Interstellar Matter and Star Formation

Chapter 13

Courtesy of W

olfgang Brandner (JP

L/IPA

C), E

va K. G

rebel (Univ. W

ashington), You-

Hua C

hu (Univ. of Illinois, U

rbana-Cham

paign), and NA

SA

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.

Courtesy of E

SO

Figure 13.01b: The black cloud

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 and carbon in the form of graphite.

– and an important family of organic molecules (polycyclic aromatic hydrocarbons).

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.

Figure 13.05a: A composite image of the N44's central region taken at the H-alpha, singly-ionized, and doubly-ionized oxygen lines.

Courtesy of ESO

Figure 13.05c: The Tarantula nebula is the largest emission nebula in the sky.

Courtesy of ESO

(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.

Figure 13.07: The reflection nebula surrounding the Pleiades.

Courtesy of N

AS

A, E

SA

and AU

RA

/Caltech

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.

Courtesy of E

SA

& the IS

OG

AL team

Figure 13.10d: An infrared image of the Eagle nebula sees through most of the opaque areas

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.

Figure 13.11: Pillars are the result of strong ultraviolet radiation striking a giant molecular cloud

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,

– 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.

Figure 13.13: H-R diagram of protostars

 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 don’t develop the necessary internal pressure and temperature to start hydrogen fusion.

8. Recent infrared observations reveal that it’s 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.

Figure 13.15: Jets emanating from three different protostars

Courtesy of J. Morse,AURA/STScI, NASA

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.

Figure 13.16: Mass balance of jets vs disk

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.

Figure 13.20a: The Quintuplet cluster—an open cluster

Courtesy of Don Figer/STScI/NASA

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.

Figure 13.23: An H-R diagram of a very young

cluster