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The Solar System, Stars and the Universe

Learn the solar system, seasons, Moon phases, eclipses and tides, star life cycles, galaxies and the Big Bang, and radiometric dating.

Practice this skill 22 min Structure of the cosmos Leer en español
Key idea

What you'll learn

This lesson takes you from our own sky out to the edge of the observable universe:

  • the solar system: the Sun, planets, moons, asteroids and comets, and the gravity that holds them in orbit;
  • the Earth–Moon–Sun system: day and year, seasons, Moon phases, eclipses and tides;
  • stars: how they make energy, and how their life cycle depends on mass;
  • galaxies and the Big Bang: evidence that the universe is expanding and about 13.8 billion years old;
  • the age of Earth (about 4.6 billion years) and radiometric dating with half-lives.

How it shows up on the GED: expect planet tables, orbit diagrams, tide or daylight data, decay curves and short readings. You may click an orbit position, order a star's life stages, calculate an age from half-lives, or pick the evidence that supports an idea.

Words to know

Key terms

  • rotation / revolution — spinning on an axis (one day) / traveling around the Sun (one year).
  • axis tilt — Earth's axis leans about 23.5°, which causes the seasons.
  • light-year — the distance light travels in one year, about 9.5 trillion km (a distance, not a time).
  • phase — the shape of the lit part of the Moon we see (new, crescent, quarter, gibbous, full).
  • solar eclipse / lunar eclipse — the Moon blocks the Sun (at new moon) / Earth's shadow covers the Moon (at full moon).
  • spring tide / neap tide — extra-large tidal range when Sun, Earth and Moon are lined up / small range at the quarter moons.
  • nuclear fusion — joining light nuclei (hydrogen) into heavier ones (helium), releasing energy; it powers stars.
  • main sequence — the long, stable stage when a star fuses hydrogen in its core.
  • supernova — the explosion of a very massive star.
  • galaxy — a huge system of stars, gas and dust held by gravity (the Milky Way).
  • redshift — stretching of light from an object moving away, toward longer (redder) wavelengths.
  • Big Bang theory — the universe began about 13.8 billion years ago in a hot, dense state and has been expanding ever since.
  • half-life — the time for half of a radioactive sample to decay (carbon-14: 5,730 years).

All Science words in the glossary

The solar system

The solar system formed about 4.6 billion years ago from a spinning cloud of gas and dust (a nebula). Most of the material collapsed into the Sun, which holds more than 99% of the system's mass. Gravity keeps the planets in nearly circular orbits. The closer a planet is to the Sun, the faster it moves and the shorter its year.

GroupPlanetsFeatures
Inner (rocky, terrestrial)Mercury, Venus, Earth, MarsSmall, dense, solid surfaces, few or no moons
Outer (giants)Jupiter, Saturn (gas giants); Uranus, Neptune (ice giants)Huge, mostly gas, rings, many moons
The solar system in order from the Sun, not to scale. Inner rocky planets: Mercury, Venus, Earth, Mars. Then the asteroid belt between Mars and Jupiter. Outer giant planets: Jupiter and Saturn (gas giants), Uranus and Neptune (ice giants). An arrow shows that farther from the Sun means a longer year and, generally, colder temperatures. The solar system in order (sizes and distances not to scale) Sun Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune asteroid belt Inner: rocky, small Outer: giant, gas/ice, rings, many moons farther from the Sun → longer year, usually colder (Venus is the exception)

Also: dwarf planets (Pluto), the asteroid belt between Mars and Jupiter, and icy comets.

Patterns to know: farther planets have longer orbital periods and are generally colder. Venus is the exception for temperature: its thick carbon dioxide atmosphere traps heat (a runaway greenhouse effect), making it the hottest planet even though Mercury is closer to the Sun. Earth is the only planet known to have liquid water on its surface and life.

Earth, Moon and Sun: seasons, phases, eclipses and tides

Day and year. Rotation (24 hours) gives day and night; revolution (about 365.25 days) gives the year.

Seasons come from the tilt, not distance. Earth's axis tilts 23.5° and always points the same way in space. When your hemisphere is tilted toward the Sun, the Sun climbs higher, its rays hit more directly, and days are longer, so you have summer. Six months later your hemisphere tilts away, and you have winter. The hemispheres have opposite seasons. Earth is actually closest to the Sun in early January, during northern winter.

Diagram of Earth at two points in its orbit around the Sun, not to scale. Earth's axis is tilted about 23.5 degrees and points the same direction in space all year. In June (Earth on the left) the Northern Hemisphere is tilted toward the Sun, giving it summer. In December (Earth on the right) the Northern Hemisphere is tilted away from the Sun, giving it winter. Earth's tilt and the seasons (not to scale) Sun N June N. Hemisphere tilted toward Sun: summer N December N. Hemisphere tilted away: winter Axis tilt ≈ 23.5°, always pointing the same direction in space

Moon phases. The Moon reflects sunlight, and half of it is always lit. Over about 29.5 days we see different amounts of that lit half: new moon (Moon between Earth and Sun, dark) → waxing crescent → first quarter (half lit) → waxing gibbous → full moon (Earth between Sun and Moon) → waning gibbous → third quarter → waning crescent → new. Phases are not caused by Earth's shadow.

Eclipses need an exact line-up. Solar eclipse: Sun–Moon–Earth (only at new moon). Lunar eclipse: Sun–Earth–Moon (only at full moon). The Moon's orbit is tilted about 5°, so most months there is no eclipse.

Two eclipse line-ups, not to scale. Solar eclipse: Sun, then Moon, then Earth in a straight line; the Moon blocks sunlight and casts its shadow on Earth. It can happen only at new moon. Lunar eclipse: Sun, then Earth, then Moon in a line; Earth's shadow covers the Moon. It can happen only at full moon. Eclipses need an exact line-up (not to scale) Solar eclipse: Sun – Moon – Earth only at NEW moon Sun Moon Earth Moon's shadow Lunar eclipse: Sun – Earth – Moon only at FULL moon Sun Earth Moon Earth's shadow

Tides. The Moon's gravity raises two bulges of ocean water, one facing the Moon and one on the far side, so most coasts get two high and two low tides a day. The Sun adds a smaller pull (less than half the Moon's). At new and full moon the pulls line up → spring tides (biggest range). At the quarter moons they pull at right angles → neap tides (smallest range).

Spring and neap tides, seen from above, not to scale. Spring tide: at new or full moon the Sun, Earth and Moon are in a line, the pulls add, and the ocean bulges are largest, giving the biggest tidal range. Neap tide: at the quarter moons the Moon is at a right angle to the Sun-Earth line, the pulls partly cancel, and the tidal range is smallest. Spring tides vs. neap tides (not to scale) Spring tide new or full moon: pulls line up Sun Moon biggest range Earth Neap tide quarter moon: pulls at right angles Sun Moon smallest range Earth

Stars and their life cycle

A star is a huge ball of hot gas, mostly hydrogen and helium. In its core, nuclear fusion joins hydrogen nuclei into helium and releases enormous energy as light and heat. The Sun is an average, middle-aged star about 4.6 billion years old, roughly halfway through its 10-billion-year main-sequence life.

Color shows temperature: blue-white stars are hottest (above 10,000 °C), yellow stars like the Sun are medium (about 5,500 °C at the surface), and red stars are coolest (about 3,000 °C).

Mass decides everything. Massive stars burn their fuel much faster, so they live much shorter lives.

StageSun-like star (up to about 8 Sun masses)Massive star (more than about 8 Sun masses)
BirthNebula → protostarNebula → protostar
AdultMain sequence (~10 billion years for the Sun)Main sequence (only millions of years)
Old ageRed giantRed supergiant
DeathPlanetary nebula + white dwarfSupernova
RemainsWhite dwarf slowly coolsNeutron star or black hole
Flowchart of star life cycles. Every star begins as a nebula, then a protostar. A Sun-like star, up to about 8 Sun masses, spends about 10 billion years on the main sequence (for the Sun), becomes a red giant, then a planetary nebula and a white dwarf that slowly cools. A massive star, more than about 8 Sun masses, spends only millions of years on the main sequence, becomes a red supergiant, explodes as a supernova, and leaves a neutron star or a black hole. Life cycle of a star: mass decides its path Nebula Protostar Sun-like star (up to ~8 Sun masses) Main sequence ~10 billion yr (Sun) Red giant Planetary nebula + white dwarf Massive star (more than ~8 Sun masses) Main sequence only millions of yr Red supergiant Supernova → neutron star or black hole Bigger stars burn fuel much faster, so they live much shorter lives.

