Galaxy es a grouping of stars, celestial bodies and cosmic matter that is concentrated in a certain region of space by the effect of gravitational attraction.

The cover image is the Andromeda Galaxy, also known as the Spiral Galaxy M30 or NGC 224. (Credit: Wikipedia). Every galaxy constitutes a unit in the universo.

The number of stars that make up a galaxy varies, from 10 million stars the smallest galaxies, to 10 billion stars the giant galaxies.

It is estimated that there are more than 100 billion galaxies in the observable universe.

Intergalactic space is composed of a thin gas, whose average density does not exceed one atom per cubic meter.

Before continuing reading, to see another wonderful image of the Andromeda galaxy published by NASA, created with 300 photographs taken from the Swift space telescope.

Edwin Hubble measured distances to galaxies

In 1925, the American astronomer Edwin Hubble (1889–1953) measured the distance from Earth to what was called the Andromeda Nebula.

He made similar measurements to other spiral nebulae and showed that these nebulae were outside the Milky Way.

Edwin Hubble (1889 -1953) Credit: web “”

From all his observations and measurements it was deduced that what was observed were not nebulae but galaxies independent of ours and that they were at enormous distances from the Milky Way.

It was crystal clear that the Universe was much larger than had been believed until then.

Since then we have known that the Earth and the entire solar system are in one of the many galaxies that populate the universe.

George Lamaitre measured velocities of galaxies

Shortly after, the Belgian astrophysicist, George Lamaître (1894-1966), measuring the speeds of such galaxies and comparing their distances, concluded that all of them were moving away from each other.

George Lemaître en 1930 Credit: Wikipedia

Lemaître interpreted these measurements as the result of the expansion of the universe and, by solving Einstein’s equations of general relativity, laid the foundations of the Big Bang theory.

Nowadays we are getting used to contemplating the immensity of stars and galaxies that populate the skies.

Click here to browse a map of the Universe, in which you can zoom in and out little by little to visualize our place in this world.

It is one of the best on the internet to have an overview. It is a magnificent work by British astrophysicist Richard Powell.

Parts of a galaxy

Galaxy (from the Greek galakt “milky” in reference to our own Milky Way) is a system of stars, gas clouds, planets, cosmic dust, dark matter, and perhaps dark energy.

The galaxies are divided into three distinct parts:

  • The halo, a spheroidal structure that surrounds the galaxy and where the presence of a large amount of dark matter is detected, the existence of which was deduced from anomalies in the galactic rotation.
  • The disk is the part of the galaxy that contains more gas and where there are still processes of formation of new stars. The most characteristic of the disc are the spiral arms.
  • The nucleus, located in the center, has a flattened spheroidal shape and is the area with the highest density of stars.

    Galaxia sombrero
    Galaxy OM104 called hat galaxy. Credit: Wikipedia

Kinds of galaxies

 Spiral galaxies are circular in shape but with a structure of curved arms wrapped in dust.

Galaxia NGC 1300
NGC 1300 is a barred spiral galaxy. Credit: Wikipedia

There is evidence to suggest the existence of supermassive black holes in the nuclei of some galaxies.

Galaxia MT 74
Messier 74 also known as the Fan Galaxy or NGC 628, is a spiral galaxy. Credit: Wikipedia

Dark matter is speculated to make up 90% of the mass in most galaxies. The nature of this component is not well understood.

Irregular galaxies are called irregular shaped galaxies and are almost always the result of disturbances caused by the gravitational pull of neighboring galaxies.

These interactions between neighboring galaxies (which can cause galaxies to merge) can induce intense star birth.

Small galaxies that lack a coherent structure can also be called irregular galaxies.

Galaxy NGC 1427
Irregular galaxy NGC 1427. Credit: Wikipedia

Most galaxies are between one hundred and one hundred thousand parsecs in diameter and are usually separated by distances on the order of a million parsecs. (1 parsec = 31 trillion km).

The image of the galaxy Messier 101 (M101) is a photographic composition of the images obtained by the Hubble telescope, with the infrared vision of Spitzer and the X-ray vision of Chandra.

It was released by NASA in February 2009 as part of the celebration of the International Year of Astronomy.

Galaxy NG 5457
Galaxia  M101 or Galaxy NGC5457 or Pinwheel Galaxy. Credit: web “”

The beautiful galaxi NGC 7331

The galaxy NGC7331 is almost 50 million light years distant from ours, in the direction of the constellation of Pegasus.

Galaxy NGC 7331
The spiral galaxy NGC 7331, called pinwheel galaxy. Credit: “Instituto de Astrofisica de Canarias”  Author: Daniel López

At that distance, it is bright enough to be seen with small telescopes on dark nights as a diffuse spot. In terms of stellar mass and shape, it is very similar to the Milky Way.

The image shown is a color composite made with three OSIRIS + GTC broadband filters with a total integration time of just 200 seconds.

Despite this, star-forming regions and different (dark) dust structures in the visible disk of the galaxy can easily be identified.

Most galaxies are arranged in a hierarchy of aggregates, called clusters, which in turn can form larger aggregates, called superclusters.

These larger structures are arranged in sheets or filaments surrounded by vast areas of void in the universe.

Famous women astronomers

Much of what we now know about galaxies is due to the painstaking and self-sacrificing work of some clever women.

Some are well known: Miriani Pastoriza, Margaret Burbidge and Henrietta Leavitt.

The largest spiral galaxy

In 2013 the news was released that it has been discovered the largest spiral galaxy in the Universe.

It is called NGC 6872, is 212 million light years from Earth and is, so far, the largest known spiral galaxy. Its dimensions are truly enormous: 522,000 light years from end to end, five times larger than the Milky Way.

The finding was just made public during the annual meeting of the American Astronomical Society.

Galaxy NGC 6872
Spiral galaxy NGC 6872.  Credit: NASA

The spectacular galaxy NGC 6872, a barred spiral of great beauty, was already among the largest assemblages of stars known to astronomers.

But now, thanks in part to chance, a group of researchers from the United States, Chile and Brazil, have realized that its dimensions were almost twice what was believed, which has served to crown it as the largest spiral galaxy known throughout the Universe.

The first complete history of a galaxy

A group of Spanish researchers, from the Calar Alto Observatory (Almería), has managed to reconstruct for the first time the complete history of how galaxies form and grow, continuously manufacturing new stars.

Galaxy m81m82
M81 and M82 face-off, a mere 12 million light-years away. Credit: web “”, Author: Ivan Eder

This information is the result of the CALIFA project (Calar Alto Legacy Integral Field Area), in which researchers from the Calar Alto Observatory and the Max Planck Institute of Astronomy collaborate.

The method used is based on observing in maximum detail a large number of galaxies that are in different stages of their existence.

The oldest galaxy in the Universe, seen so far

In November 2012, an international team of astronomers announced the discovery of what is, so far, the most distant galaxy ever seen by man.

In fact, it lies at the incredible distance of 13.28 billion light years. Or what is the same, only 420 million light years from the moment when the Big Bang occurred.

The finding, announced by NASA, will be published in December 2012 in the “Astrophysical Journal”.

Posibly the first galaxy formed after the Big Bang

Also in 2012, “Nature magazine” announced the discovery of a galaxy that may be one of the first to form after the Big Bang.

An international team of researchers, including several from the Higher Council for Scientific Research (CSIC), has discovered one of the most distant galaxies known, formed about 13,500 million years ago, only about 200 million years after the Big Bang.

This galaxy is a holdover from the unexplored era when the light of the first stars began to emerge from a dense cold hydrogen fog, ending a dark era.