Universe¶
The universe is an immense expanse of space that contains all of everything—stars, planets, galaxies, and all forms of matter and energy. The prevailing theory about its origin is the Big Bang, which suggests that the universe has been expanding from a hot, dense state for about 13.8 billion years.
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The Big Bang¶
The Big Bang theory describes the universe's beginning as a singularity around 13.8 billion years ago. From this point, the universe has been expanding and cooling, leading to the formation of subatomic particles and eventually atoms.
Galaxies¶
Galaxies are vast collections of stars, dust, and dark matter, bound together by gravity. Our Milky Way is just one of billions in the universe, each hosting millions to trillions of stars.
Star Birth¶
Stars form in regions of high density within galaxies, known as nebulae. Gravity pulls gas and dust together, forming a protostar. As it accumulates more mass, its temperature and pressure increase, eventually triggering nuclear fusion in its core.
Star Death¶
Stars die when they exhaust their nuclear fuel. Smaller stars like our Sun end as white dwarfs, while massive stars explode in supernovae, leading to either a neutron star or a black hole.
The Sun¶
Our Sun, a middle-aged star, provides the necessary energy to sustain life on Earth. It's in the main sequence phase of its life, where it fuses hydrogen into helium in its core.
The Solar System¶
The Solar System comprises the Sun and all the celestial bodies gravitating around it, including planets, moons, asteroids, and comets. It's a small part of the Milky Way galaxy.
Inner Planets¶
The inner planets—Mercury, Venus, Earth, and Mars—are rocky and relatively small. They're closer to the Sun and have few or no moons.
Outer Planets¶
The outer planets—Jupiter, Saturn, Uranus, and Neptune—are gas giants (except Neptune and Uranus, which are ice giants). They're larger, have rings, and many moons.
The Moon¶
Earth's Moon is a natural satellite that impacts our planet's tides, stabilizes its rotation axis, and has been a key focus for human space exploration.