Lecture 13 - The Interstellar Medium (2/25/99)

Stellar Structure II --- | --- Star Formation

Reading:
Chapter 11-4, 15-1, 15-2 (ZG4)

Notes:
pages 51 - 54

	Key Question:	What are the dark nebulae made of?
	Key Principle:	The 21-cm HI line
	Key Problem:	Mass-Luminosity Relation

Investigations:

1. Interstellar Dust
   • What does interstellar dust do to starlight?
   • Why is reflected light blue, and transmitted light reddened?
   • How is dust extinction related to optical depth?
   • What is the typical size of dust grains?
   • What does A_v = 1 mag of extinction imply for optical depth?
   • What is the typical visual extinction in mag per kpc in the galactic plane?
   • What is the color excess in B-V corresponding to 1 mag A_v?
   • What is the reddening vector in the H-R diagram?
   • What effect does extinction have on Cepheid distances?
   • What effect does extinction have on star counts?
   • What sort of chemical reactions can take place on dust grains?
   • What is the interstellar medium (ISM) and what is it composed of?
2. H II Regions
   • Why do hot O and B stars ionize the gas around them?
   • What is an H II region?
   • What wavelengths are relevant for ionizing photons to make an H II region? (For O9 star, n_ion ~ 10^49/s)
   • How does equilibrium between ionization and recombination determine the size of an H II region?
   • What is a Stromgren sphere?
   • Why do we need only consider recombination to levels n > 1 above the ground state? (a2 ~ 10^-19 m^3/s)
   • What is the typical size for an H II region at T = 8000 K with n_H ~ 10^3/m^3 and n_e ~ 10^9/m^3
3. Atomic and Molecular Lines in the ISM
   • Using the Boltzmann equation, what energy level do you expect the atoms to be in for a cold cloud with T < 100 K? Do you expect strong Balmer lines, for example?
   • What is a hyperfine transition?
   • Why are there two spin states for neutral hydrogen?
   • What is the energy difference corresponding to the 21-cm line at 1.420406 GHz?
   • How does H I emission in the 21-cm line allow us to map the gas in the galaxy?
   • What are the vibrational and rotational transitions in molecules?
   • How do molecular lines allow us to probe the compositions of cold molecular clouds?
   • How many species of molecules have been discovered in the ISM?
   • What is the most complex molecule seen?
   • How do the rotational level populations for a transition at frequency v (eg. CO 1-0 at v=118 GHz) follow the Boltzmann distribution?
   • Why is CO an important tracer in the ISM?

