2 August 2005 AST 2010: Chapter 19 1

Between the Between the Stars:Stars:

Gas & Dust in Gas & Dust in SpaceSpace

2AST 2010: Chapter 192 August 2005

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 matter

The entire collection of interstellar matter is called the interstellar medium

Some interstellar material is concentrated into giant clouds, called nebulae (the Latin for “clouds”)Interstellar gas and dust can produce colorful displays when lit by the light of nearby stars

Animation: flight thru nebula

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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 and helium

The remaining 1% of interstellar matter is in the form of solid interstellar dust grainsThe density of interstellar matter is very low

It has about one atom 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 exceedingly large

Consequently, its total mass is humongousThe total mass of interstellar matter in our Milky Way Galaxy has been estimated to be about 20% of the total mass of its stars

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Interstellar GasInterstellar GasDepending on where it is located, interstellar gas has temperatures ranging from a few kelvin (only a few degrees above absolute zero) to more than a million kelvinSince hydrogen (H) is the main constituent of interstellar gas, astronomers often characterize a region of space according to whether its hydrogen is neutral or ionizedA cloud of ionized hydrogen is usually called an H II region

In contrast, the hydrogen atoms in an H I region are not ionized and hence are neutral

In an H II region, the hydrogen is ionized by ultraviolet radiation from nearby stars

The proton, however, will not remain alone for long, but will quickly recombine with one of the electrons near itThe resulting neutral atom can then absorb UV radiation again, and the process is repeated

H II regions are not very common because they require very hot stars, which are rare

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H II RegionsH II RegionsThese regions have temperatures ~10,000 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 hydrogen atoms

Excited states emit light The reddish glow is characteristic of H, corresponding to the red Balmer line in its emission-line spectrum

One way to excite an atom

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Example of H II RegionsExample of H II RegionsDusty Nebulae in the Sagittarius 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 gas is cold and hence not ionized

It consists mostly of hydrogen and heliumOther atoms and molecules are also seen: Ca (calcium), Na (sodium), CN, CH, etc.

The cool gas located between the Earth and the stars can yield absorption-line spectra

Although cold hydrogen does not produce spectral lines in the visible range, some of the other elements do produce strong spectral lines when they are cold

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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) radiation

Strong visible radiation is produced by the some of the other elements in the gas

The first evidence for absorption by interstellar clouds in H I regions came from the analysis of spectroscopic binary stars

The Doppler effect was expected to cause the spectral lines to move But some of the lines did not move Explanation: the stationary lines were absorption lines produced by cold gas located between the binary stars and the Earth

X X

interstellar gasinterstellar gas

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The Hydrogen 21-cm LineThe Hydrogen 21-cm LineA hydrogen atom consists of a proton (p) & an electron (e)

Both p and e have “spin”, which could be “up” or “down”In the ground (lowest energy) state, p is up and e downIn the slightly excited state, both p and e are up

The electron can move between the spin states by emitting or absorbing a photonSuch a photon has a wavelength of 21 cm, a radio wave

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21-cm Line From Cold H-I Regions21-cm Line From Cold H-I RegionsThe “spin flip” in neutral hydrogen was predicted to produce 21-cm-long radio wavesThe prediction was confirmed by observation in 1951 using sensitive radio telescopesThis indicates that neutral-hydrogen clouds must be cold, having temperatures of about 100 KMost of the 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

Stellar QuestionStellar Question

What’What’s a s a

supersupernova?nova?

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Ultra-Hot Interstellar GasUltra-Hot Interstellar GasAstronomers were surprised to discover very-hot interstellar gas in some regions of space, even though there was no visible source of heat nearby

The hot temperatures are about 1 million K!

Theoretical calculations have now shown the source of energy that can yield such extreme temperatures is a supernova, the explosion of a massive starSome stars, nearing the end of their lives, become unstable and literally explode (to be discussed in more detail in Ch. 22)

Supernova remnant Cassiopeia A Supernova remnant Cassiopeia A

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Cosmic DustCosmic DustThere are dark regions on the sky that are seemingly empty of stars

They actually are not voids, but are clouds of dark dust

The dust betrays its presence byblocking the light from distant stars making distant stars look redder and fainter than they really arereflecting the light from nearby stars

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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Interstellar Interstellar ExtinctionExtinction

Interstellar dust particles are very tiny, just slightly smaller than the wavelength of visible light

Consequently, they readily interact with visible light

Since interstellar dust grains both absorb and scatter the starlight that they intercept, they reduce the amount of light from distant stars that can reach us, making the stars look dimmer

This dimming effect is called interstellar extinctionThe situation is similar to that of the dimming of light by fog or smoke

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Blue Sky & Red Blue Sky & Red SunsetSunset

Blue 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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Like air particles in the Earth’s atmospheric, the grains of interstellar dust interact with the different colors of visible light differentlyConsequently, interstellar dust particles also make the distant stars look redder

This is called interstellar reddeningStrictly speaking, this process should more properly be called “de-blueing” because the blue and related colors have been removed (scattered) by the dustInterstellar reddening can even make some stars that are extremely hot (and hence should look bluish) appear reddish

Interstellar ReddeningInterstellar Reddening

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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), A reflection nebula (NGC 1999), illuminated by a star, which is illuminated by a star, which is visible just to the left of centervisible 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 30 LY in diameterThe reddish H-II region is surrounded by a blue reflection nebula

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Detecting Interstellar Dust in the Detecting Interstellar Dust in the InfraredInfrared

While interstellar dust clouds are too cold to radiate measurable amount of visible light, they emit heat radiation and hence glow brightly in the infrared

InfraredInfraredVisibleVisible

Horsehead Nebula in OrionHorsehead Nebula in Orion

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

They have nearly the same composition as ordinary interstellar gasBut they behave very differently from the gas

Most cosmic rays are hydrogen nuclei (protons)About 9% of cosmic rays are nuclei of helium and heavier elementsPositrons (anti-electrons) are also found

Many cosmic rays are probably produced in supernova explosions

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Recycling of Cosmic MaterialRecycling of Cosmic MaterialMuch of interstellar matter may have been ejected by old and dying stars

The most massive stars end their lives with the giant explosions called supernovae

The ejected gas and dust will likely become part of the raw material for the formation of future stars

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The dust filaments in the Trifid Nebula are supernova debris