In the night sky in clear weather you can see many small luminous lights - stars. In fact, their size can be huge and hundreds, or even thousands of times greater than the size of the Earth. They can exist apart, but sometimes form a star cluster.
What are stars?
A star is a massive ball made up of gas. It is able to be held at the expense of the force of its own gravity. Stellar mass, as a rule, is more than planetary. Inside them, thermonuclear reactions occur that 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 temperature 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 to the Earth we see only during the day - this is the Sun. It is located at a distance of 150 million kilometers. The star closest to our solar system is called Proxima Centauri.
The birth of stars and clusters
Dust and gas present in unlimited quantities in interstellar space can be compressed by gravitational forces. The denser they are compressed, the higher the temperature is formed inside. Condensing, the substance gains mass, and if it is sufficient to carry out a nuclear reaction, a star will appear.
Several stars are often formed from a gas-dust cloud at once, which capture each other in a gravitational field and form star systems. Thus, there are binary, 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 the galaxy move as a whole. They are divided into ball and scattered. In addition to stars, clusters may 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 from ancient times watched the night sky. However, for a long time it was believed that the celestial bodies are evenly distributed in the vastness of the universe. In the eighteenth century, astronomer William Herschel threw another challenge to science, saying that in some parts of the stars there are clearly more stars than in others.
A little earlier, his colleague Charles Messier noted the existence of nebulae in the sky. Watching them through a telescope, Herschel 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 if you look at them with the naked eye. He called the discovered "heaps." Later, a different name was invented for these phenomena of the galaxy - star clusters.
Herschel managed to describe about two thousand clusters. In the 19th century, astronomers determined that they differ in shape and size. Then, globular and open clusters were identified. A detailed study of these phenomena began only in the 20th century.
Open clusters
Between themselves clusters differ 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 can be only a few million years. Such a star cluster does not have clearly defined boundaries, usually it is in spiral and irregular galaxies.
About 1100 clusters have been discovered in our galaxy. They do not live long, because their gravitational connection is weak and can easily break due to passage near gas clouds or other clusters. "Lost" stars become solitary.
Clusters are often located on spiral arms and near galactic planes - there is a greater concentration of gas. They have irregular shapeless edges and a dense, clearly visible core. Open clusters are classified according to their density, differences in the brightness of inner stars, and distinctness compared to their surroundings.
Globular clusters
Unlike open, globular star clusters have a clear spherical shape. Their stars are connected by gravity much more closely, and revolve around the galactic center, acting as satellites. The age of these clusters is many times greater than open, ranging from 10 billion years or more. But in terms of quantity, they are significantly inferior; in our galaxy, about 160 globular clusters are still open.
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 have been formed for a long time. All stars of globular clusters are approximately at the same stage of development, which means that they formed, like the open ones, at about the same time.
The high density of stars in the cluster often leads to collisions. As a result, unusual classes of luminaries can form. For example, when members of a binary star system merge, a blue lagging star appears. She is much hotter than other blue stars and cluster members. In the course of collisions, other exotic outer space may arise, such as low-mass x-ray binary stars and millisecond pulsars.
Stellar associations
Unlike clusters, star associations 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 ten million years.
Star associations exceed the size of 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.
A weak gravitational field does not allow stars to be 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 star clusters were discovered, some of them visible to the naked eye. Closest clusters of the Pleiades (Stozhary) and Hyades located in the constellation Taurus are closest to the Earth . The first contains about 500 stars, without special optics, only seven of them are distinguishable. Hyades are located next to Aldebaran and contain about 130 bright and 300 slightly burning participants.
The open star cluster in the constellation Cancer is also one of the closest. It is called a Nursery 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 one gas and dust cloud.
The star cluster in the constellation Veronica Hair in the northern hemisphere is easily distinguishable through binoculars. This is the globular cluster M 53, discovered back in 1775. It is located at a distance of more than 60,000 light years. The cluster is one of the most distant from the Earth, although it is easily distinguishable with binoculars. A huge number of globular clusters located in the constellation Sagittarius.
Conclusion
Star clusters are large groups of stars, interconnected by gravity. They number from ten to several million stars that are of common origin. Globular and open clusters are generally 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 the formation of ordinary single stars.