Into the clouds where stars are born

Looking at the sky in the daytime, weather permitting, it is possible to see the Sun, the star which provides us with light and heat, thus making life on Earth possible. Looking at the sky during the night, one can see millions of stars: the firmament is abundant in large and small suns, located here and there in our galaxy, the Milky Way. Looking further away, with the aid of a binocular or a telescope, it is possible to perceive other galaxies, each of them containing from tens of millions to hundreds of billions of stars. Stars, stars, and even more stars. Where do all these stars come from? How did they form? They all derive from a mixture of gas and dust, known as interstellar medium, which is another important component of galaxies. The interstellar medium is rather homogeneous, but regions occur where the gas is particularly dense: it is exactly in these clouds, or nebulae, that stars are born.

When the gas within a nebula is so dense that its pressure cannot balance its own weight, the so-called gravitational collapse begins: the cloud starts breaking down into small fragments, which keep contracting and become rotating spheres of gas, increasingly denser and hotter. Collapse continues for a long time (up to a million years), until the temperature achieved within each “sphere” is so high that nuclear reactions set it, turning hydrogen into helium and producing energy: the gas “sphere” has now become a star. From the collapse of a single nebula, hundreds to thousands of stars with different masses are formed: the most massive ones will only live a few million years, whereas the smallest ones will go on burning for billions, even hundred billions of years.

This image of the Eagle Nebula is an example of a “stellar nursery”: the elongated structures visible in the centre are huge pillars of gas and dust, larger than our own Solar System, where stars are born. The bluish light permeating the central regions of the nebula comes from the first, young stars which formed: with their energy, they heat up the surrounding gas, thus making the formation of new stars even more likely. This image, obtained with the telescope on Kitt Peak in Arizona, USA, has an extremely high resolution, which allows to study star formation in detail; however, the Eagle Nebula can be observed even with a binocular, in the constellation of the Serpent, visible from Europe from May to September. New stars are formed all the time: in order to see them, we only need to lift our eyes up to the sky.

In the image, the Eagle Nebula, an open window onto the processes of star formation. Credits: T. A. Rector & B. A. Wolpa, NOAO, AURA.

Translated from Il Denaro, 12.02.2009