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All About Astronomy
Where are we in the universe? We live on Earth, the third planet of our solar system. Our solar system is located in the Milky Way Galaxy, a collection of 200 billion stars (together with their planetary systems). The Milky Way Galaxy is located in a group of 30+ galaxies we call the Local Group. The Local Group is a part of a local supercluster of 100+ galaxies (called the Virgo Supercluster). This supercluster is one of millions of superclusters in the universe.
Our Solar System
Our solar system consists of the sun, planets, dwarf planets (or plutoids), moons, an asteroid belt, comets, meteors, and other objects. The sun is the center of our solar system; the planets, over 61 moons, the asteroids, comets, meteoroids and other rocks and gas all orbit the Sun. The Earth is the third planet from the sun in our solar system.
The nine planets that orbit the sun are (in order from the Sun): Mercury, Venus, Earth, Mars, Jupiter (the biggest planet in our Solar System), Saturn (with large, orbiting rings), Uranus, Neptune, and Pluto (a dwarf planet or plutoid). A belt of asteroids (minor planets made of rock and metal) orbits between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; this dwarf planet has an elliptical orbit tilted over 17° from the ecliptic).
The inner planets (those planets that orbit close to the Sun) are quite different from the outer planets (those planets that orbit far from the Sun).
- The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.
- The outer planets include: Jupiter, Saturn, Uranus, and Neptune. They are mostly huge, mostly gaseous, ringed, and have many moons (plus Pluto, which is a dwarf planet that has one large moon and two small moons).
There are other smaller object that orbit the Sun, including asteroids, comets, meteoroids and dwarf planets.
- Asteroids (also called minor planets) are rocky or metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter.
- Comets are small, icy bodies that orbit the sun. They have very long tails.
- Meteoroids are small bodies that travel through space. They are stony and/or metallic and are smaller than asteroids. Most are very tiny.
The Milky Way Galaxy
Our solar system is located in the outer reaches of the Milky Way Galaxy, which is a spiral galaxy. The Milky Way Galaxy contains roughly 200 billion stars. Most of these stars are not visible from Earth. Almost everything that we can see in the sky belongs to the Milky Way Galaxy.
From the Earth, our Milky Way Galaxy is visible as a milky band that stretches across the night sky. It is easier to see when you are far from bright city lights.
The sun is about 26,000 light-years from the center of the Milky Way Galaxy, which is about 80,000 to 120,000 light-years across (and less than 7,000 light-years thick). We are located on on one of its spiral arms, out towards the edge. It takes the sun (and our solar system) roughly 200-250 million years to orbit once around the Milky Way. In this orbit, we (and the rest of the Solar System) are traveling at a velocity of about 155 miles/sec (250 km/sec).
To reach the center of the Milky Way Galaxy starting from the Earth, aim toward the constellation Sagittarius. If you were in a spacecraft, during the trip you would pass the stars in Sagittarius one by one (and many other stars!).
The Milky Way Galaxy is just one galaxy in a group of galaxies called the Local Group. Within the Local Group, the Milky Way Galaxy is moving about 300 km/sec (towards the constellation Virgo). The Milky Way Galaxy is moving in concert with the other galaxies in the Local Group (the Local Group is defined as those nearby galaxies that are moving in concert with each other, independent of the "Hubble flow" expansion).
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