Eyes in the Sky E-Book

Andrew May

• Herausgeber: Icon Books
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

Seitenzahl: 206

Beschreibung

Over 50 years ago, astronomers launched the world's first orbiting telescope to gaze further into outer space to examine anything that appears in the sky above our heads, from comets and planets to galaxy clusters and stars. Since then, almost 100 space telescopes have been launched from Earth and are orbiting our planet, with 26 still active and relaying information back to us. As a result of these space-based instruments, such as NASA's iconic Hubble Space Telescope, we know much more about the universe now than we did half a century ago. But why is Hubble, orbiting just 540 kilometres above the Earth, so much more effective than a ground-based telescope? How can a glorified camera tell us not only what distant objects look like, but their detailed chemical composition and three-dimensional structure as well? In Eyes in the Sky, science writer Andrew May takes us on a journey into space to answer these questions and more by looking at the development of revolutionary instruments, such as Hubble and the James Webb Space Telescope, exploring how such technology has helped us understand the evolution of the Universe.

Leseprobe

Hot Science is a series exploring the cutting edge of science and technology. With topics from big data to rewilding, dark matter to gene editing, these are books for popular science readers who like to go that little bit deeper …

AVAILABLE NOW AND COMING SOON:

Destination Mars:The Story of Our Quest to Conquer the Red Planet

Big Data:How the Information Revolution is Transforming Our Lives

Gravitational Waves:How Einstein’s Spacetime Ripples Reveal the Secrets of the Universe

The Graphene Revolution:The Weird Science of the Ultrathin

CERN and the Higgs Boson:The Global Quest for the Building Blocks of Reality

Cosmic Impact:Understanding the Threat to Earth from Asteroids and Comets

Artificial Intelligence:Modern Magic or Dangerous Future?

Astrobiology:The Search for Life Elsewhere in the Universe

Dark Matter & Dark Energy:The Hidden 95% of the Universe

Outbreaks & Epidemics:Battling Infection From Measles to Coronavirus

Rewilding:The Radical New Science of Ecological Recovery

Hacking the Code of Life:How Gene Editing Will Rewrite Our Futures

Origins of the Universe:The Cosmic Microwave Background and the Search for Quantum Gravity

Behavioural Economics:Psychology, Neuroscience, and the Human Side of Economics

Quantum Computing:The Transformative Technology of the Qubit Revolution

The Space Business:From Hotels in Orbit to Mining the Moon – How Private Enterprise is Transforming Space

Game Theory:Understanding the Mathematics of Life

Hothouse Earth:An Inhabitant’s Guide

Nuclear Fusion:The Race to Build a Mini-Sun on Earth

The Science of Music:How Technology has Shaped the Evolution of an Artform

Biomimetics:How Lessons from Nature Can Transform Technology

Consciousness:How Our Brains Turn Matter into Meaning

Hot Science series editor: Brian Clegg

 

 

Published in the UK and USA in 2024 by

Icon Books Ltd, Omnibus Business Centre,

39–41 North Road, London N7 9DP

email: [email protected]

www.iconbooks.com

ISBN: 978-183773-127-5

eBook: 978-183773-128-2

Text copyright © 2024 Icon Books

The author has asserted their moral rights.

No part of this book may be reproduced in any form, or by any means, without prior permission in writing from the publisher.

Typeset by SJmagic DESIGN SERVICES, India.

Printed and bound in the UK.

ABOUT THE AUTHOR

Andrew May is a freelance writer and former scientist, with a PhD in astrophysics. He is a frequent contributor to How It Works magazine and the Space.com website and has written five other books in Icon’s Hot Science series: Destination Mars, Cosmic Impact, Astrobiology, The Space Business and The Science of Music. He lives in Somerset.

CONTENTS

1 Space and Telescopes

2 Hubble

3 Probing the Big Bang

4 Exoplanet Hunters

5 Mapping the Galaxy

6 Webb

7 High-energy Astronomy

8 The Future

Further Reading

SPACE AND TELESCOPES

1

Telescopes and space: the two are virtually inseparable. Most of what we know about the universe beyond our own planet is down to telescopes. To say there are craters on the Moon, or that the planet Jupiter – no more than a bright star to the naked eye – is a giant world with moons of its own, would have sounded unbelievable before these facts were revealed by the first telescopes in the 17th century. Yet today, they have been so thoroughly absorbed into our culture that everyone takes them for granted.

