por What is a galaxy?
A galaxy It is a conglomerate of astronomical objects and matter, such as clouds of gas and dust, trillions of stars, nebulae, planets, asteroids, comets, black holes, and even a lot of dark matter, all structured thanks to the force of gravity.
Our solar system is part of a large spiral galaxy called Milky Way. This Greek-derived name can be translated as “milk path”, due to its resemblance to a dimly lit band that crosses the celestial sphere.
On clear summer nights it can be observed very well between the constellations of Scorpio and Sagittarius, since the nucleus is located in that direction and where the density of stars is much greater.
History of the discovery of galaxies
The great Greek thinker and mathematician Democritus of Abdera (460-370 BC) was the first to suggest – in his time there were no telescopes – that the Milky Way was actually made up of thousands of stars so far away that one could not be distinguished. other.
It took some time before Galileo (1564-1642) agreed with him, when when he aimed his telescope he found that there were more stars in the sky than he could count.
It was the German philosopher Immanuel Kant (1724-1804) who speculated that the Milky Way was made up of thousands of other solar systems and that the whole had an elliptical shape and rotated rhythmically around a center.
Furthermore, he also suggested that other sets of stars and planets like the Milky Way existed and called them island universes. These island universes would be visible from Earth as small, faint spots of light.
Messier Catalog
20 years later, in 1774, the Messier catalog appeared, a compilation of 103 deep space objects visible to date and made by the French astronomer Charles Messier (1730-1817).
Among these were some candidate island universes, which were known simply as nebulae. The M31 nebula was one of them, being known today as the neighboring galaxy of Andromeda.
William Herschel (1738-1822) would expand the list of deep space objects to 2,500 and first described the shape of the Milky Way. However, scientists had not yet realized that certain nebulae like M31 were themselves huge clusters of stars similar to the Milky Way.
modern telescopes
A telescope with sufficient resolution was needed, and it could be acquired in 1904 when the huge telescope at the Mount Wilson Observatory in California was built with a 100-inch diameter mirror. It was not until then that the size of the universe became apparent, because the already immense Milky Way is barely a galaxy, among innumerable conglomerates of them.
In 1924, Edwin Hubble (1889-1953) managed to measure the distance to one of these spiral nebulae, observing the type stars cepheids in the object M31, the most remarkable spiral-shaped nebula, called Andromeda.
Cepheids are stars that periodically change their brightness and this is proportional to the period. The brighter ones have longer periods.
By then, Harold Shapley (1885-1972) had estimated the size of the Milky Way, but it was so large that he was convinced that the Andromeda Nebula was inside the Milky Way.
However, Hubble determined that the distance to the Cepheids of Andromeda was much greater than the size of the Milky Way and therefore could not lie within it. Andromeda, like the Milky Way, was a galaxy in its own right, although it continued to be called an «extragalactic nebula» for a long time.
Characteristics of the galaxies
mass and shape
Galaxies have shape and can be classified according to this criterion. In addition, they contain mass and are not static entities at all, since they have movement.
Measurement units
There are giant and very bright galaxies, such as the Milky Way and Andromeda, and also galaxies called «dwarfs», up to a thousand times less bright. To familiarize yourself with the sizes, it is convenient to know some units of measurement used in astronomy. First we have the light-year.
A light-year is a unit of distance that is equal to the distance light travels in one year. Being that the speed of light is 300,000 km/s, multiplying by the number of seconds in 365 days, the result is approximately 9 and a half trillion kilometers.
For comparative purposes, the distance from the Sun to the Earth is 8.5 light-minutes, about 150 million kilometers, which is approximately equivalent to one AU or astronomical unit, useful for measurements within the Solar System. The next closest star to the Sun is Proxima Centauri at 4.2 light-years.
The AU gives rise to another widely used unit: the parsec or parallax of a second of arc. That a point is at the distance of one parsec, means that its parallax is equal to 1 second of arc between the Earth and the Sun. The following figure clarifies it:
Size
The sizes of the galaxies are extremely varied, from so small that they barely have a thousand stars, to the giant elliptical galaxies that we will talk about in detail later.
