GALAXIES:

A galaxy is a huge mass of stars, nebulae, and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest galxies contain up to 3,000 billion stars. there are three main types of galaxy, classified according to their shape: elliptical, which are oval shaped; spiral, which have arms spiralling outwards from a central bulge; and irregular, which have no obvious shape. sometimes, the shape of galaxy is distorted by a collision with another galaxy. Quasar (quasi-stellar objects) are thoght to be galactic nuclei but are so far away that that thier exact nature is still uncertain. They are conpact, highly luminous objects in the outer reaches of the known Universe: while the furthest known "ordinary" galaxies are about 10 billion light years aways, the futhest known quasar is about 15 billion light years away. Active galaxies, such as Seyfert galaxies and radio galaxies, emit intense radiation. In a Seyfert galaxy, this radiation comes from the galactic nucleus; in a radio galaxy, it also comes from huge lobes on either side of the galaxy. The radiation from active galaxies and quasars is thought to be caused by black holes.



THE MILKY WAY:

The Milky Way is the name given to the faint band of light visible at night that stretches across the night sky. The name Milky Way is derived from Greek mythology, in which the band of light was said to be milk from the breast of the goddess Hera. Its hazy appearance results from the combined light of stars too far away to be distinguished individually by the unaided eye. All of the individual stars that are distinct in the sky lie within the Milky Way Galaxy or simply "the Galaxy".

The Galaxy is shaped like a spiral, with a dense central bulge that is encircled by four arms spiralling outwards and surrounded by a less dense halo. Our own solar system exists within one of the spiral arms, the Orion Arm (also called the Local Arm). From our position, the centre of the galaxy is completed obscured by dust clouds; as a result, optical maps give only a limited view of the Galaxy. However, a more complex picture can be obtained by studying, radio, infra-red, and other radiations. The central bulge of the Galaxy is a relatively small, dense sphere that contains mainly older red and yellow stars. The halo is a less dense region in which the oldest starts are situated; some of these stars may be as old as the Ga;axy itself (possibly 15 billion years). the spiral arms contain mainly hot, young, blue stars, as well as nebulae (clouds of dust and gas inside which stars are born). The Galaxy is vast, about 100,000 light years across (a light year is about 9,460 billion kilometres); in comparision, the Solar System seems small, at about 12 light hours across (about 13 billion kilometres). The entire Galaxy is rotating in space, although the inner stars travel faster than those further out. The Sun, which is about two-thirds out from the centre, completes one lap of the galaxy about every 220 million years.

This false-color image looks toward the center of the Milky Way, located 30,000 light-years away. Bright star clusters are visible along with darker areas of dust and gas.


References:
http://struckbyenlightning.files.wordpress.com/2009/10/milky-way-galaxy-sun.jpg
Book: DK Ultimate Visual dictionary

The Universe comprises everything that physically exists, the entirety of space time, all forms of matter and energy, and the physical laws and constants Universe may be used in slightly different contextual senses, denoting such concepts as the cosmos, the world, or Nature. and that govern them. However, the term Current interpretations of astronomical observations indicate that the age of the Universe is 13.73 (± 0.12) billion years, and that the diameter of the observable Universe is at least 93 billion light years, or 8.80 × 1026 metres. It may seem paradoxical that two galaxies on opposite sides can be separated by 93 billion light years after only 13 billion years, since special relativity states that matter cannot be accelerated to exceed the speed of light in a localized region of space-time. However, according to general relativity, space can expand with no intrinsic limit on its rate; thus, two galaxies can separate more quickly than the speed of light if the space between them grows. It is uncertain whether the size of the Universe is finite or infinite. According to the prevailing scientific model of the Universe, known as the Big Bang, the Universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable Universe was concentrated. Since the Planck epoch, the Universe has been expanding to its present form, possibly with a brief period (less than 10−32 seconds) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of the dark energy, and that most of the matter and energy in the Universe is fundamentally different from that observed on Earth and not directly observable. The imprecision of current observations has hindered predictions of the ultimate fate of the Universe. Experiments and observations suggest that the Universe has been governed by the same physical laws and constants throughout its extent and history. The dominant force at cosmological distances is gravity, and general relativity is currently the most accurate theory of gravitation. The remaining three fundamental forces and all the known particles on which they act are described by the Standard Model. The Universe has at least three dimensions of space and one of time, although extremely small additional dimensions cannot be ruled out experimentally. Spacetime appears to be smooth and simply connected, and space has very small mean curvature, so that Euclidean geometry is accurate on the average throughout the Universe. Conversely, on a quantum scale spacetime is highly turbulent. The word Universe is usually defined as encompassing everything. However, using an alternate definition, some have speculated that this "Universe" is just one of many disconnected "universes", which are collectively denoted as the multiverse. For example, in Bubble universe theory, there are an infinite variety of "universes", each with different physical constants. Similarly, in the many-worlds hypothesis, new "universes" are spawned with every quantum measurement. These universes are usually thought to be completely disconnected from our own and therefore impossible to detect experimentally. Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greeks,[citation needed] who proposed that the Universe possesses infinite space and has existed eternally, but contains a single set of concentric spheres of finite size – corresponding to the fixed stars, the Sun and various planets – rotating about a spherical but unmoving Earth. Over the centuries, more precise observations and improved theories of gravity led to Copernicus' heliocentric model and the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the characterization of the Milky Way, and the discovery of other galaxies and the microwave background radiation; careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology.

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