Astr – Chapter 18a

Interstellar gas is composed of
90% hydrogen, 9% helium by weight.

What effect does even thin clouds of dust have on light passing through them?
It dims and reddens the light of all more distant stars.

Some regions along the plane of the Milky Way appear dark because
stars in that region are hidden by dark dust particles.

Due to absorption of shorter wavelengths by interstellar dust clouds, distant stars appear

Which statement is true about the interstellar medium?
We know more about the gas than the dust.

The density of interstellar dust is very low, yet it still blocks starlight because
the dust particles are about the same size as the light waves they absorb.

What is the primary visible color of an emission nebula?
red due to ionized hydrogen atoms

What do “forbidden” lines reveal about interstellar space?
The density or pressure of this gas is much lower than can be produced in a laboratory.

Ionization of hydrogen in HII regions is most visible at:
656.3 nm, Balmer alpha line for 3-2 electron transition.

A large gas cloud in the interstellar medium that contains several type O and B stars would appear to us as
an emission nebula.

Interstellar dust clouds are best observed at what wavelength?
Radio and infrared

The “Local Bubble” is:
about 100 pcs. across and transparent to extreme ultraviolet radiation.

Neutral hydrogen is most obvious in the electromagnetic spectrum at:
21 cm in the radio region.

What color is a H I region?
colorless; it emits in the radio region

Why is 21-cm radiation so important to the study of interstellar matter and the Galaxy?
Emitted by hydrogen, it passes through interstellar dust and lets us to map the entire Galaxy.

Which of the following have not yet been observed in space?

What are the very cold (about 20-K), dense clouds of gas thought to be the most massive objects in the Galaxy called?
molecular clouds

Most interstellar clouds are
much bigger than our solar system.

What feature of interstellar dust is inferred by the polarization of starlight?
its presence in molecular clouds

A reflection nebula is caused by
starlight scattered by dust particles

Why are dark dust clouds largely misnamed?
They contain much more gas than dust.

Which of the following describes the shape of dust particles, based on polarization of light?

The spectra of interstellar gas clouds show that they have the same basic composition as

Emission nebulae like M42 occur only near stars that emit large amounts of
ultraviolet radiation.

What two things are needed to create an emission nebulae?
hot stars and interstellar gas, particularly hydrogen

The average temperature of the typical dark dust cloud is about
100 K.

The Local Bubble was probably created by:
a nearby supernova perhaps 300,000 years ago, brighter than the Full Moon.

Complex molecules in the interstellar medium are found
primarily in the dense dust clouds.

The relative density of dust to gas is least in which place?
the atmosphere of Earth

When an electron in H changes its spin from the same to the opposite direction as the proton, it
emits a radio wave photon.

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