The galaxies of the universe: Classes, clusters of galaxies, nebulae and their types, the milky way


For most of our history, human beings could only see galaxies as diffuse patches in the night sky. However, we now know that they are huge groupings of stars and other materials.
In fact, our Solar system is part of a Galaxy, the only one we have seen from the inside: the milky way. We have always known it although, of course, in ancient times nobody knew what it was. Appears as a whitish stripe that crosses the sky and, hence, takes its name: milk road.
Within the milky way, we can find different formations of stars and interstellar dust. The most noteworthy are nebulae and star clusters. It is expected that also exist in other galaxies.

The galaxies of the universe

Galaxies are huge clusters of stars, gas and dust.
There are hundreds of billions of galaxies in the universe. Each one can be formed by hundreds of billions of stars and other stars.
In the center of the Galaxy is where more stars are concentrated.
All the bodies that are part of a Galaxy move because of the attraction of the others due to the force of gravity. In general there are, in addition, a much broader movement that causes all together to rotate around the Center.
Here is a list of galaxies that we have "closer": 

Neighboring galaxies Distance (years light)
Magellanic Clouds 200,000
The Dragon 300,000
Ursa minor 300,000
The sculptor 300,000
The stove 400,000
Leo 700,000
NGC 6822 1,700,000
NGC 221 (M32) 2.100.000
Andromeda (M31) 2.200.000
The triangle (M33) 2,700,000

Sizes and shapes of galaxies

There are huge as Andromeda, or small as M32 neighboring galaxies. They exist in globe, lens shape, flat, elliptical, spiral (like ours) or irregular shapes. The galaxies are grouped together forming "clusters of galaxies".
The nearest large Galaxy is the Andromeda.
You can see with the naked eye and seems a bright spot of hazy appearance. Arab astronomers had already seen it. Currently it is known under the name M31. It is about 2,200,000 years light from us. It is twice as large as the Milky way.

Galaxies have an origin and evolution

The first galaxies started to form one billion years after the Big Bang. The stars that formed them have a birth, life and death.
The Sun, for example, is a star formed by accumulation of materials that came from above, dead stars.
Many nuclei of galaxies emit strong radiation, which indicates the likely presence of a black hole.
The movements of galaxies provoke sometimes violent clashes. But, in general, galaxies move away from the other, as drawn on the surface of a balloon that is inflated.

Kinds of galaxies

When using powerful telescopes, most of the galaxies is only detected mixed all stars light; However, the closest show individual stars. Galaxies are a variety of ways.
In 1930 Edwin Hubble classified the galaxies in elliptical, spiral, or irregular. The first two classes are more frequent.

Elliptical galaxies

Some galaxies have a globular full profile with a bright core. These galaxies, elliptical calls, normally contain a large population of old stars, little gas and dust, and some stars of new formation. Elliptical galaxies have great variety of sizes, from Giants to dwarfs. In the photo, the elliptical Galaxy of the hat.
Hubble symbolized the elliptical galaxies with the letter E and subdivided them into eight classes, from the E0, almost spherical, up to the E7, usiformes. In elliptical galaxies star concentration diminishes from the core, which is small and very bright, towards its edges.

Spiral galaxies

Spiral galaxies are flat discs that contain not only some old stars but also a large population of young stars, enough gas and dust, and molecular clouds that are the birth place of the stars.
Generally, a weak old star halo surrounds the disk, and is often a smaller nuclear bulge that emits two jets of energy in opposite directions.
Spiral galaxies are designated with the letter S (spiral). Depending on the minor or major development holding each arm, is assigned a letter a, b or c (Sa, Sb, Sc, SBa, SBb, SBc).
There are other intermediate between elliptical and spiral galaxies, called lenticular or normal lenticular, identified as OS and classified in groups SO1, SO2 and SO3. At the same time, are the barred lenticular (SBO), which are classified into three groups, according to submit more or less defined and bright bar.

