The Interstellar Medium
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00:00
Centre Party (Germany)Cardinal directionCosmic dustHochfrequenzübertragungStarCosmic rayFACTS (newspaper)GalaxyYearMorningAstronomerSpiral galaxyCrystal structureLightTelephoneDigital electronicsSizingAzimuth thrusterCocktail party effectHyperbelnavigationFunkgerätInterstellare WolkeGasMassHydrogen atomMaterialMicrowaveDirect currentMonthMagnetizationDayNightAlcohol proofRutschungForgingGround stationSolar energyGreyDrehmasse
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Centre Party (Germany)Cardinal directionGalaxyHydrogen atomDensityHochfrequenzübertragungUniverseCosmic microwave background radiationH-alphaTemperatureMicrowaveRoll formingMagnetspuleCosmic rayMagnetizationField strengthRadiationSunlightRutschungChemical substanceVisible spectrumWavelengthGround (electricity)Interstellare WolkeGasMagnetic momentDrehmasseParticleHose couplingCosmic distance ladderFundamental frequencyEmissionsvermögenStarSwitchElectronWolkengattungFACTS (newspaper)AtomismAudio frequencyMinuteCosmic dustHot isostatic pressingRail profileWind waveAngeregter ZustandPhotographyField-effect transistorAlcohol proofAccess networkRefrigeratorLightAutumnPhotonRelative datingAtmospheric pressureAerodynamicsRailroad carWeather front
10:19
Cardinal directionCentre Party (Germany)Interstellare WolkeCosmic dustElectronic componentGasBahnelementGalaxyDampfbügeleisenMapRutschungBasis (linear algebra)MeasurementArc lampCocktail party effectFACTS (newspaper)Spiral galaxySpare partAstronomical objectElectric power distributionPlanetary nebulaPhotographyRadiationCartridge (firearms)DayFirearmYearHydrogen atomGround stationUltraviolett BSpectrographBrightnessWeather frontStarWind waveAbsorption (electromagnetic radiation)AstronomerSpecific weightWavelengthRadio astronomyAmplitudeAngeregter ZustandPlane (tool)GruppensteuerungWatercraft rowingZündanlageBand gapPlain bearingNightGlassSpread spectrumWolkengattungEisengießereiCommand-line interfaceUltraRelative datingTransmission (mechanics)Nyquist stability criterionVisible spectrumCylinder headSuitcaseSchmidt cameraLightSource (album)
15:34
Cardinal directionCentre Party (Germany)WolkengattungPlanetary nebulaLightStarParticleSizingCosmic dustHydrogen atomRutschungThermometerAtomismAlcohol proofGround stationGalaxyInterstellare WolkeKelvinAugustus, Count Palatine of SulzbachGround (electricity)GasTemperatureStock (firearms)
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Centre Party (Germany)Cardinal directionCollisionBlackReaction (physics)Wind waveMetreSizingAntenna (radio)TunneldiodeAlcohol proofFACTS (newspaper)Source (album)RutschungLuftionisationChemical substanceRoll formingRemotely operated underwater vehicleTemperatureIonMicrowaveHypothetisches TeilchenThermodynamic equilibriumCosmic dustTiefdruckgebietFunkgerätVisible spectrumOrder and disorder (physics)Refractive indexRF engineeringHydrogen atomSpannungsmessung <Elektrizität>Interstellare WolkeUltraviolett BRadio astronomyFormation flyingNanotechnologyWavelengthGentlemanEngineWoodturningRoman calendarAtomismBarrelAtmospheric pressureStarYearEisengießereiRulerHourGradientFuelDrehmasseCosmic rayWeekKelvinCartridge (firearms)Club (weapon)LungenautomatHose couplingMeeting/Interview
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Cardinal directionCentre Party (Germany)LightUltraviolett BSchubvektorsteuerungDark nebulaFACTS (newspaper)Interstellare WolkeMagnetizationCosmic dustAM-Herculis-SternField strengthRadiationDirect currentMaterialDashboardParticleSpiral galaxyStarVideoSeries and parallel circuitsRutschungGalaxySchwache LokalisationSheet metalYearAtomismRelaxation (physics)Plane (tool)SizingScale (map)Line-of-sight propagationOrder and disorder (physics)WolkengattungDampfbügeleisenDensityElectronic componentDrehmasseCOMPASS experimentMinuteElektrisches DipolmomentCosmic rayChemical substanceLuftionisationFunkgerätApparent magnitudePulsarForceFiling (metalworking)ElectronEffects unitCrystal structureWind waveBook designAlcohol proofArray data structureAudio frequencyHypothetisches TeilchenMagnetGasIceFerryPlatingCaliperPhase (matter)TurningProtectionHot isostatic pressingMobile phoneStonewareFlightSunlightKickstandGentlemanVisible spectrumRailroad carWeather frontIPadTransmission (mechanics)Field-effect transistorGirl (band)SoundTypesettingMicrophoneMonthAschenwolkeWoodturningProzessleittechnikReaction (physics)
