Video Transcript: Spectra

Now, we've already seen this diagram of Wein's law.  We saw how colder objects tend to be more on the red side, hotter objects are brighter and bluer.  This represents all objects in space, but especially stars.  In a sense, this mathematical relationship is kind of an idealized approach for an object we refer to as a black body. 

Now a black body is really just any object which is purely glowing in light because of its temperature.  It's not reflecting any light; it's not having some artificial source of other energy that's shining on it.  It's just hot and glowing.  And that's pretty close to what stars are like, they're just hot because of what's happening inside this star and they're glowing as a result.  So they follow this pattern pretty closely, these spectra where they get brighter and bluer as they get warmer and warmer.   Now, because we have cameras and we have telescopes, we can actually capture a spectrum of a star.  

So consider with me, for example, this top spectrum, the one that is a star that's 5500 Kelvin.  If I could take the light from that star and send it through a prism and see the full rainbow of light, and then I use something like a camera to capture that rainbow, well, then I can measure, because I have a camera, I can measure the brightness at each one of those wavelengths of light and each color.  I can see how much blue light is there, how much green light, how much yellow, how much red, in fact at every wavelength I could measure its brightness.  And by doing that, I can build a spectrum just like these hypothetical spectrums.  Remember, the axis in this first picture is intensity versus wavelength.  Brightness versus color.  And I can do that if I were to take a picture of a spectrum. Well, naturally, astronomers have done this ever since the invention of photographic film.  And certainly with digital cameras, they do the same thing.  When you do that, you see spectrum that look like this. 

In the second picture in the gallery, you see many different spectra of many different stars. Let's just look at the top one as an example. You see it goes all the way from blue to green, yellow, all the way across to red.  Now there's many other stars labeled here and they're organized roughly by temperature.  So the hottest stars are at the top, and the coldest stars are towards the bottom.  And you'll notice the same kind of pattern I was telling you about.  The hotter stars, the brightest part of their spectrum is in the blue.  And the cooler stars these K and M stars towards the bottom, their brightest part of their spectrum is towards the red.  So this is exactly what we would expect for this black body, this Wein’s law that we've seen.  

So this spectrum is an extremely useful tool to capture one of these because with this observation, we can literally measure the temperature of a star.  Think about that.  Just by looking at it, and if we looked at it with our eyes we couldn't tell what its temperature was, but by carefully capturing a spectrum of the star we can determine its temperature.  That's amazing.  

Now, as you look at this picture, you notice undoubtedly, these dark lines in the spectrum.  You might be thinking, well, what's going on?  What are those dark lines that are in there?  Those are called absorption lines and they're caused by the atoms that make up the star.  So atoms make up everything; you and me and everything around us.  And they also make up stars.  Atoms can interact with light.  In fact, they can absorb light, but only at very specific wavelengths.  That's what you're seeing.  These absorptions are happening at very specific colors.  And that is true of atoms in general; they can only absorb very specific wavelengths of light.  Let's see why that is. 

The third picture in our gallery shows a model for what an atom would be like.  This is a model of a hydrogen atom.  And as you may have learned in the past, our simplest model for how an atom behaves is that electrons orbit around the nucleus of the atom in like a little circle.  But the key thing about this model and about how atoms behave is that electrons can only orbit in very specific orbitals, or very specific circles.  So, for example, you have this N equals 1 orbit, where the electrons are really close to the nucleus of the atom.  Well, the atom that the electron could orbit there, or it could be orbiting in N equals 2; a little bit higher energy a little bit further away.  

And it begs the question, though, can the electron be anywhere in between?  I mean, how does the electron get from 1 to 2?  It must certainly, kind of like, slide across from one to the other.  But that's not what our current understanding of physics suggests.  The electron can only be ever located at these specific orbits.  It instantly jumps, in a sense, from one to the other.  And when it makes that jump, it's either absorbing or giving off energy.  So when an atom absorbs energy, what's happening is that electron is jumping up to a new energy level, a new orbital, it's jumping from one orbit to the other.  

Because these orbits are fixed and they don't change for a given element, the amount of energy that it absorbs is always the same.  And that corresponds to a very specific wavelength of light. So that's what this this diagram is illustrating.  It's illustrating that any of these jumps like from 1 to 2, that jump corresponds to 122 nanometers, that's the wavelength of light that would have to be absorbed for an electron to make that jump.  Or if something jumped from 3 all the way to 6, it would absorb 1094 nanometers, that wavelength of light.  

So these have a range, like 1094 - that's infrared light.  122 nanometers - that's kind of ultraviolet light.  And if we get in the middle here, like 656 - that's red light.  Or 410 - that's blue light.  So these correspond to very specific colors of light. 

Now I have to push pause here and kind of step back because this is really an astounding discovery that this is how atoms behave.  And it was at the very beginning of the discoveries in quantum mechanics which is that the universe, the laws of nature, at the smallest scales are very bizarre.  You know, in ordinary life, like with you and me, I can put my hand here, I can put my hand here, and I can put it anywhere in between.  Space is continuous.  I can choose any location here, here, here, here, here, here, anywhere.  But in the very small world of atoms, it can only be at specific places and it's almost like a teleports between those places.  It's a very bizarre rules.  

