Gas in the ISM:
A Point-Form Summary.
This section of the course notes, and the associated PowerPoint presentation, makes the following critical points:
interstellar gas is very thinly distributed, and you might expect it to have little effect on the passing starlight. But its presence can be inferred from the appearance of 'extra' absorption lines in the spectra of some stars. These lines are typically very narrow, because the clouds of gas which give rise to the lines are very cool
we also see hot gas in emission, in large clouds which are fluorescing because they are being heated by the ultraviolet light of hot stars within them
the most abundant gas species in the ISM is hydrogen, and much of it is neutral (not ionized). During the war, when no observing could be done in occupied Holland, van de Hulst predicted a process by which neutral hydrogen might emit a detectable radiation that would penetrate the interstellar gas and dust, thereby allowing us to map out the structure of our galaxy
the mechanism predicted by ven de Hulst was a 'spin-flip' transition in which an electron spinning parallel to the spin of the proton 'flips over' to spin in the opposite sense, losing a little bit of energy in the process. His prediction was borne out after the war, although an American team made the first discovery
infrared detectors allow us to see protostars, object which have not yet turned into full-blown stars but which are heating up as they contract. The long-wavelength infrared light passes easily through the gas and dust, so these detectors allow us to peer into the clouds within which stars are forming
molecules of various kinds can emit radiation at millimeter wavelengths as they change their rotational and vibrational rates. But astronomers did not expect to find molecules in the ISM, because the bonds that hold molecules together are rather weak. In interstellar space, passing energetic (ultraviolet) photons would tear molecules to pieces
astronomers were surprised to discover millimeter-wave emissions that reveal that there are large amounts of some very complex molecules in the ISM. These can only survive deep in the heart of big clouds of gas and dust, where they are shielded from the dangerous radiation
it is now realised that our galaxy contains many thousands of Giant Molecular Clouds, cool dense clouds within which active star formation is going on
Associated Readings from the Text.Please look at: Chapter 17, pages 545-547. Chapter 19, pages 605-611.So Much for Dust: How About the Gas?So far we have considered only the dust, mostly because it has some quite dramatic effects. But we noted before that 99% of the ISM is in fact made up of the atoms of gas, both in moderately dense clumps called `clouds' and in a more tenuous distributed form between the clouds. The existence of hot clouds has been known for a long time. The Orion Nebula is quite conspicuous, for instance, and as soon as spectroscopy became a usable tool, astronomers were able to determine that this was indeed a cloud of hot glowing gas. But we begin our discussion with a look at the cool clouds.Gas in Cool Clouds: Absorption.If you had a cool cloud of gas in interstellar space, how would you hope to detect it? A century ago this would have been essentially impossible, since the gas is not hot enough to give off visible light. But the development of spectroscopy proved the existence of these clouds as follows: In the spectra of some stars, we see a fairly broad absorption line (a darkish region in the spectrum) which might, for example, be caused by calcium atoms in the atmosphere of the star. But right beside that broad feature we may also see a very narrow absorption line. It can be shown that these features are also caused by calcium atoms. But why are these lines so fantastically narrow? It may help to turn the question around. Let us ask why the absorption lines caused by atoms in the atmosphere of the star are so broad. Well, the principal reason is that the star's atmosphere is fairly hot, which means that the atoms within it are moving around rapidly. Thus, at any given instant, lots of the atoms in the atmosphere are moving towards you, while many others are moving away, and so on. In such circumstances, when the absorption of photons takes place, the range in speed of the atoms shows up as slightly different Doppler shifts. In other words, the absorption line, instead of being at a single well-defined wavelength, is somewhat broadened, over a modest range of wavelengths. Now what does this suggest? It tells you that the newly-identified very narrow absorption lines in the spectrum must be formed by a clump of very cool gas, at a temperature very much lower than that of the star's atmosphere. In fact, it is possible to determine that the gas must have a temperature of perhaps one hundred Kelvins (that is, 100 degrees above absolute zero). The conclusion is that this gas is not part of the star. At this temperature, it cannot be in the atmosphere of the Earth either, so the inescapable conclusion must be that there is a clump of cold gas between us and the star, out there