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) T (K) Density a/cm3 20-100 size: a few mm 50-500 1-1000 103-104 0.01 105-106 10-4-10-3 20-50 103-105 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

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.

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The Stellar Cycle New (dirty) molecular clouds are left behind by the supernova debris. Cool molecular clouds gravitationally collapse to form clusters of stars Molecular cloud The hottest, most massive stars in the cluster supernova – heavier elements are formed in the explosion. Stars generate helium, carbon and iron through stellar nucleosynthesis

The ISM Main Components (Phases) Dust HI Clouds Intercloud Medium Coronal Gas Molecular clouds T (K) Density a/cm3 20-100 size: a few mm 50-500 1-1000 103-104 0.01 105-106 10-4-10-3 20-50 103-105

The Milky Way

Dust – a hindrance to our study of the Milky Way A view at visible wavelengths of the galactic plane. 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.

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, visible light is absorbed (or scattered) and only the longer wavelengths get through.

Blocked by Interstellar Dust Radio Microwave Infrared Visible UV X-ray longer wavelength (redder) shorter wavelength (more blue)

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 reconstruct the distribution of objects in the galaxy.

Radio observations help map the galactic disk You are here 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

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