SITES of STAR FORMATION

STARS FORM NEAR THE ARMS OF OUR GALAXY

How do we know this to be true?

(1) We must first argue how we can determine whether spiral arms and star formation are somehow connected. How will we do this? (2)We must then demonstrate how we can show where star formation occurs in our Galaxy (which means that we must know how to recognize regions where star formation is ongoing).
We address the first issue now. The spiral arms in a disk galaxy (as seen in the Whirlpool Galaxy, M51) are, in many instances, compression waves which move through the disk of the galaxy. Many different mechanisms have been proposed as mechanisms for the formation of spiral arms; the panel on the right shows one plausaible model where an interaction with a nearby galaxy excites spiral arms in the galaxy (which may be the mechanism which formed the spiral arms in M51). Spiral arms are wave-like in nature and thus not solid structures; they move with respect to the underlying gas and dust in the disk of the Galaxy (say, like a sound wave moves through the air in this room). In our Galaxy near the location of the Sun, spiral arms move through the disk at around 20 kilometers per second.
So, in 1 million years, a spiral arm moves around 65 light years with respect to the disk material. This is nothing compared to the size of our Galaxy; the stellar disk of our Galaxy is around 100,000 light years in diameter.
We know that massive stars have short lifetimes--for example, a 20 Solar mass star lives for only a few million years. Thus, such massive stars indicate recent star formation and thus mark star formation regions. If we see O and B stars distributed uniformly throughout the disk of our Galaxy this would mean that star formation occurs everywhere in our Galaxy. Is this true?
It is observed that massive stars cluster around the spiral arms of our Galaxy ==> star formation is associated with the spiral arms of our Galaxy.
How did we infer that hot, massive stars (O & B stars) cluster around spiral arms? Well, in some instances, we see clusters of stars known as O & B Associations. O & B associations are young stellar clusters (a few million years or so in age) composed of 10-100 stars with diameters on the order of a few hundred light years. There are around 70 OB associations in our Galaxy. In addition, we identify signatures of star formation and then check to see if star formation regions cluster around the spiral arms of our Galaxy.

What are the Signatures of Star Formation?

Star Formation Occurs in objects known as Giant Molecular Clouds (GMC).
Giant Molecular Clouds (GMCs) are large gas clouds composed primarily of H₂, molecular hydrogen gas. They have huge masses, 10,000 to 10,000,000 Solar masses and large sizes, tens of light years in diameter. GMCs are fairly dense 100 to several million molecules atoms per cubic centimeter. A cubic centimeter is roughly the volume of a sugar cube. For comparison, the density of air in this room is roughly 10¹⁸-10¹⁹ molecules per cubic centimeter, and the average density of particles in our Galaxy (outisde of these dense clouds) is around 1 atom per cubic centimeter. GMCs are thus much more rarefied than the air in this room, but they are much denser than the average region in the Mikly Way Galaxy. GMCs are also very cold T ~ 10-20 Kelvin. GMCs also contain dust. Although there are only tiny amounts of dust, the dust is an efficient absorber and scatterer of visible light and so prevents us from seeing star forming regions inside of GMCs and it is difficult to study GMCs in the optical; the dust does not affect IR and microwaves and so they offer the best views of the star formation process. The most nearby example of a GMC is in the constellation of Orion, the Orion Molecular Cloud.

The primary markers of star formation are H II regions (gas clouds of ionized hydrogen), O & B star associations and reflection nebulae, and the Giant Molecular Clouds (GMCs) themselves. (Also found near spiral arms are H I clouds, neutral hydrogen gas clouds). To get a handle on how these clouds fit together, I next describe the material which resides between the stars of our Galaxy, the material known as the INTERSTELLAR MEDIUM, the ISM.

The Interstellar Medium (ISM)

The Milky Way galaxy has a total mass > 100 billion Solar masses. The material in-between the stars (the Interstellar Medium) contains ten or so billion Solar masses -- the gas and dust are thus about 10-15 % of the visible mass of our Galaxy. This gas and dust are, however, quite important as they are the material out of which stars form.

Make-Up of the ISM

The ISM material is primarily gas:

it is roughly 90 % hydrogen and 10 % helium, with just a touch of everything else.

The dust is very rare. Dust particles are complexes of several billion atoms; they are usually composed of carbon (graphite) and silicates. This means that dust makes a very small contribution to the mass of the ISM (< 1 %) and that it makes an even smaller contribution to the number of particles in the ISM.

On average, there are 10⁶ hydrogens per cubic meter (1 hydrogen per cubic centimeter) in the ISM. (The air in this room has roughly 10¹⁸-10¹⁹ molecules per cubic centimeter.) There is fewer than a single dust particle per cubic football field in the ISM!!! Dust is exceedingly rare.

