THE UNIVERSE
The term “universe”
typically refers to all of space, time, matter, and energy that exist. It encompasses everything, from the smallest subatomic particles to the largest galaxies and beyond. The observable universe is estimated to be about 93 billion light-years in diameter.
The universe contains galaxies, stars, planets, black holes, dark matter, and dark energy. It is governed by the laws of physics and has undergone various evolutionary stages, including the formation of galaxies, stars, and planets.
The Big Bang theory is the prevailing cosmological model that describes the origin of the universe. According to this theory, the universe began as an extremely hot and dense state about 13.8 billion years ago and has been expanding ever since.
Galaxy
The term "galaxy" typically refers to a massive, gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. Galaxies come in various shapes and sizes, ranging from dwarf galaxies with just a few billion stars to giant galaxies containing trillions of stars. They are the fundamental building blocks of the universe, and the one we inhabit is called the Milky Way.
Galaxies can be broadly classified into several types based on their shapes, including spiral galaxies, elliptical galaxies, and irregular galaxies. The Milky Way is a barred spiral galaxy, characterized by a central bar-shaped structure and spiral arms extending outward.
The study of galaxies is a crucial aspect of astronomy and astrophysics, as understanding their formation, evolution, and properties contributes to our broader comprehension of the cosmos. The universe contains billions of galaxies, each hosting a vast array of stars, planetary systems, and other celestial objects.
Star
A star is a massive, luminous celestial body primarily composed of hydrogen and helium undergoing nuclear fusion in its core. The process of nuclear fusion involves the conversion of hydrogen into helium through a series of nuclear reactions, releasing a tremendous amount of energy in the form of light and heat. This energy generation is what makes stars shine.
Stars come in various sizes, masses, and temperatures, leading to different spectral types and colors. The life cycle of a star is determined by its mass. Generally, stars form from regions of dense interstellar gas and dust, known as molecular clouds. Gravitational forces cause these regions to collapse, forming protostars. When the core temperature of a protostar becomes high enough for nuclear fusion to begin, it becomes a main-sequence star.
The main sequence is a phase in a star's life where it fuses hydrogen into helium in its core. The duration of this phase depends on the star's mass. Eventually, a star exhausts its hydrogen fuel and may undergo changes depending on its mass. Smaller stars, like our Sun, may become red giants, while more massive stars can undergo more dramatic transformations, such as supernova explosions or the formation of compact objects like neutron stars or black holes.
Planet
A planet is a celestial body that orbits a star, is spherical in shape due to its self-gravity, and has cleared its orbit of other debris. In our solar system, planets orbit the Sun, which is a star. There are eight recognized planets in our solar system
The definition of a planet was refined in 2006 by the International Astronomical Union (IAU), which established three criteria for an object to be considered a planet:
1. It must orbit the Sun.
2. It must be spherical in shape due to self-gravity.
3. It must have cleared its orbit of other debris.
Objects that meet the first two criteria but not the third are classified as dwarf planets. Pluto, formerly considered the ninth planet, was reclassified as a dwarf planet based on the third criterion.
