Figure 1 A nebula captured by the James Webb telescope
This extraordinary picture was splashed across websites and news pages throughout the world in July 2022. It was one of the first images relayed back from the remarkable James Webb Space Telescope, launched by an international collaboration in December 2021. Sitting out in space, thousands of kilometres from the Earth, with its back to the Sun, this uniquely powerful new telescope is able to see further across the universe than ever before and, as a consequence further back in time.
The image in Figure 1 shows a region of space where new stars are being formed, known as a nebula. The part of the image with a clear blue background shows newly formed stars shining as bright spots. The red-brown irregular-shaped area shows the edge of a vast region of dust and gas from which stars are gradually developing. The pressure of intense radiation from the fully-formed stars in the blue region, and a wind of particles blowing from them, are pushing back on the dust, effectively sculpting the red-brown region into the extraordinary shapes, that look like mountains and valleys.
Images from the James Webb Space Telescope and its predecessor, the Hubble telescope, bring a new sense of wonder to our understanding of the universe – and a whole set of new questions. How can something looking as solid as a mountain scene be made of dust and gas? Is the universe full of dust? How do stars emerge out of it? How are these new telescopes able to bring us such dramatically improved images of the distant universe? How can we make sense of the distances and times involved?
Nebulae – the birthplace of stars
The so-called ‘dust’ in these images consists of minute solid particles, less than a thousandth of a millimetre across, much smaller than everyday house dust. The accompanying gas in the same regions is mainly molecules of hydrogen. Yet the size of the dust- and gas-filled region captured in this image is simply enormous. The ‘mountain-like’ features are several light years across – that means it takes light many years to cross them – a distance of tens of millions of millions of miles. The area captured in this image is not an arbitrary region of outer space, it is a specific kind of celestial feature: a nebula. The name, based on the Latin word for cloud, was given to a variety of bright objects which were seen through earlier telescopes to be diffuse. By studying the light emitted from nebulae, it became clear in the last century that they in fact comprised a multitude of stars and it was light from these that illuminated the apparent cloud. Many of them turned out to be galaxies – vast clusters of many stars, like the Milky Way. Today’s telescopes are revealing these stars as being embedded in vast regions of dust and gas. But, solid though these dusty regions may appear in the image above, the density of matter in them is, in fact, extraordinarily low – just a few kilograms of it would take up the space occupied by Earth. Although this means nebulae are amazingly diffuse, they are nevertheless a lot denser than the endless regions of the universe between the galaxies and stars– the so-called interstellar medium. Here the molecules of gas are so sparse that they constitute a vacuum ten thousand times greater than the best vacuum we can make on Earth. Space is a very empty place!
The extremely low density of matter in a nebula is hard enough to imagine; even more remarkable is the process that leads to the formation of stars within them. Out in space these freely moving molecules of gas and minute particles of dust simply roam around in each other’s remote presence, unaffected by anything other than the gravitational pull of each other. Miniscule though the gravitational pull of one molecule is on another, it is the only influence acting on them out there. As a result, over millions of years the sparse molecules and particles gradually join together to form tiny chunks of matter. These in turn generate larger gravitational forces of attraction, clumping together into ever larger assemblies. These eventually collapse under ever increasing gravitational attraction forming dense regions within the clouds destined to become stars. Speeding up, as they collapse, the molecules form an ever hotter gas. The rising pressure and temperature of the collapsing gas eventually reaches such extreme values that the nuclei at the centre of the atoms fuse together in nuclear fusion reactions, releasing energy on a prodigious scale. It is this energy that fuels the new star, enabling it to radiate out light, infrared and radiation of other frequencies – just as our Sun does. It is this that our telescopes capture once it reaches us, years or millions or even billions of years later.
Infrared wavelengths
The James Webb Space Telescope has been particularly designed to pick up invisible infrared radiation, which has a wavelength longer than the visible light that human eyes are able to see (Figure 2). We sense it as heat.
Figure 2 Spectrum of wavelengths of electromagnetic radiation
The visible light emitted within a dust cloud is mostly absorbed by particles in the cloud itself and so doesn’t reach us here on Earth. Infrared wavelengths, on the other hand, do get through these clouds and can be picked up by appropriately designed receivers on a telescope. It is this that makes the extraordinary ‘landscape’ of dust and gas apparent when images from the telescope are processed to produce colourful images such as that in Figure 1.
By picking up infrared radiation rather than more usual visible light, the James Webb telescope has another important advantage over its predecessors. The wavelength of radiation from distant stars is lengthened on its journey to us as it pulls away from the mass of the nebula in which it originated. This stretching, that occurs when light passes by a huge mass, was predicted by Einstein in his 1915 General Theory of Relativity. It means that the wavelength of light that started out from its source in the visible range (i.e. the colours of the rainbow), gradually shifts towards (and beyond) the red end of the spectrum. For very distant objects this so-called redshift goes further: the wavelength of the radiation is shifted beyond the visible region of the spectrum altogether, into the invisible infrared zone. By analogy, when an electric fire cools down the wavelength of its radiation gradually lengthens, shifting its colour from yellow, to orange to red, to invisible infrared (detectable by us as heat). For this reason, the most distant stars and galaxies are not visible at all to us in the ordinary sense: neither our eyes nor our optical telescopes can see them. They can only be detected by instruments capable of picking up radiation in the infrared zone of wavelengths.