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Centre Party (Germany)Cardinal directionPaperOrbitRoll formingLightRulerDampfbügeleisenAsymmetric digital subscriber lineZeitkonstanteGround stationTemperatureRutschungVan-Vleck-ParamagnetismusCapacitanceEffects unitLadungstrennungK-EinfangMagnetizationSpannungsmessung <Elektrizität>GasRotationPhotoelectric effectInertiaSpin (physics)Absorption (electromagnetic radiation)ProzessleittechnikCosmic dustOrder and disorder (physics)Rail transport operationsAtomismHomogeneous isotropic turbulenceYearMinuteCyclotronInterstellare WolkeUltraviolett BRelaxation (physics)SolidOrbital periodPhotonIceIonAlcohol proofFrictionMonthHourForceOceanic climateMeasurementMassRailroad carSpaceflightHose couplingParticleProgressive lensKelvinRail profileVideoSolar SystemAccess networkTARGET2AerodynamicsPhotographyGround (electricity)FACTS (newspaper)Blood vesselStream bedSkyAngeregter Zustand
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Cardinal directionCentre Party (Germany)ElectronGalaxyDensityDampfbügeleisenMagnetic momentDrehmassePaperKilogramControl rodEffects unitGasUniverseSizingMint-made errorsRotationYearInterstellare WolkeRutschungSteckverbinderRelaxation (physics)Roll formingLastHot workingLeadFACTS (newspaper)EnergiesparmodusMeasurementRailroad carData conversionKopfstützeStonewareMagnetizationWoodturningSpeckle imagingRapid transitAccess networkCocktail party effectMassQuality (business)EmissionsvermögenSoundTransmission (mechanics)Big CrunchSleeve valveBauxitbergbauForce
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Cardinal directionCentre Party (Germany)Volkswagen GolfKopfstützeOrder and disorder (physics)Ground stationStagecoachGasCosmic dustRoll formingBlack holeMassBallpoint penAerodynamicsFACTS (newspaper)Dark nebulaStarStar formationStriking clockCluster (physics)Access networkIceTiger (cryptography)Angeregter ZustandHydrogen atomTARGET2Avalanche diodeWeather frontWolkengattung
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Centre Party (Germany)WoodturningStarCosmic dustStardust (novel)GasPaperHot isostatic pressingBackpackProof test
Transcript: English(auto-generated)
00:12
The galaxy in which we live consists mostly of stars. In fact, about 90% of the matter in the galaxy is gathered into stars.
00:24
My subject this morning was the other 10%, the stuff between the stars. If I were a regular astronomer, I would probably hesitate to talk on such a broad
00:41
physicist who somehow wandered into that subject, found some of the physics that was relevant, extremely fascinating, and found some problems unsolved that physics could still be brought to bear on. So I want to give you a very broad view of the interstellar medium, introduce you
01:05
to a few, only a few of the special problems in physics that are still intriguing and unsolved, and then try to suggest, in my own view at least, what are some of the big problems
01:20
that remain to be worked on, suggesting that in the hope that some of the younger people here will find that, will be tempted to turn their own physical ingenuity and imagination on those fields. Well, in our galaxy, of course, for many reasons, some of which will become clear
01:43
presently, we can't see what the structure of the galaxy is, but we can always begin in a survey like this by looking at ourselves as if we were in another galaxy. And if I may have the first slide now, could I have the lights off on the first slide then?
02:05
This is how our galaxy might look. This will be a view that many of you have seen before, if we can get it. Could you kill these lights up here too, please?
02:26
The difficulty. Next slide, please. I put that in for focusing, and it's serving its purpose. Well, this is a big spiral galaxy, very much like our own.
02:45
This is Messier 81. About the size of our galaxy, about the same number of stars, namely one and a half times ten to the eleventh, and it's about 30 million light years away. If this were ourselves, we would be somewhere out here, and I call your attention to these
03:07
dark places in here, which are actually obscured by interstellar dust, which will be one of the topics I'll come to a little later. Now the interstellar medium, let me say, if we could see with radio eyes, not when
03:24
looking at visible light, but let us say with radio, the stars here would not be bright at all, and we'd see the whole thing filled with a gently glowing gas. The interstellar medium, that is to say, everything but the stars, consists mainly of the following items.
03:41
The next slide, please. Here's our galaxy in a very schematic sketch. It's about ten to the twenty-third centimeters in diameter, although of course the edge is a little bit hard to define, and we'll be talking about the gas, which amounts to about ten percent of the mass, the equivalent of ten to the tenth solar masses.
04:02
It's mostly hydrogen and helium, of course, and about 90 percent of it is quite cold, neutral hydrogen and neutral helium. Then there's some solid material, which is of extreme interest to us in some respects, electromagnetic waves, both starlight and the ubiquitous microwaves that Professor Dirac
04:25
mentioned just a few moments ago, cosmic rays and magnetic fields. Now, this medium is extremely empty. I tried to show that on the next slide. Next slide, please.
