An astronomer looks at the absorption lines in the spectrum of light coming from a distant galaxy. He identifies the absorption lines of hydrogen, which makes up most of the galaxy. He then compares these lines to the same absorption lines from a laboratory sample. His results are shown in the diagram. How is the galaxy moving in relation to Earth?
Okay, to get started on this question, let’s first look at this diagram. And in particular, let’s look at the top half of it, the one that shows the laboratory source spectrum. 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. Here’s how we will get a spectrum that looks like this.
Say that we have our sample of hydrogen right here. And then we shine a visible light of every colour on to this hydrogen sample. 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. In large part, there is nothing unusual about this spectrum. We see all these purples and blues and greens and yellows and reds. In other words, in large part what we’re seeing is the visible light spectrum.
But notice that there’re these dark bands that also appear in the spectrum. There are four of them, a particular wavelength locations. Physically, what’s happening in order to generate these dark bands is that our sample of hydrogen is absorbing those wavelengths of light. And because the hydrogen absorbs them, they’re not passed on to the observer. That’s why the spectrum that we’re seeing is call hydrogen’s absorption spectrum. It shows us the light that the sample absorbs. And that absorption is demonstrated in these four dark bands that we’re seeing. So that’s the meaning of the spectrum that we’re seeing in the top half of our diagram, the laboratory source spectrum.
Getting back to our question, we’re told that this astronomer is looking at light that’s coming from a distant galaxy. And it’s the spectrum of light from that galaxy that we’re seeing in the bottom half of our diagram. We’re told further that this particular galaxy is made up mostly of hydrogen. And this means we will expect to see similar absorption lines as we saw in our laboratory source of spectrum. After all, we’re talking about the same element, hydrogen. 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.
But 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. 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. It’s at this point that we can recall the definition of red shift. And as we do that, let’s clear some space on screen. Here’s how we can define this term. A redshift is when the light that’s emitted by a distant object, such as in our case a galaxy, when that light moves toward the red end of the visible spectrum. And this is due to the fact that the object itself is moving away from the observer.
So here’s the idea. Let’s say that this here is our observer and that this is our object. Now, if our observer and our object are not moving relative to one another, then the light given off by the object will look just the same to the observer. It won’t be shifted any which way. But on the other hand, if our object is getting further away from the observer, if it’s moving away from it, then the light emitted by the object, though it itself hasn’t changed, will be observed to be more red than it was before. This is what we mean when we say that a redshift has taken place.
If we look back at the spectrum that we’re observing from this distant galaxy, we see that indeed a shift is going on. If there had been no shift, then we would see these absorption lines of hydrogen in the absorbed spectrum appearing here and here and here and here, in the same place as they were from our laboratory source. But that’s not what we’re seeing. Instead, we’re seeing the lines shifted towards the red end of the spectrum. In other words, redshift is taking place.
This fact tells us just how this galaxy is moving in relation to the Earth. That’s because in the instance of our astronomer and this absorbed spectrum from a distant galaxy, the astronomer on earth is the observer. And so if the galaxy is moving further away from Earth, then we would expect a light from this galaxy to be redshifted. And we see, based on the spectral analysis, that it is. Based on this, here’s that we can write. We can say that the galaxy is moving away from the Earth. In other words, the distance between the Earth and this distant galaxy is increasing.