7.3.1 Evolution of stars similar to the Sun

Balance of gravity and pressure

1. Two opposing effects decide what a star does at every stage of its life, gravity pulling all of its material inwards and thermal expansion pushing it outwards.
2. The outward push comes from the hot gas in the core, because nuclear fusion keeps the core at millions of degrees and hot gas exerts a very large pressure.
3. When the two effects are balanced the star neither collapses nor swells, so its size stays steady for a very long time.
4. When fusion slows the core cools, the outward push weakens, gravity wins and part of the star contracts.
5. When fusion speeds up the core gets hotter, the outward push strengthens and the star expands.
6. Every stage in the life of a star is one of these situations, so the whole life cycle can be read as the balance being made, held, lost and remade.

From nebula to main sequence

1. A star begins as a nebula, a cold cloud of dust and hydrogen gas spread thinly through space.
2. Gravity pulls the material of the cloud together, and the denser parts pull in more material still, so the collapse speeds up as it goes.
3. As the material falls inwards its gravitational store is transferred to the thermal store of the gas, so the centre of the cloud gets steadily hotter.
4. The hot dense ball at this stage is a protostar, which glows from that heating alone because no fusion has started yet.
5. Once the core passes about $10$ million $K$ the hydrogen nuclei move fast enough to fuse together into helium, and a star is born.
6. Fusion releases energy, the core pressure rises, and the collapse stops when that outward push exactly balances gravity.
7. The star is now a main sequence star, and it stays that way for as long as there is hydrogen in the core to fuse.
8. This is by far the longest stage, lasting about $10$ billion years for a star the size of the Sun, which is currently about halfway through it.

The red giant stage

1. Eventually the hydrogen in the core is used up, fusion there slows and the outward push is no longer enough to hold gravity off.
2. The core contracts under gravity, and that contraction raises its temperature high enough for helium nuclei to fuse into larger nuclei.
3. The extra energy released heats the outer layers, and those layers expand enormously.
4. Spreading the same energy over a far larger surface leaves that surface cooler, so the star glows red rather than yellow or white.
5. The result is a red giant, a star with a small hot core and a huge cool outer envelope.
6. When the Sun reaches this stage its surface will swell out past the orbit of the Earth, and the stage will last only a few hundred million years.

The white dwarf stage

1. Fusion in the core eventually stops for good, because the core is not massive enough to reach the temperature that heavier nuclei need.
2. With no energy released there is nothing to hold the outer layers, and they drift gently away into space.
3. Gravity pulls what is left of the core into a body roughly the size of the Earth, and this hot dense remnant is a white dwarf.
4. A white dwarf shines only from the energy stored during its earlier life, so it slowly cools and fades over billions of years.
5. The full order for a star of similar mass to the Sun is therefore nebula, main sequence star, red giant and white dwarf.

7.3.2 Evolution of massive stars

Why mass changes the outcome

1. A star with far more mass than the Sun has a far stronger inward pull of gravity acting on its core.
2. Balancing that stronger pull needs a much greater outward push, which the star can only supply by running its core at a higher temperature and pressure.
3. A hotter core fuses hydrogen far more rapidly, so a massive star gets through its fuel much faster even though it started with more of it.
4. The main sequence stage of a massive star therefore lasts a few million years rather than the billions of years a star like the Sun manages.
5. The high core temperature also lets fusion continue on to heavier elements that a smaller star can never reach.
6. Mass is the single quantity that decides the whole later life of a star, which is why two stars formed in the same nebula can end so differently.

Life of a massive star

1. A massive star begins in exactly the same way as any other, with gravity pulling a nebula together until the core is hot enough for fusion.
2. It then spends a short main sequence stage as a very hot, very bright star, burning blue or blue white rather than yellow.
3. When the core hydrogen is gone the outward push drops, the core contracts under gravity, and the outer layers swell into a red supergiant.
4. This is the same process that makes a red giant, but on a far larger scale, so a red supergiant can be hundreds of times wider than the Sun.
5. Inside it the contracting core keeps reaching new temperatures, so helium fuses into carbon and then heavier nuclei form in a series of stages.
6. Fusion stops once the core is made of iron, because fusing iron nuclei takes energy in rather than giving energy out.
7. With no energy released the outward push collapses, and gravity is left completely unopposed.