Irregular galaxies

Irregular galaxies are symbolized with the letter I or go, although they tend to be dwarf or common. Those galaxies that do not have well-defined structure and symmetry are included in this group. They are classified as irregular type 1 or Magellan, which contain large amounts of young stars and interstellar matter, and irregular galaxies of type 2, less frequent and whose content is hard to identify.
Irregular galaxies are usually next to larger galaxies, and often contain large amounts of young stars, gas, and cosmic dust.

Active Galaxies

Almost all of the galaxies have a black hole at its Center. While the black hole is active, catches and swallows all the matter that surrounds him, as a swirl. When it has no capacity to swallow more, matter continues to rotate around it, but no longer falls within. Galaxies whose black hole is still active are called Active Galaxies.
Active Galaxies are distinguished by its shape and by the large amount of radiation they emit. The black hole of the nucleus is surrounded by a bright disk of matter, dust and hot gas. Accretion disk is called, and spiral tour while emits high energy radiation. From the Poles, the black hole space launches huge jets of particles, which can measure thousands of years light.
The milky way was also an active galaxy in the past.

Types of active galaxies

Four types of active galaxies are known: the Blazars, quasars and Seyfert galaxies, the radio.
Quasars and the Blazars are the objects more distant and higher energy are known. They are thousands of millions of years Earth light. We see them as they were in the past, when galaxies were still forming. They are the most brilliant of the universe objects, even though they are so far away that their light reaches us very weakly. Almost all of the active galaxies that are known are quasars.
The radio and Seyfert galaxies are closer and also very bright objects. They emit rays X, infrared radiation and radio waves. Its radiation is so great, that they are the main source of all Cosmos radio waves.

Nearby galaxies

The closest to the milky way galaxies are which belong to the so-called Local Group. They are easily seen with an amateur telescope. Some, such as Andromeda, and the Magellanic Clouds, can be seen even at a glance.
Orbit some dwarf galaxies around the milky way. In 1994 it was discovered the elliptical dwarf galaxy Sagittarius or SagDEG (Sagittarius Dwarf Elliptical Galaxy in English) to 70,000 light years away, and in 2003 the Galaxy's greatest Can 25,000 light years. They are the closest two discovered so far.

Major nearby galaxies

Andromeda: 2.5 million light years from Earth. It is a giant spiral, twice the size of the milky way. It is the largest of the Local Group Galaxy. It contains hundreds of billions of stars and large number of nebulae. There is a supermassive black hole at its Center. It is very bright and is the most distant object that can be seen with the naked eye. It is estimated that in about 6 billion years, Andromeda and the milky way will collide.
Small and large cloud of Magellan: are two satellite galaxies of the milky way. This means that Via Lactera attracts them with severity, and in the future will be part of it. They are so called because Magellan was the first European Explorer who noted, in the 16th century. The great cloud is 170,000 years light-years away, and the small cloud 210,000 light years. They are irregular, and dwarf galaxies with many nebulae and young stars. In the southern hemisphere are seen with the naked eye, as two white clouds that are isolated from the milky way that crosses the sky.
Triangle: is the third Galaxy in the Local Group, behind Andromeda and the milky way. It is 3 million years light. It is only with telescope. Shaped spiral, similar to our galaxy. It is believed that Andromeda attracts it with its gravity, and could even orbiting around it. In the Galaxy of triangle is the emission nebula largest referred to: NGC 604.

Galaxy Distance (light-years) Diameter (light years)
SagDEG 88.000 10,000
Large Magellanic Cloud 170,000 20,000
Small Magellanic Cloud 210,000 15,000
Sculptor 300,000 5,000
Leo 750,000 4,000
Andromeda 2,500,000 250,000
Triangle 3,000,000 50,000


The Local Group

Nearby galaxies are attracted by its gravity effect and are grouped in clusters. Smaller clusters are called groups. Our Galaxy belongs to one of these groups: the so-called Local Group.
The Local Group has a diameter of 4 million years light and brings together about 30 galaxies. It is a still young cluster that is part of one still greater structure, called the Virgo supercluster.
The Local Group has three main galaxies in a spiral: Andromeda, milky way and Galaxy of the triangle. The rest are dwarf galaxies orbiting around these big three. Are called satellite galaxies.
Andromeda is the largest and most brilliant of the Local Group Galaxy. It is a 2'5 millions of years Earth light and you can see with the naked eye from the southern hemisphere. It is the most distant observable object with the naked eye from Earth. Its gravity attracts the Galaxy of the triangle, which could be spinning her around. It also attracts the milky way, and in the future they are destined to collide.