Thus, we have our Milky Way of about 100,000 light-years in diameter, being a large galaxy, but not the largest. NGC 6872 has a diameter of 520,000 light-years, about 5 times the diameter of the Milky Way, and is the largest spiral galaxy known to date.
have movement
Galaxies are not static. In general terms, stars and clouds of gas and dust have rotational movements around the center, but not all parts of a galaxy rotate with the same speed. The stars in the center rotate faster than the outer ones, in what is called differential rotation.
Chemical composition
The most common elements in the universe are hydrogen and helium. Inside the stars, as a nuclear fusion reactor, the heaviest elements that we know through the periodic table are formed.
Color and lightness
The color and luminosity of galaxies change over time. Younger galaxies are bluer and brighter than older ones.
Ellipse-shaped galaxies tend towards red, with many old stars, while irregular ones are the bluest. In spiral-shaped galaxies, blue is concentrated towards the center and red towards the outskirts.
components of galaxies
When observing a galaxy, structures such as the following can be identified, which are present in the Milky Way, which has been taken as a model because it is the best studied:
disk and halo
The two basic structures of our galaxy are the disk and the halo. The disk is in the mean plane defined by the galaxy and contains a large amount of interstellar gas that gives rise to new stars. It also contains old stars and open clusters -unstructured grouping of stars-.
It must be noted that not all galaxies have the same rate of star formation. Elliptical galaxies are thought to have a much lower rate, unlike spiral ones.
The Sun is located in the galactic disk of the Milky Way, on the plane of symmetry and, like all the stars in the disk, it orbits the galaxy following a roughly circular trajectory perpendicular to the galactic axis of rotation. It takes about 250 million years to complete one orbit.
The halo covers the galaxy with a less dense spheroidal volume, since it is a region with much less dust and gas. Contains the globular clustersstars grouped by the action of gravity and much older than the disk, individual stars and also the so-called dark matter.
Dark matter is a type of matter whose nature is unknown. It owes its name to the fact that it does not emit electromagnetic radiation and its existence has been proposed to explain the fact that stars in the exterior move with greater speeds than expected.
The speed at which a star moves relative to the center of the galaxy depends on how matter is distributed, since it is the gravitational attraction due to matter that keeps a star in orbit. Higher speed means that there is more matter that cannot be seen: dark matter.
The bulge, the galactic nucleus and the bar
Apart from the disk and the halo, in the galaxy there is a bulge, the central bulge or galactic nucleus, where there is a greater density of stars, being therefore very bright.
Its shape is approximately spherical -although that of the Milky Way is rather shaped like a peanut- and in its center is the nucleus, made up of a black hole, a fact that apparently is common in a good part of galaxies, especially in those of spiral form.
Objects that are close to the nucleus rotate, as we have said, much faster than those that are further away. There the speed is proportional to the distance from the center.
Some spiral galaxies like ours have a bar, a structure that runs through the center and from which spiral arms arise. There are more barred than unbarred spiral galaxies.
The bars are thought to allow transport of matter from the ends to the bulge, thickening it by fostering star formation in the core.
types of galaxies
The galaxy classification system is based on the shape they have and the most used currently is the tuning fork or Hubble sequencecreated around 1926 by Edwin Hubble, and later modified by himself and other astronomers, as new information appeared.
Hubble designed the scheme in the belief that it represented a kind of evolution of galaxies, but today it is known that this is not the case. In the sequence, letters are used to designate the galaxies: E for elliptical galaxies, S for spiral galaxies, and Irr for irregular ones.
elliptical galaxies
On the left, on the neck of the tuning fork, are the elliptical galaxies represented with the letter E. The stars that make it up are distributed more or less uniformly.
The number that accompanies the letter indicates how elliptical the galaxy is -ellipticity-, starting with E0, which is the most spherical, up to E7, which is the most flattened. Galaxies with ellipticity greater than 7 have not been observed. Denoting this parameter as є:
Є = 1 – (β/ɑ)
With α and β as the apparent major and minor semiaxes respectively of the ellipse. However, this information is relative, because we only have the view from Earth. For example, it is not possible to tell if an edge-on galaxy is elliptical, lenticular, or spiral.
Giant elliptical galaxies are among the largest objects in the universe. They are the easiest to observe, although the much smaller versions, called dwarf elliptical galaxies much more abound.
lenticular galaxies
Lenticular galaxies are disk-shaped, without spiral arms, but can be barred. Their nomenclature is S0 or SB0 and they are right at the fork…