The Solar System, Stars, and the Universe

You spawned on a raw planet, dug through the mine, survived the surface, explored the deep, and studied the sky. Now for the biggest map of all. Fair warning: the universe is so big that the numbers stop making sense. That's okay. Scientists struggle with it too.

The solar system is the Sun plus everything held by its gravity: eight planets, their moons, dwarf planets, asteroids, and comets. It formed about 4.6 billion years ago from a spinning cloud of gas and dust (Chapter 24).

Distances are measured in astronomical units (AU). One AU is Earth's average distance from the Sun, about 150 million km.

The inner (terrestrial) planets are small, rocky, and dense:

• Mercury: closest to the Sun, smallest planet, no real atmosphere. Scorching days, freezing nights.
• Venus: the hottest planet (about 465 °C), thanks to a thick CO₂ atmosphere and a runaway greenhouse effect. It also spins backward.
• Earth: the only planet known to have liquid water on the surface and life.
• Mars: the red planet, colored by iron oxide (rust). It has the tallest volcano in the solar system, Olympus Mons, and evidence of ancient rivers.

The asteroid belt, between Mars and Jupiter, holds millions of rocky leftovers.

The outer planets are giant and mostly gas or ice, with rings and many moons:

• Jupiter: the biggest planet. You could fit over 1,300 Earths inside. Its Great Red Spot is a storm bigger than Earth that has lasted for centuries.
• Saturn: famous for its rings of ice and rock. It's so low in density it would float in a big enough bathtub.
• Uranus: an ice giant tipped on its side, so it basically rolls around the Sun.
• Neptune: the windiest planet, with winds over 2,000 km/h.

Beyond Neptune is the Kuiper Belt, home to icy bodies and dwarf planets like Pluto. In 2006, Pluto was reclassified as a dwarf planet because it hasn't cleared other objects out of its orbit.

• Asteroids: rocky or metal chunks, mostly in the asteroid belt.
• Comets: "dirty snowballs" of ice and dust. As they near the Sun, the ice vaporizes, forming a glowing head (coma) and a long tail that always points away from the Sun.
• Meteoroids: small bits of space rock. One that burns up in our atmosphere is a meteor (a shooting star). One that hits the ground is a meteorite.

A star is a massive ball of hot gas that makes energy through nuclear fusion. Every star you see at night is a sun, many much bigger than ours.

Scientists learn about stars by studying their light. A spectroscope splits starlight into a rainbow called a spectrum. Dark lines in the spectrum act like fingerprints, revealing which elements are in the star. That's how we know stars are mostly hydrogen and helium.

A star's color shows its temperature: blue stars are hottest, then white, yellow (like our Sun), orange, and red stars are coolest. Yes, the opposite of what faucets taught you.

The brightness of a star as seen from Earth is its apparent magnitude. Its actual brightness is its absolute magnitude. A dim-looking star might actually be super bright, just very far away.

Scientists plot stars on the Hertzsprung-Russell (H-R) diagram, a graph of brightness vs. temperature. Most stars, including the Sun, fall in a band called the main sequence.

Space is so huge that we measure it in light-years, the distance light travels in one year: about 9.5 trillion km. The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away. When you look at a star, you're seeing light that left it years, even thousands of years, ago. Looking out into space is literally looking back in time.

Stars are born, live, and die. Their mass decides their fate.

1. Nebula: a star begins in a giant cloud of gas and dust. Gravity pulls a clump together.
2. Protostar: the clump heats up as it collapses.
3. Main-sequence star: when the core gets hot enough, hydrogen fusion begins. A star spends most of its life here. Our Sun is about halfway through its 10-billion-year main-sequence life.
4. Red giant: when hydrogen in the core runs low, the star swells up huge and cools.

Then the paths split:

• Low- to medium-mass stars (like the Sun): shed their outer layers as a glowing planetary nebula, leaving a small, hot core called a white dwarf, which slowly cools.
• High-mass stars: become supergiants, then explode in a supernova, one of the brightest events in the universe. What's left becomes a super-dense neutron star, or, for the biggest stars, a black hole, where gravity is so strong not even light can escape.

The Big Bang made mostly hydrogen and helium. Every heavier element, including the carbon in your body, the oxygen you breathe, the calcium in your bones, and the iron in your blood, was forged inside stars by fusion and scattered by supernovas. Making new elements in stars is called stellar nucleosynthesis. You are literally made of recycled stars.

A galaxy is a huge system of stars, gas, and dust held together by gravity. Our galaxy is the Milky Way, a spiral galaxy with 100 to 400 billion stars, about 100,000 light-years across. On a dark night away from city lights, like at Craters of the Moon (an International Dark Sky Park), you can see it as a hazy band across the sky.

Galaxy types: spiral (pinwheel arms), elliptical (oval blobs), and irregular (no clear shape). There are an estimated 2 trillion galaxies in the observable universe.

The Big Bang theory is the scientific explanation for how the universe began. It says that about 13.8 billion years ago, the universe was incredibly hot and dense, and it has been expanding and cooling ever since. It wasn't an explosion in space. It was the expansion of space itself.

Three big pieces of evidence:

1. Redshift and the expanding universe: when a light source moves away from you, its light waves stretch toward the red end of the spectrum (like a siren's pitch dropping as it drives away, called the Doppler effect). In the 1920s, Edwin Hubble found that almost all galaxies are redshifted, moving away from us, and the farther they are, the faster they're moving. The universe is expanding.
2. Cosmic microwave background (CMB) radiation: a faint glow of leftover heat from the early universe, detected in every direction in space. It was discovered by accident in 1965.
3. The composition of the universe: the Big Bang predicts the universe should be about 75% hydrogen and 25% helium by mass. That's exactly what we observe.

It's important to know what the theory does and doesn't do. It explains how the universe developed from a hot, dense early state using evidence. Questions like why there is a universe at all go beyond what science can test, and people find answers to those in many places, including faith and philosophy.

And there's still mystery: scientists think most of the universe is made of dark matter and dark energy, things we can detect by their effects but don't yet understand. Maybe you'll be the one to figure it out.