The Interstellar Medium

Assigned Reading

• Chapter 10

The ISM

• Space between stars not empty• Gas, dust

• Physical status of the gas characterized by: • Temperature

• Density

• Chemical composition

• ISM and stars are the components of the “machine” that makes the universe evolve: the cycle of star formation and death, and the chemical enrichment of the cosmos.

• ISM also “disturbs” observations, since it absorbs light and modifies (reddens) colors

The ISM Main Components (Phases)

• Phase• Dust

• Present in all phases

• “Metals”• Everything that is not

hydrogen or Helium is a metal

• HI Clouds• Inter-cloud Medium• Coronal Gas• Molecular clouds

• This what forms stars

• T (K) Density a/cm3

• 20-100 size: a few m

• 50-500 1-1000

• 103-104 0.01

• 105-106 10-4-10-3

• 20-50 103-105

How did a star form?

• A cloud of hydrogen gas began to gravitationally collapse.

• As more gas fell in, it’s potential energy was converted into thermal energy.

• Eventually the in-falling gas was hot enough to ignite nuclear fusion in the core.

• Gas that continued to fall in helped to establish gravitational equilibrium with the pressure generated in the core.

O

Molecular cloud

Molecular cloud

Cool molecular cloudsgravitationally collapseto form clusters of stars

Stars generatehelium, carbonand iron throughstellar nucleosynthesis

The hottest, mostmassive stars in thecluster supernova –heavier elements areformed in the explosion.

New (dirty) molecularclouds are leftbehind by thesupernova debris.

The Stellar Cycle

The ISM Main Components (Phases)

• Phase• Dust• HI Clouds• Intercloud Medium• Coronal Gas• Molecular clouds

• T (K) Density a/cm3

• 20-100 size: a few m• 50-500 1-1000• 103-104 0.01• 105-106 10-4-10-3

• 20-50 103-105

The Milky Way

Dust is generated in the late stages of low and high mass stars, when carbon and silicon is dredged up from the cores and ejected in stellar winds, planetary nebulae, and possibly supernova remnants.The blocking of visible light by dust is called dust extinction.

Dust – a hindrance to our study of the Milky Way

A view at visible wavelengths of the galactic plane.

Effects of Dust on Radiation

• Attenuation:• Dimming of the intensity of light as it propagates

through dust

• Reddening:• Preferential dimming of blue wavelengths relative to

red ones:• Blue photons more likely to be destroyed• Blue photons more easily scattered

• As a result, radiation emerging from dust cloud is redder than when it entered

A blue haze over the mountains of Les Vosges in France.

A multi-coloured sunset over the Firth of Forth in Scotland.

A Reminder About Scattering

If the dust is thick enough, visiblelight is absorbed (or scattered) and only the longer wavelengths get through.

Radio Microwave Infrared Visible UV X-ray

longer wavelength(redder)

shorter wavelength(more blue)

Blocked by Interstellar Dust

So, to examine our own galaxy, we must use Radio, mm-wavelength, infrared, and X-ray telescopes to peer through the interstellar

medium.

Very Large Array

Chandra X-ray Observatory

Infrared view of the sky

Radio/IR Observations are key to understanding the gas/dust Disk.

• As a result of dust extinction, most of what we know about the disk of our galaxy has been learned from observations at radio and IR wavelengths.

Very Large Array

Interstellar hydrogen emits strongly at 21cm wavelengths.

A full sky image of hydrogen (21 cm emission)

By looking at the Doppler Shift of the 21 cm emission, we can reconstructthe distribution of objects in the galaxy.

• Looking for 21-cm wavelengths of light …

• emitted by interstellar hydrogen

• as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts

• this allows the velocity and mass of interstellar hydrogen to be mapped

Radio observations help map the galactic disk

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A Map of the Milky Way Based on 21-cm wavelength light mapping