Astronomy for the layman; Analyzing starlight

The National Association of educational broadcasters with the cooperation of the California Academy of Sciences and radio station KPFA in Berkeley presents a program in the recorded series astronomy for the layman on today's program. Leoni Solomon a lecturer in astronomy at the Morrison Planetarium of the academy and John Hopkins an inquisitive layman discussed some methods of analyzing starlight. Mr. Hopkins begins the discussion. Leon we've discussed many characteristics of stars as well as their numbers and distances. Now all of this information is brought to us by light and I very much like to discuss light just going forward so today. Very good John light is one year to everyone of course as the agency by which we see if it were not for radiation in space that travels from so-called luminous are reflecting objects our eyes would be of no use to us whatever that very interesting property of

light connected with colors generally known to people through that beautiful rainbow. This is as close as most people ever get to perceiving or seeing a spectrum. The spectrum that I believe was the was at first seen by Newton or was it simply I suppose other people must have seen it before his time but what he's the first would have thought about it. Well I think we can say that Newton was the one who first demonstrated how a spectrum may be produced from white light in the laboratory situation. That is to say without the agency of raindrops as in a rainbow Newton allowed a little beam of sunlight to come through a crack or hole in the curtain and he introduced a wedge shaped piece of glass called a prism into the path of this light. He observed in the first place that the direction of the light mean was altered bent and that the pattern which fell upon the far wall was not the same white spot that came through the shutter

Windowshade but was spread out into a beautiful band of color starting with deep red on one end and finishing up with the violet on the other. He surmised correctly that white light is there for the synthesis or a combination of all colors. I realized that once having become acquainted with the spectrum I find that in everyday life they often appear in where light shines through the edge of a piece of glass or through a bottle or something like that apparently the spectrum must have been a long time not as a spectrum I suppose. Well the relationship to light in general probably wasn't appreciated and we say in that case that the optics of the science of optics particularly that of the analysis of the spectrum. I really had it start with Newton but he didn't do anything with it that would make possible it would have made possible the wonderful advances that we'll find later.

The interesting thing which we find in the application of the spec into astronomy is that what ordinary folk would call color is reduced to a much more exacting concepts which may be measured rather than just evaluated burbling measured in terms of what how does one measure color. The basic way is wavelength. Or one can choose frequency which leads us to remark that when studying light you have simultaneously to consider its velocity frequency and wavelength which your words that it might be well to delve into. John I was just going to break in there and ask you what about when you say wave length wave length of what or frequency of what. Well perhaps we'd better backtrack and consider first of all the analogy of light with sound. These are both supposed to be wave motion and that is to say

that something in the space between that thing and ourselves is set into wave motion and this transmits energy to our ears have it sound or our eyes if it's light and we perceive by this which you receive. And many times analogies are brought out between light and sound but one must be careful. The sound requires air for its propagation. Yes I realize I've heard heard mention of the other material which originally was thought to be required for light isn't that what's called the ether or what was called the. Yes and by analogy there should be something in space which transmits light. And this was called ether for perhaps the want of a better name. It is interesting to consider what the ether would have to be like sound in air travels at a relatively modest speed that something like 750 miles per hour given to about 11 hundred feet per second.

This is done in the air at ordinary pressure by ordinary compression and expansion along the path of the sound. This is what gives the name compressional waves to that which transmits sound. On the other hand it can be demonstrated by certain experiments that light waves. So whatever they are are transverse like like in a vibrating string. But their speed is enormous measured at one hundred eighty six thousand three hundred miles per second or nearly. But by the way is equivalent to going seven times around the world in between the beats of a clock. Well in order to transmit energy at that rate the ether must be a marvelous substance with great rigidity much taller than any piano string and at the same time incapable of offering any resistance to the motions of celestial bodies. Here is a combination of circumstance that seemed so hard to satisfy. Nevertheless it was thought quite seriously up to 50 or 60

years ago that such a thing must exist of necessity. It's been pretty well resolved by modern physics and what's known as the quantum theory that radiant energy may proceed from source to a receiver that is to say from the start of our eyes without anything in space itself. When you say radiant energy the thing that comes to mind is heat. Yes well heat is a manifestation of so-called long wave radiation. We don't see heat but if we hold our hand up then the presence of a hot stove lid even in a vacuum. But we would still feel a warmth coming from Millard in a sense it is light but it's light that our eyes don't receive. Yes as a matter of speaking in visible light there could be such a thing. Well now having brought out the notion of velocity we have the two other concepts out of frequency and wavelength. These are not independent they

