Unveiling the Cosmos: Universe, Galaxies, and Solar System

Posted on Jun 16, 2025 in Physics

The Universe

The science that studies the Universe is astronomy.

What is the Universe (Cosmos)?

The Universe, also known as the Cosmos, is everything that exists, encompassing all matter, energy, space, and time.

Origin of the Universe: The Big Bang Theory

The most widely accepted theory regarding the origin of the Universe is the Big Bang Theory:

  • Initially, all matter was concentrated in a single point with an extremely high temperature.
  • Approximately 13.7 billion years ago, a colossal explosion occurred, leading to an expansion in all directions.
  • The first atoms, primarily hydrogen, were formed, followed by the creation of the first molecules.
  • Matter and energy subsequently began to group together.

The Theory of Inflation further explains events one second after the explosion: the rapid expansion caused cooling, leading to the formation of fundamental particles such as electrons, neutrons, and protons.

Earth’s Position in the Universe: Historical Models

Throughout history, different models have attempted to explain Earth’s position:

  • Geocentric Model: This model proposed that Earth was at the center of the Universe. The Sun, Moon, and planets were believed to revolve around Earth, while stars were fixed in a revolving dome.
  • Heliocentric Model: This model posited the Sun as the center of the Universe, with all planets revolving around it. The stars were considered fixed in a stationary dome.

Measuring Distances in the Universe

Due to the vastness of space, specialized units are used to measure cosmic distances:

  • Light-Year: This unit represents the distance light travels in one year, approximately 9.5 trillion kilometers (9.5 x 1012 km). Light-years are used for measuring long distances between stars, galaxies, and other celestial objects.
  • Astronomical Unit (AU): Equivalent to the average distance between Earth and the Sun, which is about 150 million kilometers (1.5 x 108 km). Astronomical Units are primarily used for measuring distances within our Solar System.

Cosmic Structures: Galaxies and Nebulae

  • Galaxies: These are immense groups of stars, nebulae, and interstellar dust. Galaxies exhibit various shapes, including elliptical, spiral, and irregular, and often cluster together.
  • Nebulae: Clouds of gas, primarily hydrogen and helium, mixed with interstellar dust. Some nebulae are stellar nurseries where new stars are formed.

Our Galaxy: The Milky Way

Our galaxy is known as the Milky Way. It is a spiral galaxy located within a cluster called the Local Group. The Milky Way comprises billions of stars and has four main spiral arms. Our Sun is situated in one of these arms, specifically the Orion Arm.

Stars

Stars are massive spheres of gas, predominantly composed of hydrogen and helium. Inside stars, a process called nuclear fusion generates immense amounts of energy in the form of light and heat.

Stars are distinguished by several characteristics:

  • Color: Depends on their surface temperature (e.g., blue, white, yellow, orange, red).
  • Size: Varies greatly from dwarf stars to supergiants.
  • Brightness: Influenced by their distance from the observer, the quantity of energy they emit, and their intrinsic size.

Other Celestial Bodies

  • Planets: Spherical bodies that orbit a star, following paths (orbits) that are cleared of other significant celestial objects.
  • Dwarf Planets: Rocky or icy bodies that orbit a star but have not cleared their orbital path of other debris.
  • Asteroid Belt: A region between the orbits of Mars and Jupiter containing numerous irregularly shaped rocky bodies (asteroids). A second, less dense asteroid belt exists beyond Neptune, known as the Kuiper Belt.
  • Comets: Bodies composed of ice, rock, and dust, which orbit the Sun in highly elliptical and elongated paths.

The Solar System

The Solar System consists of the Sun and all the celestial bodies that orbit it, including planets, dwarf planets, asteroids, satellites (moons), and comets. The orbits of the planets are elliptical, lie approximately in the same plane (the ecliptic plane), and generally follow an anticlockwise direction.

Planetary Movements: Revolution and Rotation

  • Revolution: This is the movement of a planet around the Sun. The time a planet takes to complete one revolution defines its “year.”
  • Rotation: This is the movement of a body around its own axis. The time a planet takes to complete one rotation defines its “day.”

Most planets rotate in an anticlockwise direction, with notable exceptions being Venus (which rotates in the opposite direction) and Uranus (which rotates on a nearly horizontal axis).

Planets in Our Solar System

Planets in our Solar System are broadly categorized into two groups:

  • Inner (Rocky) Planets: These planets are relatively small, have little to no atmosphere, and possess a solid, rocky surface. They typically have few or no satellites. This group includes Mercury, Venus, Earth, and Mars.
  • Outer (Gaseous) Planets: These planets are very large and characterized by a thick, gaseous atmosphere. They are often surrounded by rings and have numerous satellites. Their “surface” is essentially gaseous. This group includes Jupiter, Saturn, Uranus, and Neptune.

Earth’s Movements

Earth’s Revolution

Earth takes approximately 365 days and 6 hours to complete one revolution around the Sun.

  • To account for the extra 6 hours, an additional day (February 29th) is added to our calendar every four years, creating a leap year.
  • Earth’s axis is tilted at an angle of 23.5 degrees. This tilt causes variations in the amount and intensity of sunlight received on different parts of Earth throughout the year, leading to the distinct seasons:
    • During summer in the Northern Hemisphere (NH), the Sun’s rays strike the NH more directly, resulting in greater heating. Conversely, the Southern Hemisphere receives sunlight at a more oblique angle, leading to less heating.
    • In spring and autumn, the Sun’s rays hit the Equator perpendicularly, ensuring both hemispheres receive relatively equal heating.
  • The length of days also varies throughout the year:
    • Summer Solstice: Marks the longest day of the year.
    • Winter Solstice: Marks the shortest day of the year.
    • Spring and Autumn Equinoxes: Days and nights are of approximately equal length.

Earth’s Rotation

Earth takes 23 hours and 56 minutes to complete one rotation on its axis.

  • Rotation causes the cycle of day and night.
  • Living organisms detect variations in daylight hours and temperatures, adapting their behaviors accordingly. Examples include trees losing their leaves, flowering cycles, mating and breeding seasons, and animal migrations.

The Sun, Earth, and Moon

The Moon is believed to have formed approximately 4.5 billion years ago when a protoplanet, sometimes called Theia, collided with Earth. The Moon revolves around Earth and rotates on its own axis, with both movements having the same period of about 28 days. This synchronous rotation is why we always observe the same side of the Moon from Earth.

Phases of the Moon

The phases of the Moon are the different illuminated portions of the Moon visible from Earth as it orbits our planet. These phases include: New Moon, First Quarter, Full Moon, and Last (or Third) Quarter.

Eclipses

An eclipse occurs when one celestial body passes directly in front of another, temporarily obscuring it from view. Eclipses can be either total or partial.

Tides

Tides are the periodic rising and falling of sea levels, primarily caused by the gravitational pull of the Moon and, to a lesser extent, the Sun.

  • During a typical day, there are two high tides and two low tides.
  • When the Earth, Sun, and Moon are aligned (during a Full Moon and New Moon, approximately twice a month), their combined gravitational pull results in stronger tides known as Spring Tides.