The Vast Scale of Space
Describe the scale of the universe in nested layers: Earth is one planet in our solar system, the Sun is one star among billions in the Milky Way galaxy, and the Milky Way is one galaxy among billions in the universe
What a learner can do afterwards
- State that the Milky Way is our galaxy and it contains billions of stars
- Explain the hierarchy: planet → solar system → galaxy → universe
- Use a comparison to convey cosmic scale (e.g. if the Sun were a football, Earth would be a peppercorn 26 metres away)
1 · Read
You know Earth is a planet, and it travels around the Sun with seven other planets. That whole family of planets and the Sun is called the solar system. Now let's zoom out even further.
The Sun is a star, just like the tiny sparkly dots you see in the night sky. But the Sun is not alone. It belongs to a huge group of stars called a galaxy, and our galaxy has a name: the Milky Way. It holds billions of stars.
Imagine shrinking the Sun down to the size of a football. On that same tiny scale, Earth would be a peppercorn sitting about 26 metres away. That huge gap is mostly empty space, and it is just our tiny corner of one galaxy.
The Milky Way is huge, but it is only one galaxy. Scientists have counted billions of other galaxies out there, each packed with its own billions of stars.
Planet, solar system, galaxy, universe: each one holds many of the one before it, all the way out to a universe of billions of galaxies.
More to read
The Universe
The universe is everything. All space, time, matter, and energy. It began about 13.8 billion years ago with the Big Bang. It has been expanding ever since. The universe contains billions of galaxies. Each galaxy has billions of stars. Our galaxy is called the Milky Way. It is a spiral galaxy about 100,000 light-years across. The Sun is one of 100 to 400 billion stars in the Milky Way. We live in a quiet suburb about 26,000 light-years from the center. The universe is so big that light from the most distant galaxies takes billions of years to reach us.
What Is a Galaxy?
A galaxy is a huge collection of stars, gas, dust, and dark matter held together by gravity. Galaxies come in different shapes. Spiral galaxies (like the Milky Way) have flat disks with winding arms. Elliptical galaxies are round or oval blobs. Irregular galaxies have no clear shape. The Milky Way is a barred spiral. It has a central bar of stars with arms sweeping out. Galaxies are not scattered randomly. They cluster together in groups and superclusters. The Milky Way belongs to the Local Group, which includes Andromeda and dozens of smaller galaxies.
Formation and Evolution of Galaxies
Galaxies formed from small clumps of matter in the early universe. Gravity merged them into bigger and bigger structures. The first galaxies were small and irregular. Over billions of years, they grew by swallowing neighbors and pulling in gas. The Milky Way is still eating small galaxies today. The Sagittarius dwarf galaxy is being torn apart and absorbed. In about 4 billion years, the Milky Way will collide with Andromeda. They will merge into a giant elliptical galaxy. The universe's expansion carries distant galaxies away faster than light. We will never reach them.
How to Calculate a Light Year: 10 Steps
A light-year is not a year. It is a distance. Light travels about 300,000 kilometers per second. In one year, it goes about 9.46 trillion kilometers (5.88 trillion miles). That is one light-year. The nearest star system, Alpha Centauri, is 4.37 light-years away. The center of the Milky Way is about 26,000 light-years away. The Andromeda galaxy is 2.5 million light-years away. The most distant galaxies we see are over 13 billion light-years away. Looking far away means looking back in time. We see those galaxies as they were when the universe was young.
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2 · Watch
به انگلیسیTake it off screen
Where it sits
Learn first
Optional connections
This opens up
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.