in the ISM. We now know that such interstellar clouds are very common. Indeed, a single star may show absorption lines from more than one cloud. We can tell them apart because the clouds themselves are moving through space at different velocities, so the Doppler shift (the position of the whole pattern of lines) is slightly different from cloud to cloud.Gas in Hot Clouds: Emission.Look again at the picture of the Orion nebula on page 610. (This is an object you can see for yourself on any clear night in the wintertime.) What is going on within it? Spectroscopy again comes to our rescue: a spectrum shows that the light from this object consists almost entirely of emission lines. That is, this is a hot low-density gas, as you should remember from our discussions of Kirchhoff's second law in Physics 015. But the Orion Nebula isn't dense enough, or hot enough, to have nuclear reactions going on within it. What keeps the gas hot? After all, it is emitting a lot of radiation, which means it is losing energy continuously. Why doesn't the Orion nebula `go out' just as the fluorescent lamp in your ceiling quickly goes out when we turn it off? The answer is that there must be an energy source which replenishes the energy. Can we identify the energy supplier for the Orion Nebula? To make progress on this, let us reconsider first how an ordinary fluorescent lamp operates - the `neon lamps' you find in advertising signs, for instance. In simplified terms, these work in the following way: When you turn on the switch, you are completing an electric circuit which permits electrons to flow through the tube, which is filled with a low density gas of a particular sort: neon, in our example. The electrons collide with the atoms in the tube, and some of the energy of the collision goes into `bumping' the electrons in the atoms up into higher orbits. In just a fraction of a second, these electrons drop back down to lower orbits, giving off photons of particular wavelengths. If they get bumped up to orbit 4, they may drop straight back to level 1; they may instead drop to level 3, then to level 1; and so on. There are various ways they can drop back down, each jump producing a photon of a well-defined energy and wavelength. This produces the familiar emission-line spectrum. The important thing to note is this: through the collision process, the fluorescent tube converts the energy of motion (the kinetic energy ) of the electrons and to the radiant energy (i.e. the emitted photons ). This phenomenon is called fluorescence. The Orion nebula is also fluorescing, but the fundamental energy source is not a big electric current through the gas. Instead, the process works as follows: There are several massive `O' stars at the heart of the Orion nebula. These stars are very hot, and give off most of their light in the ultraviolet part of the spectrum. Since ultraviolet photons have enough energy to ionize hydrogen, many of th eneutral hydrogen atoms sitting in the nebula will absorb a photon and lose their electrons, which now move independently within the nebula, leaving behind the protons (the hydrogen nuclei). Here and there, the protons and electrons come together and recombine. That is, they join up again to form a complete atom, losing some energy in the process and emitting a photon. The atom may spit out a photon of exactly the same energy and wavelength as the one which ionised it first, but more often it does not. For instance, the electron may be recaptured into a moderately high orbit - say, the fifth, for example. From there, it quickly drops back down to the lowest orbit, just as in my example of the fluorescent lamp, emitting a series of photons as it does so until all of the original energy is accounted for. The hydrogen atom is not left in peace for long, however: it is quickly re-ionized by yet another ultraviolet photon, and the whole process is repeated. So this is indeed a fluorescence mechanism. What happens in the Orion Nebula is essentially that the energetic ultraviolet photons get `repackaged' into a number of less-energetic photons before they are able to leave the gas cloud. This means that all the light which is coming from the nebula was originally starlight. (The gas cloud would not glow if it did not have hot stars within it!) Although individual atoms go through episodes of being ionized and being neutral, a lot of the hydrogen in such a nebula is ionized at any given time. That is why such nebulae are called HII ("H-two") regions.New Instruments Yield New Insights.What I have told you so far about the ISM summarizes the state of observation up to about the middle the twentieth century. Post-war developments led to breakthroughs in three areas. If you remember that the interstellar medium is especially effective at blocking short-wavelength radiation (blue light, for instance), it may not surprise you to learn that the developments that mattered were in detectors which worked at radio, infrared and millimeter wavelengths - all much longer than the wavelengths of visible light, and thus able to penetrate the interstellar dust.Radio Telescopes: The 