Chapter 11 The Interstellar Medium

Units of Chapter 11

Interstellar Matter

Star-Forming Regions

Dark Dust Clouds

The Formation of Stars Like the Sun

Stars of Other Masses

Star Clusters

Summary of Chapter 11

11.1 Interstellar MatterThe interstellar medium consists of gas and dust.

Gas is atoms and small molecules, mostly hydrogen and helium.

Dust is more like soot or smoke; larger clumps of particles.

Dust absorbs light, and reddens light that gets through. This image shows distinct reddening of stars near the edge of the dust cloud.

Dust clouds absorb blue light preferentially; spectral lines do not shift.

11.2 Star-Forming Regions

This is the central section of the Milky Way Galaxy, showing several nebulae, areas of star formation.

These nebulae are very large and have very low density; their size means that their masses are large despite the low density.

“Nebula” is a general term used for fuzzy objects in the sky.

Dark nebula: dust cloud

Emission nebula: glows, due to hot stars

Emission nebulae generally glow red – this is the Hα line of hydrogen.

The dust lanes visible in the previous image are part of the nebula, and are not due to intervening clouds.

How nebulae work

There is a strong interaction between the nebula and the stars within it; the fuzzy areas near the pillars are due to photo-evaporation.

Emission nebulae are made of hot, thin gas, which exhibits distinct emission lines.

11.3 Dark Dust Clouds

Average temperature of dark dust clouds is a few tens of kelvins.

These clouds absorb visible light (left), and emit radio wavelengths (right).

This cloud is very dark, and can be seen only by its obscuration of the background stars. This image is the same cloud, but in the infrared.

The Horsehead Nebula is a particularly distinctive dark dust cloud.

Interstellar gas emits low-energy radiation, due to a transition in the hydrogen atom.

This is a contour map of H2CO near the M20 Nebula. Other molecules that can be useful for mapping out these clouds are carbon dioxide and water.

Here, the red and green lines correspond to different rotational transitions.

11.3 Dark Dust Clouds

These are carbon monoxide-emitting clouds in the outer Milky Way, probably corresponding to regions of star formation.

21-cm radiation has yielded important information about

A. the density of helium in the universe.

B. the physical structure of our galaxy.

C. the prevalence of water in the universe

D. .the spin-flip propensities of methyl alcohol (ch2oh) .

Dark clouds are best studied through examination of

A. interstellar absorption lines in the spectra of distant stars.

B. Balmer emission lines.C. radio waves emitted by

molecules.D. ultraviolet radiation emitted by

the gas.

As an object contracts, its rate of rotation

A. stays the same.B. slows down.C. speeds up.D. may or may not change.

We cannot see the nucleus of our galaxy because

A. over 32,000 light years, the photons are too diffuse for us to receive a coherent picture.

B. it has been consumed by a gigantic black hole.

C. it is obscured by clouds of dust and gas.

D. it spins too fast.

Interstellar 21-cm radiation is emitted by

A. water.B. methyl alcohol.C. helium.D. hydrogen.

What effect does interstellar dust have on the magnitudes and colors of a star?

A. Dims the star only.B. Makes the star appear redder

only.C. Makes the star dimmer and

redder.D. No change.

How can interstellar dust be detected?

A. Dark regions of fewer stars in the milky way.

B. Stars that look redder than their spectral type.

C. Bluish nebulas around hot stars.

D. All of the above.E. None of the above.

Star formation happens when part of a dust cloud begins to contract under its own gravitational force; as it collapses, the center becomes hotter and hotter until nuclear fusion begins in the core.

11.4 The Formation of Stars Like the Sun

Stars go through a number of stages in the process of forming from an interstellar cloud.

Stage 1:

Interstellar cloud starts to contract, probably triggered by shock or pressure wave from nearby star. As it contracts, the cloud fragments into smaller pieces.

When looking at just a few atoms, the gravitational force is nowhere near strong enough to overcome the random thermal motion.

Stage 2:

Individual cloud fragments begin to collapse. Once the density is high enough, there is no further fragmentation.

Stage 3:

The interior of the fragment has begun heating, and is about 10,000 K.

The Orion Nebula is thought to contain interstellar clouds in the process of condensing, as well as protostars.

Stage 4:

The core of the cloud is now a protostar, and makes its first appearance on the H–R diagram.

Planetary formation has begun, but the protostar is still not in equilibrium – all heating comes from gravitational collapse.

The last stages can be followed on the H–R diagram:

The protostar’s luminosity decreases even as its temperature rises because it is becoming more compact.

At stage 6, the core reaches 10 million K, and nuclear fusion begins. The protostar has become a star.

The star continues to contract and increase in temperature, until it is in equilibrium. This is stage 7: the star has reached the main sequence and will remain there as long as it has hydrogen to fuse in its core.

These jets are being emitted as material condenses onto a protostar.

These protostars are in Orion.

11.5 Stars of Other MassesThis H–R diagram shows the evolution of stars somewhat more and somewhat less massive than the Sun. The shape of the paths is similar, but they wind up in different places on the main sequence.

If the mass of the original nebular fragment is too small, nuclear fusion will never begin. These “failed stars” are called brown dwarfs.

11.6 Star ClustersBecause a single interstellar cloud can produce many stars of the same age and composition, star clusters are an excellent way to study the effect of mass on stellar evolution.

This is a young star cluster called the Pleiades. The H–R diagram of its stars is on the right. This is an example of an open cluster.

This is a globular cluster – note the absence of massive main-sequence stars, and the heavily populated red giant region.

These images are believed to show a star cluster in the process of formation within the Orion Nebula.

The presence of massive, short-lived O and B stars can profoundly affect their star cluster, as they can blow away dust and gas before it has time to collapse.

This is a simulation of such a cluster.

When a star first appears on the H-R diagram, it moves

A. up.B. down.C. to the right.D. to the left.

A forming star is first detectable as

A. a new star in an ordinary field of stars.

B. a bright region in an otherwise dark cloud.

C. an infrared emitting region in an interstellar cloud.

D. a contracting cloud of gas.

A star reaches the main sequence when

A. it starts to collapse.B. it is a proto star.C. nuclear reactions start.D. it starts to shine.

When a star first appears on the H-R diagram it is

A. cool and faint.B. cool and bright.C. hot and faint.D. on the main sequence.

As a star is forming by the condensing of gases, the gases

A. cool as they fall.B. heat up as they fall.C. stay about the same

temperature.D. any of the above, depending

upon the mass involved.

As a new star evolves from cool dust and gas to a hot star, the peak wavelength of its spectrum of electromagnetic radiation will

A. increase from visible to infrared wavelengths

B. .remain the same.C. change from the infrared to the

visible wavelengths.D. change from the ultraviolet to the

visible range.

Summary of Chapter 11

• Interstellar medium is made of gas and dust.

• Emission nebulae are hot, glowing gas associated with the formation of large stars.

• Dark dust clouds, especially molecular clouds, are very cold. They may seed the beginnings of star formation.

• Dark clouds can be studied using the 21-cm emission line of molecular hydrogen.

• Star formation begins with fragmenting, collapsing cloud of dust and gas.

Summary of Chapter 11, cont.

• The cloud fragment collapses due to its own gravity, and its temperature and luminosity increase. When the core is sufficiently hot, fusion begins.

• Collapsing cloud fragments and protostars have been observed.

• Mass determines where a star falls on the main sequence.

• One cloud typically forms many stars, as a star cluster.