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The Milky Way that we can see in the night sky is actually just one of the spiral arms of our own galaxy, which takes, by extension, the same name.
Our galaxy is a grouping of some 300,000 million spiral-shaped or spinning stars, whose dimensions are estimated at around 100,000 light-years and whose central disk is 16,000 light-years in size.
The Milky Way, also called in Spain Santiago's road, can be seen with the naked eye as a band of light that runs through the night sky, which Democritus has already attributed to a set of innumerable stars so close to each other that they are indistinguishable.
In 1610 Galileo, using the telescope for the first time, confirmed the observation of Democritus. By 1773 Herschel, counting the stars he observed in the sky, built an image of the Milky Way as a star disk into which the Earth is immersed, but could not calculate its size. In 1912 the astronomer Henrietta Leavitt He discovered the relationship between the period and the luminosity of the stars called Cepheid variables, which allowed him to measure the distances of the globular clusters.
Several years later Shapley showed that the clusters are distributed with a more or less spherical structure around the center of the disk, in what he called the galactic halo. He also showed that it is not centered on the Sun, but at a point distant from the disk in the direction of the constellation Sagittarius, where it correctly positioned the center of the galaxy.
This structure was confirmed when it was observed from the Monte Wilson Observatory in California that the spiral object called Andromeda was made up of individual stars and was not a mere gas nebula as previously believed.
By 1930 Trumpler discovered the effect of galactic darkening produced by interstellar dust, thereby correcting both the size of the Galaxy and the distance at which the Sun is to the values accepted today. According to this data, the Solar System is located at a distance between 7,500 and 8,500 parsecs away from the galactic center, approximately two thirds away.
All the stars that make up the Milky Way are rotating around the nucleus, which is believed to have a black hole inside. Astronomical observations referring to distant galaxies show that the speed of rotation of the Sun around the galaxy is about 250 km / s, using approximately 225 million years to make a complete revolution. Stars close to the Sun make a relatively similar orbit, but those closest to the center of the galaxy rotate faster, a fact known as differential rotation.
The age of the Milky Way is estimated at about 13 billion years, a figure that emerges from the study of globular clusters and that agrees with the result obtained by geologists in their study of the radioactive decay of certain terrestrial minerals.
The observation of the star map has allowed the reconstruction of the spiral arms of the Galaxy, areas in which the number of star clusters or areas of star formation is abundant. These are named by the constellations found in them. The arm closest to the galactic center is called Centaur or Norma-Centaur. The next outward arm is Sagittarius. The Orion arm is our local arm, also called the Swan, and the contiguous outward arm is known as the Perseus.
The stars found in the Milky Way They are usually grouped into two large groups, commonly called populations. The so-called population group I is made up of relatively young, solar composition stars that are distributed in approximately circular orbits in the galactic disk, inside their arms. Population stars II are rich in hydrogen and helium, with a shortage of heavy elements, are older, and have orbits that are not within the galactic plane.
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