The advent of space travel in the 20th century gave us a new perspective on outer space – or at least the nearby part of it represented by our own Solar System. Humans themselves have only ventured as far as the Moon, but robotic probes have travelled much further. Several have visited Jupiter and its moons, including the Juno spacecraft currently in orbit there, while New Horizons is flying through the Kuiper Belt far beyond the orbit of Neptune. Everyone has marvelled at the high-definition images sent back by these missions, such as Juno’s panoramic views of Jupiter’s swirling, multicoloured clouds and the intriguing glimpses of the 4.5-billion-year-old ice world Arrokoth captured by New Horizons. What’s rarely mentioned is the fact that the probes obtained these images using – you guessed it – telescopes.

A composite image of Arrokoth produced by New Horizons using its LORRI (Long Range Reconnaissance Imager) telescope in 2019.

NASA

As for the vast universe beyond the Solar System, no human-built spacecraft is going to travel there for a very long time. Yet, if you’ve ever been blown away by breath-taking photographs of distant nebulae or galaxies – and who hasn’t? – they were almost certainly taken by a spacecraft. Not an outward-travelling space probe in this case, but one orbiting our own planet at an altitude of just 540 kilometres. This, of course, is the Hubble Space Telescope – probably the most famous juxtaposition of the words ‘space’ and ‘telescope’ of all.

Hubble and its successor the James Webb Space Telescope (JWST) are operated by NASA, the same organisation responsible for the Apollo lunar landings and interplanetary probes like Juno and New Horizons. An American government agency, NASA stands for National Aeronautics and Space Administration – and strictly speaking, the word ‘space’ in its name refers to space travel. The wider use of the word to encompass the whole of astronomy and cosmology wasn’t originally part of the agency’s remit – but thanks to Hubble and other space-based telescopes, NASA has become a world authority in those areas too. In fact, the two uses of the word – to mean space travel and the study of the cosmos – have become intertwined. Arrokoth, for example, was discovered by the Hubble telescope in 2014, while it was searching for suitable destinations for the already-launched New Horizons spacecraft.

Yet unlike New Horizons, Hubble has never been anywhere near Arrokoth, which is around 6.6 billion kilometres from Earth. Hubble, at the best of times, is a mere 540 kilometres closer to Arrokoth. And when you turn to the majestic, star-studded galaxies that Hubble is most famous for photographing, the distances become unimaginably big – a trillion times a trillion kilometres or more – so it’s not immediately obvious that Hubble has any advantage over an Earthbound telescope at all. Understanding why it does, and why the same is true of JWST and other space-based telescopes like the planet-hunters Kepler and TESS (Transiting Exoplanet Survey Satellite), is one of the aims of this book. The other is to take a closer look at some of the many amazing discoveries made with these telescopes.

But before that, it’s worth taking a moment to consider a couple of even more basic questions. Just what is a telescope, and why is it such a useful tool for astronomers?

Telescope basics

Telescopes have been associated with astronomical observation ever since the time of Galileo Galilei, one of the great pioneers of experimental science, in the early 17th century. But the basic idea of the telescope wasn’t his. It started out as a kind of toy called a ‘spyglass’, made by placing two spectacle lenses at either end of a long tube. Looking through the tube made distant objects appear closer than they really were. In 1608, an enterprising Dutchman named Hans Lipperhey applied for a patent on just such a device, only to have the application turned down on the grounds that the idea was already common knowledge. In fact, similar toys were available for purchase in several European countries by that time.

Lipperhey-style spyglasses only really had novelty value, as opposed to any practical use, since they could only magnify an image by a factor of three or so. The amount of magnification was set by the strength ratio of the two lenses used, and since people were using ready-made spectacle lenses for the purpose, three was about the best they could do. One of Galileo’s first innovations was to produce a much stronger, custom-built lens for the eyepiece end of his telescope, allowing him to achieve a much higher magnification.