The Interstellar Medium in Outline

1. Gravitational Collapse
   • Gravity is always attracting, there is no "anti-gravity" repulsion.
   • What do you think happens if you start with a bunch of matter (of any type) distributed far across space, and let gravity do its work?
   • Unless you make the matter distribution exactly uniform and infinite then it will always collapse eventually, unless something stops it.
   • What can stop gravitational collapse?
   • The matter can hit something first, namely the rest of the matter that is falling in also. These collisions cause heat and pressure which resists the collapse. This is called gas pressure.
   • If the collapsing cloud of matter becomes hot enough, and starts emitting photons like stars do, then the light pressure can stop the collapse. This is called radiation pressure.
   • Some other sort of pressure, like that from magnetic fields, can stop the collapse. This is called magnetic pressure.
   • We know our solar system is stable and not collapsing, but the planets are orbiting. They possess momentum, specifically, angular momentum that keeps it spinning instead of collapsing.
   • It turns out that stars (and gas giant planets like Jupiter and Saturn) use gas and radiation pressure to support themselves against gravity. This is what defines the size of a star.
   • It also turns out that in the early stages of star formation, when large gas clouds have to collapse by many orders of magnitude in size to form stars, that magnetic pressure (and to some extent gas pressure) and angular momentum must be overcome.
   • We will discuss the star formation process in more detail in the next lecture.
   • For now, where is the stuff from which stars are made?
2. A View of the Interstellar Medium
   • Early in the history of telescopic astronomy, it was noticed that in addition to lots and lots of stars, there appeared to be fuzzy patches, or clouds, in the celestial sky.
   • These were called nebulae, or nebula in the singular, which is Latin for "cloud".
   • Catalogs for nebulae were made so that people would not confuse them with fuzzy comets, which they were more interested in.
   • What are the nebulae? What different kinds do we see? This is the usual first step in observational science - classify the objects that we find!
   • Diffuse Nebulae - amorphous bright fuzzy blobs, sometimes with one or more bright stars in the center. These turn out to be gas clouds ionized and lit up by very bright young stars.
   • Planetary Nebulae - shells or rings of bright nebulosity surrounding a more or less hollow center. These turn out to be shells of matter thrown off of a dying star.
   • Supernova Remnant - veil like wisps, filamentary rings, or a filled shell with lots of tendrils (often similar in appearance to diffuse nebulae). These turn out to be the result of titanic stellar explosions (supernovae) marking the death of a massive star.
   • Dark Nebulae - dark clouds that block out the stars behind them, and redden those few stars that manage to be seen through them. Often look like "holes" or "gaps" in the Milky Way. These are relatively dense clouds of gas containing dust grains that absorb visible light. Some times dark nebulae are seen in conjunction with bright diffuse nebulae, which seem to live on the edges of dark clouds.
   • Diffuse nebulae can shine by their own light if they are hot, then being called emission nebulae. Lines of hydrogen, carbon, oxygen, calcium, and other atoms can be seen their spectrum. Most bright diffuse nebulae are emission nebulae.
   • Some diffuse nebulae shine by reflected starlight, and are thus called reflection nebulae. Faint blue wisps around the stars of the Pleiades, a star cluster in the constellation Taurus visible to the naked eye are a refection nebulae caused by the bright stars.
   • The nebulae that we are interested in for star formation are the dark nebulae, which contain large amounts of cold gas primed for gravitational collapse and the birthing of stars!
3. What is the Interstellar Medium Made Of?
   • We call all this gas and nebulosity the interstellar medium, the stuff "between the stars", like interstate means between states. We abbreviate interstellar medium as ISM.
   • From spectra of the nebulae, we know they are made of gas, just like the photospheres of stars like our Sun.
   • The gas in the emission nebulae often have compositions very similar to the Sun's: mostly hydrogen and helium, with traces of other elements like carbon, nitrogen, oxygen, and iron. In fact, it is as if they came from stars (...hmmmmm...).
   • There is also dust in the ISM, small grains about 1 micron (10^-6 m) in size. About 1% of the ISM is in dust grains.
   • Scattering of light off of dust grains is what reddens the light of stars seen through dust clouds. The short wavelength blue photons are more easily scattered than the longer wavelength red photons. This same effect reddens the Sun at sunset - and after dust storms or volcano eruptions or big fires sunsets are spectacularly red.
   • The dust is made of solids, like ice, carbon compounds, silicates, and iron.
   • Dust is formed in low temperature regions, below 100K or so, since high temperatures will cause collisions and sputtering of the grains thus destroying them.
   • The surfaces of dust grains can act as a matrix to hold molecules close together and allow chemistry to occur. Some very large molecules have been detected in space, with more than ten atoms. These tend to be chains of hydrocarbons and other organic molecules, and there are claims of seeing some proteins, like those that make up DNA, in some molecular clouds! These complex molecules are believed to be made on the surfaces of grains. In the past decade, the field of cosmochemistry has grown to study the formation of these molecules.
   • The densities in the molecular clouds are as high as 10^5 atoms/cm^3. This is still very tenouous, these dark clouds are still better vacuums than anything we can make here on Earth!
   • Molecular clouds range in masses from 100 up to 10^8 solar masses! You can make lots of stars out of that much stuff.
4. Atomic Hydrogen Clouds