frequency is the number of vibrations or waves passing a point in a second and the wavelength is the distance between the troughs or the crests of the way as it were speaking of the vibrations such as light which is so called transverse. The case of sound waves we have the alternate partial vacuum and compression along the path of the sound and so called A longitudinal type of wave in this case who would measure the distance from one partial vacuum to the next or from one compression point to the next. In any case and multiplying the frequency number by the wavelength number is always quantity equal to the last thing I look at it this way if you were to open up a shutter and let a little light through for one second and it would move out in one second a distance equal to little numerically to the velocity. And in that distance there would be so many waves each occupying such and such a wave length and the product of the number of waves and their wave length would be the distance the

light would travel in one second. Well then from what you have previously said the color being dependent upon the wavelength. If it's a color which has a greater wavelength that take as many of them to fill a given spaces and yes say in an allergy with sound is again helpful in the case of the so called a note which is 440 vibrations per second. We have a wavelength of 40 inches. On the other hand in considering a light what we ordinarily call green is represented by a vibration of 600 trillion per second and a wavelength of about 1 50000. Part of an inch. Well now you spoke previously of being able to measure these. How do you measure a 50000 and age. It's difficult and I have to be done with very special devices constructed in the Physics

Laboratory. We will find little later down the line I trust that a prism is most often used in a spectroscope. But when it is a matter of wearing wavelength as so-called diffraction grating is far better. Suffice it to say here that a diffraction grating is a piece of glass or metal which is ruled with many equal distant fine lines. In fact thousands per inch. And the way in which light is dispersed or broken into a spectrum. One reflection from such a thing can be fairly simply related to its color. I say fairly simply. That is after you've gone through all the preliminaries. I presume also you mean fairly simply mathematically. Yes actually if the great German physicist from off or over like so almost 75 years ago if I recall I had this pretty well worked out and the precise measurement of wavelength dates from his time. I remember reading somewhere that somebody by name of Roland

had something to a precise measurement of wavelength. Well he simply advanced the original discoveries are from Harvard to a high degree of perfection. I'm not minimizing Roland's contributions by any at all but I say that the principles of the measurement of wave length I think can be attributed to from or for the unit of wavelength for light is conveniently chosen as the end of the straw which is equal to the 10 millionth part of a millimeter. That is to say it takes 10 million Angstrom units to equal a millimeter. There then on the basis of previous discussion of color what we would call green would be expressed by a wavelength of something like a 5000 or nearly 6000 strong units. It cannot be precisely expressed you see because green is a sort of indefinite range in the

spectrum while the wavelength is a perfectly precise thing. I suppose as you get toward the yellow green that's up toward the 6000 you mentioned and you get to the blue green it's down toward four thousand or perhaps five. Simplest thing to remember is that green is in the middle of the spectrum where the wavelength is about a 50000 of an inch or something over 5000 Angstrom you don't draw fully. And if you go into the Reds you're going in direction of longer wavelength while going down to the violet end of the spectrum represents a shorter wavelength. They have this so-called visible spectrum that is the light which compresses the eye is generally speaking within the range from 4000 units to 7000. This is an almost insignificant fraction of the total range of such radiation known to physicists starting from the very shortest cosmic rays to the very longest radio waves. They're all part of the same

phenomenon of nature. As you said before all kinds of radiation. Yes technically they're known as electromagnetic radiations being a manifestation of electrical and magnetic fields in space. This is worked out by the great English physicist and Maxwell. Well you've spoken of the range of colors visible the must be. And the fact that it can be that the light can be broken up with a prism. But there must be some way of making more accurate measurements and just setting up a prism in front of the holy shade as Newton did. The prism spectroscope is the instrument which is most frequently thought of in that connection. It consists of a narrow slit little opening between two finely ground pieces of metal set parallel or nearly touching. And then comes a lens a prism another lens and an eyepiece or a magnifier. Perhaps we

should repeat that and then discuss what each part does. The slit allows like to come through in just a very narrow line which then diverges in direction of the first lens. The arrangement between the slit in the lens is such that light coming out of the lens from the slit is in parallel beams. This gives them an A in column major to render parallel for this first. You know we've spoken out well them this process of passing through a slit through a lens is prior to the passage of the light through a prism. Yes for the prism comes after the first lens the light leaves the first lens having come from the slit and then goes into the prism. It's in the prism the light is bent. This is called refraction. But more important the different wavelengths are refracted at different angles short wavelengths rather large amount longer wavelengths less the actual amounts depend

on the nature of the glass. Well I can see that seems reasonable if the shorter wavelengths in a given length of glance they're going to be more of them and I suppose the action is so much bending per wavelength. Well that's a picturesque way of looking at it which is probably as effective as any is it to remember how it goes. To most people the sort of a hit and miss. You either get it right or you get it wrong. And one of these you have a 50/50 chance. Yes I have it. So after passing through the prism that the light dispersed then moves in to the second lens which then refocuses the father on toward the eyepiece. But now when the light is refocused. You see it is broken into a pattern of color. Made up of the of the various colors that have fallen upon the original slit. I think this is just speculation. Go ahead. So the spectrum is really a spread out image of the original slit in the light of whatever colors