Galileo’s other great breakthrough was to use his telescope to look at astronomical objects that, up to that time, had only ever been seen with the naked eye. He used a telescope with a magnification of 30× to observe the Moon, for example, and described the results in a short treatise called Sidereus Nuncius (Latin for ‘Starry Messenger’) that was published in 1610:

It is a most beautiful and a very pleasing sight to look at the body of the Moon, which is removed from us by almost 60 terrestrial radii, and to see it as if it were only two radii away. This means that the Moon’s diameter looks almost 30 times larger … Anyone can grasp for himself that the Moon’s surface is not smooth and polished but rough and uneven. Like the face of the Earth, it is covered all over with huge bumps, deep holes and chasms.

Only parts of Galileo’s 30× telescope survive today, but it’s believed that it had a length of around 1.7 metres and a diameter of 40 millimetres at the main lens. These measurements effectively correspond to the two most important parameters of any telescope, its focal length and aperture. The first is the distance at which the main lens (or mirror, in a reflecting telescope) brings incoming light to a sharp focus, while the aperture is simply the diameter of that lens (or mirror). If you see a telescope characterised by just a single dimension, then it’s the aperture that’s being referred to. So an amateur astronomer boasting, as they’re wont to do, of having a ‘ten-inch reflector’ isn’t saying that it’s all of ten inches long (about 25 centimetres) but that it has a mirror of that diameter.

When I said a telescope has two important parameters, you may wonder why I didn’t mention the one that was so important to Galileo: magnification. But magnification isn’t really a property of a telescope per se, so much as the eyepiece that’s used with it – and that’s why we don’t need to worry about it in this book. Space telescopes don’t have eyepieces, they have sensors similar to the ones used in digital cameras. For that matter, the same is true of virtually all the ground-based telescopes used by professional astronomers these days, and many amateur astronomers too.

In a camera, the equivalent of increasing the magnification is ‘zooming in’. A zoom lens is simply one that has a variable focal length, and making this longer causes the object you’re looking at to fill a larger portion of the field of view. Inside the camera, the image of the object spans a greater number of sensor pixels at high zoom than it does at low zoom.

This effect is the digital counterpart to the magnification produced by Galileo’s eyepiece, and the technical term for it is ‘resolution’. In order to explain what this is, and how it differs from traditional magnification, we’re going to have to get to grips with one of the more brain-twisting concepts in this book – but one that’s absolutely fundamental to the way telescopes are used in astronomy. I’ll start by asking a rhetorical question: which is bigger, the Sun or the Moon? I’m sure everyone knows the answer: the Sun is by far the bigger of the two – but it’s also further away by roughly the same factor. This means the apparent sizes of the Sun and Moon in the sky are pretty much the same (that’s why total eclipses work out as neatly as they do). If you imagine looking at the full Moon (I don’t want you looking straight at the Sun, even in a thought experiment) and drawing straight lines to opposite edges of it, then the angle between the two lines would be around half a degree. That’s what astronomers call the ‘angular size’ of the Moon – and it’s the angular size of the Sun, too.

That’s the ‘brain-twisting concept’ I warned you about: astronomers like to measure objects in terms of angles rather than linear measurements. To complicate matters further, they’re usually talking about very small angles, so they divide a degree into 60 arcminutes, and an arcminute into 60 arcseconds, by analogy with minutes and seconds of time. An arcsecond is small, but not unimaginably small – about the same as a five-pence coin (or an American dime) viewed at a distance of four kilometres.

Now let’s go back to the question of magnification versus resolution. When Galileo described his 30× magnified view of the Moon, he used words anyone can understand. He said that his telescope made the Moon look 30 times closer than it really is, or 30 times bigger. He didn’t need to say anything about angular sizes, even though that’s what he was talking about. He meant that when viewed through the eyepiece of his telescope, the Moon appeared to be fifteen degrees across, compared to just half a degree when seen without a telescope. But he didn’t need to say that, because the magnification is just the first number divided by the second.

Unfortunately, we can’t apply such simple logic to a photograph taken with a digital camera. If you point your smartphone or digital camera at the Moon, set it to maximum zoom and take the best picture you can, what magnification is that? It might look tiny on the device’s screen, but what if you take it indoors and look at it on a 50-inch TV screen? What matters here isn’t how big the image looks, but how many of the camera sensor’s pixels the Moon spans. If the answer is, say, 1,000, that means you’ve got around half a degree, or 1,800 arcseconds, spanning those thousand pixels. This gives you a resolution of 1.8 arcseconds per pixel – and it turns out that’s the most meaningful analogue of magnification for a digital system.