Main Galaxy Satellite galaxies
Andromeda Andromeda I-VII, M32, M110, NGC185 NGC147
Milky way SagDEG, Magellanic Clouds, Ursa Minor, and more can
Draco, Carina, sculptor, Fornax, Tucana, Sextans, Leo
Triangle PISCES

The milky way in the Local Group

The milky way is the second largest Local Group Galaxy. Even so, it is half the size of Andromeda.
The galaxies of the Local Group closest to ours are more Can and elliptical dwarf galaxy Sagittarius or SagDEG.
The small and the large cloud of Magellan are two dwarf galaxies that are currently colliding with the milky way and in the future will become part of it.

The milky way

A path in the sky

On calm nights we can see a white stripe that crosses the sky from side to side, with many stars.
They are only a small part of our neighbors. Together we form the milky way, our galaxy. The Romans called it "Way of milk", which is what is meant in latin via lactea.

The milky way is our Galaxy

The Solar system is in one of the spiral arms, about 30,000 years light from the Center and about 20,000 of the end.
The dairy is Via a large Galaxy, spiral and may have about 100 billion stars, including the Sun. In total about 100,000 light-years in diameter wide and has a mass of more than two trillion times that of the Sun.
Every 225 million years ago the Solar system complete a rotation around the center of the Galaxy. It moves some 270 km. per second.
We can not see the bright Center opaque materials, cosmic dust and cold gases, which do not let light stand. It is believed to contain a powerful black hole.
The milky way has a convex lens. The kernel has a central area of ellipse-shaped and about 8,000 light-years in diameter. The stars of the core are more clustered than the arms. To its surroundings there is a cloud of hydrogen, some stars and star clusters.

The milky way is part of the Local Group

Together with galaxies of Andromeda (M31) and the triangle (M33), clouds of Magellan (satellites of the Milky Way), galaxies M32 and M110 (satellite of Andromeda), galaxies and smaller nebulae and other minor systems, form a group bound by gravity.
In total, there are about 30 galaxies that occupy an area of about 4 million years diameter light in the Local Group.
All group orbit around the large cluster of galaxies in Virgo, about 50 million light years.

What is the milky way?

If we could see the milky way from outside of it, we would see Centre bulky, yellow and bright, with rugby ball shape, and a thin disc of bluish turning around.
The milky way is spiral shaped barrada, as a grinder. In the Center there is a black hole, which scientists call Sagittarius A. The Center is not round, but somewhat elongated. Near it are old, red, and yellow stars.
The Center give birth to four arms: Perseus arm, arm of Orion, Sagittarius arm and arm cross Centaur. They form a disc that rotates slowly spiral. On the arms are younger stars, white and blue. There are also many nebulae; in most of these new stars are formed. The Sagittarius arm is the brightest of all.
The milky way is a large galaxy. It measures diameter light 100,000 years and contains more than 200 billion stars. Their gravity is so powerful, that attracts other smaller nearby galaxies.
Earth is 25,000 light-years from the center of the Galaxy, in an area that is sparsely populated in the Orion arm. Our Solar system takes 225 million years on a full ride to the milky way.

Why is it called milky way?

At night, the milky way looks like a white stripe that crosses the sky. In latin, Milky means milk road. According to Greek mythology, the god Zeus had a son with a mortal. When Hera, his wife, learned, ripped the baby from his mother's arms while you feeding it. The milk was spilled and fell through the sky.
Also called the Milky the Camino de Santiago, as formerly it served as a guide to the pilgrims going to Santiago de Compostela. Compostela means field or road star.

Star clusters

The stars do not appear in isolation, fate forming groups that we call "clusters". A cluster of stars, is a group of related stars held together by gravitation effect.
Star clusters are classified into two groups: open clusters, which do not have definite shape, and globular clusters, which are spherical or nearly spherical. The open are formed by a few hundred young stars, while globular clusters contain more than a thousand times that amount, and are generally very old stars.
Globular clusters form a halo around our Galaxy, the milky way, while the open are located in the spiral arms.
Open clusters are much more numerous than the Globular: about 1,000 in our Galaxy are known while there are only 140 globular.

