The Secret Life of Stars

Astrophysics for Everyone

Lisa Harvey-Smith, Eirian Chapman

• Éditeur: Thames & Hudson Australia
• Année: 2020
• ISBN:
  9781760761363
• Sujet: Ciencias físicas
• Sous-sujet: Astronomía y astrofísica

Astrophysics made fun!'Pam Melroy, former astronaut and space shuttle commander'The most enjoyable stroll through the cosmos'Gerry Griffin, former flight director, Apollo Mission ControlWe all know the Sun, the powerhouse of our solar system, but what about Luyten's Flare, the Rosino-Zwicky Object or Chanal's variable star? For those whose curiosity takes them far beyond Earth's atmosphere, The Secret Life of Stars offers a personal and readily understood introduction to some of the Galaxy's most remarkable stars.Each chapter connects us to the various different and unusual stars and their amazing characteristics and attributes, from pulsars, blue stragglers and white dwarfs to cannibal stars and explosive supernovae. With chapter illustrations by Eirian Chapman, this book brings to life the remarkable personalities of these stars, reminding readers what a diverse and unpredictable universe we live in and how fortunate we are to live around a stable star, our Sun.

1

Starry starry night

There are more than 7 billion human beings on planet Earth. We are blessed with a large variety of skin tones, eye colours and hair colours, not to mention a fascinating diversity of earlobe shapes (if you don’t believe me, do an internet search).

We come in many shapes and sizes and have different abilities, interests and personalities. Some characteristics are encoded in our DNA from birth, but many aspects of our personalities and physical make-up change profoundly as we age.

Some of us find solace in the arts, music, poetry and language; others’ minds are fed and fascinated by the sciences. My chosen field of astrophysics is a discipline rooted in both art and science, for the beauty and unpredictability of the cosmos stimulates every part of our brains. After all, the stars in our night sky make up the greatest and most detailed artistic canvas in the universe.

There are probably around a billion trillion stars in the observable part of the universe – that’s 1,000,000,000,000,000,000,000 for you visual types. We have no idea how many there might be in the rest of our cosmos because the expansion of it means that the light from those stars will never reach the Earth.

Of those billion trillion stars, only about 3000 are visible to the naked eye. To our gaze, whether fleeting or devoted, it seems there is little to separate them. From the brilliant white Sirius to the intense orange stare of Aldebaran to the countless anonymous faint ones whose names we don’t know, there is not much (apart from their brightness and colour) to separate each tiny point of light.

Luckily, humans are smart, and we have developed tools to magnify, deconstruct and analyse the light that comes from the heavens. An avalanche of new information is continually gathered, from their temperature to their chemical make-up and their age. The past behaviours and tantrums of stars (think explosions and eruptions) are scrutinised from records of long-dead scientists, and their likely future actions are predicted by the patterns of deeds exhibited by other stars.

Astrophysics is a detailed science. When we adopt such a scientific approach, many of our stellar subjects that seem normal and predictable when we glance up on a clear night reveal their true nature. The fact is, our universe is home to a whole host of temperamental personalities. We see stable dwarf stars, unpredictable giants, and many in between. We see kind stars, devious stars, selfish and just plain weird stars.

Some live in families, yet many destroy their relationships or even kill and eat their partners. During a midlife crisis a star can disappear completely, or reincarnate in a colourful cloud of gas. Stars are born and they age, just like us, before slowly succumbing to the inevitable, their ashes returned to the cosmos.

As we travel the universe using the vehicle of science, we discover incredible new things we could never have imagined. We peer through their keyholes to see how they live when nobody’s looking. Nothing is sacred, except the laws of physics – and even they can sometimes be negotiable.

Welcome to the secret lives of stars.

2

So erm, what exactly is the Sun?

Unless you’re less than six months old and live at the South Pole, I’m guessing you’re familiar with the Sun, our nearest star. At around 4.6 billion years old, the Sun is in the middle age of her life. And before you ask, yes, the Sun is a woman. How do I know? She holds down a steady job (heating and lighting the solar system), provides for a family of eight and hasn’t ever taken a holiday.

Our star is made up of the lightest gas in the universe, called hydrogen, as well as bits and bobs of other chemicals that she collected from outer space when she was moulded into a ball by the force of gravity.

Hydrogen is the most common element in the universe. If you zoom in millions of times, ‘Honey-I-shrunk-the-kids’-style, each atom of hydrogen is made from one tiny particle called a proton and one even tinier particle called an electron. Protons have a positive electrical charge and electrons have a negative charge, and opposites attract – so the force between the proton and electron keeps the atom together.