04:42
The emptiness of the galaxy is illustrated in two ways. Here are some stars. The typical size of a star is ten to the eleventh centimeters, and the nearest star is about ten to the ninth centimeter, nineteen centimeters away, a few light years, perhaps ten. The gas that's between the stars is almost, is empty to almost the same degree, curiously
05:06
enough. Here is a hydrogen atom in the interstellar gas. The nearest hydrogen atom is about a centimeter away. In fact, there are a few atoms typically per cubic centimeter, and the ratio of the
05:21
diameter of this to the distance is again a factor of ten to the eighth. In fact, the, I think that's an accidental, I don't think there's anything fundamental about the coincidence of those two factors. The galaxy is so empty that if two galaxies collided and passed through one another, it
05:42
would be likely, but not very likely, that one star and the whole thing would hit another star and the other one. If you draw a straight line through the galaxy, the chance that it hits a star is exceedingly minute, something like ten to the minus ten or twelve.
06:00
Nevertheless, in this emptiness, there's a lot going on, as we'll now try to explain. The next slide shows the electromagnetic radiation in the galaxy, plotted here as a function of wavelength in centimeters. This is the starlight, and this is plotted on a scale in such a way that area of the
06:25
plot truly represents energy. And the fact that these two forms of electromagnetic radiation, the starlight here and the microwave background discovered by Penzias and Wilson, are roughly the same energy density, namely
06:45
a few times ten to the minus thirteen thirds per cubic centimeter. But of course, we remember that this is only in our own galaxy. This is local. This is everywhere, filling a million times more space, so that in terms of energy density
07:02
in the universe as a whole, there's a million times more of that than that. Of course, if I get close to a bright star, this is different. I'm taking a typical place between the stars. And some of you may recognize here a few features I just sketched in. There's Lyman alpha sticking up on the spectrum, and there's, I guess, H alpha up there.
07:24
Whereas over here, as far as we know, we do have the black-body spectrum appropriate to a temperature of two point eight degrees, as Professor Dirac mentioned, although there's now some evidence from recent experiments at Berkeley that it doesn't fit quite as well, and there may be a little bit of excess at the middle of the spectrum, which is perhaps
07:45
trying to tell us something. Now, the energy here, I'd like to make... People think of the microwave spectrum as being extremely weak and very hard to detect. The following is a surprising but true statement.
08:03
At all wavelengths longer than a few millimeters, the earth receives more radiation in this form than it does in direct radiation from the sun. I'll leave that as an exercise for the student to check.
08:21
Now, the next slide compares some of these substances in energy density just to make a rather general point, which is that if we look at the mean energy density in starlight, cosmic microwaves, magnetic field, cosmic ray particles, and the gas, curiously enough,
08:43
these are all in the range of ten to the minus thirteen, ten to the minus twelve herbs per cubic centimeter. That I've already, in saying that, I've already tacitly assumed the value of the magnetic field, and in a moment I shall discuss what we know about the magnetic field strength.
09:02
Some of this equality is no doubt a kind of equipartition, since the magnetic field and the gas and the cosmic ray particles are coupled together, and the energy density, which is equivalent to a pressure, it expresses that dynamical coupling.
09:21
Other coincidences there must be said to be accidental. Now, let's go back and talk about the gas a little bit. What we know about the cold gas is largely determined by observing it in the microwave spectrum, and the next slide just reminds you of the way that is done.
09:44
Using the radiation given off by neutral atomic hydrogen in the ground, hyperfine ground state, the change in energy involves a switching of the relative orientation of the magnetic moment of the electron and the magnetic moment of the proton, and this well-known
10:06
frequency, 1,420 megacycles, is emitted by all the cold hydrogen in the galaxy. An interstellar hydrogen cloud is thus identified by its emission of this 21 centimeter electromagnetic
10:21
wave. Now, using this over the past 25 years, astronomers have, radio astronomers have gradually built up a picture of the galaxy, they're able to do so because the galaxy as a whole is totally transparent to this radiation, whereas as we shall see it is not to visible
10:40
light, and out of this one has galactic maps of which I give a recent example on the next slide. Next slide, please. This is a composite map based on measurements by Kerr and Verskur and others showing this is our location out about 30,000 light years from the center of the galaxy, and now we're
11:06
looking down on the galactic plane, and these are arcs which are believed to represent the locations of the hydrogen and thus show us that we are in fact living in a spiral, a galactic spiral.
