The Physics of the James Webb Space Telescope

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James Webb The space telescope, also known as JWST, finally launched on December 25, traveling 930,000 miles from Earth. It is the next generation that will replace the famous Hubble Space Telescope. Hubble has been photographing great for more than 30 years, but it’s time for something better. JWST will be tasked with using its infrared sensors to explore some of the most distant and hard-to-see parts of the sky, help search for exoplanets, and explore the early days of the universe. So it seems like a good time to move on to the most important scientific concepts related to space telescopes.

Why put a telescope in space?
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You can see all kinds of cool things from Earth, like nebulae and comets, with just a few binoculars or consumer binoculars. But if you want research-quality images of distant galaxies, you’ve got a problem: wind. You might think that air is transparent, but that is only partially true.

Light is an electromagnetic wave, and it can have different wavelengths. People can only see a narrow range of wavelengths from 380 nanometers (1 nm is 10).-9 m) to about 700. Our brain interprets the tall ones as red and the short ones as violet. These wavelengths are able to pass through the atmosphere without much reduction in brightness – so we can say that the air is transparent to visible light.

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However, for other wavelengths of light that we cannot detect with our eyes, air is not so transparent. If we consider the infrared region of the electromagnetic spectrum (or wavelengths longer than red), most of this light can be absorbed by both water vapor and carbon dioxide in the atmosphere. (Yes, it’s the same thing that happens with global warming: When visible light hits the Earth’s surface, the temperature rises and it emits infrared radiation. The carbon dioxide in the air tends to raise the temperature of the atmosphere further. Absorbs some of the infrared. This can lead to bad things for humans.)

This light absorption is also a particular problem for ground-based infrared telescopes. It would be like trying to look at the sky through the clouds—it just won’t work.

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One solution to this problem is to place the telescope where there is no wind: in space. (Of course, with every solution come more challenges. In this case, you’d actually have to put a highly sensitive scientific instrument on a rocket and launch it, which is a bold move.)

Why does JWST see infrared light?

JWST actually sees two Categories of infrared light: near-infrared and mid-infrared. Near-infrared light is very close to visible red light with wavelengths. It’s the wavelength that your TV remote uses (if you can find it—it’s probably under the couch cushion).

Mid-range infrared is often associated with heat, and this is mostly true. It turns out that everything produces light. Yes, you are sitting there lighting up. The wavelength of light emitted by an object depends on its temperature. The hotter it is, the shorter the wavelength of the light. Therefore, while you cannot see emitted light in the mid-infrared range, sometimes you can see perception This.

Try this: Turn on the stovetop in your kitchen, and place your hand on the burner but don’t touch it. As the element heats up, it generates infrared light. You can’t see this light, but when it hits your hand, you can feel it as heat.

Although you can’t see this kind of light, an infrared camera can. Check out this infrared image of me pouring a hot cup of coffee:

Photo: Rhett Allen

This is a false color image. Basically, the camera mapped colors — from yellow to violet — at different wavelengths of infrared light. The bright yellow parts (like the coffee pot) represent hot things, and the dark purple parts are cold. Of course, the reality is much more complicated than that (you can even reflect infrared light), but you get the idea.

Great. But Why Does JWST see infrared light? The reason for this is the Doppler effect.

You already know about the Doppler effect. You can hear it when a train or car passes you at high speed: the sound frequency changes as the source is first moving towards you, and then later away from you. Vehicle sound has a shorter wavelength, and therefore a higher pitch when coming towards you, and then a longer wavelength and lower pitch when moving away. (Here’s an older post with more details).)

It just so happens that you can get the Doppler effect with light too—but since the speed of light is very fast (3 x 10)8 m/s), the effect is not noticeable in many situations. However, due to the expansion of the universe, most of the galaxies we see from Earth are moving away from us. So it appears to us that their light has a longer wavelength. We call this redshift, which means that wavelengths are redder because they are longer. For very distant objects, this red shift is so great that interesting things are in the infrared spectrum.

There’s actually another good reason to use infrared light for JWST: It’s difficult to get an unobstructed view of distant celestial bodies because of gas and dust, which are dirt emanating from older stars. These can scatter visible light more easily than infrared wavelengths. Essentially, infrared sensors are able to see through these clouds better than visible light telescopes.

Since JWST is observing in the infrared spectrum, scientists will need to get as dark as possible around the telescope. This means that the telescope itself must be extremely cold to avoid emitting its own infrared radiation. This is one reason it has a sunshield. This will block sunlight from the main appliances so they can stay cool. It will also help remove excess light so that the telescope can pick up comparatively dim light from exoplanets as they orbit their more bright host stars. (Otherwise, it would be like trying to see in the dark while someone shines a flashlight on your face.)

How does JWST look back in time?

Light is a wave that travels really fast. In just one second, light can travel around the circumference of the Earth more than seven times.

When looking at celestial objects, we have to take into account the time taken for light to travel from that object to our telescope or eyes. For example, light from the nearby Alpha Centauri star system takes 4.37 years to reach Earth. So if you look at it in the sky, you’re looking at literally 4.37 years ago.

(Actually, everything you see is in the past. You see the Moon about 1.3 seconds early. When viewed closest to Earth, Mars is three minutes earlier.)

The idea is for JWST to be able to see more than 13 billion years into the past, up to the point in the evolution of the universe when the first stars were forming. That’s great, if you think about it.

What is Lagrange Point?

The Hubble Space Telescope is in low Earth orbit, which is good because it has become possible for astronauts to serve it when they need it. But the JWST is going to be too far at the L2 Lagrange point. But what is the Lagrange point?

Let’s consider Hubble orbiting the Earth. Any object moving in a circle must have a centripetal force or a force that pulls it towards the center of the circle. If you spin a ball around your head on a string, the force pulling it toward the center is the tension in the string. For Hubble, this centripetal force is the force of gravity due to its interaction with Earth.

As an object moves away from the Earth, the strength of this gravitational force decreases. Therefore, if the telescope moves to a higher orbit (a larger circular radius), the centripetal force will decrease. To remain in the circular orbit, Hubble would have to take more time in orbit. (We would say that its angular velocity is less.)

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