Reviewing Spectral Lines

00:00 In the previous lesson, I gave an overview of the course. In this lesson, I’ll give you a quick introduction to the science behind the data you’ll be interacting with.

00:09 On your left here, you have our sun. Isn’t it pretty? Don’t look directly at it. It might hurt your eyes.

00:17 Consider a sunbeam traveling to Earth. To give you a sense of the vastness of even our own solar system, at the speed of light, it still takes eight minutes for that sunbeam to travel from the sun to you on Earth.

00:30 If the sunbeam passes through a prism, it gets split up into its constituent colors. White light, like our sunbeam, is made up of many different wavelengths.

00:40 When the light hits the glass, it refracts. That’s a fancy word for bends, and the different wavelengths bend at different amounts. On the other side of the prism, you see the wavelength separately because of this bending.

00:52 Now, although space is mostly empty, the important part of that statement is mostly—your sunbeam can interact with things along the way. In fact, a lot of those things it’s interacting with are the atmosphere of the sun itself.

01:06 Consider this lonely hydrogen atom. The electron in the atom is quantized. That means it can only exist at specific energy levels, which I’ve represented here by three rings.

01:17 Hydrogen actually has more than three levels, but I’m trying to keep the diagram simple. When the sunlight hits the hydrogen atom, it energizes it, causing the electron to jump up one or more levels.

01:29 Since the electrons are quantized, that jump absorbs a specific wavelength of light, which means that wavelength is missing from our spectrum on the other side of the prism.

01:39 By examining the output spectrum, you can see which wavelengths got absorbed and know what level changes in what elements absorbed those wavelengths. By doing this, you can determine just what the sunbeam passed through on the way to your prism.

01:54 In fact, this is how Sir Joseph Norman Lockyer discovered helium. The element helium is named after Helios, Greek for sun, and that’s because Sir Lockyer was observing these spectral lines and discovered an element that wasn’t yet known on Earth.

02:10 Now, history is messy, and although Lockyer gets the credit, someone else saw the lines a few months before, but they thought the lines were sodium and didn’t realize it was a new element.

02:20 Also, Lockyer was working with a team, but let’s stick with the simplified version of history and give Sir Joseph the credit.

02:28 What goes up must come down, and the same goes for our excited electrons. After that sunbeam pushed it up a level or more, it eventually has to come back down.

02:38 When it does, it emits the same wavelength as was absorbed. If you’re looking at the blackness of space, you might see these wavelengths and know they came from an excited atom.

02:49 Most of the time with stars, you look at absorption lines as you’re looking at their light directly. With quasars, you might look at the absorption or you might look at their emissions.

03:00 Quasars can be pointy and produce strong directional jets. If the jets happen to be pointed at Earth, you can look at the absorption lines. If the jet isn’t pointed at Earth, it still excites the local atoms.

03:12 When the levels of those atoms reset, their emissions can be pointed at Earth. Either way, you can see absorption and emission, but for the sake of this course, I’ll be sticking with emission lines for our quasars. It isn’t just hydrogen that behaves the way I described.

03:28 It’s all elements. Each element has its own absorption and emission patterns for each of the electron levels within it. And if that’s not complex enough, ionization also has its own patterns.

03:40 An ionized atom is one that’s missing one or more electrons. Space is a pretty harsh place, and quasars are where some really hardcore physics happen, sometimes stripping atoms of their electrons.

03:52 A shorthand is used in astronomy to indicate an element and its ionized versions. For example, N-I means nitrogen with no ionization emits light with a wavelength of 653 nanometers.

04:06 Astronomers often speak of these in angstroms instead, but that’s an easy conversion as you just have to multiply by 10.

04:13 N-II is nitrogen with one electron removed. See, if astronomers were programmers, they would’ve started counting at zero, and then the number would correspond to the number of missing electrons, but they aren’t, and so you have the danger of an off-by-one error.

04:28 N-V, say it with me, is missing four electrons and that emits light at a wavelength of 1,240 angstroms or thereabouts. I did some rounding here. Because an atom has multiple electron levels, it can have multiple lines.

04:44 The Balmer and Lyman sets are collections of hydrogen transitions. The Balmer set uses Greek letters where H alpha is a specific hydrogen line. The Lyman series also uses Greek letters.

04:56 Don’t ask me what is in either of these sets or why you’d need both. That’s past my level of understanding. Just know for later on, when you see Ly and some Greek letters, that’s a hydrogen line. Emission and absorption lines are measured experimentally, which means if you look them up in various tables, they might not agree with each other exactly. This has to do with precision and experimental error.

05:20 For this course, I’m using the lines in a table on this web page. You’ll see more on that later.

05:27 I’ve got a bit more science to cover before I show you the dashboard, so this continues in the next lesson.