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1.4. Composition of the universe

Composition of the universe

The Big Bang with which the universe was formed was a great explosion that launched matter in all directions. The great clouds of gases and matter were concentrated to form the stars (if they had enough mass), and the planets and planetoids (if they had less mass).

Matter that was not part of stars or planets, formed nebulae , which can be hot and bright, or cold and dark. The nebulae are large regions that are among the stars, formed by gases (mainly hydrogen and helium), and solid particles (chemical elements) in the form of cosmic dust.

Galaxies: the Milky Way

The stars, gas, planets, cosmic dust, dark matter and energy are grouped by the force of gravity forming galaxies.

There are an estimated 2 trillion galaxies in the universe, and their size ranges from hundreds of  light years to millions of light years.

Depending on their shape, galaxies can be:

  • Elliptical galaxies.
  • Spiral galaxies. They are circular in shape but with curved arms surrounded by interstellar dust.
  • Lenticular galaxies.
  • Irregular galaxies.

The Earth, the Sun, and therefore the Solar System, are located in the Milky Way. It is a spiral galaxy containing about 200 billion stars that move around the center like a whirlpool.

The Milky Way has four arms, in one of which, close to the edge, is the Sun, which takes between 225 and 250 million years to make a complete revolution of the galaxy (galactic or cosmic year). Furthermore, the Milky Way also moves relative to the other galaxies in the Universe.

The Milky Way has the following structure:

  • The central nucleus or bulb. It is in the central part and contains the oldest stars.
  • The galactic disk. Formed by four spiral arms, and contains the youngest stars.
  • The halo. It surrounds the nucleus and the galactic disk, and contains some isolated stars.

The galaxy closest to the Milky Way is called Andromeda.

Several hundred or thousands of galaxies are grouped together forming stellar clusters, more or less dense.


The stars we see in the sky are just some of the stars in the Milky Way. Although it may seem that they are together, they are located at different distances from our planet. But that feeling of proximity allows us to group them following different imaginary figures called constellations. Not all constellations are visible from anywhere on Earth or at any time of the year.


The nebulae are large gaseous masses of the interstellar medium formed by hydrogen , helium and interstellar dust.

The stars are born nebulae, by condensation and aggregation of matter that causes an increase in temperature.


The stars are massive gas of gases (hydrogen and helium, primarily) which release energy and emit light as a result of nuclear reactions (nuclear fusion) taking place inside.

The stars are differentiated by three properties:

  • Color. The color of a star is a good indicator of its surface temperature.
  • Size. The size of the stars is highly variable. The size of the Sun is always taken as a reference. For example, supergiant stars have diameters 400 times that of the Sun, while white dwarfs have a diameter 100 times smaller than the solar. But despite their size, giant stars can only have a mass 40 times that of solar mass, while white dwarfs, which are very small, have a high density.
  • Brightness. The brightness of a star depends on its distance from Earth, its size, and its luminosity (amount of light energy produced).

Discovering Words: Galaxy and Milky Way

Surely you already know why our galaxy is called the Milky Way. Both words come from Greek and Latin mythology.

Milky Way comes from Via (sometimes circulus lacteus ) or path, and from lacteus, derived from lac, milk.

Galaxia means the same thing, but in Greek. It comes in gala, gálaktos, which means milk. Maybe it seems weirder to you, possibly the galactose in milk or dairy sounds more familiar to you.

It is named for the appearance it presents, which Greek mythology attributes to the milk spilled from the breast of the goddess Hera (Juno for the Romans).

As you can see, galaxy and Milky Way have the same origin and meaning. This made sense when the only known galaxy was ours, the Milky Way. We currently use the word galaxy to refer to them in a generic way, while we use the Milky Way to refer to our galaxy.