These rules were so weird. These laws were so weird that the leading scientists thought that they're impossible; it can't possibly be true.  Most notably, Einstein himself felt like these discoveries of quantum mechanics must surely be wrong because they just don't jive with making sense of our universe. Time and again through experiment though, we see that these laws are, in fact, the way our universe works in ways we would have never guessed; in ways that are still not really understood in any sort of philosophical way.  We have the math that describes them accurately and makes predictions but our intuition just doesn't work in this small world. 

I want to share with you a quote from this, The Hand of God book that I received, which is from Einstein, about this issue and about the mystery that goes along with it.  And in a sense, he's referring to these, not only the laws of deep space, but these laws that govern the very small as well.  I love this analogy that he uses.  Again, this is from Einstein.  He says, “The human mind is not capable of grasping the universe.  We are like a child entering a huge library.  The walls are covered to the ceiling with books, in many different tongues.  The child knows that someone must have written these books.  It does not know who or how.  It did not understand the languages in which they are written.  But the child notes a definite plan in the arrangement of the books, a mysterious order, which it does not comprehend, but only dimly suspects.”  

That's a beautiful analogy from one of the greatest scientists of all time about how we have a sense of how things are but we only barely understand and comprehend.  But one thing we do know is that there is an order someone has designed and put all of this together with a careful plan.  I love that.  I love that.  

Okay, so this is our basic model of the atom and this is why we see these dark lines in a star spectrum.  The light is being absorbed at very specific wavelengths, but only at those wavelengths. 

Now, when the electron gets absorbed, it doesn't stay at that higher energy for very long.  Pretty quickly, that electron jumps back down, making its way all the way back to the lowest energy.  It might not jump straight down, although it could.  It might kind of jump to here and then jump down to there.  It could make different paths to kind of make its way back to the energy level.  But it always hits these fixed orbitals.  So what happens then the light gets absorbed, but then the light gets re-emitted.  The atom will take the light, and then it will eventually get rid of that energy by shining light in a new direction.  And that's important. 

You know, between you and I, if there was some gas here, floating and the light that's coming from me to you would be absorbed by that gas and you would see kind of a dark line in the spectrum.  But that light is going to get re-emitted in a new direction, kind of randomly.  Now, that light might happen to come back towards you.  But it could go up this way, or out that way, or out that way; it could go in all sorts of different directions.  So what causes that, and I guess it causes the star - me shining - to look darker in that spot.  But it can also mean that this gas here, which could be cold gas, it looks like it's giving off light.  So imagine someone's looking from over here, like far that way. They see this cold gas, and it's shining in visible light, like red and blues and yellows. And they might think, well, it must be really hot, because only things shine and red and blue in the visible spectrum, if they're really hot, but actually the gas is quite cold. So it's a little tricky.  But a cold gas can actually shine in visible parts of the spectrum, because it's absorbed some radiation, some energy, and then it's giving it off in a new direction.  We see this in many places in the sky. 

Here's a beautiful example. This is a planetary nebula.  We'll learn more about these later, but it's essentially a star that has exploded.  And inside the middle is what's left of the star; it's called the white dwarf, and it's still shining brightly, giving off lots of energy.  It's very hot, it shines blue, and around it is all kinds of cold gas which is expanding out into space.  Now we can see the star but we can also see all these beautiful colors.  And these colors are, we're seeing this light because the energy from that star is shining out in all directions.  Some of it is absorbed by this cold gas and then this cold gas is re-emitting the light at those very specific wavelengths, and it's going in all directions, and we can see it. 

And the cool thing about these nebulas, and these spectra of nebulas, is that the precise wavelengths of light that we might happen to see tell us about the types of atoms that are inside this nebula.  So the model of the atom is that light can only be absorbed and emitted at very specific wavelengths.  And that depends on where the electron can be around the nucleus. 

Well, different elements have different orbitals.  So helium, or carbon, all those different elements of the periodic table, they will have different orbital configurations and they absorb and emit different wavelengths of light.  So when we look at a spectrum of something like a planetary nebula, that's what the last picture in our gallery shows, we can actually see all the different elements that are present inside that nebula. 

So the top spectrum illustrates what the spectrum of a nebula would look like.  It has lots of bright lines of different sorts.  These are the emission lines, where the atoms are giving off that light re-radiating that energy.  We see lots of different lines, but from that we can deduce what types of elements are inside the nebula.  

So below that spectrum, you see spectrum of just hydrogen.  If you only have hydrogen gas, what lines would you see?  You would only see those bright lines corresponding to those energy transitions.  If you had just helium, you would see only those lines.  If you had just sodium, you would see just those.  If you had just neon, you would see just those.  But when you look at the spectrum of the nebula, you see that it has many of those different transitions in it.  It has not only the bright green line that we associate with hydrogen, but it has all this forest of lines that we associate with neon.  It has some of the bright lines of helium and it also has some of the lines of sodium.  So we can deduce then that the planetary nebula must have all these different elements inside that cold gas.  

So by looking at a spectrum, we can not only tell how hot something is, but we can also figure out what it's made out of.  Just by looking at a star or a nebula and capturing its spectrum, we can actually pull it apart and look at what's inside.  What elements is that object made of? Truly extraordinary. 

All right, we'll see you next time.