
Most astronomers believe the universe began in a Big Bang about 14 billion years ago. This big bang theory states that the universe exploded and grew from smaller than a single atom to bigger than a galaxy. Stable particles called protons and neutrons started to form when the universe was one second old. After 300 000 years, the universe had cooled to about 3000 degrees. Atomic nuclei could finally capture electrons to form atoms.

We cannot see anything that happened during the first 300,000 years of the universe. Scientists try to work it out from their knowledge of atomic particles and from computer models. The only direct evidence of the Big Bang itself is a faint glow in space. The Hubble Space Telescope and powerful ground-based telescopes are now beginning to find galaxies that were created about one billion years after the Big Bang. These small galaxies were much closer together than galaxies are today. Since they were so close, galaxies merged into bigger galaxies, and this is how our galaxy was formed.

Outer space is a vacuum. There is no air. Since there is no air to scatter sunlight, space appears black. Space is usually regarded as being completely empty. But this is not true. The vast gaps between the stars and planets are filled with huge amounts of thinly spread gas and dust. Even the emptiest parts of space contain at least a few hundred atoms or molecules. Space is also filled with many forms of radiation that are dangerous to astronauts. Much of this infrared and ultraviolet radiation comes from the Sun. High energy X-rays, gamma rays and cosmic rays arrive from distant star systems.

We live in a galaxy called the Milky Way galaxy. The Milky Way is a barred spiral galaxy that is about 100,00 light-years across. That's gigantic! One light-year is the distance light travels in a year. The spirals making up our galaxy contain dust, gas, and stars. These are not the only components to our galaxy though. Let's take a closer look at some of the objects floating around in outer space!






Our galaxy has planets, dwarf planets, moons, asteroids, meteors, black holes, debris, stars, the Sun, and many other things we will discuss in more detail.
A star is a luminous sphere held together by its own gravity. Stars produce heat, light, ultraviolet rays, x-rays, and other forms of radiation. They are composed largely of gas and plasma, a super-heated state of matter composed of subatomic particles.Astronomers estimate there are about 100 thousand million stars in the Milky Way galaxy alone. With astronomy, we study and name stars. When we group and name stars, they are called constellations. One of the most energetic explosive events known is a supernova. These occur at the end of a star's lifetime, when its nuclear fuel is exhausted and it is no longer supported by the release of nuclear energy. If the star is particularly massive, then its core will collapse and will release a huge amount of energy.

The Sun is the star at the center of our solar system and is responsible for the Earth’s climate and weather. The Sun has the largest mass of anything in our solar system. The Sun is a mix of all the colors which is why it appears white to our eyes. The Sun is almost a perfect sphere and has a very strong magnetic field. The Sun generates solar wind. Solar wind occurs where the magnetic field of the Sun extends into space instead of following its surface. The Sun emits light and energy and is the beginning of every food chain or food web. The Sun also can create solar eclipses. This is an eclipse in which the sun is obscured by the moon.


The atmosphere of the Sun is composed of three areas: the photosphere, chromosphere, and solar corona. The corona is the Sun’s outer atmosphere with temperatures that range from a few thousand Kelvin to a few million Kelvin. Sunspots appear on the surface of the Sun and look dark because of the lower temperatures near it.Solar flares from the Sun are sudden bursts of brightness that happen in places near the sunspots. The internal structure of the Sun is composed of: the inner core, radiative, core, and the radiative core. The inner core is the hottest part of the Sun.
There are many moons in our galaxy. The one we are most familiar with though, is Earth's moon. We have sent twelve astronauts to our moon. The moon and it's gravitational pull is what causes the rise and fall of the tides on Earth. The Moon is the Earth’s only natural satellite. A natural satellite is a space body that orbits a planet or a planet-like object. The surface of the Moon features a huge number of impact craters from comets and asteroids that have collided with the surface over time.




During each lunar orbit (a lunar month), we see the Moon's appearance change from not visibly illuminated through partially illuminated to fully illuminated, then back through partially illuminated to not illuminated again. Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages, called phases. The phases designate both the degree to which the Moon is illuminated and the geometric appearance of the illuminated part. As the moon waxes, the lunar phases progress through
new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The moon is then said to wane as it passes through the gibbous moon, third-quarter moon, crescent moon and back to new moon.

The Planets
There is a total of eight planets in our solar system. The closest planets to the sun, also known as the inferior planets, are Mercury, Venus, Earth, and Mars. Then there is an asteroid belt that separates the inferior from the superior planets. The planets furthest from the sun, or the superior planets, are Saturn, Jupiter, Uranus, and Neptune.
All of the planets orbit around the sun. They move around the sun in an elliptical, which is similar to an elongated oval. As the planets revolve around the sun, they are also rotating on their axes. To rotate is to spin around, and to revolve is to go around. Earth, for example, takes 24 hours to complete a full rotation, which equals one day. It takes 365 days to complete one full revolution around the sun, which equals one year. Rotations lead to day and night, and revolutions lead to our four seasons.



Mercury
Mercury is the first planet from the sun. It is the second densest planet and second hottest planet. Mercury also does not have any moons. It is the most cratered planet in our solar system.
Mercury has a diameter of 4,879 km and its mass only 5.5% of Earth's. Mercury has just 38% the gravity of Earth, this is too little to hold on to what atmosphere it has which is blown away by solar winds.




Venus
Venus is the second planet from the Sun and the sixth largest.Venus has been known since prehistoric times. It is the brightest object in the sky except for the Sun and the Moon.
Venus spins slowly in the opposite direction most planets do. Its thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead.
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Most astronomers believe the universe began in a Big Bang about 14 billion years ago. This big bang theory states that the universe exploded and grew from smaller than a single atom to bigger than a galaxy. Stable particles called protons and neutrons started to form when the universe was one second old. After 300 000 years, the universe had cooled to about 3000 degrees. Atomic nuclei could finally capture electrons to form atoms.

We cannot see anything that happened during the first 300,000 years of the universe. Scientists try to work it out from their knowledge of atomic particles and from computer models. The only direct evidence of the Big Bang itself is a faint glow in space. The Hubble Space Telescope and powerful ground-based telescopes are now beginning to find galaxies that were created about one billion years after the Big Bang. These small galaxies were much closer together than galaxies are today. Since they were so close, galaxies merged into bigger galaxies, and this is how our galaxy was formed.

Outer space is a vacuum. There is no air. Since there is no air to scatter sunlight, space appears black. Space is usually regarded as being completely empty. But this is not true. The vast gaps between the stars and planets are filled with huge amounts of thinly spread gas and dust. Even the emptiest parts of space contain at least a few hundred atoms or molecules. Space is also filled with many forms of radiation that are dangerous to astronauts. Much of this infrared and ultraviolet radiation comes from the Sun. High energy X-rays, gamma rays and cosmic rays arrive from distant star systems.
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