Technological advances
The extraordinary clarity with which we are now able to see structures in the universe so far from us is the result of two major leaps in technology: the placing of satellites out in space rather than on mountain tops, well beyond our distorting atmospheres, and the deployment of a large diameter mirror capable of gathering very feint light.
The Earth’s atmosphere complicates the detection of infrared radiation from distant stars and galaxies. The molecules in it absorb this radiation, blocking out signals from very distant objects. In addition, being warm, the atmosphere confuses the picture by emitting infrared radiation itself. To overcome these problems, telescopes have been put into orbit in outer space, well away from the Earth’s atmosphere. The earlier Hubble Telescope sits approximately 500 km above the Earth’s surface; the James Webb one is much further away – hundreds of thousands of kilometres distant.
The new telescope has another vital advantage over its predecessor: the size of its mirror. The ability of any telescope to detect feint objects depends crucially on the diameter of its mirror. The larger the surface area of the mirror, the greater the quantity of light it picks up. As with the aperture of a camera or the pupil of your eye, a bigger diameter opening allows dimmer objects to be detected. Putting a large mirror out in space, however presents a knotty problem: how to get it there within the confines of a rocket. The diameter of the mirror in the earlier Hubble telescope was limited to 2.4 metres so it would just fit inside the tube of the rocket.
The ingenious idea for launching a larger mirror was to make it out of smaller pieces, which could be assembled later. The JWST mirror comprises eighteen hexagonal pieces (Figure 3) which fold up like the wings of a swan, inside the rocket that launched it.
Figure 3 Model of the James Webb Space Telescope
This beautiful design idea resulted in a mirror 6.5 metres across when unfolded – nearly three times wider than its predecessor, seven times greater area and power.
The engineering challenge was to package up the parts of the telescope within the launch rocket (Figure 4) and to attach electric micromotors capable of driving the pieces into their precise final positions.
Figure 4 JWST folded up for insertion in the rocket.
Temperature control
An even greater engineering challenge lay in the problem of temperature control. Any object that is itself warm will emit infrared radiation – that’s how thermal imaging cameras work when they detect animals at night or heat losses from your home. A telescope that is warm and giving off infrared radiation will clearly be useless at imaging the same kind of radiation emanating from distant stars. To avoid this kind of contamination, the whole apparatus has to be cooled dramatically: to below -220°C. Outer space is pretty cool, but heat radiating from the Sun and Earth, impinging on the telescope, would quickly warm it well above this limit. To counter this, five aluminium-coated reflective layers, each as thin as a human hair, were placed between the Sun and the mirror to protect it from the glare (Figure 2). The effect of these protective layers is to maintain the mirror and instruments at a steady -233°C. It’s hard to imagine such a temperature, more than 200 degrees below the freezing point of water; but of course there’s no water up there to freeze, just very cold metals and silicon chips.
Extreme numbers
Mention of extreme temperatures, reminds us of a recurring problem for the lay person trying to grapple with astronomical information: how does one make sense of the extreme numbers bandied about so freely by experts? To be told that a galaxy is 8,500 lightyears away and a lightyear is the distance travelled by a beam of light in a year – ten million, million kms – just leaves you dumbfounded. Similarly for the vast populations involved – a single galaxy may contain hundreds of billions of stars, and the number of galaxies in the universe may be of the order of trillions. The length of time periods in cosmology is equally incomprehensible. How does a human being, brought up in a world of days, weeks and years, respond to the fact that the universe is about 13.5 billion years old or the Sun will run out of hydrogen fuel in five billion years?
Although experts and their followers appear to talk easily about such extreme numbers, my experience of talking with lay people is that such numbers can leave them initially stunned and, ultimately, disengaged. It seems impossible to respond meaningfully to them. My suspicion is that, for most of us, trying to grasp a new scientific idea involves drawing on some kind of visual metaphor, an image of something familiar. Electric current flows like a river; DNA is like a blueprint; atoms are little hard balls. It seems impossible to apply this kind of reasoning to things on the cosmological scale, by using imagery from human experience.
So how can we make sense of these numbers? A starting point is to develop an imaginative sense of relative size, even if the absolute value is unimaginable. This is what enables scientists to talk in an everyday way about extreme numbers. The discoveries of the James Webb telescope provide good examples of this.
Figure 5 A galaxy cluster captured by the James Webb telescope
The image in Figure 5, taken with the JWST is of a galaxy cluster – a large group of galaxies. Although it shows hundreds of apparently similar looking spots, the light from one spot may have spent 3.4 billion years reaching us whereas that from another has taken 6.8 billion years. Although these huge numbers may be difficult to visualise in themselves, the difference between them tells us clearly that one is twice as close as the other. Information such as this gives us a clearer picture of the layout of the universe.