Despite its rarity, dust is the primary agent which attenuates visible light. Dust is the reason we cannot see into the interiors of the ISM clouds (in visible light). Because of this, we must study the star formation process in other parts of the EM spectrum. The most fruitful observations have been made in the infrared (IR) and radio. (Further, dust is also the reason we cannot see more than a few thousand or so light years of the Milky Way galaxy in visible light [the visible disk of the Milky Way galaxy has a diameter of around 100,000 light years] and why there are dark patchs in glowing gas clouds ( horsehead nebula, N81 in LMC.)
Why is dust such an efficient blocker of starlight (compared to the vastly more abundant gas-- recall)?

Structure of the ISM

The gas and dust are not spread uniformly throughout the Milky Way. They are mostly confined to the disk of the Galaxy. Furthermore, they are not spread uniformly throughout the disk. The ISM is lumpy. We have:

Coronal gas -- dilute gas at temperatures of around 3x10⁵ K, which fills roughly 70 % of the volume of the disk. The coronal gas has very low density -- around 0.01 % to 1 % of the average ISM density -- and so does not make a significant contribution to the gas and dust mass of our Galaxy (~ 0.1 %). The coronal gas is heated by supernova shocks.

Neutral hydrogen Clouds (H I regions). The H I clouds have sizes of ~15 light years, masses of ~ 100 M(sun), T ~ 50 - 100 K, and densities of tens of millions of particles per cubic meter. They make up roughly 40 % of the mass in the gas and dust but only a couple of percent of the volume of the ISM. HI regions do not shine by starlight. They produce something known as 21-centimeter Radiation: radio emission from hydrogen which arises when the electron flips its spin. This is a critical technique for seeing the ISM which is dominated by neutral hydrogen, HI.

Giant Molecular Clouds (GMC) -- It is well-established that GMCs are the clouds in which star formation occurs. (Giant Molecular Clouds: Orion, Orion nebula, Orion mosaic, , Video 1, You tube Flythrough).

GMC's are actually complexes of smaller clumps of material:

small dense clumps of material composed mainly of hydrogen molecules (H₂) with:

T ~ 10-100 K
densities of 100 million to biilions of particles per cubic meter
sizes of 10s of light years
masses of 10⁴ to 10⁷ M(sun) -- 10⁵M(sun) is typical
Although striking in appearance, GMCs fill only several hundredths of 1 percent of the volume of the ISM. Where the interstellar medium is cold and dense, molecules form (hence the name GMC). Molecules play an important not only in the structure of the ISM but also for the study of the ISM.
Molecules may emit radio signals, the wavelength of which indicates from which molecule it comes. For example,
Hydroxyl OH
Water H₂O
Ammonia NH₃
Formaldehyde H₂CO
spectra indicate formaldehyde molecules exist around M20
contour map of amount of formaldehyde around M20

Carbon monoxide CO - useful for spiral arm study
Molecular hydrogen H₂
Hydrogen isoCyanide HCN - see cloud around center of Milky Way

Hydrogen is the overwhelmingly dominant component, but it is harder to observe, and the other molecules are the observed tracers of the accumulation
These radio signals, in addition to the IR that arises from the dust, serve as sensitive probes of star forming regions in GMCs.

Ionized Hydrogen Clouds (HII regions). (Eagle nebula, Eagle nebula HST). H II region mark sites of massive star formation. Massive stars (O & B stars) are very hot and so produce large amounts of UV radiation. UV radiation is capable of ionizing hydrogen gas. Lower mass stars are not hot enough to produce large amounts of UV and so do not form large H II regions. Associated with spiral arms and GMCs. These are the red, glowing pictures that you see in the text. They glow red because they radiate large amounts of hydrogen balmer alpha lines (which fall in the red portion of the spectrum) which forms as the ionized hydrogen gas (H II) recombines to form neutral atoms and radiates. H II regions have temperatures on the order or 8,000-10,000 Kelvin with densities of 0.1 to 10,000 particles per cubic centimeter. They fill much less than 1 % of the volume of the ISM.

Reflection Nebulae (e.g., Pleiades), form around massive, hot stars.
One sees the reflection of blue light by the dust particles in the cloud.

The dust scatters blue light toward us while the red light streams straight through the dusty region.

Dark clouds are regions of high dust concentration. The dust very effectively blocks out the stars which sit behind them making what appears to be a hole in the sky.

The Sun and the Local Bubble

Ultraviolet radiation has helped in the study of the ISM allowing a map of our local cosmic neighborhood to be formed. Ultraviolet radiation can only be studied by instruments placed in orbit above the Earth's atmosphere The International Ultraviolet Explorer (IUE) found a number of regions of interstellar space that are much thinner (5000 atoms/m³) and hotter (500,000 K) than expected The Sun resides in the so-called Local Bubble, a region

about 100 pc across, containing about 200,000 stars
thought to have been carved out by multiple supernova explosions several hunderd thousand years ago
Such supernovae would have been spectacular events in the sky