Solar System Models, Planets, and Galactic Structures

Posted on Feb 13, 2025 in Geology

Models of the Solar System

  • Geocentric: Earth is motionless in the center of the universe, and all planets revolve around it.
  • Heliocentric: All the planets revolve around the sun.

Key Figures in Astronomy

Tycho Brahe (1546-1601)

  • Observed a supernova (1572) and a comet (1577).
  • Made the best position measurements of stars to date, with errors less than 1 minute of arc.
  • Recorded the position of the planet Mars.
  • In 1600, Johannes Kepler was hired to analyze the data from 20 years of planetary measurements.

Johannes Kepler (1571-1630)

  • Worked on the basis of Brahe’s recordings.
  • Formulated his three laws describing the elliptical movements of the planets around the sun.

Kepler’s Laws

First Law: Every planet revolves around the sun in an elliptical orbit, with the sun occupying one focus.

Second Law: The radius of a focal point to a planet with the sun sweeps equal areas in equal times.

Third Law: The squares of the periods of revolution of the planets are proportional to the cubes of the radii of their orbits. (T2 / r3 = K)

Galileo Galilei (1564-1642)

  • Observed with a small telescope he made (confirmed heliocentrism).
  • Discovered sunspots.
  • Noted that Jupiter has moons.
  • Noted that Venus has phases like the Moon.

Isaac Newton (1643-1727)

  • Found that the motion of bodies can be described by three laws.
  • Formulated the law of universal gravitation: (F = G · M1 · M2) / r2
  • These laws mark the beginning of astronomy as a science.

Origin of the Universe (Big Bang Theory)

Between 12,000 and 15,000 million years ago, all matter in the universe was concentrated in an extremely small area of space, which exploded, causing the expansion of matter in all directions. Clashes and disorder caused some matter to clump together and concentrate more in some parts of space, producing the formation of the first stars and galaxies.

Stars are huge spheres of gas, mostly hydrogen and helium, at very high temperature and pressure, kept in perfect balance and cohesion due to gravity.

Galaxies

A galaxy is a cluster of stars, gas, and stardust, held together by gravity. Each galaxy can be formed by hundreds of billions of stars and other celestial bodies.

Types of Galaxies

Elliptical Galaxies

They have the same appearance as a diskless kernel, with apparently uniform brightness. They lack gas and dust and are composed of old stars, yellow and low in metallicity.

Spiral Galaxies

They have a core or bulb formed by old stars, yellow-orange, and low metal content, and a disk with large amounts of interstellar gas and dust, indicating the formation of young stars, blue and very metal-rich.

Lenticular Galaxies

They have the appearance of a nucleus with a disc, but without spiral arms. They consist of old stars, little metal, and without interstellar gas and dust.

Irregular Galaxies

These are galaxies that have no symmetry of any kind; there is no defined core or a disk. The most notable examples are the two satellite galaxies of our Milky Way: the Magellanic Clouds.

The Speed of Light (300,000 km/s)

  • It circles the entire Earth in 0.02 s.
  • It travels to the moon in 1.3 s.
  • It reaches the sun in 8.3 min.
  • It reaches the nearest star in 4.2 years.

A light year is the distance light travels in one year, i.e., 1 light year = 9.46 million million kilometers (9.46 × 1012 km)

The Milky Way

It is our galaxy. The Romans called it “the way of milk.”

  • It’s big, spiral, and may have some 100,000 million stars, including the sun.
  • Type of galaxy: spiral.
  • Number of arms: two plants with branches.
  • Brightness: 14,000 million solar luminosities.
  • Total mass: 1 trillion solar masses.
  • Diameter: 100,000 light years.
  • Disk thickness: 2,000 light years.
  • Thickness of the central bulge: 6,000 light years.

The core stars are closer together than in the arms. All around there is a cloud of hydrogen, some stars, and star clusters.

The Solar System

Located in one of the spiral arms of the Milky Way, about 30,000 light years from the center and about 20,000 from the end.

It takes 225 million years for the solar system to complete a rotation around the galaxy. It moves about 270 km/s.

There are eight planets in the solar system:

Here they are numbered according to their proximity to the sun:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune