Our Place in the Cosmos: A Matter of Scale
Understanding the universe requires grasping the immense scales involved.
Star: A large, luminous ball of plasma held together by its own gravity, which produces energy through nuclear fusion in its core. The Sun is our local star.
Solar System: A star and all of the objects that travel in orbit around it, including planets, moons, asteroids, and comets.
Galaxy: A vast, gravitationally bound system containing billions of stars, along with gas, dust, and dark matter. We live in the Milky Way Galaxy.
Universe: All of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy.
Origin of a Solar System: The Nebular Theory
Solar systems form from the gravitational collapse of a giant interstellar cloud of gas and dust called a nebula.
1.The cloud collapses and begins to rotate, flattening into a protoplanetary disk.
2.The vast majority of mass collects in the center, forming a star (like the Sun).
3.In the disk, particles of dust and ice stick together, gradually accreting into larger bodies called planetesimals, which eventually form planets.
Origin of the Universe: The Big Bang Theory
This is the leading cosmological model for the observable universe.
Core Idea: The universe began approximately 13.8 billion years ago in an extremely hot, dense state. It was not an explosion in space, but an expansion of space itself.
Key Evidence:
1.Universal Expansion: Observations (Hubble's Law) show that galaxies are moving away from each other, meaning the universe is expanding.
2.Cosmic Microwave Background (CMB): The faint, uniform glow of microwave radiation filling the universe is the 'afterglow' of the Big Bang.
Measuring Cosmic Distances: The Light-Year
Because distances in space are so vast, astronomers use a unit called the light-year.
Definition: A light-year is the distance that light travels in one year in a vacuum.
It is a unit of distance, not time.
1 light-year ≈ 9.46 trillion kilometers or 5.88 trillion miles.
Looking at an object that is one million light-years away means we are seeing it as it was one million years ago.