Galaxies and the Big Bang

Stars are grouped into galaxies of billions of stars. The Milky Way is a spiral galaxy about 100,000 light-years across. The Sun sits in one of its arms, and there are hundreds of billions of other galaxies.

Evidence for the Big Bang (an expanding universe that began about 13.8 billion years ago):

  1. Redshift: light from distant galaxies is shifted toward red, so they are moving away. The farther the galaxy, the faster it moves (Hubble's discovery). Every galaxy sees every other moving away, just like dots on an inflating balloon, so there is no center.
  2. Cosmic microwave background: faint microwave radiation fills the whole sky. It is the cooled leftover glow of the hot early universe.
  3. Abundance of light elements: the amounts of hydrogen and helium in the universe match what the theory predicts.

The age of Earth and radiometric dating

Relative dating puts events in order. In undisturbed rock layers, lower layers are older (the law of superposition), and fossils help match layers in different places.

Radiometric (absolute) dating gives an age in years. Radioactive parent isotopes decay into daughter isotopes at a fixed rate, described by the half-life. After each half-life, half of the remaining parent is left: 100% → 50% → 25% → 12.5% → 6.25% …

Decay curve: percent of the original parent isotope remaining (y-axis) versus number of half-lives passed (x-axis, 0 to 5). Values: 0 half-lives 100%, 1 half-life 50%, 2 half-lives 25%, 3 half-lives 12.5%, 4 half-lives 6.25%, 5 half-lives 3.125%. The curve drops quickly at first and then levels off. Radioactive decay of a parent isotope 0 1 2 3 4 5 0 20 40 60 80 100 100% 50% 25% 12.5% 6.25% 3.125% Number of half-lives that have passed Parent isotope remaining (%)
IsotopeHalf-lifeGood for
Carbon-145,730 yearsOnce-living things, up to about 50,000 years
Potassium-401.25 billion yearsVolcanic rock
Uranium-2384.5 billion yearsThe oldest rocks and meteorites

Carbon-14 cannot date dinosaur bones: almost none would be left. Dating meteorites gives Earth's age, about 4.6 billion years. The universe is about three times older, 13.8 billion years.

Formula

Half-life formulas

Fraction remaining after n half-lives:

\text{fraction remaining} = \left(\tfrac{1}{2}\right)^{n}

Age of a sample:

\text{age} = n \times \text{half-life}

where n is the number of half-lives that have passed.

Example: 25% remains → (\tfrac{1}{2})^2 = \tfrac{1}{4}, so n = 2. For potassium-40: 2 \times 1.25 = 2.5 billion years.

The Science test has no formula sheet; questions give the half-life.

Worked example

Worked example 1: Moon phase reasoning

Stimulus: On March 1 the Moon is new.

Question: About what phase will it be on March 8, and where is it in its orbit?

Think-aloud:

  1. A full cycle is about 29.5 days, so each quarter of the orbit takes about 29.5 \div 4 \approx 7.4 days.
  2. Seven days after new moon, the Moon has moved a quarter of the way around Earth.
  3. A quarter orbit from new moon puts the Moon at a right angle to the Sun–Earth line, and we see half of the lit half: first quarter.
  4. Trap: a "half moon" is not halfway through the cycle; that is full moon.
View from above the North Pole of the Moon orbiting Earth, with sunlight coming from the right. On March 1 the Moon is at new moon, between Earth and the Sun, and its lit half faces away from us. About 7.4 days later, one quarter of the 29.5-day orbit, on about March 8, the Moon is at a right angle to the Sun-Earth line: first quarter. Insets show what we see: a dark new moon, and a first quarter moon with its right half lit. New moon → first quarter: one quarter of the orbit sunlight Earth Mar 1: new moon (between Earth and Sun) about Mar 8: first quarter 29.5 ÷ 4 ≈ 7.4 days orbit What we see: new first quarter hatched = unlit (dark) half

Answer: first quarter, a quarter of the way around its orbit.

Worked example

Worked example 2: Seasons from daylight data

Stimulus: Hours of daylight on June 21.

CityLatitudeDaylight
Anchorage, USA61° Nabout 19.4 h
Miami, USA26° Nabout 13.7 h
Buenos Aires, Argentina35° Sabout 9.8 h

Question: Which statement is best supported?