fell upon the slit from the source which was somewhere out in space or on the laboratory table. This is a very important thing to recall. Yes that's what I was going to ask you about was if you had a lens which rendered the light rays parallel not another lens wouldn't you just get back to an image of the slit. You will but since the prism is in between the image of the slit is separated into several. Oh I see I was running to the various colors. Yeah I see. This becomes very important when we discuss the types of spectra and the analysis of the spectrum. Well an actual working spectroscope may be more complicated because frequently the eyepiece is removed in the camera attached for photography. Getting a permanent record or the thing is mounted in such a way as can be attached to a telescope that is of course our interest in it. But these details I think we can pass by and next inquire as to the type of spectrum which we might meet on examining certain light sources.

Well I presume the historical way to go at this is perhaps for the first time suspect or observed I guess again. The main Well not quite because Newton's contraption was so crude that he couldn't very well separate the different kinds of spectra he had. Just a blob of light coming from a hole in the wall which is like having a slit spread out so that it did not give a fine line of light but just a big circle or other irregular some or some irregular shape. It was. A man named Wallace Dunn who first thought of using the slit and immediately upon applying his improved spectroscope to sunlight he decided discovered there were certain dark lines across the spectrum. However this idea wasn't carried to completion until a phone call from the same one we mentioned a little while ago made a really complete spectroscope with all the trimmings as we've described it above

and he saw it definitely then thousands of fine clear dark lines across the spectrum of sunlight. In other words sunlight is not just a continuous band of color but it's complicated by the presence of so many fine breaks. Dark lines across the ice to say at right angles to the spreading out of the colors you see the colors are spread out. Let's think of it horizontally for looking at the Spectrum red on one side blue and the other on the dark lines are vertical and there are simply places in the spectrum where light isn't so to speak. In other words there are dark images of the slit Exactly. They may not be 100 percent dark as there may be some light coming through at that wavelengths but they are relatively dark so they are quite clearly black when viewing. It's a pity we don't have color television we could produce such a thing right here now. Well if the lines are can't be located

specifically within a given color then how does one locate any specific line. Well without going into think complications introduced by the disco called to freshen gratingly might just say that once wavelengths have been established for a different spectrum lines we mean we may measure the angle at which the prism bends these particular radiations and we can draw a graph in which the angle of deviation is plotted against the wavelengths and then some one known case we simply measure the angle through which the light is bent and then read off the graph what its wavelength must mean and doing that what do we know having identified an identical line into spectra. Well we don't know very much until we investigate a little further the properties of the spectrum. We've already hinted that the sun spectrum is a so-called continuous band of color crossed by dark lines. This is what is typically called simply the dark line spectrum. The astronomer and the physicist.

This is to be Contrast that with the spectrum that would be obtained by pointing our instrument to some luminous source such as a neon tube tube with rarefied neon carrying electrical current is an excellent example. In this case all colors would not be present. In fact nothing much except reds and oranges as witness the ordinary. Yes color is the way it looks when you look at it. But then this is then when you look at a neon lamps that you see a band of red or you see a series of dark lines assortment of images of the spectroscope slit in distinct separate wavelengths. Oh it isn't that I'm told it was band of red there are just certain kinds of red you know as we say bright line spectra. And the interesting thing is that if we were to do the same stunt with a tube filled with argon or neon or some other paper we would find a very different pattern. And this is the basis of one of the most important laws of the

spectrum. An incandescent gas at high temperature luminous when viewed through a spectroscope gives a pattern of lines representing an assortment of wavelengths of radiation which is absolutely distinct and unique for that kind of gas. Well Miles the same thing true then if we point a spectroscope. See an electric ordinary electric light bulb. The filament must be made of some particular metal. Ordinarily the electric light bulb would give no clue as to its chemical identity for the filament is simply an incandescent solid. I say ordinarily because some tubes are filled with a certain amount of gas and there might be some of the bright line spectrum but just an ordinary run of the mill incandescent lamp they would expect to find just a continuous spectrum and it wouldn't matter whether it was incandescent platinum or tungsten or gold or carbon that made up the film. So no chemical analysis is possible. Well in other words it does it has to be a gas as you mentioned yes or now if there is a layer of gas between us and such a