11:21
I must, however, say that the picture here is not neat, and in a way it is less neat now than it was at the beginning. So complicated is the actual distribution of the gas that the location of the spiral arms and so on by this indirect method is really very uncertain, and it's in fact
11:42
I think recognized to be more uncertain now than was believed to be the case in the early days of 21 centimeter astronomy. So that I really, I don't put too much faith actually, I would not like to have to go out here and guarantee to find one of these arms when I get there. The situation is often, in astronomy, is more complicated than it appeared to be at
12:05
first. Moreover, the gas is very lumpy and distributed in clouds, and there's a lot of things going on. Now there is one other method for studying the gas from the Earth, and that is to avoid
12:25
the atmospheric absorption by going into orbit, and much of what we are learning now about the interstellar gas comes from the orbiting telescope, in particular from the Copernicus telescope, which is operated and conceived actually by Professor Spitzer
12:45
at Princeton and others, and I show you on the next slide first a view of the Copernicus telescope, which has been doing wonderful observations now for several years, it's still running fine. This is basically a flying ultraviolet spectrograph looking at the stellar radiation which never
13:05
reaches in Earth, and on the next slide you will see a spectrum, next slide please, this is a spectrum, one of the hundreds of spectra taken by the Copernicus telescope, looking at a particular bright star, Zeta Ophiuchus, as an ultraviolet source, and
13:27
in front of that star we see absorption of specific wavelengths by the interstellar gas. The particular importance of these observations is emphasized in this slide when we look at peaks that are absorptions by molecular hydrogen, H2, and indeed by the molecule HD over here,
13:47
since the H2 molecule unfortunately has no radiation and no absorption that we can see from Earth, it first is seen in the ultraviolet, and in fact these observations
14:00
represent the first really fruitful observations of molecular hydrogen, which is as it turns out a very important component of the gas. In this way too one measures the abundance of other elements, one has already found for instance that elements like magnesium, silicon, iron are depleted in the gas, that
14:28
is to say one sees much less of those elements than one would expect to on the basis of the general relative abundance of the elements. This already is a hint of the things that we will learn when we talk about the interstellar
14:44
dust. Well, so much for the gaseous components, now I'd like to move ahead and talk some about the interstellar dust, which I must tell you will get rather more attention in my talk than it perhaps should, because that is the, happens to be the part of this
15:04
thing that got me intrigued in the beginning, and to a certain extent I'm still stuck working on certain problems with the interstellar dust. Let me first show you some interstellar dust on the next slide. This is a beautiful astronomical object called the trifid nebula in Sagittarius, one of
15:26
the photographs I think from the famous Schmidt, 60 inch Schmidt, and these dark clouds in here in this nebula which are obscuring the light from the very bright star in the
15:40
middle are just clouds of literally solid particles more or less the size of dust or cigarette smoke or any things like that. Now, the, it is the dust which obscures our own view of our galaxy.
16:00
There's so much of it that when we try to look toward the center of the galaxy, we most, we can't see it at all, it's like looking through a big smokescreen. The next slide shows how the, this emphasizes the point I made about the hydrogen being in clouds.
16:23
The dust is associated with the gas, and in general where we find the gas, we also find the dust, and so if you have the whole business compressed into a smaller cloud, that's darker and harder to see through, even though it's the same fraction of dust. So that we have a typical cloud that most of the hydrogen is in clouds like this.
16:47
The cloud might have 20 atoms per cubic centimeter, its temperature, local thermometer would read something like 70 degrees Kelvin, starlight would be able to go through it with some absorption, perhaps 80% would come out the other side.
17:07
If I compress this cloud down to this size, I get a cloud which is so dark that starlight can't get through it, it now has 2,000 atoms per cubic centimeter, it's somewhat colder, and it's in clouds like this that much of the chemistry goes on
17:25
for reasons that I think will become apparent when we turn to that. There's also believed to be an inner cloud medium, well there must be an inner cloud medium if we have the clouds, and it's thought that in between this space is typically very much hotter with perhaps only a fraction on the average of an atom per cubic centimeter.
17:45
Incidentally, I meant to remark when we were talking about how empty the gas is, that a hydrogen atom at this density experiences a collision with another hydrogen atom about once every 30 years. It goes in a straight line, absolutely straight line,
18:02
hits another hydrogen atom 30 years, off again, traveling about the distance of the Earth's orbit between collisions. It's this rather weird situation that a laboratory physicist is bound to find intriguing because one's instincts are all unreliable when those situations exist.
18:24
Now, how do we measure something quantitatively about the extinction of light? The next slide explains that very quickly. Astronomers do that by picking out two stars, which they can tell from other evidence are very similar stars, one of them not obscured and the other obscured by clouds.
18:44
The effect of the obscuration is to, of course, weaken the light but also to reden it. In fact, the dust clouds in the galaxy have precisely the same effect as dust clouds on Earth. They make the sunset red. They do so by absorbing the blue light more effectively than the red light,
19:06
and the next slide shows how that kind of information is actually plotted out and analyzed. Here, I plotted against wavelength in a logarithmic scale, wavelength in micrometers,
19:21
something called the extinction. When that's high, it means that the light is being absorbed. Never mind exactly how that's defined. And here, we see the extinction as a function of wavelength. The entire range of the visible spectrum is just spanned in there so that when we had only that information, one really didn't know very much.
