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Components of the universe

Grade 8 Science Worksheets

What’s the Scale of the Universe?

Everything that exists anywhere in space and time constitutes the Universe. What’s the scale of the entire universe? Let’s start with our own star, the Sun. Our sun is 865,370 miles in diameter. In comparison, our Earth is only 7,917.5 miles in diameter. Our planet is on average 93 million miles away from our sun. We call this distance 1 Astronomical Unit (1 AU). Now that’s a long way off already. Let’s notch it up a bit…

Our sun, earth, moon, the other planets and their moons, and many more heavenly bodies like asteroids and comets – indeed, every bit of matter that goes around our sun – together constitutes our Solar System. The solar system is estimated to be about 178.62 billion miles, or 1921.56 AU, in diameter. Let’s notch it up a bit more…

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Hundreds of billions of such Stars including our own sun are grouped together to form one Galaxy, which we call the Milky Way Galaxy. Our sun is an inconspicuous speck in this vast galaxy. The Milky Way galaxy is shaped like a spiral that is 100,000 – 150,000 light-years across. (One light-year is the distance traveled by light in one year, which is about 6 trillion miles. Do the math). Let’s notch is up one more time …

There are estimated to be 100 billion such galaxies, could even be twice as many or more, in the entire universe. The observable universe – those parts of the universe that we have been able to see with our most sophisticated telescopes – is estimated to be 93 billion light-years in diameter. That’s the scale of the universe we know!


A galaxy consists of stars, dust, gas and other matter held together by the forces of gravity. Galaxies have a diameter anywhere from a few thousand to several hundred thousand light-years. Galaxies are flung far apart from one another, often measured in Parsecs and Megaparsecs, where one parsec is 3.26 light-years and a megaparsec is a million times that!

On a dark night in the northern hemisphere you might be able to discern the shape of our own galaxy, the Milky Way, in the form of a spiral. You may also spot the galaxy nearest to us, the Andromeda Galaxy, which is 2.5 million light-years away. But the billions of other galaxies are too far away to spot without powerful telescopes.

The Hubble telescope focused on a small patch of ‘empty sky’(marked as XDF – see picture on left) for about 100 hours and collected light emitted by thousands of undiscovered galaxies (See picture on right).

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Galaxies are classified by their appearance and shape. The most common shapes are Spiral, Elliptical and Irregular. Others can be sausage-shaped or flattened balls, or like rugby balls.

Our Milky Way galaxy and the nearby Andromeda galaxy are spiral. The Small and Large Magellanic Cloud are examples of irregular galaxies. Most other galaxies simply have alphanumeric names, as seen in the pictures below

Check Point

  1. One Astronomical Unit (AU) is –
    1. Roughly 93 million miles
    2. The average distance between the sun and earth
    3. A unit of distance in space
    4. All of the above
  2. The shape of the Milky Way Galaxy is –
    1. Irregular
    2. Spiral
    3. Elliptical
  3. Distances between galaxies can be measured in –
    1. Parsecs
    2. Megaparsecs
    3. Light-years
    4. All of the above
  4. The Milky Way galaxy is about –
    1. 100,000 to 150,000 light-years across
    2. 93 million miles across
    3. 93 billion light-years across
    4. 93 million light-years across
  5. The galaxy nearest to our galaxy is –
    1. Milky Way
    2. Messier
    3. Andromeda
    4. NGC 3923

Answer Key

  1. d) All of the above
  2. b) Spiral
  3. d) All of the above
  4. a) 100,000 to 150,000 light-years across
  5. c) Andromeda

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