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All eyes on the Milky Way

Anyone admired the night sky will perhaps surprising to believe that our eyes are really blind than most of the "light" which comes from the universe. What we perceive is, in fact, only a small fraction of the energy that stars, galaxies and other astronomical sources produce.

The so-called light "visible" is but a fraction of what scientists call the "electromagnetic spectrum: radio waves, microwaves, infrared light correspond to lower energies than visible light, ultraviolet light, X rays and rays range rather have higher energies, but it's still light. The atmosphere

Earth, also contributes to our "isolation" taking some of this radiation and allowing only visible light and radio waves. If there were no atmosphere, and if our eyes have evolved in some of the countless other ways, probably the night sky would appear in a different way.

Thanks to technological advances of recent decades, astronomers have learned to look at the sky but also in other energy, thus creating innumerable discoveries. Using space telescopes that scan the universe from a privileged location outside the atmosphere, in orbit around the Earth, have been able to open "new eyes" on cosmos, observed by light infrared and ultraviolet, through X-rays, gamma rays and microwave .

In particular, the images of the same object made of different "bands" of energy reveal the wide range of physical phenomena that occur within it. This image, released last Tuesday by NASA, it is a shining example.


For being portrayed is the center of our galaxy, the Milky Way, which is hidden inside a supermassive black hole with a mass equal to four million times that of the Sun, whose presence you can not directly observe only sense in an indirect way. The different colors in the image correspond to observations made in different energy bands, and reflect different physical phenomena.

what he sees Hubble Space Telescope in visible light and the so-called "near-infrared, is shown in yellow: it is hundreds of thousands of stars, some of which are emerging while others shine, burning the their nuclear fuel. In red, however, shows the observations made by the infrared Spitzer Space Telescope. The infrared light corresponds to lower energies than visible objects Spitzer is so sensitive to colder than to Hubble, and can "see" the filamentary structure of the clouds of cosmic dust , forged by winds generated by nearby stars, and from which new stars are born in the future. Blue and purple, finally, to the observed data are reproduced by the Space Telescope Chandra X-ray: X-rays are emitted by gas at very high temperatures, over a million degrees, which is located near the galactic center, so warm thanks to 'energy released from the nearby stellar explosions and energy jets from the black hole hidden in the heart of the Milky Way.

combining the images obtained with different telescopes, astronomers have been able to discover previously unknown details about the phenomena violent and impetuous that take place in the center of our Galaxy, about 26.000 years-light from us.

CLAUDIA MIGNONE

This image, about the size of the full moon in the sky, shows the rising stars, explosions, clouds of gas and dust in the Milky Way's center. The different colors correspond to observations in different bands of the electromagnetic spectrum: the yellow is in the visible and near infrared observations (Hubble), red is infrared observations (Spitzer), the blue-violet shows the X-ray observations (Chandra). Image released by NASA, ESA, CXC, SSC, STScI.

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The energy gamma rays

All amateur astronomers are accustomed to seeing images of the same object in different energy bands from radio observations, optical, infrared, ultraviolet, X-ray It is not so common, however, hear about most of the energy of the electromagnetic spectrum: gamma rays.

This light has an energy about 10 billion times greater than the light to which the human eye is sensitive. Unfortunately it is not possible to directly observe this radiation as the Earth's atmosphere is not transparent at these energies. The problem can be circumvented in part through the use of space telescopes , but what about an important part of gamma rays, high energy ones, this is not possible for purely logistical reasons: these photons are so rare that the dimensions of the telescope are prohibitive for an experiment to be sent into orbit.

Another way to observe these photons uses the same atmosphere of the Earth as a large particle detector. The gamma rays that pass through the atmosphere interact with the atoms thereof and lose energy, producing a cascade of particles. The particles in the cascade have a speed faster than the speed of light in the atmosphere (but still less than the speed of light in vacuum!) And produce the so-called Cherenkov radiation: this is a cone of light that, when it reaches the ground, it can illuminate an area of \u200b\u200babout 100 meters range!

This light is then gathered by telescopes and analyzed. Among the latest experiments in this field include HESS (High Energy Stereoscopic System), located in Namibia, MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov Telescope), located in La Palma, Canary Islands, and VERITAS (Very Energetic Radiation Imaging Telescope Array System), Arizona, United States. These experiments take advantage of the stereoscopic technique, or use multiple telescopes spread over an extensive area, to cover the cone of light produced by the waterfall: this allows us to determine more certainty the direction from which the photons and their energy.


Each telescope is not fundamentally different from a traditional optical telescope. The main difference is the size of the mirror: the two telescopes MAGIC experiment, for example, have a diameter of about 17 meters each, while that of the telescope to be built during the next phase of the experiment HESS is even over 20 meters ! Of course it is not monolithic mirrors, which consist of a single block, but made up of many small pieces.

Many and various are the astronomical objects that can be studied with this technique: an example is jets of particles emitted by the nuclei of so-called active galaxies, which hide within them a supermassive black hole that devours the surrounding matter, or nebulae of energetic particles ejected from the stars when, at the end of their lives, explode in the form of supernovae.

GIOVANNA PEDALETTI

picture, the four telescopes currently comprising the HESS experiment in the Plateau of Khomas, Namibia. The experiment is fully operational since 2004, will be enhanced in the coming years with more telescopes to achieve even better performance. (Credits: HESS Collaboration)