19:44
Nevertheless, I must say people were very busy fitting this curve with the rather elaborate theories which had three or four adjustable constants, and needless to say, these theories all were able to fit it. The result of the excursion into space in the ultraviolet telescope is to add the blue curve up there
20:06
with this conspicuous hump at about 2,000 angstroms, a hump whose actual origin we are not yet able to explain, but that is certainly trying to tell us something. In the infrared, there are some very interesting parts of the spectrum
20:25
which are already interpreted as indicating that the dust is at least partly silicates. Now, how much dust is there? It turns out that that's one thing we can answer rather definitely.
20:42
The next slide gives an idea of that. If you took the whole galaxy, let me say in a big disk like this, the galaxy, and I put a big piece of sheet of paper under the whole thing, then the amount of dust that you would precipitate on that piece of paper
21:02
is given there, but I'll tell you what it would look like. It would look like this. If I take a piece of white paper and a soft pencil and just slightly gray the paper, that's what the paper would look like under the galaxy. On the other hand, if I were to try to look through the galaxy edge-wise, through all that dust,
21:23
then it would look like this, and that's why we can't see the center of the galaxy in visible light. On the other hand, if we go into the infrared or even further into the radio, then the whole galaxy becomes visible, and that is the way in which today people are learning a great deal
21:43
about the very center of the galaxy where a great deal is going on where there's an enormous concentration of mass. Now, what the dust is is another question. May I have the next slide? We really don't know what the dust is,
22:01
but if one had to guess today and make a bet, I think it is fairly certain that a large part of it, some part of it, is in the form of silicates. There's very likely some graphite, and the rest of it might be called dirty ice where ice is used in a rather general way
22:22
to include not only H2O but NH3 and CH4. These abundant elements, condensable, are probably there. Now, there are many other suggestions, some rather exotic. Some people think that dust might partly be or largely be polymers.
22:43
Even cellulose has been suggested as a constituent, but that there are silicates and graphite and dirty ice, I think, is almost inescapable, but apart from that, we know very little. In fact, curiously, we know two things about the dust rather certainly.
23:05
One is its total amount, which I've already indicated, and just for the physicists, let me remark that the way we know the total amount so well is by applying the Kramers-Kronig relation to the attenuation curve. We also know the temperature of the dust.
23:22
Rather accurately, we know what it must be, and the next slide shows why we can make a statement about that. This is a nice point of elementary physics. I like explaining to my students. If I had a large black object out in the interstellar space, it would absorb starlight,
23:40
and then it would have to come to equilibrium by radiating at the appropriate temperature, and if you work that out, you'll find that its temperature would come to be something like 3 degrees Kelvin, a little higher because, of course, actually, it's also coupled to the microwave radiation at 2.8. Well, a dust grain, however, can't be black
24:02
because this thing is radiating at millimeter wavelengths, and a dust grain being only 10 to the minus 4 centimeters in size makes an extremely poor antenna. In fact, radio engineers know that you cannot make a good antenna for long waves in a short space,
24:20
and this means the dust grain has to get hot, and when one does the calculation, one finds that it will end up somewhere between 10 and 20 degrees Kelvin, and that is a fact about the dust that I think I really would be willing to bet fairly high odds on. The dust is important in many ways,
24:42
but one of the most interesting ones is that it is the primary catalyst in interstellar chemistry, about which I would now like to say a few words. The next slide shows the most important chemical reaction, namely the formation of molecular hydrogen from atomic hydrogen.
25:01
If the whole system were in chemical equilibrium, it would all be molecular hydrogen, despite the very low pressure because of the low temperature, but in order to form molecular hydrogen, when two hydrogen atoms hit, you have to get rid of about four volts of energy, and there's simply no way to do so.
25:20
In the laboratory, normal pressure, you could do it in a three-body collision, but in the interstellar gas, this practically never happens. By practically never, I mean that in a volume the size of this hole, in the interstellar gas, there is not one such collision in the life of the universe,
25:44
so that doesn't go, and the way this reaction is catalyzed is by an event in which a hydrogen atom hits a dust grain, sticks to it, perhaps wanders around, tunneling over the surface, held on by van der Waals forces,
26:00
another hydrogen atom sticks, the two get together, and then they have no trouble making a hydrogen molecule that flies off. This is the primary reaction of the interstellar chemistry. From there on, the picture changes because once one has molecular hydrogen and a source of ultraviolet light or ionization as cosmic rays,
26:22
then, as a number of people shown in particular, my colleague at Harvard, William Klemperer, the chemist, has shown that the other reactions will go in the form of ion molecule reactions right in the gas, and that in this way he can explain the buildup of the extraordinary number of molecules
26:42
that the radio astronomers have now identified. The next slide gives a list, well, not a list because I didn't want to write everything in, but the interstellar molecules of which about 50 have now been seen nearly all of them by, with the exception of H2 and HD,
27:01
by radio astronomy, spectrum in the millimeter and centimeter range, start off with HD, OH, the radical H2O, and carbon monoxide, which is an extremely important indicator and is very widely distributed, and then through lots of others whose names are listed,
27:22
then finally coming down through ethanol that you will recognize, I called up an expert in this just before I left home to say what is the biggest molecule that's been found yet, and he told me about HC9N,
27:40
which I'm not quite sure about its name, but I think it is cyanotetraacetylene. Now, you'll notice that all these molecules I put on here, in fact, every one that if I'd fill these in, would be molecules that are not symmetrical. They're molecules with electric, with the exception of hydrogen,
28:01
they're molecules with an electric dipole moment, and that's simply because in order to see them, for them to radiate radio waves at their rotational spectrum, they have to have that. There's no doubt whatever, I think, that the symmetrical equivalence of all these things, that is HC9H, for instance, also exists.
