From stars to galaxies

For most of its life a star is in a tug of war. Gravity squeezes it inward. The energy from fusion pushes outward. Neither side wins, so the star stays steady and shines. Astronomers call this stage the main sequence. Our Sun is about 4.6 billion years old and only halfway through, so there is no need to worry.

Here is a surprise: the biggest stars live the shortest lives. A heavy star burns hot, bright and fast, and is finished in a few million years. A small star sips its fuel slowly and can glow for trillions of years. You can tell which is which by colour. The hottest stars are blue-white, stars like the Sun are yellow-white, and the coolest stars are red.

One day the hydrogen in the middle runs out, and the tug of war is over. What happens next depends on how heavy the star is. Pick a star below and see how its story ends.

Some giant stars do not shine steadily. They swell and shrink, growing brighter and dimmer again and again, like slow breathing. These are Cepheid variable stars, and one full breath takes from a day to a few months.

In 1912 the astronomer Henrietta Leavitt studied these stars on photographic plates at Harvard and found a rule: the slower a Cepheid pulses, the more powerful it truly is. That turns every Cepheid into a lamp of known brightness. Time its pulse and you know how bright it really is. Compare that with how faint it looks and you know how far away it is, the same way you judge the distance of a motorbike at night from how dim its headlight looks.

Leavitt's rule gave astronomers their first way to measure distances beyond our galaxy. It is how we learned that the universe is full of other galaxies, and that it is expanding.

Remember the neutron star, the crushed core left behind by a supernova? It is only about 20 kilometres wide, and it spins. Many spin once a second; some spin hundreds of times a second. A neutron star sends out two narrow beams of radio waves, and as it spins the beams sweep around the sky like the lamp of a lighthouse. If a beam crosses the Earth, we receive a pulse, then another, then another, as regular as a clock. Such a star is called a pulsar.

Jocelyn Bell discovered the first one in 1967 with a radio telescope made of wooden posts and wire. There is more about it in Chapter 6. On metres of paper chart she noticed a tiny smudge that returned every 1.3 seconds. It was so regular that the team jokingly labelled it "LGM", for "little green men", before they understood it was a spinning star.

When stars die, they do not keep their treasures. Planetary nebulae and supernovae scatter the stars' gas back into space, now sprinkled with the heavier elements made inside them: carbon, oxygen, silicon, iron. That stardust mixes into new clouds, and new stars and planets are born from it. The rocks under your feet, the iron in your blood and the calcium in your teeth were all made inside stars that died before the Sun was born. You are made of stardust. Really.

A cloud that is turning into a star is always spinning a little. As it shrinks it spins faster and flattens out, the way a ball of dough flattens when a cook spins it. So the newborn star ends up with a wide, flat disc of gas and dust around it. Inside the disc, specks of dust bump into each other and stick. Specks become pebbles, pebbles become rocks, and rocks become lumps a few kilometres wide called planetesimals. Gravity pulls those together into planets. It takes patience: tens of millions of years.

Near the star it is too warm for ice, so only small rocky planets can form: Mercury, Venus, Earth and Mars. Farther out it is freezing cold. Ice joins the building material, and the planets grow so big that they can hold on to thick blankets of gas: Jupiter, Saturn, Uranus and Neptune.

That is why all the planets go around the Sun the same way, almost on the same flat track, like runners on one racecourse. They were all made from the same spinning disc, about 4.6 billion years ago. And the Sun is not special: astronomers have already found thousands of planets around other stars. Who knows what is out there?

The stars in a cluster are brothers and sisters. They came from the same cloud at the same time. So they are all the same age, they are all about the same distance from us, and they are made of the same ingredients. Only one thing is different: some are heavy and some are light.

That makes a cluster a perfect science experiment, set up by nature. Astronomers compare the heavy stars with the light ones and see exactly what a star's weight does to its life. Much of what you read in Chapter 1 was worked out this way. There are two kinds of cluster, and they could not be more different.

Here is a galaxy like ours, seen from the side. It has four ingredients. Tap each one to see where it hides and what it does.

