1.The solar system
The solar system consists of the Sun and the bodies that orbit it. The Sun accounts for most of the solar system's total mass. In order of distance from the Sun, there are 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was reclassified as a dwarf planet in 2006 under the International Astronomical Union's definition of a planet.
The four inner planets are terrestrial planets, small and made of rock, while the four outer planets are Jovian planets, large and made mainly of gas (or icy material), with rings and many moons. The asteroid belt lies between Mars and Jupiter.
The average distance between Earth and the Sun is about 150 million km (1.5 × 10⁸ km), and this is called 1 AU (astronomical unit). It is a convenient unit for measuring distances within the solar system.
| Terrestrial planets | Jovian planets | |
|---|---|---|
| Planets | Mercury, Venus, Earth, Mars | Jupiter, Saturn, Uranus, Neptune |
| Size and mass | Small | Large |
| Main components | Rock, metal | Gases such as hydrogen and helium, icy material |
| Average density | High | Low |
| Rings and moons | No rings; few or no moons | All have rings and many moons |
2.The speed of light and light-years
In a vacuum, light travels at about 300,000 km per second (about 3 × 10⁸ m/s), the fastest speed in the universe. Its exact value is defined as 299,792,458 m/s.
Distances to stars written in km would be enormous numbers, so we use a unit called the light-year. 1 light-year is the distance light travels in 1 year, about 9.46 × 10¹² km (about 9.46 trillion km). Note that a light-year is a unit of distance, not time. Even Proxima Centauri, the closest star after the Sun, is about 4.2 light-years away.
Light also takes time to travel, so looking at a distant object means seeing it as it was in the past. The sunlight you see now left the Sun about 8 minutes 20 seconds ago, and the light from a star 4.2 light-years away set out about 4.2 years ago.
- Time = distance ÷ speed
- 1.5 × 10⁸ ÷ (3 × 10⁵) = 0.5 × 10³ = 500 seconds
- 500 seconds = 8 minutes 20 seconds (8 × 60 = 480, with 20 seconds left over)
- 1 year in seconds: 365.25 × 24 × 3600 = 31,557,600 seconds ≈ 3.16 × 10⁷ seconds
- Using a value close to the exact speed of light, 299,792 km/s: 299,792 × 31,557,600 ≈ 9.46 × 10¹² km
- Using the rounded value 3 × 10⁵ km/s gives about 9.47 × 10¹² km, almost the same.
- 3.8 × 10⁵ km ÷ (3 × 10⁵ km/s)
- ≈ 1.27 seconds
3.The life of a star
Stars are born when clouds of gas and dust in space (nebulae) clump together under gravity. Once the temperature and pressure at the center are high enough, nuclear fusion begins, in which hydrogen nuclei combine to form helium, and the star shines with the energy this releases. The stage in which a star spends most of its life steadily burning hydrogen like this is called the main sequence. The Sun is in this stage now, and its age is estimated at about 4.6 billion years.
A star's fate is determined by its mass. When a star like the Sun uses up the hydrogen at its center, it swells into a red giant, then blows its outer layers off into space (a planetary nebula), leaving a small, hot white dwarf at the center. Stars much more massive than the Sun go through a red supergiant stage, explode as supernovae, and leave behind neutron stars or black holes.
Many of the elements heavier than iron are thought to be made in events like these stellar explosions or collisions of neutron stars. The elements on the periodic table in Lesson 5, including the carbon and oxygen that make up our bodies, were made inside stars.
4.The phases of the Moon
The Moon gives off no light of its own; it reflects sunlight. Half of the Moon is always lit by the Sun, but as the Moon orbits Earth, the angle from which we see that lit side changes, so its shape appears to change. This is called the changing phases of the Moon, and one cycle takes about 29.5 days.
