Introduction
Our galaxy, the star system of our solar system, is called the Milky Way. The Milky Way is a grand cluster of stars visible in the sky as a bright band of fog. In the ancient Greek, the word "galaktikos" means "milk", "milky", so the Milky Way and stellar systems similar to it are called galaxies. In our galaxy, there are more than 200 billion stars of very different luminosity and color ("The Milky Way Galaxy - 200 billion stars", 2016).
The Solar neighborhood is the volume of the galaxy, where we can observe and study the stars of different types by the available means of modern astronomy (Carigi, 2011). As practice shows, the form of the Solar neighborhood is sphere, which contains about 1500 stars . The radius of this sphere is 20 parsecs. Currently, the solar neighborhood is developed for all or nearly all the stars except for very dwarf, emitting very little light.
Body
In the immediate vicinity of the Sun, in a radius of about 5 parsecs, absolutely all the stars are investigated (there approximately 100 stars in this area). Most of them (almost two thirds) are very faint red dwarf with a mass of 3-10 times smaller than that of the sun ("Red dwarfs: The most important stars in the Universe", 2016). Stars like the sun are very rare, there are only 6% of such kind. There is no more massive stars (astronomers know stars with masses up to about 100 solar masses) found in the immediate vicinity of the Sun, indicating that they are very rare. In addition to the living stars, scientists have found 7 white dwarfs in this area.
Weak red dwarf Proxima (from the Latin "immediate") - a component of a triple system of alpha-Centauri - is now considered to be the nearest star from the Sun ("Red dwarfs: The most important stars in the Universe", 2016). The distance to Proxima is 1.31 pc, its light comes to us for 4.2 years.
Our ancestors have combined all the stars in the groups - constellation. Constellations are not the physical groupings of stars, linked by common characteristics. Constellations are the areas of the sky. The stars in the constellations are united by our ancestors in order to make it easier to navigate in the starry sky, i.e., on the basis of random coincidence of their positions in the sky.
All the sky is divided into 88 constellations, which are named after mythological characters (eg, Hercules, Perseus), animals (eg, Lion, Giraffe), objects (eg, Libra, Lyra), and others ("The 88 Official Constellations", 2016).
Clusters of stars are the groups of the stars with common physical properties. This is how clusters are different from the constellations, which are the result of random coincidence of the positions of stars in the sky. Observations in the XIX century revealed that the star clusters are divided into globular clusters and open clusters. In the second half of the XX century, another class was added to these classes of stellar groups – star associations ("Star Clusters", 2016).
Globular star clusters are the oldest objects in our galaxy: they were formed at the same time with the Milky Way. The distances to these clusters are very large - thousands of parsecs. Now there are more than 150 known globular clusters in the Milky Way galaxy.
Open cluster consists of several hundreds or thousands of stars. The mass of open cluster is small and the gravitational field is not able to resist the destruction of clusters for a very long time. Having existed for nearly a billion years, they dissolve in the ocean of the galaxy. Association is a grouping of young stars, united by a common formation. They are sparser than clusters.
Many details of the structure of the Milky Way are hidden from our view. However, they are being studied by the examples of other galaxies that are similar to ours, such as the Andromeda Nebula. It was researched by German astronomer Walter Baade in the 40-ies of XX century (Gregersen, 2010).
As a result, there is an isolated lenticular flat disk in the structure of the Galaxy. It is immersed in a rarefied stellar cloud of spherical shape - halo. The galaxy has a form of lenticular lenses, similar to the lentil seed. The stars of the galactic disk are called population of type I, the stars of the halo are population of type II.
One of the most interesting areas of the galaxy is its center (or nucleus), which is located in the direction of the Sagittarius constellation. Visible radiation of the central regions of the galaxy is completely hidden from us by the huge layers of absorbent material. In the center of the galaxy, there is supposed to be an existence of massive compact object - a black hole weighing about one million solar masses. One of the most prominent structures in the galactic disks is the spiral arms.
There are galaxies, including the trillions of stars. Our galaxy, the Milky Way, is also quite large (it has more than 200 billion stars). The smallest galaxies contain a million times smaller number of stars. Apart from the stars, galaxies also include interstellar gas, dust and various exotic objects: white dwarfs, neutron stars, black holes. The closest to us and brightest galaxy is the Magellanic Clouds. It is one of the largest astronomical objects visible in the sky (Sessions, 2016). The appearance and structure of the star systems are very different. They are into the following morphological types: elliptical, spiral, irregular. Our galaxy belongs to a spiral type. The spiral structure of our galaxy is very well developed. In the vicinity of 1.5 pc of our galaxy, there are located about 40 galaxies that form a local group.
However, not all objects of the Milky Way are known to science. For modern astronomical picture of the world it turned out to be fundamentally important that there are space objects, from which it is impossible to receive radiation. Their presence can be established only through their gravitational influence on its neighbors. Invisible substance, manifesting itself in the interaction with a visible means of gravitational forces in modern astronomy is called dark matter.
For the first time the hidden mass of talking in the 30-ies of XX century, when Swiss astronomer Fritz Zwicky, measuring the redshift velocities of galaxies from the cluster in the constellation Coma Berenices, was that the velocities of the galaxies is much higher calculated. It is hypothesized that there is an unseen cluster, dark matter, which is the reason for the large velocities of galaxies. According to calculations, this invisible mass is many times greater than the mass of the visible ("The Search for Dark Matter (Sidebar) | The Kavli Foundation", 2016).
Today astronomers confidently conclude: The universe is filled with mostly invisible substance. It forms an extended halo of galaxies and intergalactic space fills, concentrating to clusters of galaxies. The issue of hidden mass nature is far from being resolved. Perhaps this is not created by the mass of elementary particles is open. Part of the missing mass may be in the objects consisting of normal atoms.
References
Carigi, L. (2011). Solar Neighborhood. Encyclopedia Of Astrobiology, 1526-1527. http://dx.doi.org/10.1007/978-3-642-11274-4_1460
Gregersen, E. (2010). Astronomical observations (p. 187). New York: Britannica Educational Pub. in association with Rosen Education Services.
Red dwarfs: The most important stars in the Universe. (2016). Armaghplanet.com. Retrieved 23 March 2016, from http://www.armaghplanet.com/blog/most-important-stars-in-the-universe.html
Sessions, L. (2016). The spectacular Large Magellanic Cloud | EarthSky.org. Earthsky.org. Retrieved 23 March 2016, from http://earthsky.org/clusters-nebulae-galaxies/the-large-magellanic-cloud
Star Clusters. (2016). Atnf.csiro.au. Retrieved 23 March 2016, from http://www.atnf.csiro.au/outreach/education/senior/astrophysics/stellarevolution_clusters.html
The 88 Official Constellations. (2016). Physics.csbsju.edu. Retrieved 23 March 2016, from http://www.physics.csbsju.edu/astro/asp/constellations.pre.html
The Milky Way Galaxy - 200 billion stars. (2016). Myastrologybook.com. Retrieved 23 March 2016, from http://www.myastrologybook.com/Milky-Way-Galaxy-200-billion-stars.htm
The Search for Dark Matter (Sidebar) | The Kavli Foundation. (2016). Kavlifoundation.org. Retrieved 23 March 2016, from http://www.kavlifoundation.org/science-spotlights/astrophysics-closing-in-dark-matter-sidebar#.VvKz6-KLTbg
