Stars and the Universe

Almost everything we know about the universe comes to us as light, so start with what light is. Light is electromagnetic radiation. It is made of waves of energy that need no material to travel through, and it crosses empty space at 300,000 kilometers per second. Page 3 of your Reference Tables charts the full electromagnetic spectrum. It runs from long radio waves with low energy, through microwaves, infrared, visible light, ultraviolet, and X rays, up to short gamma rays with high energy.

Visible light, the rainbow from long-wavelength red to short-wavelength violet, is only a thin slice of that range. Stars and galaxies shine across the entire spectrum. That is why astronomers build radio dishes, infrared space telescopes, and X-ray observatories. Each band is a different window on the same sky.

Pass starlight through a prism and dark lines show up in the rainbow at exact wavelengths. That happens because each chemical element absorbs light at its own set of wavelengths. Read the lines and you have worked out what the star is made of without leaving Earth. That is how we know the sun is mostly hydrogen and helium.

The lines carry a second piece of information, which is motion. When a light source moves toward you, its waves arrive squeezed together and shift toward shorter, bluer wavelengths. That is a blueshift. When it moves away, the waves stretch toward the red end. That is a redshift. This is the Doppler effect. You hear the same thing when a siren drops in pitch as it passes you. The size of the shift tells you the speed, and the direction of the shift tells you whether the object is coming or going. Keep this tool handy, because the rest of this section depends on it.

Set a star or galaxy in motion and watch what its spectrum does. Positive speeds move the object away from Earth. Negative speeds bring it toward us. Notice which way almost all galaxies turn out to be moving.

A star is a ball of gas with enough mass to start nuclear fusion in its core. Fusion joins hydrogen into helium and turns a little bit of mass into a lot of energy in every reaction. That is what powers the sun, and the sun powers nearly everything else in this book.

Stars differ in temperature, size, and luminosity, which is their true brightness. You can judge a star's temperature by its color. Blue stars are the hottest and red stars are the coolest, with our yellow sun in between. The H-R Diagram on page 4 of your Reference Tables plots luminosity against temperature and sorts stars into groups. The long diagonal band is the main sequence, where stars spend most of their lives. Above it are the giants and supergiants, which are cool but very bright. Below it are the white dwarfs, which are hot but dim. The sun sits in the middle of the main sequence. It is an average star, and that is exactly what makes it a good one to orbit.

Stars are born in nebulas, which are huge clouds of gas and dust. Gravity pulls a cloud together until its core gets hot enough to start fusion. What happens after that depends almost entirely on the star's mass. A star like the sun fuses quietly on the main sequence for about ten billion years. When the hydrogen in its core runs low, it swells into a red giant, pushes off its outer layers, and ends up as a white dwarf that slowly cools.

Massive stars live fast and die spectacularly. They burn through their fuel in mere millions of years, swell into supergiants, and end in supernova explosions that briefly outshine entire galaxies. What is left behind is an ultradense neutron star or a black hole. Supernovas also forge and scatter the heavy elements, which means the iron in Earth's core, and in your blood, was assembled inside dying stars. You are reading this book with star-made atoms.

Stars gather by the hundreds of billions into galaxies, held together by gravity. Ours is the Milky Way, a spiral roughly 100,000 light-years across, with the sun orbiting its center about two-thirds of the way out. The band of light across a dark summer sky is our own galaxy seen edge-on from inside. Beyond it lie countless other galaxies, spirals, ellipticals, and irregulars, strung through space in clusters and filaments.

These distances need their own unit. A light-year is how far light travels in one year, about 9.5 trillion kilometers. The nearest star beyond the sun is more than 4 light-years away. The nearest large galaxy, Andromeda, is about 2.5 million light-years away. That means you always see distant objects as they were in the past. You see Andromeda as it looked 2.5 million years ago, because that is how long its light took to reach you.

Now point the Doppler tool at the galaxies. Almost every galaxy's spectrum is redshifted, which means the galaxies are moving away from us. The pattern is exact. The farther away a galaxy is, the greater its redshift, so the faster it is moving away. This does not mean we sit at the center of anything. In an expanding universe, every galaxy sees every other galaxy moving away from it.

Run the expansion backward and everything points to a beginning. That beginning is the Big Bang, about 13.8 billion years ago, when the universe started expanding from an extremely hot, dense state. Several separate lines of evidence point the same way. One is the pattern linking redshift and distance. Another is the faint background radiation that glows from every direction, left over from that early heat. A third is the amount of hydrogen and helium in the universe. This is the same kind of reasoning from several independent witnesses that the book started with in Chapter 1.1. Next to 13.8 billion years, Earth's own 4.6 billion years is fairly recent.

Two more lines of evidence moved the Big Bang from an idea to settled science. The first is a faint glow that comes from every direction in the sky, found in 1965 and called the cosmic microwave background. It is leftover heat from the early universe, released when space first cooled enough for light to travel freely. Billions of years of expansion have stretched it into faint microwaves. Scientists found it exactly where the theory predicted and at the temperature the theory predicted.

The second is a simple count of atoms. The theory predicts that the very early universe should have forged its ordinary matter into about three-quarters hydrogen and one-quarter helium, with only traces of anything heavier. That is precisely the composition astronomers read in the spectra of the oldest stars and gas. Redshift, the microwave background, and this hydrogen-to-helium ratio are three independent clues that all point to the same beginning.

Light carries the information. The electromagnetic spectrum runs from radio waves to gamma rays, and visible light is a thin slice of it. Spectral lines tell you which elements are in a star. The Doppler shift turns those lines into a speed measurement, with a blueshift for approaching and a redshift for moving away. Stars run on hydrogen fusion. Color tells you their temperature, and the H-R Diagram sorts them into the main sequence, giants, and white dwarfs, with the sun near average. Mass decides how a star ends. Sunlike stars become white dwarfs, while massive stars explode as supernovas and build heavy elements. Galaxies hold billions of stars and sit light-years apart. Nearly all of them are redshifted, and running that expansion backward gives the Big Bang, 13.8 billion years ago, confirmed by the cosmic background glow.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.