Exploring the Universe: Stars, Galaxies, and the Solar System

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How Did the Universe Start?

The universe is everything that exists. Scientists estimate it could contain 100 billion galaxies. A galaxy is a large group of many stars and celestial bodies.

What Are Stars?

Stars are giant spheres of very hot gas. The Sun is the closest star to Earth.


Constellations are visible at different times of the night. We see different constellations in the Northern Hemisphere and the Southern Hemisphere.

Types of Galaxies


Spiral galaxies are disc-shaped, with arms.

Elliptical galaxies are oval-shaped.

Irregular galaxies have no defined shape.

Solar System

Mercury, Venus, Earth, and Mars are inner planets. They are mostly made of rock.

Jupiter, Saturn, Uranus, Neptune, and Pluto are the outer planets. They are mostly made of gas.

What Other Objects Are There in the Solar System?

Comets are made up of ice, gas, and dust.

Asteroids are irregular-shaped objects made of rock.

Natural satellites are moons that orbit planets.

Dwarf planets are small spheres that orbit the Sun.

What Causes Day and Night?

The Earth is similar to other planets in that it moves in two different ways at the same time. The Earth continually rotates on its axis. We call this movement rotation. It takes 24 hours, or one day, for the Earth to make a complete rotation on its axis.

The Earth's axis of rotation is the imaginary line that connects the North Pole and the South Pole.

The Earth's axis is at an angle of 23.5°.

This is called the Earth's tilt.

A line divides the Earth into day and night. This line is called the terminator. The Earth orbits, or revolves, around the Sun. A complete revolution of the Sun takes approximately 365 days, or one year.

How Does the Earth's Tilt Cause the Seasons?

The revolution of the Earth and its tilted axis cause the four seasons.

In summer, the Northern Hemisphere is tilted towards the Sun. This means it receives direct sunlight. The days are hot and long. In the winter, the Northern Hemisphere is tilted away from the Sun, so it gets indirect sunlight. This means it is cooler and the days are shorter than in summer.

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