What can the James Webb Telescope see?

The James Webb Space Telescope uses infrared light to capture the universe (and black holes) like never before
22 August 2022

Interview with 

Becky Smethurst, University of Oxford


An artist's impression of a black hole


We have an absolutely incredible tool, which is allowing us to see beyond our solar system. In fact, to galaxies so, so far away, they think they are the oldest and farthest we have ever seen. That is of course the James Webb Space Telescope. Julia Ravey spoke to Becky Smethurst from the University of Oxford and author of "A Brief History of Black Holes: And why nearly everything you know about them is wrong", who gave an insight into what we're actually looking at in those beautiful images...

Becky - It's almost easier to say 'what are we not looking at the minute?' Because it feels like JWST or 'James Webb Space Telescope' feels like it's just revealing so much at the minute. But what we're seeing is essentially infrared light from all these objects in space. So this is light that has a longer wavelength than visible light that we can see with our eyes. It's redder, so that means it can see through dust because this light has a longer wavelength that essentially just kind of goes around all these dust particles rather than getting scattered off it. So we can see into star forming regions, which is really cool. And also because the universe is expanding, and all the light from galaxies at great, great distances has been red shifted by that expansion, it's been stretched out to these longer wavelengths beyond what we can see. It means that we can actually see some of the most distant galaxies in the entire universe, the most distant of these islands of stars in our universe, which is amazing because if they're the most distant, the light that's going from them has traveled the most distance as well, which means it left those galaxies when the universe was much younger,

Julia - There's been all these claims around the James Webb images that we've seen the oldest stars in our entire universe, but what more needs to be done to confirm how old these stars actually are?

Becky - People are starting to pick out what we call candidates for the most distant galaxies. You know, the light from which is the oldest light we could detect. People are claiming, oh, you know, this light left these galaxies 13.6 billion years ago therefore it's the oldest light. The thing is, that we've only found evidence for that in images. Ideally, what you would want to do is take all the light from that galaxy, split it through a prism to confirm that they actually are at that distance and also declare which one actually is the furthest away. We're gonna need to, to get the spectrum of light as well, which James Webb will have already collected. People are working on it, but there's also cool things you can do with spectrum at those wavelengths. For example, you can take the light from an exoplanet, such as a planet in orbit around another star in our galaxy, our island of stars, the Milky way. And you can say, okay, take the little bit of star light that passed through that planet's atmosphere on its way to us and we'll see how it differs from the normal star light that we receive from that star. And any differences essentially tell you what's in the atmosphere of that planet, because say there is water or carbon dioxide, or maybe methane or something in that atmosphere, it will steal away a little bit of the light. And so we can actually use this to figure out what exoplanet atmospheres are made of and if they contain water.

Julia - That is really exciting. And then if we move away from planets, can you also see black holes in these images?

Becky - Amazingly enough, Yes. <laugh>. I mean, you could tell I'm excited about this, because this is my stuff. And people always get thrown by this, the idea that you can see a black hole, because the idea of a black hole is that it's so dense that light can't escape. But the thing is the material around a black hole, and especially a super massive black hole because of the black hole's extreme gravity, it gets accelerated to huge speeds, which means that it gets hot and it starts to glow, you know, in the same way that you shove a piece of metal in a fire, right? It all starts to glow like in a blacksmith forge or something. So it starts to glow incredibly brightly, so much so that super massive black holes are some of the brightest objects in the entire universe. They literally light up like Christmas trees <laugh>. But the thing is, the centres of galaxies where we find these super massive black holes, are really dusty. So we only ever see about half of them. And they're also not that bright in visible light. But they're very bright in infrared light. So one of the images that was actually released of what was called Stephan's Quintet, it was those five nearby galaxies that were all grouped together. One of the galaxies at the very, very top, if you look in the longer wavelengths of infrared light in the image that was released, there's just this giant bright, blazing thing in the center of it. And that is the gas that's swirling around the super massive black hole. And it's amazing that we're gonna be able to pick them out at that distance as well.

Julia - Now that we have these images and we're gonna be able to analyze them and we've seen the power of this telescope, how do you think this will change our thoughts on the universe?

Becky - I don't think that's gonna be an area of astronomy that isn't touched by what JWST is gonna find. And that's the thing is that we've designed it to do various different things, to see these exoplanet atmospheres, to peer through dust where stars are forming, and to see back the oldest light in the universe. Not only are we gonna get all these observations that we plan to take, but all these observations we didn't plan to take and who knows what we're going to find. And that I think is what I'm most excited for, what you call these sort of 'unknown unknowns', The things that we didn't even know to ask as we were designing this telescope. But think in 20 years time we're going to be like, 'wow, that thing that we never knew when JWST launched that now has changed everything'. We can't call what it's gonna be, but I think it's gonna be big, whatever it is.


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