Think-aloud:

  1. Both northern cities have more than 12 hours of daylight, and the farther north city has more. The Northern Hemisphere is tilted toward the Sun.
  2. Buenos Aires, in the south, has fewer than 12 hours. The Southern Hemisphere is tilted away, so it is winter there.
  3. Reject "distance from the Sun explains it": all three cities are the same distance from the Sun.
Bar chart of hours of daylight on June 21, with a dashed line at 12 hours. Anchorage, 61 degrees north: about 19.4 hours. Miami, 26 degrees north: about 13.7 hours. Buenos Aires, 35 degrees south: about 9.8 hours. The northern cities have more than 12 hours because the Northern Hemisphere is tilted toward the Sun; Buenos Aires has fewer because the Southern Hemisphere is tilted away. Daylight on June 21 0 h 4 h 8 h 12 h 16 h 20 h 24 h Anchorage (61° N) 19.4 h Miami (26° N) 13.7 h Buenos Aires (35° S) 9.8 h 12 h North tilted TOWARD the Sun → long days (summer) South tilted AWAY → short days (winter)

Answer: In June the Northern Hemisphere is tilted toward the Sun (long days) and the Southern Hemisphere is tilted away (short days).

Worked example

Worked example 3: Star color and temperature

Stimulus:

StarColorSurface temp. (°C)
RigelBlue-whiteabout 12,000
SunYellowabout 5,500
BetelgeuseRedabout 3,300

Question: A newly observed star appears orange. What is its likely surface temperature?

Think-aloud:

  1. The table shows a pattern: blue is hottest, then yellow, then red is coolest.
  2. Orange falls between yellow and red on the color scale.
  3. So the temperature should fall between about 3,300 °C and 5,500 °C, for example about 4,500 °C.
  4. Reject values like 11,000 °C, which match blue stars.
Number line of star surface temperature from 0 to 14,000 degrees Celsius. Betelgeuse, red, about 3,300 degrees. The Sun, yellow, about 5,500 degrees. Rigel, blue-white, about 12,000 degrees. A shaded band from 3,300 to 5,500 degrees marks where an orange star most likely falls, between red and yellow. Star color shows surface temperature cooler ← → hotter 0 2,000 4,000 6,000 8,000 10,000 12,000 14,000 3,300 Betelgeuse (red) 5,500 Sun (yellow) 12,000 Rigel (blue-white) Orange is between red and yellow → about 3,300 to 5,500 °C surface temperature (°C)

Answer: between about 3,300 °C and 5,500 °C.

Worked example

Worked example 4: Half-life calculation

Stimulus: A radioactive isotope has a half-life of 1,000 years. A sample now has 6.25% of its original amount.

Question: How old is the sample?

Think-aloud:

  1. Count halvings: 100% → 50% (1) → 25% (2) → 12.5% (3) → 6.25% (4). So 4 half-lives.
  2. Age = 4 \times 1{,}000 = 4{,}000 years.
  3. Check: (\tfrac{1}{2})^4 = \tfrac{1}{16} = 0.0625 = 6.25\% ✓
  4. Trap: dividing 100 by 6.25 gives 16, which is the fraction \tfrac{1}{16}, not the number of half-lives.
Bar chart of a radioactive sample with a half-life of 1,000 years. Percent of original amount left: 100 percent at 0 years, 50 percent at 1,000 years, 25 percent at 2,000 years, 12.5 percent at 3,000 years, and 6.25 percent at 4,000 years. Each arrow between bars is one halving, so 6.25 percent left means 4 half-lives, an age of 4,000 years. Count the halvings: half-life = 1,000 years 0% 25% 50% 75% 100% Percent of original left 100% 0 yr 0 half-lives ÷2 50% 1,000 yr 1 half-life ÷2 25% 2,000 yr 2 half-lives ÷2 12.5% 3,000 yr 3 half-lives ÷2 6.25% 4,000 yr 4 half-lives 6.25% left → 4 half-lives → age = 4 × 1,000 = 4,000 years

Answer: 4,000 years.

