Astronomy and Space

Grade 6 · Science · Study Guide

Explore the structure of our solar system, the life cycles of stars, and the scale of the universe.

Astronomy is the study of objects and phenomena beyond Earth's atmosphere. Our solar system consists of the Sun, eight planets, dwarf planets, moons, asteroids, and comets. The Sun is a medium-sized star at the center of our solar system, and its gravity holds everything else in orbit. The four inner planets — Mercury, Venus, Earth, and Mars — are rocky. The four outer planets — Jupiter, Saturn, Uranus, and Neptune — are gas or ice giants.

The Moon is Earth's only natural satellite. Its gravitational pull causes ocean tides on Earth. The Moon orbits Earth approximately every 27 days and we see different fractions of its sunlit side as it moves — these are called lunar phases.

Stars are enormous balls of plasma powered by nuclear fusion in their cores, where hydrogen atoms fuse to form helium, releasing massive amounts of energy as light and heat. Stars form in clouds of gas and dust called nebulae. After billions of years, stars expand into red giants, then collapse. Medium-sized stars like our Sun eventually become white dwarfs, while massive stars can explode as supernovas and leave behind neutron stars or black holes.

Our solar system is part of the Milky Way galaxy, which contains hundreds of billions of stars. The universe contains at least two trillion galaxies. Distances in space are so vast that astronomers measure them in light-years — the distance light travels in one year, about 9.5 trillion kilometers.

Key terms

Solar system
The Sun and all the objects bound by its gravity, including planets, moons, asteroids, and comets.
Nuclear fusion
The process in stars where hydrogen atoms combine to form helium, releasing enormous amounts of energy.
Galaxy
A massive collection of stars, gas, dust, and dark matter bound together by gravity; our galaxy is the Milky Way.
Light-year
The distance that light travels in one year, approximately 9.5 trillion kilometers; used to measure astronomical distances.
Orbit
The curved path of one object around another due to gravity, such as Earth orbiting the Sun.
Lunar phases
The changing appearance of the Moon as seen from Earth, caused by the Moon's position relative to Earth and the Sun.

Flashcards

Solar system
Sun + 8 planets + moons + asteroids + comets; inner planets are rocky, outer planets are gas/ice giants.
Nuclear fusion
Hydrogen nuclei fuse to form helium in a star's core, releasing massive energy as light and heat.
Nebula
A cloud of gas and dust in space where new stars form.
Light-year
Distance light travels in one year (~9.5 trillion km); used to measure distances between stars and galaxies.
Galaxy
Billions of stars, gas, and dust bound by gravity; Milky Way has ~300 billion stars.
Lunar phases
Apparent changes in Moon's shape from Earth as it orbits; cycle repeats every ~29.5 days.
Orbit
Curved path of a body around another due to gravity; planets orbit the Sun in ellipses.
Black hole
An object with gravity so strong that nothing, not even light, can escape; forms from collapse of very massive stars.

Questions

  1. What process powers the Sun and other stars?

    1. Nuclear fission
    2. Nuclear fusion
    3. Chemical combustion
    4. Gravitational collapse
    Show answer

    Answer: Nuclear fusion

    Stars are powered by nuclear fusion, in which hydrogen atoms (specifically their nuclei/protons) fuse together to form helium under extreme heat and pressure, releasing enormous amounts of energy.

  2. Jupiter is a rocky, terrestrial planet like Earth.

    Show answer

    Answer: False

    Jupiter is a gas giant — the largest planet in our solar system — composed mainly of hydrogen and helium gas. The rocky, terrestrial planets are Mercury, Venus, Earth, and Mars.

  3. The galaxy that contains our solar system is called the ________ Way.

    Show answer

    Answer: milky

    Our solar system is located in the Milky Way galaxy, a barred spiral galaxy containing roughly 200-400 billion stars. It is about 100,000 light-years in diameter.

  4. What causes the ocean tides on Earth?

    1. Earth's rotation
    2. Solar wind from the Sun
    3. The gravitational pull of the Moon
    4. Underwater volcanic activity
    Show answer

    Answer: The gravitational pull of the Moon

    The Moon's gravitational pull creates a tidal bulge of water on the side of Earth facing the Moon (and a smaller bulge on the opposite side), causing high and low tides as Earth rotates.

  5. A light-year is a measure of time, not distance.

    Show answer

    Answer: False

    A light-year is a unit of distance, not time — it is the distance that light travels through space in one year, approximately 9.5 trillion kilometers (5.9 trillion miles).

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