Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
THE SOLAR SYSTEM The world as we know it, as well as everything beyond it, is known as the Universe. It is everything that exists. Scientists who study the stars and planets are called astronomers. They think that a ‘big bang’, which was a gigantic explosion, billions of years ago, formed the universe. It then took billions of years for galaxies to form.
The Big Bang Theory
It is estimated that there are about 100 billion, billion (100, 000,000,000,000,000,000) stars in the universe.
Galaxies A galaxy is a huge group of stars. Scientists think there are about 100 billion galaxies in the universe (100, 000,000,000). The galaxy that we live in is called the Milky Way, which formed 9 billion years after the big bang. It is a spiral shaped galaxy, made up of all the stars that you can see and millions more that you cannot see. It is called the Milky Way because sometimes we can see it and it looks like a band of milky white light in the sky. The Milky Way has about 1000 billion stars in it.
The Milky Way
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Most galaxies have numbers, rather than names, but a few tell us what the galaxy looks like, for example, the Whirlpool. The nearest galaxy to ours is called Andromeda. It would take us 2,2 million years to get there!
The Whirlpool Galaxy
The Great Andromeda Galaxy
Galaxies comes in three shapes: spiral (like the Milky Way), egg shaped and irregular (no particular shape).
A Spiral Shaped Galaxy Wikimedia Public Domain, Courtesy NASA
An Irregular Shaped Galaxy Wikimedia Public Domain, Courtesy NASA Version 1: July 2016
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
An Egg Shaped Galaxy
The Solar System The solar system includes the Sun and the planets that move around it, i.e. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. It is the force of gravity that makes the planets orbit the Sun. Stars, moons, asteroids and comets are also found in the solar system.
Illustration of Our Solar System
Stars Stars occur in groups. In the past, people who looked at them imagined that they represented something. For thousands of years, people have stared at the sky and imagined the patterns that stars make. These patterns or groups of stars are known as constellations. Each constellation has a name, related to the shape they make in the sky. There are eighty-eight constellations in modern day astronomy. People who live south of the equator can easily identify the Southern Cross on a clear evening.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
The Southern Cross Constellation
Sirius is the brightest star that we can see from Earth and is also called the ‘Dog Star’, as its constellation is in the shape of a dog. It is twice as big as the Sun.
Birth of a Star
The Sirius or ‘Dog Star’ Wikimedia Public Domain, Courtesy NASA
How does a star come into being? There is a huge amount of gas and many clouds of dust in the universe. Sometimes these are drawn together and form bigger masses, called nebulae. As they increase in size, they become very hot, hotter than 1 000 000 degrees Celsius. The gas, helium, is formed and when this happens, an incredible amount of energy and light is released.
Nebulae Being Formed Version 1: July 2016
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
This mass of gas and dust begins to sparkle as a star. So a star is not solid, it is made up of gases, just like air. Stars are mainly made up of hydrogen and helium. Stars do not really shine or twinkle, they just look like they do when we see them from Earth. This is due to the air around Earth which bends and wobbles the light from the star. They are also not star shape, although they might look star-shaped from Earth, but they are actually ball shaped. The nearest star to Earth is called Proxima Centauri.
Proxima Centauri as Taken by Hubble Wikimedia Attribution Share-Alike: ESA / Hubble and NASA Wikimedia Public Domain, Courtesy NASA
New stars are continually being born and live for a very long time. Towards the end of their life, a star swells up, gets much bigger and turns into a red giant. When the star has used up all of its gas, it shrinks down into a white dwarf. It cools down and ends it life as a black dwarf. Some stars explode when they die. These are known as a supernova.
Red Giant
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White Dwarf
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Black Dwarf Wikimedia Creative Commons Attribution Share-Alike: Michael Liu
Supernova
A black hole can be formed when a huge star dies and falls in on itself. A star that gets too close to a black hole can be sucked into it.
Black Hole
The Sun The Sun is a star in the centre of the solar system. It produces its own energy and is a huge ball of super-hot gas, mainly hydrogen and also helium. These gases radiate heat and light which support life on Earth. The Sun is much bigger than the Earth and the Moon, with a diameter of 1,4 million kilometres and is much further away from the Earth than the Moon, around 150 million kilometres away. The temperature on the outside of the Sun is 6 000 ºC and at the core is around 15 million ºC. The Sun was born 4,6 billion years ago and is thought to be halfway through its life.
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The Sun
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Life Cycle of the Sun Wikimedia Public Domain
The Sun is the only star that is close enough for Planet Earth to feel the Sun’s heat. The Sun’s light takes 8,3 minutes to get to Earth. Without the Sun’s heat and light there would be no life on Earth. Solar radiation is the light energy given off by the Sun. The ozone layer in our atmosphere is a protective barrier that filters the harmful infrared and ultra violet rays from reaching Earth’s surface. The Sun has an atmosphere, but this is very different from Earth’s atmosphere. Its atmosphere is divided into three parts: The Corona – this extends for millions of kilometres and is extremely hot. The Chromosphere – this extends about 2 400 km outwards and gets hotter as it gets further away. It can only be seen during a solar eclipse. The Photosphere – this is the part of the Sun that we can see with the naked eye.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
The Moon A moon is a rocky body that orbits a planet. It is also known as a satellite. The only planets in our solar system that do not have moons are Mercury and Venus, while some have many, for example Jupiter has sixty-seven and Saturn has sixty-two confirmed moons.
