Exploring Doppler Effect & Redshift

00:00 In the previous lesson, I explained absorption and emission spectral lines. In this lesson, I’ll be talking about redshift and quasars.

00:09 Consider a thing that is emitting sound and moving relative to you. Its motion affects the frequency that you hear. If it’s moving towards you, the pitch goes up, and if it’s moving away from you, the pitch goes down.

00:21 You can experience this by listening to the siren of a fire engine going by you on the street.

00:28 This funky little animation borrowed from Wikipedia demonstrates how the sound waves get affected. As the car moves towards you, the sound waves get compressed, making their frequency go up.

00:38 As the car moves away from you, the sound waves get elongated, making the pitch go down. This is known as the Doppler effect, or Doppler shifting.

00:48 Doppler discovered this phenomenon by observing sound waves, but the effect happens with all waves, including light. If a celestial object is moving away from you, its wavelengths shift.

01:00 This shifting of light is commonly known as redshift because the wavelength moves towards the red end of the visual spectrum. The change in question can be the object moving away from us due to its motion, moving away from us due to the expansion of the universe, and gravity can also have this effect, but let’s not open the whole general relativity can of worms.

01:21 Here on the screen, I’m showing the emissions of a quasar as they’re observed on earth. There’s a line at 490 nanometers, another at 760 nanometers, and one more at 850 nanometers. For reference, that little rainbow on top is the visual spectrum, so the 850 nanometer line isn’t something you’d be able to see with your own eyes.

01:43 Modern telescopes can observe outside the visual spectrum as well though. Quasars generally are moving away from us because quasars tend to be very old. This means they were earlier on in the formation of the universe, which means the expansion of the universe is dragging them away from earth, so their emissions get Doppler shifted.

02:02 The new set of lines on the screen are spectral lines for magnesium, missing two electrons, and two of the hydrogen lines from the Balmer set. There is a bit of trial and error in selecting the right lines and matching them to a quasar’s emissions, but take my word that these three elements are in this quasar.

02:20 Knowing that, you can determine how much shift happened.

02:25 Redshift is calculated by one plus the amount of shift times the wavelength. A redshift of 0.752 causes the diagram above to transform into the diagram below.

02:36 Finding the right emission lines can be tricky, but once you have them, you can find a quasar’s redshift. The value here translates into about 7 billion light-years.

02:48 A quick aside before getting into quasars specifically, astronomers use an angular coordinate system to specify direction. It has two terms: right ascension, that’s the degrees around the earth, and declination, which is the angle you’re looking up or down at.

03:03 This is kind of like longitude and latitude on a map, but in three-dimensional space. If you’re observing something in space, you have to normalize your position, and of course, the earth is moving, which just complicates things, but right ascension and declination are standardized, so the value in a star catalog is the same for everyone.

03:22 You just then need to translate it if you want to point your telescope at it.

03:27 In the first lesson, I gave a quick definition of quasar, but let’s drill down a bit. Quasars are luminous active galactic nuclei. Their name comes from the original term, quasi-stellar objects. Observers figured out that they weren’t quite stars, they were something else, and that something else is a hungry black hole.

03:46 The galactic nucleus part is because quasars are typically from the really big black holes found at the center of galaxies. And if you’ve ever heard of a blazar, that’s the same thing, but the jet is pointing in a different direction.

03:59 The jet caused by the black hole eating things produces a mind-numbingly large amount of energy. In fact, it can be several thousand times the emissions of the Milky Way.

04:09 Quasars are by far the most energetic things in the universe. Several million quasars have been identified with about a million fully cataloged, including their spectrums.

04:20 The closest one to Earth is 600 million light-years away, while the furthest is over 13 billion light-years away near the dawn of the universe. The picture on the right here is an artist’s rendition, but it gives you a bit of a sense as to what’s going on.

04:37 This is the graph of the emissions of a quasar. In fact, it’s the one I use to demonstrate redshift. The three peaks in the graph correspond to the emission lines of magnesium three and the gamma and beta values of hydrogen in the Balmer series. The spectral lines have been shifted, which is how the redshift got calculated.

04:56 A couple things to note: the wavelengths here are in angstroms, and the amount of energy, which is the height of the graph, is known as flux. Also, real-world data is messy.

05:05 There’s a lot of squiggly going on here, and a big part of what astronomers do is taking multiple measurements and teasing out the data. If you ignore the noise in the 400 nanometer range, although it is messy, there are several distinct peaks in the flux.

05:20 The three I’ve highlighted are those for our spectral lines.

05:24 You may vaguely recall that this is a Python course. Our goal is to use marimo to visualize spectral data like this, then create movable overlays of elemental spectral lines whose position changes based on a redshift slider.

05:39 You’ve been patient as I’ve science geeked out. Next up, I’ll show you how to graph a quasar spectrum using Matplotlib and marimo.