When the Moon is in the same direction as the Sun, its lit side faces away from Earth and it can't be seen: the new moon. When it is in the opposite direction, we see the whole lit face: the full moon. In the Northern Hemisphere, the phases go new moon → waxing crescent → first quarter → full moon → last quarter → waning crescent → new moon, filling in from the right and then shrinking from the right.
“The Moon's shape changes because of Earth's shadow” is a common misconception. Earth's shadow covering the Moon is a lunar eclipse, which happens only occasionally, around a full moon, when the Sun, Earth, and Moon line up almost exactly.
5.The Big Bang theory — what we know and what we don't
Observations that the more distant a galaxy is, the faster it is moving away from us (Hubble's law) show that the universe is expanding. Run time backward, and the universe must once have been much smaller, hotter, and denser. The theory that it expanded and cooled from that state to reach the present day is the Big Bang theory, and the age of the universe is estimated at about 13.8 billion years.
The main evidence supporting the Big Bang theory is the way galaxies are moving apart, the cosmic background radiation that arrives evenly from across the whole universe, and the fact that the proportions of hydrogen and helium in the universe match the theory's predictions well. The Big Bang is understood not as an explosion at one point in space but as space itself stretching.
There is still much we don't know. What came before the moment of the Big Bang or what caused it, and the nature of dark matter, which appears to make up a large share of the matter in the universe, and of dark energy, which is accelerating the expansion, are all under active research. Science distinguishes between what has been confirmed, what is estimated, and what is unknown.
📌 Key points
- The solar system has 8 planets — Mercury, Venus, Earth, Mars (terrestrial) and Jupiter, Saturn, Uranus, Neptune (Jovian) — and Pluto is a dwarf planet
- The speed of light is about 3 × 10⁵ km/s, and 1 light-year is about 9.46 × 10¹² km — a light-year is a unit of distance
- Sunlight takes about 8 minutes 20 seconds to arrive — looking far away means looking into the past
- A star's fate is set by its mass: Sun-like stars become white dwarfs, and very massive stars become neutron stars or black holes after a supernova
- The Moon's phases are due to viewing angle and follow a cycle of about 29.5 days — Earth's shadow on the Moon is a lunar eclipse
- The Big Bang theory is supported by cosmic expansion, the cosmic background radiation, and the proportions of light elements; dark matter and dark energy are still being researched
🤖 Try asking AI like this
Copy a prompt and replace the [ ] parts with your own situation. Don't take the answer on trust — check it against this lesson.
When you want to get a feel for the size of space
If the Sun were shrunk to a ball 1 m across, calculate a scale model showing how big Earth, the Moon, Jupiter, Neptune, and Proxima Centauri would each be and how far away they would be. Put the calculations and the reference values you used (average distances, diameters) in a table, and make clear that the values are approximations.
When you want to separate what's confirmed from what's estimated in theories about the universe
Make a table about the Big Bang theory divided into 1) what is well confirmed by observation, 2) what is estimated from theory, and 3) what is still unknown. Add one line to each item saying what observation it's based on, and mark anything uncertain as uncertain.
- General content of middle and high school science textbooks (the solar system, stars and the universe)
- The International Astronomical Union (IAU) definition of a planet (2006)
- International System of Units (SI) — the definition of the meter based on the speed of light
Reached every goal above? Mark the lesson complete.
Storage is unavailable in this browser, so this lasts only for this page.🔬 Basic Science
- 1The Scientific Method — The Skill of Checking Claims
- 2Force and Motion — Newton's Three Laws
- 3Energy — It Changes Form, but the Total Stays the Same
- 4Electricity and Magnetism — Current Makes a Magnet
- 5Matter and Atoms — How to Read the Periodic Table
- 6Chemical Reactions — Rearranging Atoms
- 7Cells and Heredity — From DNA to Protein
- 8Human Body Systems — Organ Systems and Homeostasis
- 9Earth and Climate — Plates, Atmosphere, Seasons
- 10Space Basics — From the Solar System to the Big Bang