Lecture Transcript: Research with the World’s Largest Telescopes
Okay, I’m Katharine Johnston. I’m one of the astrophysicists, along with Phil Sutton. Today I’m going to tell you about doing research with the world’s largest telescopes—interferometers—because that is something I do a lot in my research.
Outline
Here is an outline of what we are going to talk about. First, we will discuss big optical and radio telescopes and why astronomers care about making big telescopes. Then we will talk about why we would want to observe at longer wavelengths—mostly to get around the problem of interstellar dust, which creates issues if you are observing at shorter wavelengths.
I will introduce the Atacama Large Millimeter/submillimeter Array (ALMA) and interferometers in general, touching upon how they work. Then I will discuss specific results on a source called AFGL 4176, which is part of my research, looking at a disk of material around a forming massive star.
I will move on to talk about even bigger interferometers, such as the Event Horizon Telescope (EHT). Then I will discuss science results from the EHT, specifically looking at—you may have seen this in the news—supermassive black holes and the fine detail we can see with this telescope.
Big Optical Telescopes
First, if you were asked to think of a big telescope, you usually think of big optical telescopes with a big polished parabolic mirror. For instance, the Very Large Telescope (VLT), which confusingly is made of four telescopes that are eight meters in diameter. You can see what it looks like inside one of these four domes: a big mirror supported by a structure that allows it to be pointed in various directions.
The reason we care about making these giant telescopes is twofold. First, we want something called resolution, which is the detail in the image. There is a very important relationship in astronomy: Resolution = λ/D, where λ is the wavelength of the light and D is the diameter of the telescope.
You can see that if we make a bigger telescope, we get finer detail in our image. That is reason one—why we go to all the engineering difficulty to make these big things.
There is a second reason: telescopes are just big light buckets. The bigger the bucket you make, the more photons you collect, which means higher sensitivity because you have more photons. That is basically why we care about making big telescopes.
Giant Radio Telescopes
You don’t just have optical telescopes—you have giant radio telescopes, and these are even bigger. Some of them you may have actually visited, like the Jodrell Bank telescope in Cheshire, which is 76 meters in diameter. Some are up to 500 meters in diameter. Why are they even bigger?
Looking back at that relationship (Resolution = λ/D), it makes sense: for a longer wavelength, to get the same detail, we need a bigger telescope. That is why you need these huge telescopes in the radio.
You might ask: Why do we even care about making even bigger telescopes? It is quite a lot more effort to make these huge things. The reason is that we can actually get more information looking at radio wavelengths and solve a specific problem.
The Problem of Interstellar Dust
Here is a beautiful image—a fisheye image of a huge portion of the sky looking at the galactic center. This is a very different scale from the view we will see of the galactic center later in this talk. This is taken from the southern hemisphere, actually from the ALMA telescope that I will discuss later.
You can see all these black blobby patches where light is coming from the galactic center. That is due to something called interstellar dust, which is really a big pain if you are an optical astronomer. What is this stuff I’m talking about?
My duster. So this is, um, a very high, uh, uh, zoomed in image of what dust looks like.
So, um, the bar there is, um, one micron.
And actually, this is quite a big dust screen because most of them are, uh, smaller than one micron in size.
Um, and so they’re mostly, uh, uh, big conglomeration of molecules that are made mostly from, uh, carbon and or silicon based molecules.
And so, uh, the problem is that if you, uh, go in and observe, um, like we saw in the previous image, um,
with visible light of what we see with their eyes, then this, uh, these kind of, uh, particles, uh, actually block out, um, the light, right?
The, uh, the the light comes, um, photon comes along and basically, uh, the light gets absorbed or scattered by this, uh, dust.
Uh, so it’s a little bit of a problem, um, if we want to see past it and see what’s there.
Um, but what we can do instead actually is observe it longer wavelengths.
Um, so I should go back there soon. And because at that point, uh, the light doesn’t get blocked, it can just go straight through.
It’s like, um, um, I don’t know if you have very, very long wavelengths.
You can go through walls, although sometimes your Wi-Fi doesn’t. The longer the wavelength, the more you can go past these objects. At wavelengths longer than a micron, you actually don’t have any more trouble with dust.
Wien’s Law and Thermal Emission
Another benefit of observing at long wavelengths is that you can use Wien’s Law, which you have probably come across before. It says that if you have something emitting thermally (a blackbody, meaning everything it absorbs it re-emits), then the peak of the emission wavelength is related directly to its temperature.
From an astronomical standpoint, luminous massive young stars like the Pleiades (which you may have seen—visible with the naked eye) are really bright blue-tinged stars. That is because they are really hot. Blue is actually hot—you can see that the hotter it is, the shorter the wavelength.
I also show an example of an ultra-cool dwarf star, which is the star of the TRAPPIST-1 system. That is incredibly red—obviously this is just an artistic impression. In that case, you have a really cool star. You can see that stars are very close, not exactly but close, to blackbodies when they are emitting.
Taking it closer to home: people emit at about 37 degrees Celsius when we are well, and we peak in the infrared wavelengths. That is why you see people when you look through an infrared camera.
Observing at Different Wavelengths: The Horsehead Nebula
What do we see if we look at a particular object in space using different wavelengths? This is the Horsehead Nebula. On the left, we have optical wavelengths—you can see the lovely outline of the Horsehead Nebula. We cannot really see into it because the dust is blocking our view.