Common mistake

Common traps

  • Seasons from distance. Wrong: "Summer happens when Earth is closest to the Sun." Right: the 23.5° tilt causes seasons; Earth is closest in January, during northern winter.
  • Phases from Earth's shadow. Wrong: "The crescent is Earth's shadow on the Moon." Right: phases come from how much of the Moon's lit half we see. Earth's shadow touches the Moon only in a lunar eclipse.
  • Mixing up eclipses. Wrong: "A solar eclipse is at full moon." Right: solar = new moon (Moon blocks Sun); lunar = full moon (Earth's shadow on Moon).
  • Bigger stars live longer. Wrong: "More fuel means a longer life." Right: massive stars burn fuel much faster and live only millions of years.
  • Dividing instead of halving. Wrong: "25% is left, and 100 ÷ 25 = 4, so 4 half-lives." Right: halve repeatedly: 100 → 50 → 25 is 2 half-lives.
Test strategy

GED strategy for this topic

  • Orbit diagrams: find the Sun first. Lit sides face the Sun, and shadows point away from it. Then place Earth and the Moon.
  • Two-part answers ("what and why"): many distractors have the right fact with the wrong reason. Check both halves.
  • Tables of planets or stars: confirm the column and the units (AU vs. km; million vs. billion years).
  • Evidence questions: to disprove a cause, find a case where the cause is present but the effect is opposite (closest to the Sun in winter).
  • Half-life items: count halvings on your erasable note board, then multiply by the half-life. Check that the age fits the isotope's range and is less than 4.6 billion years for Earth rocks.
  • Big-picture claims: reject conclusions that claim a center, a proof, or a value the data don't show.

Check yourself

Structure of the cosmos
The Planets of Our Solar System

Eight planets orbit the Sun. Distances are given in astronomical units (AU): 1 AU is Earth's average distance from the Sun, about 150 million km. A planet's orbital period is the time it takes to travel once around the Sun.

PlanetAverage distance from Sun (AU)Orbital period (Earth years)Average temperature (°C)Type
Mercury0.390.24167Rocky
Venus0.720.62464Rocky
Earth1.001.0015Rocky
Mars1.521.88−65Rocky
Jupiter5.2011.86−110Gas giant
Saturn9.5429.46−140Gas giant
Uranus19.1984.0−195Ice giant
Neptune30.07164.8−200Ice giant

Notes: Mercury has almost no atmosphere. Venus has a very thick atmosphere made mostly of carbon dioxide, a gas that traps heat. Earth's atmosphere is much thinner than Venus's.

According to the table, which planet takes about 12 Earth years to orbit the Sun once?

Answer choices
Hint

Use the "Orbital period" column, not the distance column.

Check yourself

Structure of the cosmos
The Planets of Our Solar System

Eight planets orbit the Sun. Distances are given in astronomical units (AU): 1 AU is Earth's average distance from the Sun, about 150 million km. A planet's orbital period is the time it takes to travel once around the Sun.

PlanetAverage distance from Sun (AU)Orbital period (Earth years)Average temperature (°C)Type
Mercury0.390.24167Rocky
Venus0.720.62464Rocky
Earth1.001.0015Rocky
Mars1.521.88−65Rocky
Jupiter5.2011.86−110Gas giant
Saturn9.5429.46−140Gas giant
Uranus19.1984.0−195Ice giant
Neptune30.07164.8−200Ice giant

Notes: Mercury has almost no atmosphere. Venus has a very thick atmosphere made mostly of carbon dioxide, a gas that traps heat. Earth's atmosphere is much thinner than Venus's.

In general, planets farther from the Sun have lower average temperatures. Which planet does not fit this pattern, and what is the most likely reason?

Answer choices
Hint

Read the temperatures in order of distance. Which planet is hotter than the planet just inside its orbit? Then check the notes under the table.

Summary

  • Gravity holds the planets in orbit. Farther planets have longer years and are usually colder (Venus is the greenhouse exception).
  • Seasons come from Earth's 23.5° tilt, not its distance from the Sun; the hemispheres have opposite seasons.
  • Phases show how much of the Moon's lit half we see. Solar eclipses happen at new moon and lunar eclipses at full moon. Spring tides occur at new and full moon, when the Sun and Moon line up.
  • Stars shine by fusion. Mass sets lifetime and fate: Sun-like stars become white dwarfs; massive stars explode as supernovas, leaving neutron stars or black holes.
  • Redshift, the cosmic microwave background and light-element amounts support the Big Bang: an expanding universe about 13.8 billion years old.
  • Radiometric dating: age = number of half-lives × half-life. Earth is about 4.6 billion years old.
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