The Milky Way is one galaxy among many billions
Universe
Everything that exists, made up of many billions of galaxies together with the space between them.
- The Milky Way is one of many billions of galaxies that make up the Universe.
- That gives the Universe a three-layer structure worth holding in your head: billions of stars make a galaxy, and billions of galaxies make the Universe.
- Nothing about our galaxy is special in that picture: the Milky Way is a typical galaxy in a Universe full of them.
- Other galaxies lie far beyond the edge of our own, separated by gaps far larger than the galaxies themselves.
One hundred thousand light-years across: putting a number on the scale
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The diameter of the Milky Way is approximately 100 000100\,000100000 light-years, and that figure is worth memorising exactly.
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Because a light-year is a distance, that statement says light needs about 100 000100\,000100000 years to cross our galaxy from one side to the other.
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Converting it shows why light-years exist as a unit at all, using 1 light-year=9.5×1015 m1\ \text{light-year} = 9.5 \times 10^{15}\ \text{m}1 light-year=9.5×1015 m.
d=1.0×105×9.5×1015≈9.5×1020 m d = 1.0 \times 10^{5} \times 9.5 \times 10^{15} \approx 9.5 \times 10^{20}\ \text{m} d=1.0×105×9.5×1015≈9.5×1020 m -
Compare that with the 888 light-minutes from the Earth to the Sun, and the jump in scale from a solar system to a galaxy becomes obvious.
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The Universe is larger again by another huge factor, since it holds billions of galaxies with vast empty space between them.
Two numbers carry this topic: a galaxy holds many billions of stars, and the Milky Way is about 100 000100\,000100000 light-years across.
Getting the words in the right order
- A solar system is a star with the objects orbiting it, and ours is one of countless such systems in the Milky Way.
- A galaxy is the next level up, holding many billions of stars and their planetary systems.
- The Universe is the top level, containing all the galaxies there are.
- Ordering questions ask you to rank these by size, so run through the chain planet, star, solar system, galaxy, Universe to check your answer.
- Do not treat the Milky Way and the Universe as the same thing, because the Milky Way is one galaxy among billions.
- Do not quote the 100 000100\,000100000 light-years as a time, since it is the diameter of the galaxy.
- Do not use "solar system" as a synonym for "galaxy", because a galaxy contains billions of systems like ours.
- Give the number, not an impression: 100 000100\,000100000 light-years earns the mark where "very big" does not.
- Include the unit with that figure, since 100 000100\,000100000 on its own is meaningless.
- If asked to convert to metres, multiply by 9.5×10159.5 \times 10^{15}9.5×1015 and leave the answer in standard form.
- Multiple-choice items here usually test the size order, so check each option against the chain before choosing.
- How many galaxies make up the Universe?
- State the approximate diameter of the Milky Way, with its unit.
- How long does light take to cross the Milky Way, and how do you know?
- Put these in order of increasing size: galaxy, star, Universe, planet, solar system.
- Convert 100 000100\,000100000 light-years into metres, giving your answer in standard form.
