- NEBULAR THEORY:
The basic idea of the nebular theory is that all matter shows gravitational attraction and therfore comes together in big clumps.
When a clump of matter becomes big enough, is begins to spin or rotate, and is then called a nebula.
As it rotates and spreads out, a nebula becomes shaped like a disk.
Because of gravitational attraction, the center of the nebula is thicker.
Lots of matter colliding together produces heat, so the nebula begins to radiate (give off) energy.
The dense center of the nebula may condense to form a star. (This depends, in part, on size. Bigger nebulas become galaxies.)
Outside the center, matter is flung out as dust particles, rocks, or larger objects. These objects attract one another (by gravity),
and they may condense to form planets circling the central star.
The whole process may also happen on a smaller scale within the nebula, producing satellites (moons) around the planets.
As evidence for this theory, scientists have already found thousands of planets circling distant stars.
- Our galaxy: The Milky Way:
A Galaxy is a collections of millions of stars. It is formed as a giant nebula, but on a much larger scale than the nebulas that form individual stars.
Our galaxy is called the Milky Way because, on a very clear night, you can sometimes see it as a milky-white streak across the
sky made up of millions of stars too tiny to see individually.
Other galaxies are so far away that they usually appear in the sky as faint objects that look something like fuzzy stars.
A few galaxies can be seen with binoculars, but most can only be seen with high-power telescopes.
Most galaxies spin very fast and become disk-like in shape. From a distance, an edge-on view of a galaxy looks like a streak with a fat blob in the center.
If we can view a galaxy along its axis of spin, we would see most of the mass in the center and two or more spiral arms radiating out from the central disk.
Most galaxies are visible in a tilted view, so the spinning disk looks like the shape of a football.
Take a dinner plate and have someone hold it up across the room, but view it at an angle so that the entire outline looks elliptical.
Now, imagine it to be fatter in the center. Depending on how it is tilted, your dinner-plate galaxy may look more circular, or elliptical,
or narrower, or really narrow if viewed along the edge.
Our galaxy is rather typical in size and shape, with an overall disk-like shape and two large, spiral arms. The center is fatter in the middle.
Our Sun (and the entire Solar System) is about halfway out one of the spiral arms.
- Solar System:
Our Solar System has a central star, the Sun, containing most of the Solar System's mass.
Circling the sun are a series of planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Beyond the planets lies an area of very small objects, called the Kuiper Belt.
The largest object in the Kuiper Belt is Pluto, which used to be considered a planet.
Between the planets are an assortment of smaller objets called asteroids.
Most asteroids circle the sun in an asteroid belt located between Mars and Jupiter.
The inner planets (Mercury, Venus, Earth, Mars) are mostly "rocky" and dense; they are sometimes called Terrestrial (Earth-like) planets.
The outer planets (Jupiter, Saturn, Uranus, Neptune) are "gas giants," hundreds of times larger than Earth, and made mostly of gas.
They are sometimes called the Jovian (Jupiter-like) planets.
All the planets and asteroids circle the sun in the same direction. Their orbits lie in a plane called the ecliptic.
Most of the planets have "moons" or satellites, circulating in the same direction as the planet.
For any object in the solar system, its spin on its axis is called rotation and its orbital motion around a larger object is called revolution,
so the Earth rotates on its own axis while it revolves around the sun.
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