In the center, you can look at it in infrared, and then you start to see sources being picked out as light passes through. If we go to even longer wavelengths—what we refer to as submillimeter (just everything with a wavelength shorter than a millimeter)—at that point, the dust actually starts to emit itself. It is very close to being a blackbody, but not exactly. You actually see it emitting light.
That is really useful because now we can tell the content of that cloud, add up the mass, and determine physical properties.
Molecular Emission Lines
There is another thing you can do, which is slightly different: using molecules in space. A very common example is carbon monoxide (CO). The idea is you have a carbon and an oxygen atom in this molecule, and they are rotating. That rotation has energy.
If it starts to rotate more slowly, it will lose energy as a photon. The amount of energy lost is always quantized—it always loses a specific amount of energy. Therefore, you get a line of emission at one specific energy or frequency.
As we can see on the right, J = 1 → 0, where J is the quantum number for angular momentum of that molecule. For instance, if we go from J = 1 to J = 0, we emit a photon of wavelength 2.6 mm.
That is useful because we have emission at a very specific wavelength, which we can use to study the dynamics of objects.
Doppler Shift and Velocity Measurements
You have probably heard of the Doppler shift—it works with light as well. Stuff coming towards you is blueshifted (the light gets squished together), and stuff moving away is redshifted (you probably heard of this with the expansion of the universe—light gets stretched out).
In the case of things moving away, it has the same effect—their light looks redder (longer wavelengths).
If we look at something with dynamics, such as a rotating disc, you would see the receding side redshifted and the approaching side blueshifted. This is a lovely example of M33, a disc galaxy that is rotating. You see it color-coded by velocities, which we can measure.
Introducing ALMA
This diagram shows the range of frequencies that ALMA works at—just short of 1000 GHz—which is the millimeter wavelength range. This means we can probe really cool temperatures.
ALMA (as well as meaning “soul” in Spanish) is probably one of the nicest acronyms in astronomy. Many astronomy acronyms are relatively boring: Very Large Telescope, Extremely Large Telescope, Overwhelmingly Large Telescope. But this one sounds nice: Atacama Large Millimeter/submillimeter Array (ALMA).
It is in the Atacama Desert in Chile at 5000 meters elevation, which is incredibly high. The reason for this elevation is to avoid water content in the atmosphere. Water molecules can absorb light, and we do not want that because we are trying to observe light from space. Going as high as we can above the atmosphere helps.
ALMA comprises 66 antennas. Now we are not just looking at one telescope—we are looking at a conglomeration of many telescopes. How do we use that as one telescope? ALMA has 12-meter telescopes and 7-meter telescopes that can be configured in different arrangements.
How Interferometry Works
Here is a basic idea of how it works. Imagine we have Earth, and we put all our antennas around the North Pole (this is a simplified example). Imagine we have an object in space at the same position as the North Star, but it is a radio source. What we see is what we would observe from that source looking down on Earth.
The useful aspect of Earth is that it rotates. We can use that to fill the aperture as seen from that source we are observing. It is as if we had a giant telescope—that is basically how it works.
For ALMA, we have different telescopes that can be placed in different positions—different configurations. They can be picked up by transporters and moved to different pads. You can have them really close together or really far apart. The farther apart you put your telescopes, the bigger the effective telescope you synthesize, and the finer the detail you get in your image.
If two telescopes are separated by a certain distance, we call that a baseline—the distance between those two antennas.
The baseline B and source direction vector s allow us to work out an important distance: B · s. The path difference between signals arriving at the two antennas creates interference patterns that we can use to reconstruct images.
Research on AFGL 4176
Moving to my research: AFGL 4176 is a massive star formation region. Using ALMA, we can observe a disk of material around the forming star. The high resolution allows us to see the structure and dynamics of the accretion disk, measuring rotation velocities using Doppler shifts in molecular emission lines.
The Event Horizon Telescope
Now let us talk about even bigger interferometers: the Event Horizon Telescope (EHT). The EHT is a network of radio telescopes around the world that work together as one Earth-sized telescope. This provides incredibly high resolution—enough to image the event horizon of supermassive black holes.
You may have seen in the news the first image of a black hole’s shadow in the galaxy M87. This was a groundbreaking achievement, showing the photon ring around the black hole’s event horizon. The EHT used very long baseline interferometry (VLBI) with telescopes across the globe to achieve angular resolution of about 20 microarcseconds.
The data from all the telescopes is recorded with precise timestamps using atomic clocks, then combined later—a technique called correlation. This allows us to synthesize an Earth-sized telescope to observe the finest details around supermassive black holes.
Conclusions
To summarize: We discussed why we need big telescopes (resolution and sensitivity), why it is useful to observe at longer wavelengths (to see through dust), and how interferometers like ALMA work. We touched on my work on massive star formation, and we discussed the Event Horizon Telescope and its groundbreaking observations of supermassive black holes.
At some point, there may be diminishing returns in making interferometer baselines longer. For the Event Horizon Telescope, you need sources that are incredibly bright and compact. With very long baselines, you are sensitive to very small emission, but if you have no short baselines, you miss large-scale fuzzy emission. Perhaps at some point, there will not be enough compact bright sources to warrant making baselines even longer—you might only be able to observe one object.
Thank you for your attention. Are there any questions?