28:22
So this is even a small sample of the very rich array of chemicals out there, and goodness knows how far up the line we go, as these are discovered in size. These actually in dark clouds, the chemistry is kept alive primarily by cosmic ray ionization
28:42
since the ultraviolet light can't get in, and dark clouds are otherwise a good sight. Now, may I turn back briefly to the magnetic field? There are various lines of evidence that show that there is a large-scale magnetic field, interstellar, throughout the galaxy.
29:01
I will just mention one way in which the magnetic field can be directly measured. The next slide, please. Making use of the properties of a pulsar, which sends out pulses of radiation which are polarized, and in the radio range, one can measure two things.
29:22
If there is a magnetic field in space along the direction in which the wave is traveling, and if there are electrons out there, which there are, I should have said that even the cold interstellar gas is slightly ionized, then we have the Fairey effect operating to rotate the plane of polarization
29:42
by an amount that depends on the magnetic field strength and the density of the electrons integrated over the path. On the other hand, the electrons also cause different frequencies to travel at different speeds, namely dispersion. And by noting that the pulse arrival time
30:00
depends on the frequency at which you're observing, one can determine the dispersion, and that depends on the integral of the number of electrons. And so if you've measured those two things, and if everything is uniform, then you divide this result by that result, and you get very directly the magnitude of the magnetic field along the line of sight.
30:20
Using this, with the extraordinary number of pulsars that are now known, one can say something about the direction and magnitude of the field. The next slide is a chart, I won't spend much time on this, from a recent book of Manchester, in which this has been done. And these circles all refer to a particular pulsar,
30:46
the magnetic field measured in the manner just described. They're clustered here in the galactic plane because these are galactic coordinates. And a plus here means the field is pointing toward us, and a blank means the field is pointing away. And the size of the circle is such that that's a microgauss,
31:03
but let me just say that in general, the magnetic field, interstellar magnetic field of the large scale sort, appears to be of the order of a few microgauss in strength. That was the number I assumed way back there on that chart, which showed the energy density. Now there's another demonstration of an interstellar field,
31:24
which has been known for a long time, in fact for nearly 30 years, that I would like to spend a little more, a few more minutes on. And that is the remarkable fact that the interstellar dust seems to be capable of polarizing starlight.
31:41
In the next slide, I show you the primary observational material. No, I'm sorry. This slide is one which tells us what the magnetic field is in our neighborhood. I said a few microgauss. If this is the sun, we now believe that the magnetic field in our general vicinity, here's the center of the galaxy down here,
32:02
points in that direction. I'm not sure what that direction, which side of the galaxy I'm on, perhaps that direction to astronomer. But anyway, it points along, generally along the spiral arm in which we live. Okay, the next slide, please.
32:22
Please ignore this stuff down here. Just look at the top. Which is a plot in coordinates of the galaxy. Here's the galactic plane going clear around 360 degrees. Each one of these little dashes is an observation of a star at that position in which the starlight turned out to be
32:42
somewhat linearly polarized. The polarization has nothing to do with the star itself. It's caused by the intervening medium, as if someone had taken a sheet of polaroid and held it out between you and the star. And the direction of this shows the direction of the polarization, and the length of the line
33:00
shows something about its strength, which is not enormous, one or two percent typically, but still very definite. And now you notice here, you can hardly escape noticing that these are all combed out here in a systematic way. Here they're sort of in all directions. Here again, they're rather combed out again.
33:20
I like to call this the iron filings picture because in elementary physics, you know, we all scattered iron filings on a magnet to see the lines of force. I don't know the German equivalent for filings. I don't know what you call it, but there's really no doubt that this is showing us
33:40
the structure of a large-scale magnetic field. Just how it is showing us that, though, is a little harder to say. Here's the way we explain the polarization of starlight by the dust in the magnetic field. Next slide. The star emits unpolarized light,
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as much vibrating this way as that way. This light comes to us and perhaps passes through a cloud of dust grains. When it comes out of the dust grain cloud, one of these components has been absorbed more than the other so that we receive it as polarized light. And in fact, it turns out that
34:21
when the light has its vector like this, that's the direction of the magnetic field. The presumed explanation of this is as follows. The dust particles are certainly not spherical. They couldn't do that if they were spheres. Let us assume that they are rather elongated things,
34:43
although I really can't tell the difference between that and flakes. And these somehow are caused to be aligned with the magnetic field cross-ways so that if the magnetic field is running like that, then these, on the average, spend more time perpendicular to the field
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than parallel to it. Only in a very rough average way. I mean, their motion is quite irregular, but nevertheless, there's some physics going on here which orients these things with the magnetic field locally. And that physics has been a puzzle that's at least 25 years old.
