes - made up of - made up of molecular hydrogen and large enough to make molecular hydrogen and large enough to make millions of stars like our sunmillions of stars like our sun

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Review QuestionsReview Questions

• What is interstellar mediumWhat is interstellar medium?? • the matter between the starsthe matter between the stars• What are dark dust clouds?What are dark dust clouds?• huge clouds made primarily of gas that huge clouds made primarily of gas that

absorb starlight and are cooler than their absorb starlight and are cooler than their surroundingssurroundings

• What are molecular clouds?What are molecular clouds?• gas particles (molecules) that are in the gas particles (molecules) that are in the

dense interstellar regionsdense interstellar regions

• What are emission nebulaeWhat are emission nebulae??• glowing clouds of hot intersteallar matterglowing clouds of hot intersteallar matter