Astronomy
The universe encompasses all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and energy. It is the totality of everything that exists.
Here are some key aspects of the universe:
- Size and Expansion: The universe is vast and is continuously expanding. The observable universe, which is the portion we can see from Earth, is estimated to be about 93 billion light-years in diameter.
- Composition: The universe is composed of ordinary matter (baryonic matter), dark matter, and dark energy. Ordinary matter makes up only a small fraction of the total. Dark matter and dark energy are not directly observable and are not well understood.
- Origin: The prevailing cosmological model for the universe is the Big Bang theory. This theory suggests that the universe originated from an extremely hot, dense state about 13.8 billion years ago and has been expanding and cooling ever since.
- Structure: The universe has a hierarchical structure, with planets organized into solar systems, solar systems into galaxies, galaxies into groups, groups into clusters, and clusters into superclusters, forming large-scale structures like filaments and voids.
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The solar system is a gravitationally bound system consisting of the Sun and the objects that orbit it, either directly or indirectly.
- The Sun: A star at the center, containing 99.86% of the system's known mass and dominating the system gravitationally.
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Planets: Eight planets orbit the Sun:
- Inner, terrestrial planets: Mercury, Venus, Earth, and Mars.
- Outer, gas giants: Jupiter, Saturn, Uranus, and Neptune.
- Dwarf Planets: Such as Pluto, Ceres, Eris, Makemake, and Haumea.
- Other Small Bodies: Includes asteroids, comets, and trans-Neptunian objects.
- Belts and Clouds: Asteroid belt, Kuiper belt, and the Oort cloud.
The planets orbit the Sun in elliptical paths. The asteroid belt lies between Mars and Jupiter. Beyond Neptune lies the Kuiper Belt and the scattered disc. Farther out is the Oort cloud, which is believed to be the source of long-period comets.
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Astronaut footprints are still visible on the Moon due to several factors:
- Lack of Atmosphere: Unlike Earth, the Moon has virtually no atmosphere. This means there's no wind or weather to erode or disturb the footprints.
- Absence of Water: There is no liquid water on the Moon's surface to wash away or degrade the footprints.
- No Biological Activity: There are no living organisms (plants, animals, or bacteria) to disturb or decompose the footprints.
- Fine Lunar Soil: The lunar surface is covered in a fine, powdery soil called regolith. This material is easily compressed and holds its shape well, preserving the footprints.
Because of these conditions, the footprints and other traces left by astronauts during the Apollo missions could remain visible for potentially millions of years, unless disturbed by a significant event like a meteorite impact.
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