Chapter 4 – astronomia-iniciacion.com

Cumulus, nebulae and galaxies

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The clusters

There are also stars that form groups. In the first rung of the scale of clusters are the globular ones, they are giant clusters of stars that began to appear in the Universe about 11,000 million years ago.

On the last step are the open clusters. Next a small introduction on these celestial bodies.

The globular clusters

More organized and compact than the open clusters are the globular clusters. A globular cluster is a ball of densely packed stars that contains hundreds of thousands of individual stars. The globular clusters of our Galaxy are scattered along a spherical halo that surrounds the Galaxy, and contain some of the oldest stars in the Galaxy.

There are about 150 globular clusters in our Galaxy. Other similar globular clusters distributed in spherical halos have been identified in other galaxies, for example more than 300 in the Andromeda M31 galaxy, and approximately 6,000 in the vicinity of M87. The number of stars is so high and the relative distances so minimal that they constitute groups gravitationally linked, in a single cubic parsec of space there can be up to 1,000 stars, in which each star travels a more or less elliptical orbit around the center of the conglomerate.

The distribution of the clusters suggests that they were formed when the Galaxy was young, 15,000-18,000 million years ago, any model of the big bang should give an age of the universe of about 20,000 million years or more. Globular clusters contain mainly Population II stars, many of which have evolved into red giants.

The object M5 (NGC 5904) of the image is a beautiful globular cluster of the northern hemisphere, belonging to the constellation of Serpent. Its location is very easy in the summer months.

Observed through a small telescope, they appear as small, fuzzy balls, but with instruments with a larger aperture (200 mm or more in diameter), these balls become thousands of stars.

The best globular cluster of the northern hemisphere is M13 in the constellation of Hercules, with a magnitude of 5.8, a diameter of 14 ‘and located 23,000 light years away, it has a width of 100 light years. Easy to find in the small trapeze of Hercules, in the line that joins the two stars Zeta and Eta.

 Open clusters

An open cluster is an irregular grouping or swarm of stars that appear to the naked eye as spots of light. They are also called galactic cluster, being located relatively close to us in the plane of our Galaxy.

Open clusters contain young, hot stars from Population I that have recently formed on the disk of the Galaxy. Open clusters worthy of mention are the following, all are visible within the northern hemisphere:

The nebulae that surround the open cluster M45, commonly called the Pleiades, are reflective (photo).

The nebulae

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Nebulae are clouds of gas and dust that seem foggy to the naked eye. These celestial objects are some of the most beautiful objects that can be observed in space. The word nebulous comes from the Greek and means cloud. Nebulae play an important role, since new stars are formed in their interior due to gravitational collapse. Part of the gas was formed at the beginning of the history of the universe. The dust and the heavy elements are of more recent origin, since they have been formed in stars that liberated the interstellar medium at the end of their lives in a more or less violent way (supernovas).

Nebulae are divided into three basic types:

  1. Reflection nebulae
  2. Emission nebulae
  3. Dark or absorption nebulae
  4. Planetary nebulae

1. Reflection nebulae: are clouds of dust whose atoms reflect the light of a nearby star, so they appear the same color as the stars whose light they reflect, an example of this is the bluish nebula that surrounds the Pleiades . They appear bluer than the star because of the way starlight is scattered by the dust particles in the nebula (equivalent to the scattering of light that makes the sky blue).

2. The emission nebulae: they shine because their atoms, excited by the radiation emitted by nearby stars, become sources of radiation. They are gas clouds that receive energy radiated by nearby hot stars, and they are red in astronomical photographs due to the characteristic radiation of hydrogen in the red region of the spectrum.

The North American nebula in the constellation of the Swan, is an example of an emission nebula with an absorption nebula that defines the limits we perceive. In the zone equivalent to the Gulf of Mexico, few stars are observed due to the dark absorption nebula located in it.

3. The absorption nebulae: they are vast clouds rich in dust that absorb light and are only optically visible when behind them there is a light source on which they can stand out.

The great dark line that divides the Milky Way in two in the constellations of the Swan and the Eagle is also due to a cloud of dark dust.

4. Planetary nebulae: some nebulae represent gas envelopes detached from dying stars. The term was used by Herschell because of its circular and very delimited appearance reminiscent of a planet’s disk, hence its name.

A planetary nebula shines because the light (ultraviolet radiation) from the star with which it is associated is absorbed by the atoms of the nebula and reirradiada. They are old stars that are expelling material into space (enriching interstellar matter with heavy elements) and are on the way to becoming white dwarfs, that is, red giant star nuclei that have lost their outer layers. A planetary nebula is a transition from the red giant to the white dwarf state. There are some 1,500 planetary nebulae cataloged.

The Trifid nebula, M20, in Sagittarius, is a reddish emission nebula, while the blue color is from a reflection nebula.

The galaxies

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A galaxy is a giant set of millions or billions of stars, gas and dust that are held together by gravity to form a disk galaxy of 30 kiloparsecs (1 kiloparsec = 1000 parsecs and 1 parsecs = 3.2616 light years) in diameter and surrounded by a halo of visible globular clusters. Galaxies are the most magnificent celestial objects. It is an island of matter in space.

M81 located in Uma, is a galaxy of the type Sb similar to our Galaxy, called the Milky Way.

The stars of the disk describe orbits around the center of the galaxy. The speed of each star in its orbit around the center depends on its distance from the galactic center: the stars furthest from the center move more slowly than the stars closest to it. The Sun moves in its orbit at about 250 km / s, and needs about 225 million years to describe an orbit around the Galaxy.

The parts of a spiral galaxy are:

  1. Core or protuberance
  2. Accretion disk.
  3. Globular clusters.

From the central nucleus of the spiral galaxies two or more spiral arms that unfold around the nucleus in the form of spirals emerge at two diametrically opposed points.

In the halo and in the central region there are only red and old stars (15 million years old), especially in the globular clusters, known as Population II. The young stars, Population I, are typically hot stars of the main sequence and are found in the spiral arms of the galaxies, where a continuous star formation is triggered, the stars of the spiral arms are moving and fading steadily as They age, but the spiral structure does not wear out because blue stars are constantly being born along the inner edges of the arms. Population I stars are found in open clusters and in diffuse objects such as nebulae.

Most galaxies are too pale and too far away to be seen with the naked eye or with binoculars, except the Andromeda M31 galaxy which is visible to the naked eye as a blur, but it is fascinating to study its shapes with a telescope.

Galaxies tend to occur in groups, called clusters of galaxies linked by gravity. Our Galaxy is a member of a cluster called the Local Group, which only has about 40 members, including the Andromeda Galaxy, the Magellanic Clouds and several dwarf galaxies.

The largest cluster of galaxies close to us is the Virgo cluster, with 2,500 galaxies. The galaxy clusters are grouped into superclusters, and the Local Group is part of the same Virgo supercluster.