continuous spectrum source we get the dark lines. And very common experiment is to vaporize and sodium just to put some in a spoon over a bunsen burner or something and make a cloud of sodium aper between the spectroscope slit and an incandescent lamp and then a dark line appears in the yellow portion of the spectrum which is the spectroscopic earmarks so to speak in the presence of sodium aper. These are so-called Bunsen Perkoff principles of spectrum analysis rather haphazardly stated and they are fundamental to the analysis of light sources. The beautiful thing about it you see John is that we don't have to have the source of light in the laboratory table. What we have said about the right lines works just as well as the Orion Nebula except that it seems to me that Ryan Neville is going to be a lot fainter than a source right there in the laboratory. Yes which introduces difficulties as far as a technical result. But

when the spectrum is obtained the analysis of the wavelengths. Proceed just as if this had been a faint light on the laboratory in fact our friends the physicists study light sources in their laboratories which are equally faint and may require days and days of exposure time and some to film to record. What you're really saying is that sodium is sodium whether it's in the lab or in Ryan nebula and that is the nice thing about it. And so you see the interest of the astronomer in the spectroscope is this. Change the spectrum of the distance source be a star Nebula a planet reflecting sunlight and then he seeks to find the wavelengths of the various spectrum lines and then check with the table and chemistry handbook. Where are listed the wavelengths of the various spectrum lines so we answer your previous question. It's the matching of the wavelengths and the unknown source that can be checked up against the chemical tables that just say we can identify what gaseous elements are present

in the object we're looking at providing it is in a proper condition. That is to say there have to be incandescent gases present ordinary incandescent solid would not do any good. Fortunately celestial sources tend to be gaseous. I hate to say that's universally true but ninety nine and nine tenths percent of the case the sun is gaseous and so are the stars as proved by the similarity of stellar and solar spectrum. And of course the nebulæ are they have just bright lines spectra proving that there is no continuous source in the background frequently. Well no we have this spectrum for used for identification of of the elements of the constituents of a body I presume you can find other things too. Yes I hope we'll have time to summarize just what an astronomer can do with the basic principles. But there is one very important principle that must be brought out first and

that is the famous Doppler shift. Yes I've heard that mentioned this business of the velocity I believe it is of the source yes we can put it this way the Doppler principle describes or names the apparent change of frequency and or wavelength as a result of motion and line of sight. This is to say that if we are looking at a source which is moving in our Elias site the going away from us or coming toward us. The wavelengths the radiation which it emits will not be precisely the same as if they were standing still as if the source were standing still. A simple analogy is good as far as it goes. Is that with sound. When a train is coming toward us and blows a whistle the whistle has a slightly higher note. We are receiving more waves percent and a higher frequency which could be interpreted as a shorter wavelength if we simply assume the velocity of a sound constant. On the other hand if the source are moving away from us that would be a an apparent lengthening

of the wavelength or a lowering of the frequencies so the pitch would drop. This is certainly very apparent when something like a moving train passes one of the pitch drops rapidly just as the source passes by. Yes I don't know how this will go over to our audience but I've got it in a classroom this way we hear it coming like this. If it's going away from us and then coming toward us or if it's coming toward us vs. E.. Yeah I mean I see how they're going you know that I was also heard similar things on a train listening to a bell at a crossing. Yes the pitch of the bell seems to change. Well it's precisely analogous way when we observe a light source which is moving toward us rapidly all the corresponding wavelengths in the spectrum are shipped a little to the violet. On the other hand into the red if the source is moving about away and it can be established that the change in frequency percentage change in frequency is simply equal to a fraction low velocity of

the light source relative to the velocity of light. Compare the velocity of light. Let's put it this way if something were moving at a tenth the speed of light. Why of the wavelengths and spectrum will be changed by 10 percent. Or one more cent speed of light would produce a 1 percent change in in the way of light. Course that means things are moving pretty fast to the velocity of light. Oh yes that means that it would be hopeless to measure the Doppler shift in. The headlights coming toward us. But then we have interstellar velocities of several miles per second and with the precise measurements possible under using a refined spectroscope has attached to a telescope it's can be done and it's the basis of a tremendous amount of information. Well you mentioned several things that we can learn from an analysis of a spectrum. Would you list those off again.

Well let's count them off. We can determine the chemical nature of the gases present. We can measure and measure the motion in the line of sight. We didn't discuss it but we can measure also the temperature of the bodies in any cases we can get. Presence of anyone yet it feels. Well that's another one we haven't had and you know we didn't have time to discuss that but it can be done. Also pressure and density gases can be approximately analyzed. Well it would appear that sometime in the future we will need to go into these other factors or perhaps also how we get the light to use with these with a spectroscope. Yes John we've said a very little amount of very big step. You've been listening to a discussion between Leoni Sellon a lecturer in astronomy at the Morrison Planetarium in San Francisco and John Hopkins an inquisitive layman

on analyzing starlight. This series of astronomy for the layman was recorded at radio station KPFA in Berkeley where the cooperation of the California Academy of Sciences. This is the end of the network.