THE UNIVERSE All of the objects and energy in space make up the universe.

What is a galaxy?  A galaxy is a huge family of stars. It can contain millions of stars.  Our star, the Sun, is part of the Milky Way Galaxy.

 Galaxies come in three main shapes.  Spiral galaxies – have a thick bunch of stars in the middle surrounded by stars arranged in spiral arms. The stars of the Milky Way form a thick bar-like center surrounded by a spiral of stars.  Elliptical galaxies have stars that form shapes like balls or flat discs.  Irregular galaxies have stars that do not form any particular shape. Irregular galaxies are less common. Scientists think most irregular galaxies were once spiral or elliptical galaxies, but their shape changed over time because of gravity.

Stars

When you gaze up at the night sky, you see countless points of light. Almost all of these points of lights are stars. A star is an object in space that gives off its own light and heat. Stars come in all sizes. Our sun, which is the closest star to Earth, is an average, medium-sized star. It is about 1,392,000 km in diameter.

Star Sizes  Some stars, called giant stars, are times larger than the sun. And super giant stars can be 1000 times larger than the sun.  At the other end, stars called white dwarfs may be smaller than Earth. The smallest of all stars, neutron stars, may be as small as the community where you live.

What are stars made of? Almost all stars are make mostly of two gases- hydrogen and helium. But how do we know what stars are made of? When an element burns, it gives off a set of colors. No two elements give off the same set of colors. Hydrogen gives off one set of colors and helium gives off another set of colors.

What a star looks like up close. Our Sun

Planetary system  Some stars, like our sun, are surrounded by planets and other objects. A planet is a large ball- shaped objects that moves around a star in a fixed path. Unlike stars, planets do not give off their own light and heat. The eight planets that revolve around our sun make up a planetary system called the solar system. A star and its planets are called a planetary system.

 Our solar system, includes the sun, the planets, and many smaller objects.  One way that scientists measure distance in space is by using the astronomical unit.  One astronomical unit (AU) is the average distance between the sun and Earth, or approximately 150,000,000 km.

 Another way to measure distances in space is by using the speed of light.  Light travels at about 300,000 km/s in space. This means that in one second, light travels 300,000 km.  The distance is also called light- minute.  Light from the sun takes 8.3 minutes to reach Earth.

 The average distance between the Earth and the sun is approximately 150 million kilometers.  Up until the 17 th century, the universe was thought to have only eight bodies. These bodies included the planets Earth, Mercury, Venus, Mars, Jupiter, Saturn, the sun, and Earth’s moon.

 After the telescope was invented in the 17 th century, nine more large bodies were discovered. These bodies were the moons of Jupiter and Saturn.  By the 18 th century, the planet Uranus, along with two of its moons and two more of Saturn’s moons were discovered.

 In the 19 th century Neptune, as well as moons of several other planets were discovered.  Finally, in the 20 th century, Pluto and many other bodies were discovered.

Two Main Parts of the Solar System  The solar system is divided into two main parts: inner solar system and the outer solar system.  The inner solar system contains the four planets that are closest to the sun. The inner planets are more closely spaced than the planets of the outer solar system.  The inner planets are also known as the terrestrial planets because their surfaces are dense and rocky.

Outer Planets  Unlike the inner planets, the outer planets are large and are composed mostly of gases.  Because Jupiter, Saturn, Uranus, and Neptune are composed mostly of gases, they are known as gas giants.

 Several smaller bodies, including Pluto are located beyond the orbit of Neptune.  These bodies are made of rock and ice and may be leftover material from the formation of the early solar system.

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Exit Ticket 1. In your own words, write a definition for the term astronomical unit. 2.The invention of what instrument helped early scientists discover more bodies in the solar system? 3. List the four inner planets in order of their distance from the sun. 4. List the outer planets in increasing order of their distance from Earth’s orbit. 5. Describe how the inner planets are different from the outer planets.