Download Chapter 13: Interstellar Matter and Star Formation

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 nearly a
perfect vacuum because it 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.
Dust grains make up only 1% of the mass of the interstellar medium.
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. Gas accounts for 99% of the total mass of the interstellar material.
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 cirrus 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. Astronomers estimate that interstellar matter contributes about 15% of the total visible
mass (dust, gas, and stars) of our Galaxy.
6. Observations suggest that large quantities of many kinds of prebiotic molecules are
present in the interstellar medium.
Advancing the Model: Holes in the Heavens
1. Herschel, the discoverer of Uranus, proposed that the dark patches we see in the sky
are simply large spaces between the stars that allow us to see into the dark void beyond.
2. Stars are not all at the same distance from us. For us to be able to see through gaps between the stars, the gaps thus would have to be similar to tunnels, and the tunnels would
have to be perfectly aligned with Earth. This is extremely unlikely.
3. However, one such truly empty region was discovered by the Herschel Space Observatory around a group of very young stars. The jets or powerful radiation from the young
stars may have cleared the region of dust and gas.
Advancing the Model: Blue Skies and Red Sunsets
1. The same phenomenon that explains the reddening of starlight by interstellar dust also
explains why the sky is blue and sunsets are red.
2. As in interstellar dust clouds, molecules in the Earth’s atmosphere scatter higher frequency (blue) light more than lower frequency (red) light, giving the sky a blue color.
3. Light that reaches us directly from the Sun without scattering is somewhat deficient in
these higher frequencies, and appears redder than it appears when viewed from a vantage
point in space. This reddening of the Sun is especially noticeable near sunrise or sunset,
or when the atmosphere contains significant amounts of dust.
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.
2. We now know that stars live most of their on the main sequence with very little change
in position 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 150 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.