   • Neutral hydrogen atoms (ionization state H I) make up about 22% of the interstellar medium.
   • These clouds range in density and temperature from 3000K at 0.3 atoms/cm^3 for warm clouds to 100K at 50 atoms/cm^3 for cold clouds.
   • The primary way of identifying clouds of H I is through the 21 cm line.
   • The hydrogen line at a wavelength of 21cm in the radio spectrum is caused by a slight energy difference between states of the hydrogen atom where the proton and electron spins are aligned or opposite.
   • If you remember our discussion of the hydrogen energy levels and quantum numbers, then you will remember that particles like electrons and protons have a property called spin (quantum number s = +/- 1/2) which is analogous to picturing the particle as a little spinning top.
   • Because the proton and electron are electrically charged, the spin means that they act as little magnets. Because they are charged oppositely, their magnetic poles are aligned oppositely with respect to the spin in the electron versus the proton (if their spins are aligned, then their magnetic poles are opposite).
   • If you've played with magnets, then you know that they like to be oriented oppositely - the opposite poles attract and the like poles repel.
   • Thus, if the spins of the proton and electron in the hydrogen atom are aligned (poles opposite), then there is a tiny extra attraction which is like decreasing the energy of the ground state level (making the electron slightly more bound).
   • If the spins of the proton and elecron are opposing (poles aligned) there is a tiny repulsion, which is like increasing the energy of the ground state level.
   • The energy difference between these two configurations of the ground state is tiny: the photon wavelength to excite this transition is 21cm, compared to 121.6 nm for the transition from the ground state to the next higher electron level. Q: How many times smaller is this energy difference? What is this difference in electron volts (eV)?
   • If a photon of wavelength 21cm is absorbed or emitted, the spins flip relative to each other.
   • The emission (and absorption) of the 21 cm line by clouds of hydrogen can be observed with large radio telescopes. The line was predicted by the Dutch astrophysicist van der Hulst in 1945, and first observed by Ewen and Purcell of Harvard in 1951.
   • The 21 cm emission has been used to map out the H I clouds in our galaxy, and to detect H I in other galaxies.
5. Molecular Clouds
   • If a cloud is cold enough, then the atoms have the chance to come together and form molecules. This is able to happen in clouds with temperatures of roughly 100K or less.
   • These clouds tend to be buried deep within atomic H I clouds.
   • These are the coldest (10K - 100K) and densest (10^4 - 10^5 atoms/cm^3) clouds.
   • These clouds are made up mostly of molecular hydrogen (H2), which is two atoms of hydrogen bound together. The dissociation energy for H2, the energy needed to break apart the molecule, is 4.48 eV.
   • Carbon monoxide CO is also seen in clouds. CO has a dissociation energy of 9.6 eV, and it thus hardier than H2.
   • There are many other diatomic (two-atom) molecules found in molecular clouds, such as C2, CO, CN, CH, CS, SO, SiO, OH, etc.
   • More complex molecules like H2O (water!), HCN (cyanide!), H2S, as well as alcohols and molecules with more than 13 atoms have also been seen in these clouds! As of the end of 1993, over 93 different molecules have been seen in the ISM. That number has increased in the years since then.
   • The presence of these molecules has been identified by observing molecular lines, which like the atomic lines like the Balmer series of hydrogen, correspond to energy levels in the molecule.
   • The molecular energy levels correspond not to electron orbits like in atoms, but to vibrations and rotations of the molecule.
   • Vibrational lines are the most energetic, with typical wavelengths of 0.1 mm or shorter. For H2, the series limit is 4.48 eV (277 nm), with the "alpha" transition at 2275 nm, or 2.275 microns, in the far-infrared. The CO "alpha" vibrational transition is at 4.6 microns.
   • Rotational Lines are lower in energy, and thus come out in in the millimeter wave range. It turns out that to get a rotational transition, you need two (or more) atoms of different mass in the molecule - H2 has no rotational transitions, because it is symmetric! The "alpha" rotational transition for CO is at 2.7 mm, this is one of the primary lines used to map out molecular clouds.
   • Because the molecular clouds are the coldest and densest of the gas clouds, they are the places where stars are formed. They are in many ways "stellar nurseries".
6. The Orion Molecular Cloud
   • One of the most spectacular and nearest large molecular clouds is the Orion Molecular Cloud, which covers much of the constellation of Orion, and contains the bright diffuse Orion Nebula.
   • The cloud is at a distance of 460 pc from us, and extends over 30pc in size. The total mass of molecular gas in the cloud is around 2 x 10^5 times the mass of the Sun.
   • The oldest stars formed out of the cloud are near the shoulder of Orion, and are around 12 million years old. The youngest stars that we see are in the center of the Orion Nebula itself, and are about 2 million years old. The sun is about 4.5 billion years old in comparison.
   • Star formation in Orion uses less than 25% of the gas in any given region, there rest is blown away.
   • Because there seems to be a pattern to the ages, with the oldest stars farthest away from the nebula, we believe that there is progressive star formation where the formation of the first stars triggers the next stars to form. Thus, star formation sweeps through the cloud like a forest fire!
   • Note that the Sun itself was not formed with the first stars, its formation was triggered later on! In fact, basically all of the elements heavier than helium were formed in stars, then blown out in supernovae. Our bodies are made up of stuff that was cooked in long dead stars!

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smyers@nrao.edu Steven T. Myers