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1. Solar System
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| An illustration of the Solar System, showing the Sun and its orbiting planets. Vecteezy.com |
The Solar System is our local cosmic neighborhood, centered around the Sun—a G-type main-sequence star that provides the gravitational anchor and energy source for everything orbiting it. Formed approximately 4.6 billion years ago from the gravitational collapse of a giant molecular cloud, it includes eight recognized planets: Mercury, Venus, Earth, Mars (the inner terrestrial planets), and Jupiter, Saturn, Uranus, Neptune (the outer gas and ice giants). Beyond these are dwarf planets like Pluto, countless moons (such as Earth's Moon or Jupiter's Ganymede), asteroid belts, comets from the Oort Cloud and Kuiper Belt, and interplanetary dust. The Solar System spans about 1-2 light-years in diameter if including the Oort Cloud, and it's a dynamic environment shaped by gravity, solar wind, and occasional impacts. Human exploration has reached as far as the outer planets via probes like Voyager, but crewed missions remain limited to low-Earth orbit and the Moon.
2. Milky Way Galaxy
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| The spiral structure of the Milky Way Galaxy, highlighting its key components. ESA Standard Licence |
The Milky Way is the barred spiral galaxy that houses our Solar System, located about 27,000 light-years from its central supermassive black hole, Sagittarius A*. With a diameter of roughly 100,000 to 180,000 light-years and a thickness of about 1,000 light-years in the disk, it contains an estimated 100 to 400 billion stars, along with vast amounts of gas, dust, and dark matter. The galaxy's structure includes a central bulge of older stars, spiral arms (like the Orion Arm where we reside) where new stars form from nebulae, a surrounding halo of globular clusters, and a dark matter envelope that provides most of its mass. The Milky Way rotates, with our Solar System completing one orbit around the center every 225-250 million years. It's part of the Local Group of galaxies and is on a collision course with the Andromeda Galaxy in about 4.5 billion years, potentially merging into "Milkomeda." Observations from telescopes like Gaia have mapped billions of its stars, revealing its complex history of mergers with smaller galaxies.
3. Observable Universe
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| Photo on Pixnio |
The observable universe represents the portion of the cosmos we can theoretically detect from Earth, limited by the speed of light and the universe's age of about 13.8 billion years. It forms a sphere with a diameter of approximately 93 billion light-years, larger than expected due to cosmic expansion stretching space itself. This expanse includes about 2 trillion galaxies, each potentially containing billions of stars and planets, along with cosmic microwave background radiation, dark energy (driving expansion), and dark matter (influencing galaxy formation). Our Milky Way sits within the Laniakea Supercluster, but the observable universe's edge is a horizon beyond which light hasn't reached us yet. Telescopes like Hubble and James Webb have peered back to the universe's early epochs, revealing galaxies forming just hundreds of millions of years after the Big Bang. The observable universe is likely just a tiny fraction of the total universe, which could be infinite.
4. Undefined
This category likely refers to the vast, uncharted territories beyond our current understanding or observational capabilities, such as the unobservable universe or regions dominated by unknown physics. The unobservable universe extends infinitely beyond the observable horizon, where cosmic expansion has carried galaxies away faster than light can travel back to us (without violating relativity, as space itself expands). It could include exotic phenomena like dark flow, voids larger than expected, or areas where physical laws differ due to quantum fluctuations during the Big Bang. "Undefined" might also encompass speculative concepts like the role of dark matter (making up 27% of the universe's mass-energy) or dark energy (68%), which we detect indirectly but don't fully comprehend. Current models suggest the total universe could be at least 250 times larger than the observable part, or even infinite, leaving much to "undefined" status until new theories or technologies emerge.
5. Multiverse
The multiverse is a theoretical framework proposing the existence of multiple, perhaps infinite, universes beyond our own, each potentially with different physical constants, laws, or histories. Popularized by theories like eternal inflation (where "bubble universes" form from quantum fluctuations in an ever-expanding space) and string theory (suggesting up to 10^500 possible universes), it explains fine-tuning paradoxes—why our universe's constants allow for life. Types include Level I (infinite space repeating configurations), Level II (bubbles with varying physics), Level III (quantum many-worlds interpretation), and Level IV (mathematical structures). While untestable directly, evidence might come from cosmic microwave background anomalies or gravitational waves. The multiverse challenges our understanding of reality, suggesting our universe is just one in a vast ensemble.
I hope this expanded version aids in better understanding the scales and mysteries of the cosmos. Originaly published in 2019.
The entire Milky Way galaxy will someday be a human settlement.
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