Gravity rules the universe
Gravity is the force of attraction between all objects with mass. The more mass, and the closer the objects, the stronger the pull.
Gravity:
- keeps the Moon orbiting Earth and Earth orbiting the Sun
- holds stars together, squeezing their cores to millions of degrees
- binds billions of stars into galaxies
- formed the solar system from a collapsing cloud of gas and dust about 4.6 billion years ago
Without gravity, planets would fly off in straight lines (Newton’s first law). Gravity bends their paths into orbits.
Measuring cosmic distances
Space is so big that kilometres become awkward.
| Unit | Definition | Example |
|---|---|---|
| Astronomical unit (AU) | Average Earth–Sun distance, about 150 million km | Jupiter is about 5.2 AU from the Sun |
| Light-year (ly) | Distance light travels in one year, about 9.5 trillion km | Proxima Centauri, the nearest star, is about 4.2 ly away |
When you look at a star 100 light-years away, you see light that left it 100 years ago. Looking far into space means looking back in time.
Stars
A star is a giant ball of hot gas, mostly hydrogen and helium. In its core, nuclear fusion joins hydrogen nuclei into helium, releasing enormous energy as light and heat. The Sun fuses about 600 million tonnes of hydrogen every second.
Star properties
| Property | Meaning | Notes |
|---|---|---|
| Colour | Shows surface temperature | Blue (hottest, over 30 000 °C) → white → yellow → orange → red (coolest, about 3000 °C) |
| Brightness | How much light we receive | Depends on actual luminosity and distance |
| Size | Diameter | From city-sized neutron stars to supergiants wider than Mars’s orbit |
| Mass | Amount of matter | Determines a star’s whole life story |
The Sun is a medium-sized, yellow, middle-aged star about 4.6 billion years old.
Hertzsprung–Russell diagram
Astronomers plot stars by temperature and brightness on an H–R diagram. About 90% of stars, including the Sun, lie on a diagonal band called the main sequence, where they fuse hydrogen. Red giants and supergiants sit in the upper right; white dwarfs in the lower left.
Star life cycles
| Star mass | Life path |
|---|---|
| Low to medium (like the Sun) | Nebula → main sequence (about 10 billion years for the Sun) → red giant → planetary nebula → white dwarf |
| High (over about 8 times the Sun) | Nebula → main sequence (millions of years) → red supergiant → supernova → neutron star or black hole |
Massive stars live fast and die young. Their supernova explosions create and scatter heavy elements. The iron in your blood and the calcium in your bones were made inside ancient stars: we are literally made of star stuff.
Galaxies
A galaxy is an enormous system of stars, gas, dust and dark matter held together by gravity.
| Type | Shape | Example |
|---|---|---|
| Spiral | Flat disc with curving arms and a central bulge | Milky Way, Andromeda |
| Elliptical | Smooth oval; older stars | M87 |
| Irregular | No clear shape | Large and Small Magellanic Clouds |
The Milky Way
- Our home galaxy, the Milky Way, is a barred spiral about 100 000 light-years across.
- It contains an estimated 100–400 billion stars.
- The solar system is about 26 000 light-years from the centre, in one of the spiral arms.
- At the centre is a supermassive black hole, Sagittarius A*, about 4 million times the Sun’s mass. Its first image was released in 2022.
- On a dark night away from city lights, you can see the Milky Way as a faint band across the sky.
The Andromeda Galaxy, about 2.5 million light-years away, is the most distant object visible to the naked eye. It and the Milky Way are expected to merge in about 4–5 billion years.
Beyond
Galaxies form groups and clusters. The observable universe contains perhaps hundreds of billions to trillions of galaxies.
Tools of astronomy
- Optical telescopes collect visible light.
- Radio telescopes detect radio waves from cold gas and distant galaxies.
- Space telescopes avoid the blurring and absorption of Earth’s atmosphere. The Hubble Space Telescope (1990) and the James Webb Space Telescope (2021) see deep into the universe. Canada contributed the Fine Guidance Sensor to Webb.
Common mistakes
- Thinking a light-year is a unit of time. It’s a distance.
- Thinking all stars are the same as the Sun. They vary hugely in size, colour and lifespan.
- Thinking the Sun is the brightest or biggest star. It just looks that way because it’s close.
Try it yourself
- How long does light from the Sun (8.3 light-minutes) take to reach us? From Proxima Centauri?
- Rank these stars by temperature: red, blue, yellow, white.
- On a clear night away from city lights, find the Milky Way and the Big Dipper.
Quick recap
- Gravity holds moons, planets, stars and galaxies together.
- Distances are measured in astronomical units and light-years.
- Stars shine by nuclear fusion; their mass determines their life cycle.
- Massive stars end as supernovas, making heavy elements.
- The Milky Way is a spiral galaxy of hundreds of billions of stars.