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Chapter 18. The Interstellar Medium. The Interstellar Medium. Occupies the space among the stars. Is made up of:. cold gas , mostly hydrogen and helium. Dust grains (size ~ 10 -7 m -  10 3 x larger than gas particles). Interstellar Matter.

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chapter 18

Chapter 18

The Interstellar Medium

the interstellar medium
The Interstellar Medium

Occupies the space among the stars

Is made up of:

  • cold gas , mostly hydrogen and helium
  • Dust grains (size ~ 10-7 m - 103 x larger than gas particles)

Starlight passing through a dusty region of space is both dimmed and reddened, but spectral lines are still recognizable in the light that reaches Earth


The dimming of the starlight by dust is called “Extinction”

The blocking of the shorter wavelength (blue) light and transmission of the longer wavelength (Red) light gives the star a redder appearance. This is called “reddening”


Dust grains are generally elongated rather than spherical.

Astronomers know this by studying their polarizing effects on starlight


Emission Nebulae

Nebula – Any “fuzzy” patch (bright or dark) on the sky.

Emission nebulae – extended clouds of hot, glowing interstellar gas.

They are associated with star formation, and result when hot O and B class stars heat and ionize the surrounding gas and dust from which they have formed.


Most of these emission nebulae are catalogued in the Messier Catalogue. (Ex: M8, M16, M17, M20)

Charles Messier was an 18th century French Astronomer who was mainly interested in comets.

Messier was actually more concerned with making a list of celestial objects that might be confused with comets.


Nebular Spectra

Nebular spectra tell us a great deal about the ionized interstellar gas.

Nebula are large enough for their actual sizes to be measured by simple geometry. (unlike stars, which appear to be points of light)


The Butterfly Nebula, a glowing patch of gas a few parsecs across.

Its emission spectrum, showing light intensity over the entire visible portion of the EM spectruum from red to deep violet.


The Orion Nebula

The greenish tint of portions of this nebula are produced by an electron transition in doubly ionized oxygen.

“Forbidden” Lines since they rarely occur in a “low density” gas on Earth.


Dark Dust Clouds

Rho Ophiuchi

Dark Dust clouds are cold irregularly shaped regions in the interstellar medium that diminish or completely obscure the light from background stars.


Typical 21 –cm radio spectrum observed from several different regions of interstellar space.


Interstellar Molecules

The interstellar medium also contains cold, dark molecular clouds which are observed mainly through the radio radiation they emit.


Contour Map of the amount of formaldehyde near M20 nebula, demonstrating how formaldehyde is especially abundant in the darkest interstellar regions.