4. INTRODUCTION Galaxy ,a vast ensemble of hundreds or thousands of millions of stars, all gravitationally interacting, and orbiting about a common centre. Galaxies also contain: clusters of stars; atomic hydrogen gas; molecular hydrogen; complex molecules composed of hydrogen, nitrogen, carbon, silicon and cosmic rays.
5. The Formation of Galaxies Theoretical investigations indicate that galaxies formed from a diluted but lumpy mixture of hydrogen and helium gas - the primordial elements forged in the Big Bang. They also indicate that two vastly different scales of mass prevailed less than 100 million years after the Big Bang, which ultimately affected the formation of galaxies. (See the later discussion of dark matter and the formation of structure .)
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7. Elliptical Galaxies These type of galaxies contain a population of old stars, usually with little apparent gas or dust, and few newly formed stars. Elliptical galaxies come in a vast range of sizes, from giant to dwarf.
8. Spiral Galaxies Spiral galaxies are flattened discs containing not only some old stars but also large populations of young stars, much gas and dust, and molecular clouds that are the birthplaces of stars.
9. Properties Of Spiral Galaxies Stars and gas clouds orbit about the centre of their galaxy. In spiral galaxies, the stars move in circular orbits, with velocities that increase with increasing distances from the centre. At the edges of spiral discs, velocities of 300 km/s have been measured at distances as great as 150,000 light years.
10. Irregular Galaxies Approximately 3% of galaxies observed cannot be classified as either ellipsoidal or spirals. These galaxies have little symmetry in their structure and are termed irregular galaxies . An example is Sextans A. This irregular galaxy is a member of the Local Group, at a distance of about 10 million light years .
12. Properties of Irregular Galaxies Irregular galaxies have masses in the range 10 8 to 10 10 solar masses, diameters from 1 to 10 kpc, and blue magnitudes from -13 to -20. Other than that, they have few systematic features.
28. What is a black hole? The term "black hole" describes a location in space with extremely high gravity. There is no hole per se, just a lot of gravity. The gravity is so strong that nothing can escape its pull, not even light, so it would appear black. A rocket merely travels at 25,000 miles an hour to escape our planet's pull. That rocket could travel 671 million miles an hour, the speed of light, and still not escape from a black hole. Since nothing can travel faster than light, nothing can escape a black hole once it is in its clutches. BLACK HOLE
29. Black holes have a boundary of no return. Once an object crosses the boundary, called the event horizon, the object is theoretically lost forever. (No light can escape, hence no information about the object's whereabouts can escape.) Outside of the event horizon, or even close to the edge, an object still has a fighting chance to escape. The gravity is strong there, but light can get away. Within the event horizon, the game is over. All matter is infinitely stretched and, at the very center of the black hole, it is crushed to what physicists call a singularity. A singularity is a mathematical term to characterize matter infinitely compressed to a single point.
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31. These low-mass black holes form from collapsed stars that were once 100 times as massive as our Sun. When such a massive star runs out of energy, it throws off its outer shell and is left with a core that is still 15 times as massive as the Sun. The incredible mass collapses further to a region only a few miles across. This is this black hole, and the event horizon -- that boundary of no return -- is a few miles across . LOW-MASS BLACK HOLES
32. Low-mass black holes are more common than you might think. Scientists believe that our Milky Way Galaxy contains tens of thousands of these black holes, hence the name "galactic" black hole. They tend to live in binary star systems, with a "living" companion star losing healthy portions of its gas to the black holes extreme gravity. Scientists can see this gas as it funnels toward the black hole.
33. The large black holes are called supermassive black holes. They have the mind-boggling mass of one million to one billions Suns confined in a region no larger than our Solar System. This region marks out the event horizon. Once an object crosses the event horizon... well, you know. Singularity. Supermassive black holes reside in the center of galaxies.
34. Some scientists speculate that most galaxies, including our own, harbor a supermassive black hole. The theory is that these large-mass black holes either formed from collapsed gas when the host galaxies formed, or they formed later, growing with time as the result of galaxy and black hole mergers. The supermassive black hole is the source of large amounts of energy. All of the gas that pours into a black hole puts up quite a fight. The gas, under the force of extreme gravity, heats to millions of degrees and releases bright light across most of the electromagnetic spectrum, from radio waves through optical light and into X-ray light.
35. This is matter's final cry before it is lost forever. Some black holes produce enormous jets of matter shooting away for millions of miles at nearly light-speed via a poorly understood mechanism. We cannot see the black hole, but we can see all the fireworks shooting out from the center of galaxy with a black hole. The brightest of these galaxies are called quasars, all at the farthest reaches of the Universe and all powered by black holes. The mid-size black holes don't have a catchy name yet. This is because they were just discovered, and the scientific jury hasn't reach a decision yet as to what these may be. The massive, compact objects are 100 to 10,000 times as massive as our Sun. They are most likely produced by black hole mergers. They are also much closer than quasars, residing in several nearby spiral galaxies.
36. How do we "see" black holes? One unavoidable challenge when studying black holes is that they're almost invisible. Depending on its mass and rate of spin, a black hole exerts a noticeable influence on its surrounding environment. Stellar black holes are often part of a binary star system, two stars revolving around each other. It is orbiting around the black hole. We can infer the mass of the black hole by the way the visible star is orbiting around it. The larger the black hole, the greater the gravitational pull, and the greater the effect on the visible star.
37. Another way we can "see" a black hole is by observing X-rays generated around it. Because a black hole has such a powerful gravitational force, it can capture material that wanders nearby, like a spider that lives hidden until an unlucky insect stumbles into its nest. A galactic black hole in a binary system can literally tear apart its companion star. Gas from the companion swirls into the black hole like water down a drain. The swirling gas is called an accretion disk. As the gas gets closer to the black hole, it heats up from the friction of ever-faster moving gas molecules. Just outside the black hole's event horizon, the gas heats to temperatures in the range of millions of degrees. Gas heated to these temperatures releases tremendous amounts of energy in the form of X-rays.
38. PROJECT MADE BY SACHIN GUPTA AMBUD SHARMA XII SCIENCE 2001-2002 THANK YOU