Planet with a Moon Orbiting
On most evenings, Earth’s Moon, which is our nearest neighbour in space (384 000 km away), lights up our night sky. It is a satellite of the Earth and completes one full revolution or orbit of the Earth in approximately 27,3 days. It is the second brightest object in the sky after the Sun.
The Earth’s Moon
The Moon is about a quarter of the size of the Earth with a diameter of 3 476 km and is made of rock.
Comparison of Size of Earth and Moon Version 1: July 2016
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
The Moon is covered in craters, dust and mountains. The craters were formed when asteroids crashed into the Moon a very long time ago. As there is no atmosphere on the Moon, there is no wind. We know what the Moon looks like because photographs have been taken from satellites and astronauts have also taken photos on the Moon. The lighter areas shown on photos are mountains and darker areas which are flat plains. Moons do not give out heat or light.
Craters on the Moon
Water has been found to be present on the Moon in the form of water molecules as well as in lunar craters. Signs of frozen water have also been found. Apart from Planet Earth, other planets also have moons, which are natural satellites orbiting around them. The only two planets that do not have moons are Venus and Mercury. Mars has two moons, Demios and Phobos. They both have deep craters and are much smaller than Earth’s Moon.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Jupiter has sixty-seven known moons.
Saturn has sixty-two known moons.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Uranus has twenty-seven moons.
Neptune has fourteen moons.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Planets There are eight planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, as well as an asteroid belt, that orbit the Sun. The features and sizes of the planets differ and so does the amount of time they take to orbit the Sun. All the planets are at different distances from the Sun and have different orbits, i.e. they take a different path around the Sun. Planets with smaller orbits are closer to the Sun.
The Solar System, Showing the Position and Relative Size of Planets
The Inner Solar System The Inner Solar System comprises of four planets, i.e. Mercury, Venus, Earth and Mars. They are also known as the Terrestrial Planets or the Inner Solar System. The planets nearest to the Sun are generally small and rocky. Some have a thin layer of gas.
Relative Size of Terrestrial Planets, Wikimedia Public Domain, Courtesy NASA From left to right: Mercury, Venus, Earth, Mars Version 1: July 2016
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Outer Planets The planets furthest away from the Sun, i.e. Jupiter, Saturn, Uranus and Neptune are known as gas giants and are also known as the Outer Planets. They are found on the outer part of the solar system, beyond the asteroid belt. These planets are not composed of rock or other solids, but are made up of gases or frozen gases, with a relatively small rocky core. They do not have a solid surface. Jupiter and Saturn are much larger and have a different composition to Neptune and Uranus. The outer planets are all large and cold as they are far away from the Sun. They also rotate very quickly, but revolve very slowly around the Sun.
The Four Gas Giants and the Sun Wikimedia Public Domain, Courtesy NASA
The photo above shows the four gas giants: from left to right, Jupiter, Saturn, Uranus and Neptune. The Sun is in the background.
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
All of the planets are tiny compared to the Sun. Version 1: July 2016
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
Relative Size of Planets (to scale) Wikimedia, Public Domain, Courtesy NASA N.B. Distance from Sun is not to scale
Asteroids Millions of asteroids orbit the Sun in a ‘belt’ between Mars and Jupiter, i.e. between the inner solar system and the outer solar system. Asteroids are huge pieces of rock that orbit the Sun at a very fast rate, up to 25 km per second and are too small to be classified as planets. Some are tiny and others are the size of countries. They have no atmosphere. Ceres is the largest asteroid (now considered to be a ‘dwarf planet’) and was the first to be discovered in 1801. Eros is also a large asteroid near Earth. The Vesta asteroid is the second largest asteroid in the belt and is the brightest asteroid that is visible from Earth.
Comparison of Size of Asteroids Wikimedia Public Domain Courtesy NASA
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Natural Science & Technology: Grade 6 Earth and Beyond: The Solar System
In order to travel to Jupiter, spacecraft would need to travel through the asteroid belt. Pioneer 10 was the first spacecraft to successfully navigate the asteroid belt and travel to Jupiter in 1973. Although it did not land on Jupiter, it took photos of the planet.
Comets Comets are chunks of ice, rock and dust. They are mainly found on the edge of the solar system but sometimes they get close to the Sun. They can grow huge dust and gas tails when the heat of the Sun starts to melt them.
Hale – Bopp Comet 1997 Wikimedia Creative Commons Attribution Share-Alike: Philipp Salzgeber
Meteors Meteors are also called ‘shooting stars’ and are lumps of rock that burn up as they enter the Earth’s atmosphere.
Bolide Meteor Wikimedia Commons Public Domain
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