Astronomy & Space
Astronomy is the study of everything beyond Earth — planets, stars, galaxies, and the universe itself.
The Solar System
8 planets orbit the Sun: Mercury, Venus, Earth, Mars (inner rocky planets) | Jupiter, Saturn, Uranus, Neptune (outer gas/ice giants). The asteroid belt lies between Mars and Jupiter.
Stars & the Sun
The Sun is a medium-sized star — a giant ball of plasma powered by nuclear fusion (hydrogen → helium + energy). Star life cycle: nebula → main sequence → red giant → white dwarf (or supernova → neutron star/black hole for massive stars).
Galaxies
A galaxy is a massive system of stars, gas, and dust held by gravity. Our galaxy is the Milky Way — a spiral galaxy containing ~200–400 billion stars. Nearest large galaxy: Andromeda (~2.5 million light-years away).
Black Holes
A black hole forms when a massive star collapses. Its gravity is so strong that not even light can escape. The boundary is called the event horizon. At the center: the singularity.
Scale of the Universe
A light-year is the distance light travels in one year (~9.46 × 10¹⁵ m). The observable universe is ~93 billion light-years in diameter — a number too large to truly imagine.
Why the Inner and Outer Planets Differ
The split is not a coincidence of naming. It comes from where each planet formed relative to the young Sun.
The frost line
Close to the Sun it was too hot for ice to survive, so only rock and metal could condense. That produced four small, dense, rocky planets.
Beyond a boundary astronomers call the frost line, water and other volatiles froze solid. There was far more of that material available, so those planets grew massive enough to hold onto hydrogen and helium — becoming the gas and ice giants.
| Inner planets | Outer planets | |
|---|---|---|
| Composition | Rock and metal | Gas and ice |
| Size | Small | Large |
| Moons | None or few | Many |
| Rings | None | All four have them |
| Orbit length | Short | Long |
Why Stars Shine
A star is a permanent standoff between two forces. Gravity pulls its enormous mass inward; the energy from fusion pushes outward. As long as those balance, the star is stable.
Fusion in the Sun's core
Pressure and temperature at the centre are extreme enough to force hydrogen nuclei together into helium. The helium produced weighs slightly less than the hydrogen that went in, and that missing mass is released as energy.
Light leaving the Sun's surface takes about eight minutes to reach Earth — so you always see the Sun as it was eight minutes ago.
What ends a star depends entirely on its mass. Sun-sized stars swell into red giants, shed their outer layers, and leave a white dwarf. Much heavier stars collapse and explode as supernovae, leaving a neutron star or a black hole.
Looking Out Is Looking Back
Because light travels at a finite speed, distance in space is also distance in time. Andromeda is 2.5 million light-years away, so its light left before modern humans existed. Telescopes are, quite literally, instruments for observing the past.
Moons, Seasons, and Tides
- Seasons come from Earth's 23.5° axial tilt, not from distance to the Sun. Earth is actually closest to the Sun in January.
- Moon phases come from the changing angle at which we see the Moon's sunlit half — not from Earth's shadow.
- Tides are caused mainly by the Moon's gravity pulling on Earth's oceans, with the Sun adding a smaller effect.
- Eclipses happen when Sun, Earth, and Moon line up precisely, which is rare because the Moon's orbit is tilted.
Common Misconceptions
- Summer is not when Earth is nearest the Sun. It is when your hemisphere tilts toward it.
- There is no true "dark side" of the Moon. The same face always points at us, but every part receives sunlight during the month.
- Astronauts float because they are in continuous free fall around Earth, not because gravity has disappeared.
- A light-year measures distance, not time.
Summary
- Four rocky inner planets and four giant outer ones, split by where ice could form.
- Stars balance gravity against fusion energy, and their mass determines how they die.
- Galaxies are gravitationally bound systems of billions of stars; ours is the Milky Way.
- Light's finite speed means distant objects are seen as they were in the past.
- Tilt causes seasons; geometry causes moon phases.
FAQ
Is Pluto a planet? No — reclassified as a "dwarf planet" in 2006. It doesn't clear its orbital neighborhood.
What is a light-year exactly? The distance light covers in one year, roughly 9.46 trillion kilometres. It measures distance, despite the word "year".
Why do stars twinkle but planets do not? Stars are so distant they act as single points of light, easily disturbed by Earth's atmosphere. Planets appear as tiny discs, and the disturbances average out.
Could we live on Mars? Not without sealed habitats. The atmosphere is thin and mostly carbon dioxide, there is no magnetic field to deflect radiation, and surface temperatures average about −60°C.
What is the Big Bang? The leading explanation for the universe's origin: everything expanded from an extremely hot, dense state about 13.8 billion years ago. The observed expansion of galaxies is the primary evidence.
Quick Quiz
Test what you just learned. Choose the best answer for each question.