It’s the same with telescopes, up to and including Hubble. At this point, if you’ve been following closely, you may be getting the germ of an idea. ‘What if I just buy a super-high-resolution sensor that’s millions of pixels across?’ you might be thinking. ‘Will that make my backyard telescope just as powerful as Hubble?’ Unfortunately, it’s not that simple. You might be able to get as many pixels across the image as a giant professional telescope, but the result isn’t going to be any clearer than it was with a fraction of those pixels. The laws of optics put a natural limit on a telescope’s resolution that’s inversely proportional to its aperture,* so a small telescope is never going to be as good as a large one. That’s one of the reasons that serious astronomical telescopes are designed with the biggest aperture that circumstances allow (the other reason being the obvious one that a large aperture can collect more light than a small one).

The other feature common to virtually all the telescopes used in professional astronomy, besides their enormous size, is the fact that light is collected and brought to a focus by a mirror rather than a lens. At first sight this may seem puzzling. A lens-based telescope – technically called a refractor, from the word ‘refraction’, describing the bending of light as it passes from one medium to another – was good enough for Galileo, and it’s still by far the commonest arrangement in telescopes designed for non-astronomical use. Cameras, too, have lenses at the front rather than mirrors at the back. So why do astronomers do things differently?

The biggest disadvantage of a lens stems from a basic property of refraction called chromatic aberration. This means that as white light passes through a lens, its constituent colours are bent through different angles. This can be a useful effect when we want it to happen, for example, when a prism is used to produce a spectrum of light, but it’s extremely irritating when trying to create a sharp image with a lens. Different colours from the same object come to a focus in slightly different places, creating a blurred image. In cameras and small telescopes, the effect can be lessened by using multiple lens elements, but this becomes prohibitively expensive for the larger apertures needed by astronomers.

A better solution is to use a convex mirror to focus the light instead of a lens. Mirrors, of course, work by reflecting light from a surface, rather than refracting it through a material, so they don’t suffer from chromatic aberration. A mirror also has the advantage of only needing to be shaped on one side, so it’s easier to make. The latter point made reflecting telescopes popular with amateur astronomers back in the days before space became a hot topic, when astronomy was such a niche hobby that you couldn’t buy a good-quality, ready-made astronomical telescope in the way you can today. Instead, people had to make their own telescopes, using a simple design invented by Isaac Newton a few decades after Galileo’s time.

Despite their different optics, Newtonian reflectors have one very basic thing in common with refracting telescopes: the length of the tube is approximately the same as the telescope’s focal length. But if you look up the specs of any large astronomical telescope, whether in space or on the ground, you’ll find that they generally have a focal length that’s much longer than the physical length. That’s because instead of Newton’s design, they use one developed by Laurent Cassegrain, a French contemporary of Newton. He came up with a clever way to ‘fold’ the light rays inside a telescope tube that results in a more compact and practical arrangement than Newton’s. The downside of Cassegrain telescopes is that they’re more complicated to build, which is why amateurs traditionally stuck to Newtonians,* but the Cassegrain has always been the design of choice for professional astronomers.

A simplified diagram illustrating the basic principle of a Cassegrain reflecting telescope.

If the purpose of a telescope was simply to produce an image of a distant object – which, for most amateur astronomers, that really is all there is to it – then we’ve already covered all the basic physics we need to know about the subject. In professional astronomy, however – the kind that’s done with Hubble and the like – there’s much more to using a telescope than creating pictures. In reality, astronomers don’t spend any more time than the rest of us looking at all those stunning images the Hubble press office puts out. Those are really just ‘outreach’ for the general public’s benefit.

Science is much more about measurements – putting hard numbers on things – than images. There are some measurements, such as the diameter of a galaxy or the way brightness varies across it, that can be gleaned directly from a digital photograph, but astronomers need to know more than that. If you’ve ever read an article about a distant galaxy, for example, it may have referred to the speed at which the stars inside it are moving, or what chemical elements they’re composed of. You can’t get that kind of information by simply looking at a photograph, so where does it come from?

To answer this question, we need to look in a bit more detail at the nature of light. It’s a common enough word, and one that I’ve used multiple times in this chapter already, but what is light, exactly? Confusingly, there are two possible answers, both of which are true even though they sound like they contradict each other. Ultimately, it all depends on what the light is doing. If it’s landing on a digital sensor inside a camera or telescope, or the retina of your eye, then you can think of it as a stream of discrete particles called photons, each of which carries a specific amount of energy that the sensor detects. When the light was emitted from its source, whether that was a light-emitting diode (LED) in your room or a star in a distant galaxy, it behaved like a stream of discrete photons too.