21-cm Radiation of Neutral Hydrogen.During the Second World War, while the Netherlands was occupied, Dutch astronomers could not carry out any active research. In 1944, Jan Oort (whose name you may remember from Physics 015, when we discussed the `Oort cloud' of comets) suggested to a young astronomer named van de Hulst that he should try to think of a way of observing what was probably the most important constituent of the Milky Way galaxy other than the stars: neutral hydrogen gas (HI, or "H-one"). After some considerable thought, Van de Hulst worked out that neutral hydrogen should produce a special kind of radiation by what is known as a `spin-flip transition.' To understand the origin of this emission, we return to an earlier analogy in which we think of the hydrogen atom as something like a solar system with one planet. (The proton can be thought of as something like the massive sun, with the electron orbiting it like a planet of much smaller mass.) Now, in the solar system there are other motions in addition to the orbital motions. In particular, bodies are also spinning (or rotating ). For instance, the sun is slowly spinning, once every twenty-five days; the Earth is spinning more quickly, once every twenty-four hours; and so on. Within the solar system, these spins are often (but not always) nearly parallel. The Earth, for instance, spins in the same sense as it orbits the Sun, and the sun itself spins in that same sense. If, however, you consider a planet like Uranus, which is tipped pretty much on its side, you can see that the spin can be oriented in almost any direction. Within the atom, the proton and electron also each possess a `spin.' But the spins are quantized, and in the hydrogen atom the spin axes of both the electron and proton must point up or down. (The analogy is not perfect, but this is a simple way to visualise the situation.) Thus the electron can spin in the same sense as the proton, or in the opposite sense, but not at some arbitrary orientation in between. If the atom started with the spins in parallel, they would become opposite if the electron were to `flip over.' Indeed, van de Hulst realised that this is exactly what should happen. An isolated hydrogen atom in which the two spins are aligned contains a little more energy that an atom in which they are opposite. Such an atom might spontaneously flip its electron over with the net loss of a little bit of energy. The lost energy could not simply vanish, of course. Instead, it would come off as a photon. This photon carries so little energy compared to that in a photon of visible light that it is far out in the radio part of the electromagnetic spectrum, at a wavelength of 21 cm. This is particularly helpful because photons of that wavelength will pass completely unhindered through the gas and dust in the ISM. As a consequence, we will be able to detect such photons from very remote clouds of gas, right on the other side of the Galaxy in fact. Indeed, as we will see later, we can use this kind of radiation to study the distribution of gas in the whole Galaxy: that is how we can `see' the spiral arms of the Milky Way. It is also useful in that there is lots of hydrogen gas out there, so there are many such photons. Sad to say, van de Hulst was `scooped' in the actual detection of the 21-cm radiation. An American group headed by Purcell knew of his prediction and, in a spirit of friendly competition, developed detectors to try and find it. They succeeded in 1952, shortly before van de Hulst's group did likewise. Incidentally, the `spin-flip transition' is a very improbable event in atomic physics. If you were to focus your attention on any particular hydrogen atom with the spins parallel, you would probably have to wait many millions of years before it made this jump and emitted the 21-cm photon. But there are so many hydrogen atoms in the ISM that at any given moment there are still lots of these photons being produced, and radio telescopes can pick up that radiation.Infrared Detectors: Star Formation.A subsequent breakthrough came with the development of infrared detectors. There are two important things to note: Since it is of longer wavelength than visible light, the infrared radiation,will pass more readily through the intervening dust clouds. Thus, like the 21-cm radiation, this will allow you to see objects on the other side of any obscuring clouds of gas and dust. But even more important is the fact that it is within the big cool clouds that we expect to find stars in the process of formation. As they contract under their own gravitation, they begin to heat up, and so before they become real stars (supported by nuclear fusion deep within) they will be big and moderately warm - in other words, they will produce prodigious amounts of infrared radiation! Thus these infrared detectors are ideal: when we look towards the big clouds, we can see right into their hearts, where active star formation is now likely to be taking place.Millimeter-wave Detectors: Interstellar Molecules Again.Radiation which has a wavelength of some millimeters is longer than that of infrared