What is a galaxy?
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Updated June 01, 2013
A galaxy is an assembly of stars, interstellar matter and possibly large amounts of dark matter, whose cohesion is ensured by the forces of gravity.
The mass of a galaxy is an order of magnitude greater than one hundred million solar masses.
Galaxies interact with their neighbors and change their shape these interactions.
Two galaxies sometimes merge. Galaxies are of three types: elliptical, spiral, irregular.
Like the stars, which are grouped into galaxies, most galaxies are gravitationally bound together. The structure containing up to fifty galaxies is known group of galaxies.
The structure containing thousands of galaxies grouped in a few megaparsecs sector is a cluster of galaxies. Groups and clusters of galaxies are themselves grouped into superclusters, giant collections containing tens of thousands of galaxies.
At very large scale distribution of galaxies is not uniform, but organized in sheets or filaments.
The Hubble Space Telescope routinely sends images that show the diversity of galaxies.
The Milky Way Galaxy or with a capital G, is also the name given to our galaxy is a spiral of stars huge wheel with a diameter of approximately 100 000 light years. What appears from Earth, is a continuous white band of the Milky Way.
This galaxy is actually composed of three spiral arms (Sagittarius arm, the arm of Orion and Perseus arm). The mass of our Galaxy is 2x1041 kg or 1011 solar masses. Many clues suggest that the center of many galaxies is occupied by a black hole.
The Large and Small Magellanic Clouds are nearby galaxies to ours, visible in southern latitudes.
They are with the Milky Way and Andromeda, the only galaxies visibles to the naked eye.
Image: Our galaxy is a spiral galaxy like the M83 (NGC 5236) on the image below cons.
Center of the Milky Way
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The Milky Way is the central region of our galaxy. Infrared image below shows the cons exact center of our galaxy, known as the Central Molecular Zone and purple, the radio arc of the galactic center.
A number of emission nebulae are visible from the young massive stars that illuminate the interior. Like almost all galaxies, our galaxy contains at its center, a black hole. This black hole of several million solar masses, is called Sgr A.
The galactic center is also home to the region of star formation, the most active of the galaxy.
The observations in Namibia, with the HESS telescope, have revealed the presence of very energetic gamma rays from the galactic center.
These gamma rays are produced by the collision of protons at very high energy protons with lower energies.
It can occur at that time, a reaction in which neutral meson decays, produce gamma photons. These photons when they collide with atoms in Earth's atmosphere, producing an avalanche of particles of various kinds are known as cosmic ray showers.
Image: In this picture we see the exact center of our galaxy, known as the Central Molecular Zone and purple, the radio arc of the galactic center. Besides its scientific interest, this image won first prize in the photographic AUI / NRAO in 2008. Credit: A. Ginsburg (U. Colorado - Boulder) et al. SPMO team, team GLIMPSE II.
Infrared image of our Galaxy
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This color infrared composite image from the center of the Milky Way reveals a new population of massive stars and new details in the complex structures in the hot ionized gas that swirls over a distance of 300 light years. This broad view is the clearest picture taken in infrared light on the galactic nucleus. It is a laboratory for understanding how massive stars shape and influence the environment of the nuclear regions, often violent, other galaxies. This image combines the NICMOS imaging (Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer) imaging with IRAC (Spitzer Space Telescope Infrared Camera). The galactic nucleus is obscured in the spectrum of visible light, but infrared light penetrates dust. NICMOS shows a large number of massive stars spread throughout the region. This view allows astronomers to see that massive stars are not confined to the three known groups of massive stars in the Galactic center (the core, the Arches cluster and the cluster of Quintuplet). In the NICMOS image, these three groups are seen as tight concentrations of massive stars.
The stars were scattered or formed individually or originate in clusters that have been disturbed by strong gravitational tidal forces. Winds and radiation from these stars form the complex structures observed in the nucleus, which in some cases, may trigger new generations of stars.
Image: Infrared view of the center of our Galaxy. The NICMOS mosaic was made from pictures of 2304 which required 144 rotations of the Hubble telescope around its orbit, between February 22 and June 5, 2008. Image Credits: Hubble: NASA, ESA, and QD Wang (University of Massachusetts, Amherst); Spitzer: NASA, Jet Propulsion Laboratory, and S. Stolovy (Spitzer Science Center / Caltech).nota: The Hubble Space Telescope (Hubble Space Telescope or HST) is in orbit 560 kilometers above sea level, it performs a full rotation of the Earth every 100 minutes.
Dust in the Milky Way
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Intertwined structures visible from the cold dust in our galaxy have been resolved on this image, infrared with a fineness of detail never reached by the European satellite Planck.
This image is a digital combination of three infrared images, both taken at high resolution by Planck, while the other is an older image taken by the former satellite IRAS. In this picture the red represents temperatures of about 10 degrees Kelvin above absolute zero. White corresponds to a heat of 40 degrees Kelvin.
The pink stripe at the bottom of the image corresponds to the gas confined in the plane of our galaxy.
The bright regions typically correspond to dense molecular clouds slowly collapsing on themselves to form stars. Colder regions are generally composed of diffuse interstellar gas and dust forming cirrus clouds.
Image: In this picture, we see the details of the cold dust in our galaxy, the Milky Way.
Credit: ESA, Planck HFI Consortium, IRAS