Open clusters

The two best known open clusters are the Pleiades and the Hiadas, both observable to the naked eye, in the constellation Taurus. The Hiadas cluster is located about 150 light years from Earth and has a diameter of about 15 light years. The cluster of the Pleiades has a similar diameter, but is approximately 400 light years, so it is smaller.
Open clusters are formed from clouds of gas and dust in the arms of a spiral galaxy. The most dense regions contract under their own gravity, giving rise to individual stars.
The Orion Nebula is an example of a region that still stars are forming. A group of old stars, "Orion trapezium" is located in the Centre of the nebula. The Nebula contains enough gas to form hundreds of stars of the same type.
Referred to as "Association" to a grouping of stars like a heap, but spread over one larger area. Clusters open inside of an association, in areas where the density of the gas from which the Association was formed is greater are often.
Members of a cluster are born together and continue moving together through space. This serves to find their distances. By measuring the motion of stars along the line of vision and through the line of sight, can calculate the gap that separates them from the Solar System. This technique is known as the moving cluster method.

Globular clusters

The two brightest globular clusters are Omega Centauri and 47 Tucanae, both observable to the naked eye from the southern hemisphere. The most remarkable globular cluster in the northern hemisphere is M13, in the constellation Hercules, also observable to the naked eye. The photo shows the globular cluster NGC 6388.
In globular clusters, the concentration of stars in the Middle can be 100,000 times higher than in the region of space occupied by us, and the terrestrial perspective may seem the stars merge among themselves.
Globular clusters contain some of the oldest stars of the milky way, with ages of 10,000 million years, twice as much as the Sun.
The age of a cluster is calculated by putting its stars in a Hertzsprung-Russell diagram. As the speed of evolution of a star depends on its mass, the point at which the star begins to leave the main sequence to become a giant, shows the age of the cluster.
The globular clusters were formed when the immense cloud of dust and gas that resulted in our Galaxy was collapsing. As the Sun is in the outer area of the Galaxy, most of the clusters is one half of the sky toward the center of the Galaxy.


Nebulae are gas and interstellar dust. Depending more or less dense, they are visible, or not, from the Earth.
Nebulas can be found anywhere in interstellar space. Before the invention of the telescope, the nebulous term applied to all celestial objects diffuse appearance. As a result, many objects that we now know are clusters of stars or galaxies were called nebulae.
Nebulae in almost all galaxies, have been detected including our own, the milky way. Depending on the age of the associated stars, they can be classified into two major groups:
1.- Associated with evolved stars, as the planetary nebulae and supernovae remnants.
2.- Associated with very young stars, some even still in process of formation, such as molecular clouds and Herbig-Haro objects .

Classification of the nebulae according to your light

If you use the process that causes light-emitting, nebulae are classified as:
The emission nebulae, whose radiation comes from dust and gases ionized as a consequence of the warming that is being tested by nearby hot stars. Some of the most striking objects in the sky, like the Orion Nebula, are such nebulae.
The Reflection nebulae reflect and scatter the light of nearby little hot stars. The Taurus Pléyades are an example of bright stars in a reflection nebula.
The dark nebulae are clouds, little or nothing bright, which is represented as a dark spot, sometimes surrounded by a halo of light. The reason why not emit light themselves is that the stars are too remote to heat cloud. One of the most famous is the Horsehead, Orion Nebula. Throughout the dark strip that is observed in the sky when we look at the disk of our Galaxy is a succession of dark nebulae.

Types of nebulae

One of the most notable aspects of the nebulae is its variety of forms and structures. Thanks to modern telescopes and the use of computers, have been able to be detailed digital photos which, through suitable software can be colored for spectacular images.