light (although not quite so long as the traditional radio wavelengths), so it likewise passes through the obscuring dust. But, like the infrared, it has an extra importance, this time because this kind of radiation is emitted by interstellar molecules. Molecules can emit photons in several ways. The electrons within the constituent atoms can jump from orbit to orbit. The molecule can slow from a rapid rotation to a slower rotation. (Think of a majorette's baton, spinning in the air, with each of the two lumps on the ends representing a single atom in a molecule like H 2 , molecular hydrogen. This is shown schematically on page 161 of your text.) The molecule can slow from a rapid vibration to a slower vibration. (Think of two balls joined by a spring, and visualise them virbrating back-and-forth towards and away from one another. This is also shown on page 161.) You can imagine the majorette's baton spinning at almost any speed you like, but in the micro-world of molecules we once again encounter quantum effects: the spin and vibration states of the molecules are quantized. In other words, a given molecule can rotate at certain fixed rates, but not at in-between values. This means that only certain transitions (from one spin rate or vibration rate to another) are allowed. The consquence of this, of course, is that only certain photons will ever be given off by these molecules, and we will be able to identify their presence by the spectrum of emissions produced: there is a characteristic pattern or 'fingerprint' associated with each molecule. It is at millimeter wavelengths that we can study the vibrational and rotational transitions of interstellar molecules -- a fairly recent technological development. You might be surprised to learn that we find molecular absorption lines in the spectra of the very coolest (M) stars -- for instance, the molecule of titanium oxide (TiO) is seen in such stars. It is no surprise that we don't find any evidence for this molecule in the spectra of hotter stars, because the rapid motions of the atoms in the stellar atmosphere would yield collisions of such vigour that the molecules would be completely torn apart. In the very outer parts of M stars, however, such molecules can just hang together. Historically, and for analogous reasons, astronomers did not expect to find many molecules in interstellar space: it was thought that any molecule out there would be quickly disrupted and destroyed. In this case, the problem is not the collisions, but rather the fact that interstellar space is criss-crossed with many energetic photons. Any of these running into a complex molecule would surely disrupt it. This it came as rather a shock to discover that there are indeed many molecules in the ISM. The first ones found were fairly simple - things like water (H 2 O), ammonia (NH 3 ), carbon monoxide (CO), and so on. But more and more complex ones have subsequently been found, including methanol, ethanol, some of the amino acids, and the so-called PAHs (poly-aromatic hydrocarbons). How can we explain the formation and survival of such fragile species? We have seen the answer already. If interstellar photons have the power to disrupt the molecules, then we need to shield the molecules from the photons. And this is the answer: the molecules are not distributed in the general ISM, but are found deep in some of the dark clouds, where the photons cannot penetrate through the concentrations of dust. It must also be cool there, of course, or else vigorous collisions with fast-moving particles would disrupt the molecules. This realisation led to the new science of molecular astronomy and the astonishing conclusion that our galaxy contains many hundreds or thousands of Giant Molecular Clouds (GMCs), each of which is millions of times the mass of the sun - great complexes of rather high density and low temperature. In other words, these are ideal locations for the formation of stars. One nearby example of a mid-size GMC is to be found in the Orion Nebula. Previous chapter:Next chapter0: Physics 016: The Course Notes, spring 2005. 1: The Properties of the Sun: 2: What Is The Sun Doing? 3: An Introduction to Thermonuclear Fusion. 4: Probing the Deep Interior of the Sun. 5: The Sun in More Detail. 6: An Introduction to the Stars. 7: Stars and Their Distances: 8: The HR Diagram: 9: Questions Arising from the HR Diagram: 10: The Importance of Binary Stars: 11: Implications from Stellar Masses: 12: Late in the Life of the Sun: 13: The Importance of Star Clusters in Understanding Stellar Evolution: 14: The Chandrasekhar Limit: 15: Supernovae: The Deaths of Massive Stars, 16: Pulsars: 17: Novae: 18: An Introduction to Black Holes: 19: Gravity as Geometry: 20: Finishing Off Black Holes: 21: Star Formation: 22: Dust in the Interstellar Medium: 23: Gas in the ISM: 24: The Size and Shape of Our Galaxy: 25: The Discovery of External Galaxies: 26: Galaxies of All Kinds: 27: The Expanding Universe: 28: Quasars and Active Galaxies: 29: The Hot Big Bang: 30: The Geometry of the Universe: 31: Closing Thoughts: Part 1:Part 2:Part 3: |
(Wednesday, 22 April, 2026.)
