Astroquizzical

A Curious Journey Through Our Cosmic Family Tree

Jillian Scudder

• Publisher: Icon Books
• Year: 2018
• Print ISBN: 9781785783340
• eBook ISBN: 9781785783357
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

In this enthralling cosmic journey through space and time, astrophysicist Jillian Scudder locates our home planet within its own 'family tree'. Our parent the Earth and its sibling planets in our solar system formed within the same gas cloud.Without our grandparent the Sun, we would not exist, and the Sun in turn relies on the Milky Way as its home. The Milky Way rests in a larger web of galaxies that traces its origins right back to tiny fluctuations in the very early universe.
Following these cosmic connections, we discover the many ties that bind us to our universe.Based around readers' questions from the author's popular blog 'Astroquizzical', the book provides a quirky guide to how things work in the universe and why things are the way they are, from shooting stars on Earth, to black holes, to entire galaxies.
For anyone interested in the 'big picture' of how the cosmos functions and how it is all connected, Jillian Scudder is the perfect guide.

1

A HUMAN PERSPECTIVE

The view from home

As with any hunt for our own human ancestors, we begin our journey up the cosmic family tree with ourselves. How do we see the universe from where we stand? As both the children of this vast cosmos, and the only ones (to our knowledge) who are attempting to chart our cosmic lineage and understand the rules of the universe, our own perspective is a unique one. Our sense of what the ‘big picture’ is, as well as how we might fit into that picture, is very much affected by how the stars appear to us. As we have developed more advanced ways of viewing the night sky, our sense of just how big the picture truly is has only grown in scope.

Our view of the cosmos is almost always from the surface of our parent planet, Earth. A select few members of our human race have had the privilege of observing our planet from a loftier perch, but with the exception of those astronauts, all of humanity has observed the stars and planets from the ground. It’s at night, with our atmosphere protecting us from the freezing void of space, and our own star no longer flooding our planet with light, that we can see the first glimpses of our immediate cosmic family.

Many of us, on a clear night, will look up to find ourselves briefly captivated by the shining of a bright object in the sky. Some of the brighter lights we can see in the night sky are our Earth’s planetary siblings, formed out of the exact same cloud of dust and gas that generated our own home 4.5 billion years ago (more on this in Chapter 3). But even without the planets overhead to shine extra brightly, the night sky can be dazzling, especially if you happen to find yourself away from the lights of the city streets.

Without interference from artificial lights, thousands of stars are visible even to our unaided eye, but these skies are increasingly difficult to find. According to a 2016 study by Fabio Falchi, 99% of the US and European populations live under light-polluted skies. It’s easy to forget, or to have never seen, just how many stars are visible to us from Earth.

But just as the images from the International Space Station can remind us of the curvature of the Earth, photographers who have the means to travel to the few remaining truly dark places can capture the night sky in those remote spots, reminding the rest of us of what we’re missing.

Many photographers aim their lenses at the Orion Nebula. It’s both a very aesthetically pleasing part of the sky, and a very bright one, so it’s easy to capture a number of stars in the image. Looking at some of these photographs of the night sky, and then looking at the version above your own homes, it may seem that the images have been exaggerated somehow, or that the number of stars has been digitally increased. This isn’t the case – but a camera has an advantage that our own eyes can’t access. Our eyes are relatively small light-capturing devices, and we can’t increase the exposure time in the same way that you can on a camera to catch even faint light.

Many of these photographs of the night sky (see color plate 1), instead of being exaggerated, come instead from a very long time spent observing the sky with a much larger lens than our eye. The longer you point your camera at a specific part of the night sky, the fainter the starlight you’re able to capture. Once the light has been compiled together, we arrive at an astounding view of what our night sky looks like, beyond the limitations of light pollution and the small size of the human eye.