In a similar way, estimates of the age of specific galaxies, unimaginably old though they may be, help us develop a chronology of events in the universe. From this we can begin to trace its history back to its earliest days.
To simplify the practical handling of huge numbers, scientists use powers of ten rather than long rows of digits. Thus an object 4.6 billion lightyears away and another 46 million years away can be more easily compared using the form 4.6 x 109 and 4.6 x 107 respectively. The small number (exponent) simply shows how many times the main number is multiplied by ten. It is easily seen in this example that the first object is a hundred times (102) further away. This notation enables periods of time in the universe’s history to be written simply. Thus the age of the universe is estimated to be 13.7 x109 years. Light recorded by the James Webb telescope from one extremely distant galaxy started out 13.1 x 109 years ago. Again, although such lengths of time are unimaginable in the ordinary sense, the numbers tell us in an understandable way that the JWST is able to see back to near the beginning of the universe: 13.1 units of time out of the 13.7 since time began.
It’s this telescope’s ability to detect such ultra-feint objects that is key to its importance. By seeing objects so far away, it is able to pick up light that has spent over 13 billion years travelling on its journey to us. This light will therefore have set out in the early days of the universe. The information it carries comes from the early universe. Thus, by studying the properties of this light we are in effect looking back at the universe as it was long, long ago. We will learn more about its chemical composition and structure at that time.
The structure of the universe
The unprecedented detail revealed in Figure 5 demonstrates several important features of our current understanding of cosmology. The varying colours indicate the chemical composition of the various objects in the image. For example, red indicates the presence of dust; green, of hydrocarbons. Comparing this information from different sources enables us to trace how galaxies evolve over time – first coming together, then growing and perhaps merging. The streakiness or stretching of light around a short arc, is reminiscent of the distortions that can be introduced by a lens. Indeed this is known as “gravitational lensing” and is due, like optical lensing, to the bending of light. In this cosmological case, however, it is not a glass lentil that does the bending but the very nature of space itself. Einstein had predicted in the early 1900s, as a result of his theory of General Relativity, that the dimensions of space are not just the rigid lines we expect of three dimensional space – height, width and depth – but are curved in the presence of matter. For the ordinary quantities of matter we are accustomed to on Earth, the degree of curvature is undetectably small; that’s why we understand the dimensions in our world to be straight and rigid. But huge amounts of matter, such as that contained in a cluster of galaxies, have a noticeable effect. As light (or other radiation) from behind a galaxy cluster passes through the huge mass of the cluster, it follows a slightly curved path. The result is the appearance of a streak of light where a simple spot might have been expected.
Figure 5 also shows that galaxies are not isolated and remote from one another. Research in recent years has gradually revealed that matter is not scattered randomly or uniformly across the universe but is gathered together at various levels. The simplest level is the star, of which our Sun is an example. Stars aggregate together in huge agglomerations called galaxies, as we can see with our own eyes when we look up at our local one: the Milky Way. It alone contains hundreds of billions of stars. Recent observations reveal that structure occurs at higher levels than this, too. Galaxies themselves are grouped together in vast numbers, forming what are known as ‘galaxy clusters’. Figure 5 is a depiction of such a cluster. Amazingly, the patch of sky covered by this image is as small as that of a grain of sand held at arm’s length – it’s amazing how many galaxies can be seen in such a tiny speck of space in the night sky! This give one a sense of just how many galaxies must populate the whole dome of our heavens and also of just how powerful the James Webb telescope is, in being able to separate out light from so many objects in such a tiny patch.
To complete this description of cosmological structure, recent research shows that galaxy clusters are themselves grouped into even more massive ‘super clusters’ and these in turn are arranged non-randomly in so-called ‘filaments’. The image in Figure 6, taken in 2015, confirms the idea that matter is not evenly distributed across the universe. It shows how radiation remaining from the earliest stages of the universe (known as the Cosmic Microwave Background) is distributed unevenly across the universe. At the largest scale, matter appears to be clumped together in patches across the universe with voids in between. The hunt for an explanation of this is leading scientists to conjecture that some kind of invisible matter – dubbed ‘dark matter’ – permeates all space and may provide a kind of network of lines along which visible matter is clustered.
Figure 6 map of cosmic background radiation across the universe
The nature of ‘dark matter’ is just one of the many new questions thrown up by this exploration of cosmological research. How did the universe begin, is another; what are black holes, yet another. But there’s surely a limit to how many impossible things one can think about in one sitting. These further questions will have to await a further blog, in the months to come.
An amusing footnote to this story appeared on social media as this blog was going to press.
A French physcist posted this image of what he claimed to be a photo of our closest star Alpa Centauri, taken by the James Webb Telescope. It was in fact a slice of chorizo, the Spanish saugage.
Though a fairly harmless joke, it’s a useful reminder to keep on our toes when judging social media comments, even from apparent authorities!
© Andrew Morris 4th August 2022
Getting to Grips with Science 