Inside the Sun, things are a little different. The solar gas is so hot and the atoms are shaken so ferociously that they are broken into pieces, with protons and electrons left whirling about as if in a savage dust storm. Where all the atoms are broken like this, the resulting gas is called a plasma. So really, the environment of the Sun is like the inside of one of those fun zappy plasma balls where you press the glass sphere with your finger and a tiny bolt of lightning touches it, discharging a purple arc of light. But never touch the Sun, y’all – it’s hot.

We depend on the Sun more than we can ever imagine. As the source of all heat and light on Earth, our star is completely essential to life on our planet. She doesn’t burn like a fire but shines with her own power – a power that has barely diminished in thousands of millions of years. Her life force comes from deep within her, as if she were a radiant Buddha. Instead of karmic energy, though, what she releases is more like a violent explosion of heat, light and dangerous radiation that, if you got too close, could burn you to a crisp faster than you can say ‘Pass the factor 50’.

Why does the Sun shine? Well, it’s a real melee inside her, with gas crushed together causing a rumbling and grumbling that is symptomatic of something a little stronger than heartburn. In the middle of our star, the gas is all squished together by the massive weight of material above it. It’s a bit like the pressure we feel due to the weight of the atmosphere. At sea level, we have around 100 kilometres of atmosphere on our heads. As a result of this enormous mass bearing down on us, every square centimetre of our body feels a force equivalent to 1 kilogram from the weight of the air. At the top of Mount Everest, where most of the atmosphere is below your feet, this weight of air drops to less than 300 grams.

Since the Sun is made of gas, or, strictly, plasma, it’s basically one big atmosphere all the way through. Don’t think clear blue skies, though. The radius of the Sun is around 700,000 kilometres from the core to the surface, and the weight of all this gas causes an enormous pressure in the core of about 3.8 trillion PSI (a million million times your car’s tyre pressure). In the Sun’s core, the so-called ‘gas’ has around 150 times the density of water, an amount that is denser than lead!

Early on in the Sun’s life, this colossal pressure squashed and heated up the plasma to such a level that it triggered a spectacular subatomic cooking show that has continued unabated ever since. The temperature in the Sun’s core is 15 million degrees Celsius -so it’s not a great place for a holiday. This unimaginable heat is generated by the nuclear reactions that produce all the Sun’s energy and light. In the raging fire of this dense and hot furnace, tiny particles crash into each other and stick together. Quickly, pieces of atoms are formed along with lots of heat and light. It’s like an atomic jigsaw puzzle.

Interested in the gory details? Here goes. The Sun shines by a process called nuclear fusion, which happens when a bunch of protons and neutrons crash into each other and decide to make a complicated, messy group.

We know that the proton is the building block at the centre of a hydrogen atom; it’s also called hydrogen’s atomic nucleus. In a series of collisions, six protons combine to make a pair of protons and a helium nucleus made from two protons and two neutrons. So that’s six protons in, and four protons and two neutrons out. This reaction is happening 100 trillion trillion trillion (100,000,000,000,000,000,000,000,000,000,000,000,000) times every second in the Sun and in most of the 1 billion trillion (1,000,000,000,000,000,000,000) other stars in our universe.

How does it work? Where did the other two protons go, and where did the pair of neutrons come from?

It turns out that particles are pretty flexible creatures. They can be converted into other types, or even into energy, so long as a bunch of rules are followed. For every nuclear reaction that happens in the Sun, a shower of smaller particles is created too. They include subatomic weirdos like neutrinos (particles thought to have no mass) and positrons (particles of antimatter – yikes!) and cosmic rays. Many of these particles are coursing through your body right now, harmlessly emerging out the other side of your skin, but some of the more dangerous could potentially cause damage to your DNA. By a stroke of luck, these baddies are mostly deflected by the magnetic force field of the Earth and never enter our ‘hood.

The other important thing produced by nuclear fusion in the Sun is LOADS of energy, in the form of gamma rays. That’s where sunshine really comes from.

Gamma rays are the most energetic (and, to humans, deadly) type of radiation that exists on the spectrum of light and colour. The nuclear reactions in the Sun give out only gamma rays, and no light at all. The more friendly and familiar forms of radiation – heat and light – that we see from the Sun come later, when the gamma rays bump into particles inside the Sun, are absorbed and are re-emitted in a random direction with a slightly lower energy as the collision ‘steals’ some of the energy of each incoming ray. The process is so haphazard that it takes a gamma ray emitted by nuclear fusion around 30,000 years to get from the core of the Sun to the surface as heat and light. We call the process, amusingly, ‘random walk’.

That, my friend, is how the Sun shines.

It’s amazing to realise how vast the quantities of energy being produced from the gas that makes up our Sun are. A mind-bending 4 million tonnes of hydrogen plasma is converted into energy every second inside the Sun. That’s enough to service the current power consumption of the Earth for more than 4 trillion years.