No special filters are required here; many of these photographs are taken by regular digital cameras – slightly fancier than the one in your phone. The astronauts on the International Space Station, for all they have a unique position from which they can take their photographs, have the same technique as the photographers on the ground. If you take a series of 10- to 30-second photos of the sky, you can then assemble your series of images into a single, much more detailed record. The stars that exist in all of the exposures should pop up more brightly in the assembled image, and anything that happens to show up in one 30-second window but not another, will fade.

To capture the faintest stars in an image where many thousands of other stars will appear, you have to take into account the rotation of the Earth. The Earth rotates every 24 hours, of course, and if you want to take images of a single set of stars over the course of several hours, they will be moving dramatically as the Earth rotates us into a different direction. To counteract this, many of the deepest images are taken by attaching the camera to a mount which can pivot as the Earth turns, constantly correcting for the spin of the planet. With this technique, you can take even more images to assemble together, allowing you to bring out the light from fainter and fainter stars as you spend more time taking photos.

The color of the sky

Images of our cosmic relatives also come to us from beyond our planet’s surface. Pictures from the Hubble Space Telescope, for instance, have revealed that the world beyond Earth is a vivid and highly detailed one. But this vividness can be a puzzle; fundamentally, these images are not being taken by a human eye, and in many cases they don’t represent what a human eye would see, if we could travel to experience these vistas (see color plate 2).

The human eye has a really unusual sensitivity pattern to light. We’re pretty good at seeing things in the yellow-green range, orange we can usually do, but once you get into reds and blues, our eye suddenly gets extremely bad at registering deep reds and dark purples, and our brain translates those colors into ‘black’, or more accurately, as ‘there is no light here that I can deal with’. To anything outside the range of visible light, we are completely blind. This odd sensitivity pattern means that it’s quite difficult to make a camera with exactly the same sensitivity as our eye. This is the same reason why it’s sometimes hard to get your own camera to pick up the colors you can see by eye. Most cameras have settings nowadays to help change the sensitivity towards a specific color, but they won’t perfectly replicate the eyeball’s experience.

If you want to make a color picture from an image coming from a space telescope, there is an additional challenge to overcome. All the cameras attached to telescopes are just photon* counters – if a photon makes it through the telescope and into the camera, it adds 1 to the number of photons that arrive from that patch of the sky. This means that the only images you can make are intensity maps – black and white images. For scientific purposes, astronomers are generally more interested in measuring the amount of very specific color slices of light that arrive to the telescope. In order to limit the kind of light that actually makes it to the telescope’s camera, filters are usually put in front of it. The filter works in the same way as red-blue 3D glasses and images: the red lens lets through only red light, and the blue lens lets through only the blue, so each eye gets a different picture, and your brain reconstructs the depth of the image.

An astronomical filter is usually constructed to let in light from a very specific physical process – for instance, the color of light that hydrogen produces when it is in a very hot environment. Hydrogen here produces a deep pink color, so instead of a red or blue filter, we’d have a deep pink one. This would let in only light that is produced by that hydrogen, and we can map the locations of that gas on the sky. This image is still entirely in black and white, but it is the astronomer’s map to untangling what’s happening in that part of the sky.

But to reconstruct a colorful image out of this black and white one is no simple task. Given that we’re detecting light at much better sensitivities than the human eye, and that we’re usually doing it in discrete chunks instead of one (very complex) curve as the eye does, putting these chunks of light back into a single image is a tricky business. Even when all the light is taken from the narrow range that we can see, it must still be reconstructed and tweaked to reflect the brilliance of the colors we’ve observed. Hubble has produced many beautiful images (such as the nebula in plate 2) labeled as ‘visible light images’. What this means is that the narrow ranges of colors that Hubble observed all fall within the range of light detectable by our eyes – but they have still been patched together, the colors of each set of data overlaying each other to build an image in full color. In this particular case it made a lovely and vivid image, but it is still only rendered with six colors, each color coming from its own black and white photograph of the sky. In other words, the images come out of the telescope as black and white, but each is assigned a color and then reassembled. While the general term for this style of image is ‘false color’, the colors here aren’t actually ‘false’. The deep pink glow of hydrogen will remain deep pink, and the glow of oxygen, a brilliant aqua, has stayed that color.

