James Webb Space Telescope Part 1: The Science

More astronomy today.

This week it occurred to me that we’re approaching one year to go until the launch of the James Webb Space Telescope (JWST). The JWST is very big news for anyone interested in learning about the early universe and also for anyone who has enjoyed the cornucopia of stunning imagery that the Hubble Space Telescope has brought us since its launch nearly 30 years ago.

As Hubble comes to the end of its operating life it’s easy to think of JWST as its natural replacement, but it would be much fairer to think of it as the successor. After all, we are not talking about a like-for-like replacement here. Hubble mainly operates in the visible spectrum of light with a little capability in the infrared. JWST will primarily be an infrared telescope with just a little visual ability at the red end of the spectrum.

This is a crucial difference between the two as visible light cannot pass through clouds of debris and dust whereas infrared light can. This will give JWST the ability to peer through the murk and reveal what is going on inside dusty nebulae and proto-solar systems that are still forming; indeed, learning how early solar systems develop is one of the four main science aims of the mission.

As an example of the power of this technology let’s compare the classic image of the Eagle Nebula, also referred to as The Pillars Of Creation, when viewed through visible and then infrared light.

The Pillars of Creation — visible and infrared comparison
This image compares two views of the Eagle Nebula’s Pillars of Creation captured by Hubble. On the left the pillars are seen in visible light, capturing the multi-coloured glow of gas clouds, wispy tendrils of dark cosmic dust, and the rust-coloured elephants’ trunks of the nebula’s famous pillars. The right image is taken in infrared light, which penetrates much of the obscuring dust and gas and unveils a more unfamiliar view of the pillars. Image courtesy of NASA/ESA

Both of these images are beautiful and awe inspiring, but scientifically they also compliment each other and each tells us things that the other cannot. This is why it is better to think of JWST not as a replacement to Hubble but as a successor.

This isn’t the only advantage of seeing infrared light. It was Edwin Hubble who discovered that the universe was not static but expanding back in the 1920s; literally the fabric of reality, space-time itself, is stretching apart.

Imagine a beam of white light passing through that space much as the visible light does from our own star. If an alien astronomer were surveying our star from a billion light years away that means that the light given off by our sun has taken a billion years to reach that distant observer as opposed to the eight and half minutes it takes to reach us. During that billion years the space-time that the white light passes through has been constantly stretching, this results in an equivalent stretching of the wavelength of the light. The wavelength increases from around 500-700 nanometers to 700-1,000,000 nm.

visiblelightspectrum
The electromagnetic spectrum. Image courtesy of NASA.

It might help to remember that all the light we can see along with gamma rays, x-rays, microwaves, radio waves: they are all just different parts of the same spectrum. You may have heard of the cosmic microwave background. This is light left over from about 380,000 thousand years after the big bang and has been knocking around the universe for 13.5 billion years now. It has been stretched such that it has gone even further than the infrared, it has become microwaves.

This brings us to JWST’s second and third main science objectives: scientists want to study the period from when the very first light was emitted once the universe stopped being opaque. They also want to study the very first galaxies that formed after the big bang. We’re talking about light from more than 13.5 billion years ago and so the ability to see in the infrared is imperative for such observations.

The fourth and final science goal of JWST is to look at the spectra of exoplanets. When I was a boy in the 80s we didn’t know if there were any exoplanets outside of our own solar system; now we have a catalogue of thousands. When one of those planets passes in between us and its parent star some of the light that reaches us will have passed through the atmosphere of the planet. We can look at the absorption patterns of that light to see what are the constituents of that atmosphere.

This is significant because if we find a rocky, earth-sized planet that has an atmosphere mainly composed of nitrogen and oxygen with a lot of water vapour in it and some hydrocarbons, well, that’s starting to sound a lot like home, isn’t it?

I’ve got a lot more to say about JWST but I’ll call it a day here because I know you have a lot more internet to read. Let’s call this part one: the science of JWST. Next time I’ll have a post on the engineering details of JWST like just how damn big its mirror must be. Until then.

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