Swing a ball around on a string. The faster it goes, the harder you have to hold on. Let go, and it flies away. Stars circling a galaxy are the same, with gravity as the string. The faster a star moves, the more gravity is needed to hold it, and gravity comes from matter.

So the speed of the stars tells us how much matter a galaxy has, even matter we cannot see. In the 1970s Vera Rubin measured those speeds in galaxy after galaxy. Press the two buttons to see what everyone expected, and what she found.

The answer was the same in every galaxy. To hold on to its stars, a galaxy needs several times more matter than all its stars, gas and dust put together. This extra matter cannot be seen in any kind of light. We only feel the pull of its gravity.

People often mix these two up, because both sound dark. They are very different things.

Light is the fastest thing there is: about 300,000 kilometres every second. Even so, space is so big that light needs time to cross it. Light from the Moon takes a little over one second to reach us. Light from the Sun takes about eight minutes. The light in our photograph of the Orion Nebula left it about 1,300 years ago, and the light from the Pinwheel Galaxy set out about 21 million years ago.

So a telescope is also a time machine. The farther away you look, the further back in time you see.

Light travels as a wave. The colour we see depends on the length of the wave: red light has longer waves, blue light has shorter ones. But the rainbow our eyes can see is only a thin slice of all the light there is. Beyond red there is infrared, microwaves and radio. Beyond violet there is ultraviolet, X-rays and gamma rays. Together they are called the electromagnetic spectrum, and you already use most of it every day. Slide along it below.

Each kind of light shows a different side of the same sky. A cloud that is dark and empty in visible light can be bright in infrared or radio. That is why astronomers build so many kinds of telescope, and why some of them have to fly above the air: the atmosphere blocks most of these kinds of light, and lets through mainly visible light and radio waves.

Astronomers spread light out into its colours, like a rainbow, and read it like a barcode. This spread-out light is called a spectrum. Different things in the sky leave different marks on it. Pick one below, and find it in our photograph of the Orion Nebula.

Put a cup and a wide bucket out in the rain. The bucket catches more water, simply because its opening is bigger. Light from the sky falls on us like a very thin rain, and a telescope is a bucket for it. The opening of your eye, the pupil, is at most about 7 millimetres wide in the dark. A telescope's mirror or lens is far wider, so it catches far more light and can show things much too faint for the eye.

Because the catching area grows with the square of the width, a mirror twice as wide collects four times the light. Try it.

A wider opening also shows finer detail. Astronomers call this resolution: how close together two points can be before they blur into one. It is the difference between seeing a headlight in the distance and seeing that it is two headlights.

On the ground there is a limit, and it is the air. Starlight crosses the whole atmosphere on its way down, and the air is always moving. Look at a coin at the bottom of a pond and it shimmers; stars twinkle for the same reason. This is why big telescopes are built on high, dry mountains, and why some are sent into space.

Your eye takes a fresh picture many times a second and throws the old one away. It cannot save up light. A camera can. Leave its shutter open and light keeps piling up on the sensor, second after second, until things far too faint to see appear. The night mode on a phone works the same way.

Astrophotographers go further. They take many exposures and add them together on a computer, a method called stacking. Our photograph of the Orion Nebula is eight exposures of two minutes each: sixteen minutes of light in one picture. The colours are real, but they are too faint for the eye, which mostly sees in grey when the light is dim.

You cannot hear a whisper in a noisy room. In the same way, you cannot see faint stars in a bright sky. Street lamps, signs and buildings send light upward, and the air scatters it into a glow that hides the stars. This is why Dūrbin students carry telescopes out of the cities to places like the hills of Bandarban. Slide from the city to a dark site and watch the sky fill up.

Not every telescope has a mirror or a lens. Radio waves are light with very long waves, and they are caught with metal dishes and wires, like the dish that brings satellite television to a house. A radio telescope can work in daylight and through cloud. There is one on the roof of the building at IUB.

One of the most famous radio telescopes did not look like a telescope at all. In 1967, at Cambridge in England, a student named Jocelyn Bell (now Dame Jocelyn Bell Burnell) helped build one from about a thousand wooden posts and more than 190 kilometres of wire and cable, spread over a field the size of 57 tennis courts. With it she discovered a new kind of star, the pulsar. You met it in Chapter 1.