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8.1.1Astrophysics

Objects in Space — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 2 of 8

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Part 1 of 7The Solar System

The Solar System is made up of the Sun, eight planets, and the many smaller objects that orbit the Sun. The Sun is a star, an enormous ball of hot gas that releases energy by nuclear fusion; it contains over 99% of the mass of the whole Solar System. Its gravity keeps everything else in orbit.

The Solar System
Planets
Planets are large objects that orbit the Sun, are roughly spherical, and have cleared the region of their orbit of other objects.
  • In order from the SunMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
  • The four inner planets are small and rocky; the four outer planets are large gas or ice giants.
Dwarf planets
, such as Pluto and Ceres, are roughly spherical and orbit the Sun but have not cleared their orbits of other bodies.
Moons
Moons are natural satellites that orbit planets (and some dwarf planets). Earth has one; Jupiter has dozens.

Part 2 of 7Smaller bodies: asteroids and comets

Asteroids are rocky or metallic lumps, from a few metres to hundreds of kilometres across. Most are found in the asteroid belt between the orbits of Mars and Jupiter and orbit the Sun in roughly circular paths.

Comets are balls of ice and dust, often called "dirty snowballs", that follow very elongated (elliptical) orbits taking them from the outer Solar System close to the Sun and back out again.

Smaller bodies: asteroids and comets
When a comet gets close to the Sun, its ice evaporates and forms a glowing head and a long tail of gas and dust that points away from the Sun, pushed by the Sun’s radiation and by the solar wind.
Comets travel fastest when closest to the Sun and slowest when far away.

Part 3 of 7Stars, galaxies and the Universe

Stars like our Sun are not evenly scattered in space.

Galaxies
They are grouped into huge collections called galaxies, each held together by gravity.
A galaxy contains millions to hundreds of billions of stars, along with gas, dust and, probably, large amounts of dark matter.
Milky Way
Our Sun is in the Milky Way galaxy, a spiral galaxy containing about 1011 stars with a diameter of roughly 100 000 light-years.
The nearest large galaxy to ours is the Andromeda galaxy, about 2.5 million light-years away.

The Universe is everything that exists: all of space, matter and energy, including billions of galaxies. In order of increasing size: moon, planet, star, solar system, galaxy, cluster of galaxies, Universe.

Part 4 of 7The light-year

Distances beyond the Solar System are so large that metres and kilometres become awkward. Astronomers use the light-year: the distance that light travels through space in one year. Because light travels at 3.0 × 108 m s−1 and a year has about 3.15 × 107 s:

1 light-year = 3.0 × 108 × 3.15 × 107 ≈ 9.5 × 1015 m.

A light-year is a unit of distance, not of time. The nearest star after the Sun, Proxima Centauri, is about 4.2 light-years away. Within the Solar System, distances are often compared with the average distance from the Earth to the Sun, 1.5 × 1011 m, which light covers in about 500 s (8.3 minutes).

Part 5 of 7Scale and observations

The scale of the Universe is astonishing: the Moon is about 1.3 light-seconds from Earth, the Sun about 8 light-minutes, the outer planet Neptune about 4 light-hours, the nearest star 4 light-years, the centre of our galaxy about 26 000 light-years and the nearest large galaxy 2.5 million light-years.

Scale and observations
Because light takes time to travel, looking at distant objects means looking back in time: light from the Andromeda galaxy has taken 2.5 million years to reach us,
  • so we see it as it was then.
This idea becomes important when we discuss the expanding Universe.

Part 6 of 7Putting the sizes in order

It helps to build a mental ladder of sizes.

1
The Earth is about 1.3 × 107 m across; the Sun is about 109 times wider.
2
The Solar System, out to Neptune, stretches about 4.5 × 1012 m, and the Milky Way is roughly 100 000 light-years across, which is about 1021 m.
3
Between the stars there is mostly empty space: if the Sun were the size of a grapefruit, the nearest star would be a similar object thousands of kilometres away.
4
This is why collisions between stars are extremely rare, even in galaxies that contain hundreds of billions of them.

Part 7 of 7Worked examples

  1. Calculate the distance in metres of 1 light-year, using a speed of light of 3.0 × 108 m s−1 and 1 year = 3.15 × 107 s.

  2. Proxima Centauri is 4.2 light-years from Earth. Calculate this distance in metres, and the time in years that light takes to reach us.

  3. The Sun is 1.5 × 1011 m from the Earth. How long does sunlight take to reach us?

Key terms

TermMeaning
Solar SystemThe Sun and all the objects bound to it by gravity.
GalaxyA very large collection of stars, gas and dust held together by gravity.
Light-yearDistance travelled by light in one year, about 9.5 × 1015 m.
AsteroidRocky or metallic body orbiting the Sun, mostly in the belt between Mars and Jupiter.
CometBody of ice and dust in a highly elliptical orbit around the Sun.
Dwarf planetRoughly spherical body orbiting the Sun that has not cleared its orbit.
MoonNatural satellite orbiting a planet.

Avoid these mistakes

1
Treating the light-year as a unit of time. It measures distance.
2
Confusing a galaxy with the Solar System. The Milky Way has billions of stars; the Solar System has just one.
3
Saying a comet’s tail points behind it along its path. It always points away from the Sun.
4
Saying Pluto is a planet. It is classed as a dwarf planet as it has not cleared its orbit.
5
Believing the asteroid belt is dense with colliding rocks. The asteroids are very far apart.

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