And as we do that, let’s clear some space on screen. It’s at this point that we can recall the definition of red shift. The absorption lines in this absorbed spectrum from the galaxy seem to be shifted to the right towards the red end of the visible spectrum. And indeed, we do see these four distinct absorption lines with the same spacing distance between them as we saw in our laboratory source spectrum.īut notice that these lines in our absorbed spectrum, the one we see coming from this distant galaxy, there’s been a shift that’s taking place between these two spectra. After all, we’re talking about the same element, hydrogen. And this means we will expect to see similar absorption lines as we saw in our laboratory source of spectrum. We’re told further that this particular galaxy is made up mostly of hydrogen. And it’s the spectrum of light from that galaxy that we’re seeing in the bottom half of our diagram. Getting back to our question, we’re told that this astronomer is looking at light that’s coming from a distant galaxy. So that’s the meaning of the spectrum that we’re seeing in the top half of our diagram, the laboratory source spectrum. And that absorption is demonstrated in these four dark bands that we’re seeing. It shows us the light that the sample absorbs. That’s why the spectrum that we’re seeing is call hydrogen’s absorption spectrum. And because the hydrogen absorbs them, they’re not passed on to the observer. Physically, what’s happening in order to generate these dark bands is that our sample of hydrogen is absorbing those wavelengths of light. There are four of them, a particular wavelength locations. In other words, in large part what we’re seeing is the visible light spectrum.īut notice that there’re these dark bands that also appear in the spectrum. We see all these purples and blues and greens and yellows and reds. In large part, there is nothing unusual about this spectrum. If we put an observer behind this sample, so that the sample comes between the light and the observer, then this spectrum in the top half of our diagram is what that observer will see. And then we shine a visible light of every colour on to this hydrogen sample. Say that we have our sample of hydrogen right here. Here’s how we will get a spectrum that looks like this. We’re told that this spectrum, this span of colours that we’re seen here with these dark lines in them, is the absorption spectrum of the element hydrogen. And in particular, let’s look at the top half of it, the one that shows the laboratory source spectrum. Okay, to get started on this question, let’s first look at this diagram. How is the galaxy moving in relation to Earth? He then compares these lines to the same absorption lines from a laboratory sample. He identifies the absorption lines of hydrogen, which makes up most of the galaxy. An astronomer looks at the absorption lines in the spectrum of light coming from a distant galaxy.
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