The supernova explosion

1. The unsupported core collapses in a fraction of a second, and the outer layers fall inwards behind it.
2. The infalling material rebounds off the collapsed core and is blasted back out in an enormous explosion called a supernova.
3. For a few weeks a single supernova can shine more brightly than all of the other stars in its galaxy put together.
4. The explosion supplies enough energy to build nuclei heavier than iron, including gold, lead and uranium.
5. All of those elements are thrown out into space, where they enrich the gas and dust that later collapses into new nebulae.
6. Every atom heavier than helium on Earth was made inside an earlier star or in a supernova, which is why the Solar System contains rocky planets at all.

Neutron star or black hole

1. What is left behind after the supernova depends on how much mass the collapsed core still holds.
2. A core of moderate mass is squeezed until its material is packed as tightly as the inside of a nucleus, forming a neutron star.
3. A neutron star packs more mass than the Sun into a ball only about $20km$ across, so a teaspoon of it would have a mass of billions of tonnes.
4. If the remaining core is more massive still, nothing can resist gravity and the collapse continues without limit.
5. The result is a black hole, where the gravitational field is so strong that not even light travels fast enough to escape.
6. A black hole emits no light of its own, so it is detected by its gravitational pull on nearby stars and gas rather than by being seen directly.
7. The full order for a massive star is therefore nebula, main sequence star, red supergiant, supernova, and then either a neutron star or a black hole.

7.3.3 Observing the Universe

From naked eye to telescope

1. For most of history the only instrument available was the naked eye, so astronomers recorded the positions of bright stars and planets and how those positions changed.
2. That method could reach only a few thousand objects, because the pupil of the eye collects very little light and cannot store it up.
3. The optical telescope changed that from the early $1600$s, first with lenses and later with curved mirrors.
4. A telescope gathers light over a much larger area than the eye and brings it to a focus, so far fainter and more distant objects become visible.
5. Mirrors can be built much larger than lenses, so telescope apertures grew from a few centimetres to the ten metre class instruments used today.
6. From the nineteenth century photography replaced the eye at the eyepiece, and a long exposure builds up light from a faint object over hours instead of an instant.
7. A photograph is also a permanent record that other astronomers can measure and check, which a personal sketch at the eyepiece never was.

Observing beyond visible light

1. Visible light is a narrow band of the electromagnetic spectrum, so an optical telescope reveals only a small part of what objects in space emit.
2. From the $1930$s radio telescopes began detecting radio waves from the Milky Way, which opened up objects that give out almost no visible light at all.
3. Detectors were then developed for infrared, ultraviolet, X-rays and gamma rays, so the whole spectrum could be used.
4. Each band reveals different physics, since cool dust clouds show up in the infrared while extremely hot gas and violent events show up in X-rays.
5. The cosmic microwave background could only be found once microwave receivers existed, which is a direct example of new instruments producing new evidence.
6. Modern telescopes use digital detectors rather than film, and computers combine, sharpen and store the images automatically.
7. Signals from several telescopes spread across the world can also be combined, giving detail that no single dish could produce on its own.

Why telescopes are put in space

1. The atmosphere absorbs most of the electromagnetic spectrum before it reaches the ground, so those wavelengths simply never arrive at a ground based telescope.
2. Gamma rays, X-rays and most ultraviolet are blocked completely, and much of the infrared is absorbed by water vapour in the air.
3. Visible light and radio waves pass through reasonably well, which is why those two types of telescope still work from the ground.
4. Moving air also bends light continuously, which blurs the image and makes stars appear to twinkle, so fine detail is lost even on a clear night.
5. Cloud, rain and light pollution from towns and cities cut observing time further and swamp faint objects.
6. A telescope placed in orbit sits above the whole atmosphere, so it receives every wavelength, produces far sharper images and can observe continuously.
7. The price is that a space telescope costs far more to build and launch, and repairing one is difficult or impossible once it is in orbit.
8. Ground based telescopes are therefore still built for visible and radio work, where the atmosphere causes least trouble and a much larger instrument can be afforded.