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We've thought for a long time, we know how that works from a proposal that Davison Greenstein made back in 1952 or 3, but there's still some questions about it. Let me indicate very briefly now how that goes. I don't want to go into this story
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except in one way to show you how some interesting and ancient physics came into it unexpectedly near the end. The alignment of these things in the magnetic field is not explained by saying that they are little particles of iron which orient like a compass.
36:00
Even if they were totally magnetized, that would not work because the field is too weak and the bombardment with other atoms would completely destroy the alignment. The physics is much more subtle and is believed to involve a phenomenon called paramagnetic relaxation. That's suggested on the next slide which I'll spend only a moment on.
36:23
No, I'm sorry. This I must... In going into the physics of the dust grain, I want to show you what the situation is and what kind of thing one has to work with. We assume that the dust grains are about two or three times ten to the minus five centimeters.
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That conclusion is drawn from the way they scatter and absorb the light. It's not absolutely certain. The temperature of the dust grain we know is small. The dust grain is embedded in the gas and now there are a number of time constants in the problem. If you're an atom, you hit a grain
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about once every ten to the eighth years. That's actually a short time from one point of view and very important. If you're a grain, you get hit by an atom once every five minutes. You get hit by an ultraviolet photon somewhat more often.
37:20
But the grain, after all, is in a gas at a hundred degrees Kelvin and is executing Brownian rotation so its energy is presumably something like kT and that means it's rotating at ten to the fourth revolutions per second. All these numbers are, of course, average rather than exact.
37:41
Now here is the time constant that rules this problem from the point of view of the physics is the time that the grain motion is damped by the gas. That is, if I throw a grain into the gas at high speed or spin it, it will hit gas atoms and will gradually slow down. They're going to that simple form of friction.
38:01
The time of slowing down is a few hundred thousand years. If I spin a grain and go away and come back, several hundred thousand years later, it will gradually have slowed down a little bit. This makes it possible for very small effects to have, very small causes to have big effects.
38:22
Well, the next slide shows another form of, which is interesting. The grain undoubtedly has some electric charge. Actually, the processes tending to charge it negatively, electron capture and charge it positively, photoelectric emission are different.
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It would be unreasonable for them to cancel exactly and we believe the grains have a potential the order of a few tenths of a volt. Such a grain is coupled to the magnetic field, just like a charged particle, and if you calculate the period for a cyclotron orbit, it turns out to be only ten to the fourth years.
39:03
The point is that that is short compared to this time I just mentioned, and therefore, for their dynamical progress, the grains are effectively locked to the magnetic field, just as an ion would be, and can only be pushed for a long time parallel the field and not crossways.
39:22
Well, in this situation, the next slide indicates the idea that I may assume that the grain is paramagnetic because any dirty ice in the solar system, in the interstellar medium,
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has probably got some iron atoms, and that's all I need. The magnetization, paramagnetic susceptibility, of course, is complex. There is a relaxation effect. The magnetization lags behind the field, and for a rotating grain, that means there is a viscous, there is a torque, tending to slow the rotation,
40:01
an absorption of energy from the rotating field, and that, operating in a rather indirect manner, results eventually in a partial alignment of the grain axis if the grain is not spherical. It's a very messy problem. We think we now understand the dynamics of that.
40:20
And when we finally did understand it and had reliable calculations of all the dynamics, it turned out that, in fact, we were still in difficulties. Namely, it appeared to require a magnetic field at least ten times as strong as we have assumed in order to explain the alignment. And this is the thing that's kept me at this problem for so long.
40:44
I hope the paper I'm publishing next month will be my last paper on the subject, but I'm not sure it will be. At any rate, the reason for this last paper is that some physics has come into the problem, which I had not suspected,
41:00
although I've been working on it for a long time. The physics is of two kinds. The next slide... May I have the next slide, please? If I have... This is a thing we hadn't taken into account before. If I have an asymmetric thing rotating, not a sphere, but some odd object, rotating about some odd axis,
41:22
then there is a source of internal... If there is any internal dissipation, the dynamical effect is to cause the rotation to line up with the major axis of inertia. There are two kinds of internal dissipation, both of which we had omitted and ignored for a long time.
