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CHAPTER 13
Interstellar Matter and Star Formation
CHAPTER OUTLINE
13-1 The Interstellar Medium
1. A large amount of dust and gas exists in the space between stars. But space is so vast that the
density of this interstellar medium is very small.
Interstellar Dust
1. The observed dark areas in the sky are caused by giant clouds of interstellar dust that block
light from stars behind them.
2. In the 1930s, astronomers became aware that grains of dust exist throughout space. Interstellar cirrus clouds are faint, diffuse dust clouds found throughout interstellar space.
3. Cirrus emits infrared radiation because it is warmed slightly by light that it absorbs.
4. Interstellar clouds span huge volumes of space, from parsecs to tens of parsecs across.
5. Interstellar extinction is the effect by which starlight is blocked completely by interstellar material.
6. The light from distant stars is reddened by the dust through which it passes because dust grains
scatter blue light more than red. The reddening caused by scattering is not the same as the redshift
caused by the Doppler effect.
7. Interstellar dust grains are smaller than the wavelengths of visible light.
8. Spectral analysis indicates that interstellar dust contains silicate grains, carbon in the form of
graphite, and an important family of organic molecules (polycyclic aromatic hydocarbons); PAHs
are common to daily life (they form by incomplete burning of carbon-containing fuels) and provide us with a tool to understanding the abundances of chemicals related to life.
Interstellar Gas
1. Dust in space accounts for 10% of the total mass of the interstellar material. The remainder is
gas.
2. The gas between the stars reveals its presence in several ways.
(a) An emission nebula is an interstellar gas cloud that fluoresces due to ultraviolet light from a
star near or within the nebula. Fluorescence is the process of absorbing radiation of one frequency and reemitting it at a lower frequency. A cloud is called a nebula if it is dense or bright enough
to show up in a photograph.
(b) Interstellar gas causes absorption lines in stellar spectra. These lines can be distinguished from
absorption lines of a stellar atmosphere in three ways.
(i) Absorption lines due to interstellar gas tend to be narrower than those produced by a star’s atmosphere.
(ii) Lines caused by a stellar atmosphere will have a different Doppler shift than those caused by
the interstellar gas.
(iii) Interstellar gas will generally be much cooler than the gas of the stellar atmosphere.
(c) In 1951, Purcell and Ewen used a specially built radio telescope to detect the 21-cm radiation
emitted by interstellar hydrogen. Additional radio emission lines can be detected from other interstellar gases—water (H2O), carbon monoxide (CO), ammonia (NH3), and formaldehyde (H2CO).
Clouds and Nebulae
1. Interstellar clouds contain less than 1,000 molecules per cubic centimeter.
2. A reflection nebula is an interstellar dust cloud that is visible due to reflected light from a nearby star.
3. Dust clouds can contain up to 1 million molecules per cm3, though this is still 20 trillion times
less dense than Earth’s atmosphere at sea level.
4. A dark nebula is a cloud of interstellar dust that blocks light from stars on the other side of it.
5. Within a radius of a thousand parsecs from Earth, astronomers estimate that interstellar matter
contributes about 15%–30% of the total mass of dust, gas, and stars.
13-2 A Brief Woodland Visit
1. Like observing the stages of tree development within a forest, astronomers can learn about the
life cycle of stars by observing tremendous numbers of stars in various stages of development.
13-3 Star Birth
1. Theories about star birth began with Russell (and the H-R diagram) early in the 20th century.
Russell thought that stars are born as red giants, become O- and B-type main sequence stars and
then move down the main sequence, gradually dimming as they live out their lives. We now
know that stars live most of their on the main sequence and the red-giant stage is near the end of
their lives.
The Collapse of Interstellar Clouds
1. Stars are born in the cold (20K), giant molecular clouds (GMCs) found in the Galaxy. Astronomers estimate that our Galaxy contains 5,000 GMCs.
2. The average density of a GMC is about 200 molecules/cm3; a typical cloud may be 50 pc
across and contain as much as a million solar masses of material.
3. Strong UV radiation may evaporate GMCs, but particularly dense regions called Evaporating
Gaseuous Globules (EGGs) are left behind. EGGs, sometimes become stars, as their material
sometimes collapses due to gravity.
4. The mechanism that begins the collapse of part of a GMC is not well understood. It could be
that collisions between GMCs or shock waves could provide the trigger for the collapse of parts
of a GMC to form globules.
5. There are at least 4 possible sources of interstellar shock waves: radiation from hot, newly
forming stars, bursts of stellar winds, supernova explosions, or the shock waves forming the Galaxy’s arms.
Protostars
1. A protostar is an object in the process of becoming a star before it reaches the main sequence.
2. A protostar’s energy source is gravitational—it comes from the infall of material.
3. A cocoon nebula is the dust and gas that surrounds a protostar and blocks much of its radiation.
4. Evidence for protostars is obtained from the infrared radiation emitted from the cocoon.
Evolution toward the Main Sequence
1. A star’s evolutionary track is the path it follows on the H-R diagram as its luminosity and
temperature change.
2. As the protostar shrinks, it gets hotter, emits more radiation, and gradually blows away the outer portions of its cocoon.
3. T Tauri stars are a certain class of young stars that show rapid and erratic changes in brightness in the form of enormous flares; these flares are thought to play a part in blowing away the
cocoon of newly forming stars, particularly K- and M-type stars.
4. M-class stars may remain protostars for hundreds of millions of year. G stars (like the Sun)
spend about 30 million years in the protostar phase. Massive O- and B-type stars remain protostars for only tens of thousands of years before joining the main sequence.
5. O and B stars may undergo a period of instability that is more violent than the one for stars of
low mass; during this period, they blow off material at supersonic speeds.
6. Astronomers calculate that a star with a mass greater than 100 solar masses will emit radiation
so intense that it will prevent more material from falling into the star, thereby limiting the star’s
size.
7. Protostars with masses less than 0.08 solar masses do not develop the necessary internal pressure and temperature to start hydrogen fusion.
8. Recent infrared observations reveal that it is very common for protostars to be surrounded by
disks of gas and dust; this fits well with the theory of the formation of our solar system.
9. A protostar’s mass may increase as a result of accretion (that is, particles in the surrounding
disk lose energy and spiral closer to the protostar as a result of collisions within the disk). A protostar’s mass may decrease due to the ejection of mass in the form of jets (outflows) perpendicular to the disk.
10. Outflows of material from a protostar could help reduce its angular momentum and thus its
rotational speed.
11. Whether or not a protostar will have an accretion disk and the duration of the different phases
that it goes through as it evolves depend a lot on the protostar’s environment.
Star Clusters
1. A galactic (open) cluster is a group of stars that share a common origin and are located relatively close to one another. Such clusters are found primarily in the disk of the Galaxy.
2. A globular cluster is a spherical group of hundreds of thousands of stars; they are found primarily in the halo of the Galaxy.
3. Clusters are important for two reasons:
(a) All stars in a cluster are at about the same distance from us, so their apparent magnitude is a
direct indication of their absolute magnitude.
(b) All the stars within a cluster formed at about the same time. Thus they formed from the same
GMC and have about the same chemical composition.
4. Much of our knowledge of star formation has come from examination of clusters. H-R diagrams of clusters reveal that low-mass stars spend more time in the protostar stage than more
massive stars. H-R diagrams of older galactic clusters reveal when stars end their main-sequence
part of their lives.