Alternatively, when the light is anywhere between the emitter and detector – whether it’s travelling through empty space or some other medium like the Earth’s atmosphere or a lens – it behaves more like a wave than a stream of particles. Just what it is that’s doing the waving is something we’ll come back to later, but one thing these waves have in common with photons is the speed they travel at. In empty space, that’s around 300 million metres per second – which, needless to say, is very fast. In fact, it’s worth taking a moment to look at the implications of this speed, before getting into more detail on the wave nature of light.

If an LED in your room is three metres away, the light from it takes just a hundredth of a millionth of a second to reach your eye, which is so fast as to be almost instantaneous. But it’s not literally instantaneous, as we can see if we think about a light source further away, such as the Sun. At a distance of 150 million kilometres, it takes sunlight 500 seconds, or just over eight minutes, to get to us. When we move on to other stars beyond the Sun, the light from them takes years to reach us. They’re so far away that it becomes meaningless to talk about their distance from us in kilometres, since most people’s minds start to boggle at any figure greater than a trillion. Yet a trillion kilometres doesn’t even get you to the nearest star, Proxima Centauri.

It’s much easier to say that Proxima is 4.2 light years away, meaning that light from it takes 4.2 years to reach us (or, equivalently, that when we look at it, we see it as it was 4.2 years ago). The light year is one of the most useful concepts in astronomy, and one that we’ll encounter many times in this book. It’s a great way to talk about astronomical distances without using mind-bogglingly huge numbers. For example, the centre of our own galaxy is around 30,000 light years away, while the distance to Andromeda, our closest neighbouring galaxy, is 2.5 million light years, and the most distant galaxies observed by Hubble are several billion light years distant.

The speed of light also helps to understand two properties associated with the wave theory of light, namely wavelength and frequency. These terms are in common usage in the context of radio waves, but when it comes to light, they’re much less familiar than an everyday word that’s closely related to them – and that’s colour. Take green light, for example, which has a wavelength of around 500 nanometres (500 billionths of a metre). If you picture a wave where the crests are separated by this distance, travelling past you at the speed of light (300 million metres per second), then you’ll count 600 trillion crests, or wave cycles, per second.* Since the prefix for trillion is tera, and the technical term for ‘cycles per second’ is hertz, this means the frequency of a green light wave is 600 terahertz.

Looking at the colour of light from a star or nebula is useful to astronomers because it tells them something about how that light is produced. At this point, we need to briefly switch back to the particle view of light – which, as I said a moment ago, is more relevant to the emission and absorption of light than the wave model. It turns out that what looks like frequency when you think of light as a wave corresponds to the amount of energy per photon in the particle model. The hotter an object is, the higher the energy of the photons it emits – and hence the higher the frequency when the resulting light is viewed as a wave. In terms of colour, blue light has a higher frequency (and shorter wavelength) than red light – so that means, for example, that a blue star is hotter than a red star. You might think that things should be the other way around because the standard convention – for example, on weather maps or bath taps – uses red to mean hot and blue to mean cold. But if you think of metal being heated up in a forge, you can see that the convention is actually wrong: metal glows red first, then changes to blue-white as it gets hotter.

You can see the colour of a star, and hence estimate its temperature, directly from a photograph. But a lot more information can be gained if you break the light down into a spectrum, separating out all the different colours (or frequencies/wavelengths). We already know one way to do this, via the bending of light through a lens, because it’s the source of chromatic aberration in refracting telescopes. A prism is a much more effective approach, and the first spectrum-producing instruments were indeed based on the refraction of light through prisms.

Those early instruments involved looking through an eyepiece like a traditional telescope and were called spectroscopes by analogy. The basic process of analysing spectra is still referred to as spectroscopy to this day, although the instruments used – which nowadays project the spectrum onto digital sensors – are called spectrometers or spectrographs (two terms that, as far as I can tell, mean exactly the same thing). Modern ones, such as those on board the Hubble telescope, don’t use prisms but a different physical principle called diffraction. But the end result is the same – they split light into a spectrum.