41:43
One is just due to imperfect elasticity of the solid. I won't say anything about that, except that it made us scurry off to look into the solid-state literature again. The other one is the one I really thought some of you would be amused by, what I call Barnett relaxation,
42:01
an effect which, as far as I know, has never even been predicted, let alone measured in the laboratory, which has to do with the so-called Barnett effect. The next slide... The Barnett, for most of us, know better, the Einstein-de Haase effect,
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a very important experiment done in 1915 by Einstein... by de Haase at Leiden, but Einstein suggested the experiment. The experiment is very simple. You take an unmagnetized iron rod, hang it so it can turn. You suddenly magnetize it, and you see it begin to turn.
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It does so because you've lined up the electron spins, as we now say, and then the conservation of angular momentum means the whole rod has to turn. It's an important experiment because it gives one a way of measuring the fundamental g-factor of whatever it is that causes magnetism,
43:01
which was really a key experiment. The Barnett effect is the converse, namely, if I have a freely rotating rod, it gets magnetized if it has spins in it because it's cheaper in energy to have some of the spins lined up
43:21
and take that angular momentum out of the rotator as a whole. To my surprise, I'd heard about the... I knew about both these effects, of course. I'd heard more about the Einstein-de Haase effect, and I was surprised when I went back to the literature last year to find that Barnett did his experiment first. In fact, he did it a year before Einstein-de Haase.
43:43
Not only that, he got the right answer because one of the remarkable aspects of the Einstein-de Haase story is that when they determined the g-factor, they found that it was what they, of course, then expected, namely g-factor one, and of course, we know it as really g-factor two.
44:02
Barnett got two, more or less, but didn't know what to make of it, and it was several years before the experiment, which is exceedingly difficult experiment, both of them fantastically difficult, fought with all kinds of systematic errors before the g-factor settled down too. And now it turns out that in the interstellar grains,
44:22
this ridiculously small effect, the Barnett effect, is important. It was first suggested, its role in interstellar grains was first suggested by a paper by Dolginov and Mitrofanov in Moscow two or three years ago, and they did not use it in this way.
44:42
They simply pointed out that the Barnett effect produces a magnetic moment in the rotating grain, and then it occurred to me that there must also be associated with a relaxation, and the relaxation is the thing that does the business in lining up the grain rotation with its principle magnetic moment.
45:01
The reason that a tiny effect can be so important is that the interstellar grain is not slowed down by the gas until it has made something like 10 to the 18th revolutions, and it's in that setting where a thing has a rotational Q of 10 to the 18th that you can do it.
45:21
Okay, now I would like to conclude by taking a moment to look at the extragalactic space on my last slide. The last slide shows the universe, the extragalactic space that Professor Dirac was talking about. In fact, on this slide,
45:41
we'll have something about the mean density of the universe, and here the question is, could it be that some of the mass, the missing mass, which would be required if the universe is to be closed according to the conventional footage, could that be hidden outside the galaxy in some of the forms that we've discussed about
46:03
in connection with the interstellar medium? Here is the very... The size of the universe I've taken is 10 to the 28th centimeters, and the difficulty in defining the size, of course, has already been pointed out, but when we add up all the things we see,
46:22
which are something like 10 to the 10th galaxies now, about 100 times as far apart as their size, with 10 to the 44 grams each, we find the apparent mean density of the universe, the density, if all there is is what we see, is about 10 to the minus 30th
46:42
grams per cubic centimeter, whereas the critical density, that density which would just be enough to slow the expansion, more than that would then cause it to come back, that is something like 20 times as large.
47:00
The question then is, have we seen only 5 or 10% of the total matter, the rest of it being hidden? I think we can say now fairly confidently from the observations that if it is hidden in the intergalactic space, it's not hidden as dust, it's not hidden as cold hydrogen,
47:22
it might be hidden as hot, totally ionized hydrogen, but even that prospect is rather quickly being ruled out by some of the x-ray astronomy observations. One is left of course with the possibility that it's hidden in black holes, more massive objects,
47:41
and in fact in order to hide matter in the galaxy or intergalaxy, all you need to do is to make it in large lumps and not in small pieces. Any amount of matter could be hidden in the form of golf balls and one would never be able to see it, but there is a suspicion now that if the mass is there, it's hidden as black holes
48:02
or possibly it's hidden mainly in the centers of galactic clusters, but even saying that, I've really strayed into astronomical fields that I haven't yet explored and shouldn't act as if I knew the answer in.
48:22
Within the galaxy itself, there are still some important problems. One of the most important in my view is the dynamics of the interaction between the dust and the gas when the gas is turbulent. There is some suspicion already from some curious results that in a turbulent gas, the dust may tend to clump
48:41
and in clumping make dark clouds, making dark clouds make chemistry, and also those dark clouds are really the forerunners of stars. So the whole question of star formation may hinge in a rather critical way at some stage on the behavior of dust grains in a turbulent gas.
49:01
That's a problem that has not been solved and I think may be a very exciting one. The dust, you see, is the material out of which stars are made and it's in turn made from stars and in fact, when I showed you the graphite on the paper, it may even be that the inner star dust is graphite,
49:23
but what's certain is that the dust on the paper was once in a star and will be again. We won't be around to use the pencil. Thank you.