You might think that nuclear fusion would be a great way to generate our power requirements here on Earth, and in some ways you’d be right. Nuclear fusion would have none of the dangerous by-products of nuclear fission, which is used in the ‘splitting the atom’ technology currently employed in nuclear power stations. Unfortunately, though, we haven’t yet mastered the extreme conditions required to initiate and sustain nuclear fusion reactions safely here on Earth.

The processes that generate energy in the Sun are the same as the ones going on inside a thermonuclear bomb. Perhaps you’re wondering: ‘If the Sun is a giant thermonuclear bomb, how come it doesn’t just explode?’

The answer is simply ‘Gravity keeps it in’.

Yep – believe it or not, the humble force of gravity keeps the phenomenal ‘H-bomb’ at the core of the Sun from exploding into space. The outward forces of searing-hot, ever-expanding gases are balanced perfectly by the sheer weight of the star. Our Sun is as one, balanced and centred like a yoga goddess.

That’s not to say she is completely static, though. Her outer layer, called the convection zone, is a thick layer of gas at more than 1 million degrees Celsius that rises to the surface, then cools to a mere 6000 degrees and falls back down below like globs of boiling soup on a hot stove.

This bubbling cauldron of heated gas beneath the surface of the Sun affects how she spins. A ‘day’ on the Sun (in other words, how fast she spins once on her head) ranges from 25 to 36 Earth days, depending on whether you are at her equator or her poles. This is quite unlike the spin of the Earth, which takes exactly 23 hours, 56 minutes and 4 seconds wherever you happen to be standing. The hot fluid convection cells rising from beneath the surface of the Sun promote a quicker spin at the equator and a slower rate of rotation at her poles. This weird ‘differential’ rotation causes her magnetic field, which is generated deep within her belly, to tangle up as the faster bits overtake the slower bits. The Sun, in other words, has knots in her stomach.

Where does this magnetic field come from?

We know it’s been there for a very long time, perhaps even before the Sun started shining. We also know that her magnetic field has remained strong for billions of years. We have good reason to believe that it is generated by something called a dynamo, deep within the Sun.

Ever heard of a dynamo? If you’re as old as me, you might remember dynamo lights on bicycles – environmentally friendly, with no batteries needed. (Perhaps we should go back to using them.)

Dynamo lights work by placing a small wheel against one of the bike’s tyres. As the wheel turns, a magnet inside it spins around. A coil of copper wire is placed next to the spinning magnet and, as its force field rotates, a small electrical current is created within the wire, which powers the bicycle light. Electricity and magnets are connected: when one moves, the other is induced. That’s what we call ‘electromagnetism’.

A similar effect (but in the opposite direction) is happening inside the Sun. Magnetic forces are created inside our star from the action of spinning electrical currents.

I know what you’re thinking. Why are there electrical currents inside the Sun in the first place? Has someone wired her up and plugged her in – some sort of intrepid sparkie with asbestos hands?

Well, no, but the movement of hot gases in the Sun, which spin with the rotation and convection just under the surface, actually amounts to an electrical current. Electricity is just the motion of charged particles. The gas within the Sun is so hot that the atoms are broken up into their constituent pieces, many of which (for example, protons and electrons) are electrically charged. All this positive and negative charge swirling around generates a magnetic field that is about twice as strong as our Earth’s.

Because different parts of the Sun rotate at different speeds, the magnetic field is squeezed and tangled up over time. In some places, it becomes 8000 times stronger than the Earth’s puny magnetic field. This causes some rather drastic heliospheric acne, called sunspots. They look like black dots on the surface of our star. That’s because the hot gases that usually bubble up from below are pushed under by the tremendous magnetic forces in the sunspot. These parts of the Sun are therefore much cooler, around 4600 degrees, and so appear darker than the rest of the burning orb.

Another symptom of the Sun’s magnetic field is solar flares. These gigantic eruptions, or burps, from our star originate when the tangled magnetic field of a large group of sunspots suddenly ‘snaps’ and rights itself, releasing vast quantities of hot plasma from below the surface. It’s a bit like when a rubber band gets twisted and knotted, then finally breaks.

Solar flares can be seen with special filtered telescopes as soon as they happen, but the effects on Earth are not felt until several days later. These solar eruptions – with 10 million times the energy of a volcanic eruption – spew searing-hot plasma into space. The gas escaping can reach temperatures of up to 100 million degrees.

The most powerful solar flare ever recorded happened on 1-2 September 1859. Vast globs of solar plasma rocketed towards the Earth at unprecedented speeds, taking only seventeen-and-a-half hours to reach the planet. The skies were alight with glowing green and red light as the electrically charged stuff spilled into our atmosphere and interacted with the oxygen and nitrogen in the air, causing one of the brightest auroral displays ever recorded.

Usually, the aurora would be confined to the north and south polar regions because solar plasma is guided that way by the Earth’s magnetic field. ...