Planetary nebulae

Planetary nebulae are similar to planets when they are observed through a telescope. In reality they are layers of material that are detached from a star evolved from mass media, going from red giant to white dwarf.
The Nebula's ring, in the constellation of Lyra, is a typical planet that has a period of rotation of 132.900 years and a mass of about 14 times the mass of the Sun. Thousands of planetary have been discovered in the milky way.
Most spectacular, but less in number, are the remnants of supernovas, cuta most significant representative is the nebula of the crab, in Taurus, which vanishes at a rate of 0.4% per year. Such nebulae are intense radiofuentes, because of the explosion that formed them and losrestos of pulsars in that the stars became.

Herbig-Haro (HH objects) objects

Herbig-Haro objects, which owe their name to the Mexican astronomer Guillermo Haro and the American George Herbig, are small bright nebulae that are within very dense interstellar clouds
They are probably produce jets of gas expelled by stars in formation. Molecular clouds are, moreover, extremely large, many light years wide, with an undefined profile and a Dim and Misty appearance.
Herbig-Haro objects can be studied in the infrared. These objects vary in size and brightness in a few years. They are regions of active star formation. It is believed that these nebulae correspond to flows of high-speed gas ejected by young stars hit interstellar clouds. The study of Herbig-Haro objects helps to understand the details of how stars are formed.

What is an emission nebula?

Nebulae are great clouds of interstellar dust and gas where stars are born. The emission nebulae are bright clouds of gas and dust that host young stars in its interior.
At the beginning, the nebulae are dark and cold. As the interstellar material is concentrated, the pressure and the temperature in the Centre of the Nebula rise. When they get to accumulate the sufficient amount of matter, a star is born inside the nebula.
The new star heats the nebula. The hot gas lights and dark Nebula becomes an emission nebula.
In the emission nebula new stars are formed while girls illuminate it more and more. Emission nebulas created the most varied forms, and are the most beautiful images of the Cosmos.

Emission nebulas

Some of the most beautiful and known emission nebulae are:
Orion Nebula: is the most brilliant and known. It is 1,500 light years from Earth in the constellation of Orion, and measures about 30 years light. It is the nearest emission nebula. You can see even with the naked eye, as a white patch on the left under the belt of Orion. It was the first Nebula in photo taken, almost 150 years ago.
Of the Eagle nebula: 7,000 light years away in the constellation of the snake. It has an extension of 70 light years. There are the famous pillars of creation. Three enormous pillars of material, dust and gas several years are light. Photographed them the Hubble in 1995 and they are probably the most beautiful image of the Cosmos taken until today.
Nebula NGC604: located in the Triangulum Galaxy, is the emission nebula largest referred to. It measures 1,500 years light in diameter and is more than 6,000 times brighter than the Orion Nebula. It is the area of the Local Group where more stars are born.
Carina nebula: is about 8,000 light-years, in the constellation of Carina, in the southern hemisphere. It is one of most big and bright nebulae that are known. Its expansion more than 300 years light. It contains several of the most massive stars of the milky way.

What is a dark Nebula?

In cold areas of the Cosmos, where interstellar dust and gas are abundant, the material is concentrated and is huge icy clouds. Consist above all of hydrogen, which is the fuel of the stars. Gravity causes the material group together more and more. As it increases the amount of material, in the center of the Nebula pressure and temperature increase.
When pressure and temperature are high enough, the Nebula collapses. This means that it has just been born a star inside. The new star begins to heat up the cold cloud. When heated, the dark Nebula turns red and becomes an emission nebula.

Some dark nebulae

Dark nebulae only look like black spaces in front of a light background, normally an emission nebula. Some of the most well known dark nebulae are:
Horse head nebula: in the photo above, is the dark Nebula, most beautiful and famous for its curious shape of head of seahorse. It is in the constellation of Orion, a 1,500 years light-years away from Earth.
Coal sack nebula: is the most famous dark Nebula in the southern hemisphere. Cruz del Sur is a 600-year light in the constellation. It is easily recognised at first glance, like an unexpected dark area in the white band of the milky way.
Cone nebula: located at 2,500 years light-years from Earth, in the constellation of Monoceros. It forms a dark cone on top of the Christmas tree cluster. Interestingly, it was discovered on December 26.

