All of the spacetime and its contents including the Earth, possibly being part of a multiverse, distinct from parallel universes if they exist.
The universe is a vast, seemingly infinite expanse that contains everything that exists — from the smallest particles to the largest galaxies and from the tiniest moments of time to the longest eons. It encompasses all of space and time, along with all the matter and energy within them.
The study of the universe and its contents is known as astronomy. This scientific field has a long history, dating back to ancient civilizations that observed the night sky and identified patterns in the movements of celestial bodies. Over the centuries, our understanding of the universe has evolved dramatically, from the geocentric model of the ancient Greeks, where Earth was considered the center of the universe, to the modern understanding of a vast cosmos filled with billions of galaxies.
In the universe, celestial bodies such as stars, planets, galaxies, and nebulae exist. Stars are massive celestial bodies made mostly of hydrogen and helium that produce light and heat from the nuclear reactions (nuclear fusion) in their cores. Planets are celestial bodies that orbit stars, and they do not produce their own light. Galaxies are vast collections of stars, gas, dust, and dark matter, bound together by gravity. Nebulae are enormous clouds of dust and gas in space, often serving as "stellar nurseries" where new stars are born.
The scale of the universe is truly mind-boggling. Distances in the universe are so vast that they are measured in light-years — the distance that light travels in one year, which is about 9.46 trillion kilometers. For example, our galaxy, the Milky Way, is about 100,000 light-years in diameter. Andromeda, the nearest spiral galaxy to the Milky Way, is approximately 2.5 million light-years away. These vast distances pose significant challenges for astronomers, leading to the development of various methods to measure them.
In the next unit, we will delve into one such method known as the Cosmic Distance Ladder, a crucial tool in astronomy that allows us to measure the vast distances in the universe. This tool has not only expanded our understanding of the universe but also paved the way for groundbreaking theories and discoveries, such as the Big Bang Theory and the study of galaxies.
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