In the night sky in clear weather, you can see many small luminous lights - stars. In fact, their sizes can be huge and hundreds or even thousands of times larger than the size of the Earth. They may exist in isolation, but sometimes form a star cluster.
What are stars?
A star is a massive ball of gas. It is able to be held by the force of its own gravity. The stellar mass is usually greater than the planetary mass. Thermonuclear reactions take place inside them, which contribute to the emission of light.
Stars are formed mainly from hydrogen and helium, as well as dust. Their internal temperature can reach millions of Kelvin, although the external one is much less. The main characteristics for measuring these gas balls are: mass, radius and luminosity, that is, energy.
With the naked eye, a person can see approximately six thousand stars (three thousand in each hemisphere). The closest thing to the Earth we see only during the day - this is the Sun. It is located at a distance of 150 million kilometers. The closest star to our solar system is called Proxima Centauri.
Birth of stars and clusters
Dust and gas, present in unlimited quantities in interstellar space, can be compressed under the influence of gravitational forces. The tighter they are compressed, the greater the temperature formed inside. As the matter condenses, it gains mass, and if it is sufficient to carry out a nuclear reaction, then a star will appear.
From a gas and dust cloud, several stars often form at once, which capture each other in a gravitational field and form star systems. Thus, there are double, triple and other systems. More than ten stars form a cluster.
A star cluster is a group of stars of common origin, which are connected to each other by gravity, and in the field of galaxies move as a whole. They are divided into spherical and scattered. In addition to stars, clusters can contain gas and dust. United by a common origin, but not connected by gravity, groups of celestial bodies are called stellar associations.
Discovery history
People have been watching the night sky since ancient times. However, for a long time it was believed that the celestial bodies are evenly distributed in the expanses of the Universe. In the 18th century, astronomer William Herschel challenged science again by saying that some areas clearly had more stars than others.
A little earlier, his colleague Charles Messier noted the existence of nebulae in the sky. Watching them through a telescope, HerschelI found out that this is not always the case. He saw that sometimes a stellar nebula is a cluster of stars that appear to be spots when viewed with the naked eye. He called what he found “heaps.” Later, a different name was coined for these phenomena of the galaxy - star clusters.
Herschel managed to describe about two thousand clusters. In the 19th century, astronomers determined that they differed in shape and size. Then globular and open clusters were identified. A detailed study of these phenomena began only in the XX century.
Open clusters
Clusters differ among themselves in the number of stars and shape. An open star cluster can include from ten to several thousand stars. They are quite young, their age may be only a few million years. Such a star cluster does not have well-defined boundaries, it is usually found in spiral and irregular galaxies.
About 1100 clusters have been discovered in our galaxy. They do not live long, since their gravitational connection is weak and can easily be broken due to the passage near gas clouds or other clusters. "Lost" stars become single.
Clusters are often found on spiral arms and near galactic planes - where the concentration of gas is greater. They have uneven, shapeless edges and a dense, well-defined core. Open clusters are classified according to density, differences in the brightness of internal stars, and distinctness compared to their surroundings.
Ballclusters
Unlike open star clusters, globular star clusters have a distinct spherical shape. Their stars are much more tightly bound by gravity, and revolve around the galactic center, acting as satellites. The age of these clusters is many times greater than the scattered ones, ranging from 10 billion years and more. But they are significantly inferior in number, in our galaxy about 160 globular clusters have been discovered so far.
They contain from tens of thousands to a million stars, the concentration of which increases towards the center. They are characterized by the absence of gas and dust, since they formed a long time ago. All stars of globular clusters are at approximately the same stage of development, which means that they, like scattered ones, formed at about the same time.
The high density of stars in a cluster often leads to collisions. As a result, unusual classes of luminaries can be formed. For example, when members of a binary star system merge, a blue stray star is formed. It is much hotter than other blue stars and members of the cluster. Collisions can also produce other space exotics, such as low-mass X-ray binaries and millisecond pulsars.
Star associations
Unlike clusters, associations of stars are not connected by a common gravitational field, sometimes it is present, but its strength is too small. They appeared at the same time and have a small age, reaching tens of millions of years.
Starryassociations are larger than young open clusters. They are more rarefied in outer space, and include up to hundreds of stars in their composition. About a dozen of them are hot giants.
Weak gravitational field does not allow stars to stay in association for a long time. For decay, they need from several hundred thousand to a million years - by astronomical standards, this is negligible. Therefore, stellar associations are called temporary formations.
Known clusters
In total, several thousand clusters of stars have been discovered, some of them visible to the naked eye. The closest to Earth are the open clusters of the Pleiades (Stozhary) and Hyades, located in the constellation Taurus. The first contains about 500 stars, only seven of them are distinguishable without special optics. Hyades is located near Aldebaran and contains about 130 bright and 300 low-burning members.
The open star cluster in the constellation Cancer is also one of the closest. It is called the Manger and contains more than two hundred members. Many characteristics of the Nursery and Hyades coincide, so there is a possibility that they are formed from the same gas and dust cloud.
Easily visible with binoculars is the star cluster in the constellation Coma Berenices in the northern hemisphere. This is the globular cluster M 53, discovered back in 1775. It is over 60,000 light years away. The cluster is one of the most distant from the Earth, although it is easily distinguishable with binoculars. A huge number of globular clusters are located in the constellationSagittarius.
Conclusion
Star clusters are large groups of stars united by gravity. They number from ten to several million stars that have a common origin. Basically, globular and open clusters are distinguished, differing in shape, composition, size, number of members and age. In addition to them, there are temporary clusters called stellar associations. Their gravitational connection is too weak, which inevitably leads to the decay and formation of ordinary single stars.