‘Exaggerated’ color images can be used to extend our sight much beyond what we can actually see. Perhaps a galaxy is rather unimpressive in visible light, but has a stunning brilliance in the ultraviolet or X-ray. To our eyes this is dark; but a black and white image from a telescope sensitive to that light can be added to our collection, allowing us to construct an image. In these cases, a color too blue for human eyes is often added as a vivid blue or purple, and a color too red for us is added as a bright red or purple. Sometimes these composites are scientifically instructive, but most of the time they are created to harness the power of an illustration, and are manipulated to reflect the beauty of the image.

Atmospheric problems

It’s easy to forget that from the ground, our view of our cosmic family is strongly influenced by the presence of our atmosphere. Even when the skies are clear, our atmosphere can pose some barriers to seeing the stars as clearly as we might like. We humans are not used to thinking of the atmosphere on our parent planet as much of a barrier, in part because we move so easily through it, and it’s transparent to the light most of us use to navigate through our world. But simply being transparent to light doesn’t mean that light doesn’t change as it encounters our atmosphere – and it usually does. If you’ve ever seen the stars twinkling overhead in the view from home, you’ve witnessed one of these transformations.

When the stars overhead appear to flicker and change in brightness, that’s the atmosphere at work. The stars themselves are quite stable, and aren’t changing the amount of light they produce and send towards our little planet. We can check this by waiting a few days – if you go out on a clear, still night, you should find that the stars hang quietly in the night sky, not a twinkle to be spotted. And if you were fortunate enough to observe the stars from space, you would see them as perfectly point-like pricks of light, no matter how twinkly they appeared from the ground.

Back on Earth, if the wind has picked up, you should be able to see the stars twinkling their hearts out. You might see something similar if you look to the stars near a low horizon (sorry, city dwellers). Even if the stars straight overhead seem stable, the ones closer to the horizon may appear unsteady.

Whenever light encounters a gas – and our atmosphere is all gas – the direction the light is pointed changes slightly. How much the light bends depends on a number of things, but the density of the gas is one of them. How densely packed the gas is depends strongly on its temperature, so warm air bends light to a slightly different angle than cool air does. The higher you are in the atmosphere above our planet, the cooler the temperature, but this isn’t a completely smooth transition from warm to cold. These temperature changes exist in little bubbles of air, packed against one another.

These little bubbles act like a series of lenses suspended above us, twisting and distorting the light on its way through. If the air is calm, these air-pocket lenses are relatively large, so the light travels through fewer of them, and has fewer deflections on its way down to the ground. Similarly, if the temperature isn’t changing rapidly from bubble to bubble, the light won’t have to go through as many changes in direction.

This is the same reason that stars overhead might appear to twinkle less than the stars at the horizon. Light from a star directly overhead is taking the shortest path through our atmosphere: straight down. A star close to the horizon is taking one of the longest paths possible, and so the number of air lenses it must travel through in order to reach our eyeballs is much, much higher. With more chances for the starlight to be bent into an unusual place, the likelihood that the starlight will flicker in and out of focus goes dramatically up – these atmospheric focusing problems are what we see as a twinkle in the night skies.

The technical term for this atmospheric interference with starlight is ‘seeing’. The better the seeing, the less twinkling the stars are doing. Even if it’s a perfectly clear night, you can still have bad seeing, usually due ...

Table of contents

• Cover
• Title Page
• Contents
• Dedication
• Acknowledgments
• List of illustrations
• List of thought experiments
• Timeline
• Prologue
• CHAPTER 1: A Human Perspective
• CHAPTER 2: The Moon
• CHAPTER 3: The Solar System
• CHAPTER 4: Stars
• CHAPTER 5: Stellar Deaths
• CHAPTER 6: Galaxies
• CHAPTER 7: The Universe at Large
• Index
• Plates
• About the Author
• Copyright