Formation of the stars

In our Milky Way, he is born an average of four or five stars each year.
The star formation are often called young stars.
The observation confirms that stars form in groups, from the gravitational contraction of a cloud of gas and dust, which is fragmented and contracts in several areas protostellar. Molecular clouds are composed of interstellar dust wandering left during the formation of the galaxy. The main material of interstellar clouds, is hydrogen, which is also the main constituent of the stars. Data from the Spitzer Space Telescope, on the picture against, shows the bottom, the molecular cloud Cepheus B. Young stars, purple, in and around Cepheus B, are seen by the X-ray telescope, Chandra.
It is thanks to Chandra that astronomers were able to identify young stars in and near Cepheus B, they are identified by their strong emission of X-ray As for the Spitzer Space Telescope data, they have shown that young stars have a disk "protoplanetary" around them. These discs exist only in very young systems, where the planets are still in formation, their presence is an indication of the age of a star system.
These data provide an excellent opportunity to test a model to explain how stars form. The study of this cloud, suggests that star formation in Cepheus B is primarily triggered by the radiation of a massive star bright (HD 217086), located outside the cloud.

According to this particular model, the triggering of star formation, called RDI (Radiation Driven Implosion), starts from the radiation of the massive star that generates a compression wave inside the cloud, the binder material while ejecting into the outer layer of the cloud.
However, different types of triggered star formation has been observed in other environments.
For example, the formation of our solar system was triggered by a supernova explosion, the star-forming region W5.
The blast swept violently matter that has contracted on the fronts of shock. Finally, the concentration of gas accumulated on these fronts, becoming dense enough to collapse and form hundreds of stars.
 RDI mechanism is also present in W5 and is responsible for the formation of dozens of stars.
Most of the time, star formation is triggered where a cloud of gas cools, gravity takes over, falls on the cloud itself.

A star is a aster like the sun, which shines through nuclear reactions that occur in the center.
With the exception of the Sun, the stars appear to the naked eye as a bright, shimmering due to atmospheric turbulence, without immediate apparent motion relative to other fixed objects in the sky.
All the stars are considerably more distant from Earth than the Sun.
The nearest star, Proxima Centauri, is located about 4 light years of the solar system, nearly 250 000 times farther than the Sun.
The mass of a star is of the order of 10³⁰ kg and its radius of the order of several million kilometers.
The power radiated by a star like the Sun is about 10²⁶ watts. Stars form due to the contraction of a nebula of gas and dust under the influence of gravity.
If the heating of the material is sufficient, it will trigger the cycle of nuclear reactions in the heart of the nebula to form a star. The energy from these reactions is then sufficient to stop its contraction due to the radiation pressure generated.

The number of stars in the Universe is estimated at between 10²² and 10²³. Apart from the Sun, the stars are too faint to be observed in daylight.

Image: Birth of a star image made from data from the Chandra X-ray Telescope (blue) and data from the Spitzer Infrared Telescope (red and orange). At about 4000 light years from Earth lies RCW 108, a region of the Milky Way, where star formation is active where the presence of clusters of young blue stars in the picture. That we see born, in yellow in the center of the image is deeply rooted in a cloud of molecular hydrogen. According to data from different telescopes, astronomers have determined that the birth of stars in this region is triggered by the proximity effect of young massive stars.

The dark cloud of dust around Rho Ophiuchus at a distance of about 400 light years, making it one of the closest star-forming regions.
The cloud is located both in the constellation Scorpius and Ophiuchus that of (Ophiuchus).
This remarkable giant cloud around the star ρ (Rho) Ophiuchi, is rich in molecular hydrogen.
Its exceptional vividness makes a region characteristic of star formation. In this dark cloud of gas and dust hiding most of the stars. If they do not show up in visible light, infrared studies have revealed more than 300 young stars in the central region of the nebula.

These observations have shown that most stars were between about 300 000 years. Over time, this cloud will shrink slowly under its own weight and form new stars.
This region owes its blue reflection of hydrogen from young stars forming in the dust cloud.

Image: The dark cloud of dust around Rho Ophiuchus.
credit ESO: GigaGalaxy Zoom

The Spitzer Space Telescope shows in this picture, thanks to its infrared vision, the depths of the dusty nebula in the constellation of Orion. It can be seen in visible light, in each nebula, a rising star, hidden behind a dark cloud. These two cosmic nebulae like a mask, behind which two stars, watching us.
Cataloged Messier 78, the two nebulae are actually greenish round cavities in the surrounding dark clouds of dust. Despite this very dark dust, Spitzer can see the edges bright in infrared light, bright interiors that frame. Messier 78 is easy to see with small telescopes in the constellation Orion, just northeast of Orion's belt. Spitzer's infrared eyes, which penetrate the dust, revealing an interior light, surprising. A string of young stars that have yet to burn their native shell are shown in red on the outside of the nebula.