The distant galaxies

The most distant galaxies are more than 13,000 million years light, almost on the edge of the visible universe. Its light has taken all this time to reach us. This means that we see such and as they were made more than 13 billion years, only 500 million years after the Big Bang. For this reason, more distant galaxies are also the oldest in the universe.
The most distant galaxy discovered so far is the Abell 1835 IR1916. It is located behind the cluster Abell 1835, in the Virgo constellation. It was discovered in 2007 by European scientists, from the VLT in Chile. 13.200 million years is light and increasingly more moves.


Quasars are distant galaxies with a powerful black hole at its Center. They are very young, typical galaxies in the early universe. They are more than 12 billion years light.
They are the most powerful and brilliant universe objects. Although its light comes very weak, they can be up to a trillion times brighter than our Sun. They emit an enormous amount of radiation. Eventually, his black hole ceases to be active. It is possible that many galaxies were quasars in the past.
The first quasars were discovered in the 1950s and known today more than 100,000. Although they remain mysterious objects.

Clusters of galaxies

Galaxy clusters are gigantic structures of the universe. Galaxies emit a lot of gravity. This makes the nearby galaxies to attract each other and be grouped together forming clusters. Our Galaxy, the milky way, is part of a small Local Group called cluster.
Within a cluster, galaxies rotate each winch to others, and even often they collide. The size and mass of a cluster galaxies forming it will vary, but the distance between one end and the other is always several million years light.
Clusters are not composed only of galaxies, but also of great clouds of hot gas. In general, they are remains of galaxies that die to the clash. But most of the mass of the cluster's dark matter. It is believed that each cluster can have up to five times more dark matter than visible matter.
Clusters are spherical or spiral, and revolve around a central core. The core hosts most of the hot gas, and emits a large amount of x-ray. The most dense galaxies are close to the Centre, where the gravity is greater. Thousands of clouds of gas between galaxies are scattered around.

Superclusters of galaxies

The Superclusters of galaxies are the second largest structure in the universe. Huge walls of Superclusters outweigh them. They are groupings of galaxy clusters, and are found throughout the known universe.
Galaxy clusters are joined by their ends, and form huge chains. The Superclusters gravity is so great that even slows the expansion of the universe. All matter is attracted, and that creates huge gaps between some Superclusters and others.
Our Local Group is part of the Virgo supercluster, which, in turn, is integrated into a superstructure called Laniakea ("sky immense", in Hawaiian).

Astronomical catalogues

Astronomical catalogues are lists of deep space objects that are fixed in the sky. They are very useful for astronomy fans, since they collect the major galaxies, nebulae and star clusters that can be seen with a medium telescope.
The best-known are:
Messier catalog: is the most famous and the first published. Developed it the French astronomer Charles Messier in 1.769. It brings together 110 objects numbered M1 to the M110, in the order in which Messier discovered them while observing the sky in search of comets. Only collect objects visible from the northern hemisphere.
Caldwell catalogue: was developed to complement the Messier catalogue. Collect 109 numbered objects from the C1 to the C109, by order of latitude from North to South. It includes objects in the southern hemisphere and others important in the northern hemisphere who do not appear in the Messier, as the Hyades.
Herschel 400 catalogue: was created for use by astronomy fans. Collect 400 easy to see with a telescope half objects. It is a selection of objects of the General catalogue of nebulae and clusters of stars, made in 1864 by the English Astronomer William Herschel.
New General catalogue: is currently, along with the Messier catalog used to name objects in deep space. He was published in 1880 to update the catalogue for Herschel, and included all the objects known until then. Uses the acronym NGC then the object number. The current version of the NGC gathers more than 13,000 objects.

What is the Messier Marathon?

Each year, thousands of astronomy enthusiasts gather to celebrate the Messier Marathon. It is observed in a single night the 110 objects of the Messier catalogue.
The Messier marathon held at the end of March each year, coinciding with the new early spring moon. It is the ideal time for observation.
Although attended by fans from around the globe, Marathon only can hold in the northern hemisphere, as the catalogue only collects objects visible from there.
Published for educational purposes authorized by: Astronomía: Tierra, Sistema Solar y Universo

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