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A family of stars, held together by their mutual gravitational attraction, and with a distinct identity separating it from other galaxies. Galaxy is a vast ensemble of hundreds or thousands of millions of stars, all gravitationally interacting, and orbiting about a common center. All the stars visible to the unaided eye from Earth belong to our own galaxy, the Milky Way. The Sun is just one star in this galaxy. Besides individual stars and their attendant planets, galaxies contain clusters of stars and nebulae--vast clouds of gas and dust from which new stars condense
Galaxies cover a huge range of size and mass as well as exhibiting a variety of structures and properties. The smallest galaxies known are relatively nearby dwarf galaxies containing only 100,000 stars, fewer than in a typical globular cluster . At the other end of the scale, the most massive galaxy known, the giant elliptical M87 , contains 3,000    billion solar masses, about 15 times more than our own galaxy 


Our Galaxy is a spiral, possibly a mildly barred spiral galaxy, containing of the order of two hundred billion stars as well as much interstellar matter, both dark and luminous. It is disc-shaped with an almost spherical bulge at the centre. The disc is 100,000 light years across, but much of its content is concentrated in a thin layer only 2,000 light years thick towards its outer edges, though stars are distributed through a somewhat thicker disc. The central bulge has a radius of about 15,000 light years. Studies of the dynamics of stars and interstellar material suggest that the luminous material we can see accounts for as little as 10 per cent of the total mass of the Galaxy. The rest is so-called dark matter, in a form not yet identified. The spiral arms are concentrations of stars and interstellar material appearing to wind outwards from the edge of the bulge. Regions of star formation and ionized hydrogen are concentrated in the arms. In the space between the arms, the average density of matter is a factor of two or three lower than within the arms. The Sun is located about 28,000 light years from the galactic centre, within the disc, near the inner edge of a spiral arm. The whole Galaxy is in rotation, but not as a rigid body, so it is constantly deforming. The Sun takes about 220 million years to complete a circuit, but stars nearer the centre take shorter times. Centred on the nucleus is a sparsely populated, roughly spherical region, with a radius of at least 50,000 light years, known as the galactic halo. The halo contains globular clusters and, in general, the oldest stars in the Galaxy. There is very little luminous matter in the halo compared with the disc and central bulge, though gravitational studies suggest that the invisible component of the Galaxy's mass is probably distributed in a sphere around the Galaxy rather than concentrated in the disc. This dark matter is thought to extend up to 300,000 light years, far beyond the halo defined by visible objects, into what is sometimes termed the galactic corona.
The innermost nucleus, lying in the direction of the constellation Sagittarius, is concealed from direct optical observation by dense opaque dust. However, observations in the infrared and radio regions of the spectrum, and at gamma-ray and X-ray wavelengths, suggest that the core contains a tightly packed sphere of stars and a
black hole

Milky way A band of hazy light circling the sky. It results from the combined light of vast numbers of stars in our own Galaxy. The term Milky Way is also used as a synonym for the Galaxy.
The band of light around the celestial sphere is the disc of the Galaxy viewed from within. The Sun is situated two-thirds of the way out towards the edge of the galactic disc, and the Milky Way appears brightest in the direction of the bulge around the galactic centre, which lies in the constellation Sagittarius.


Our Galaxy



Around the center of our Galaxy are found huge numbers of stars that are faint and often billions of years old, as ancient as the Galaxy itself. These stars are seen with the unaided eye as the star clouds of the southern Milky Way, especially rich in Sagittarius. Our line of sight is strongly affected by dust, which reveals itself by both dimming the starlight passing through it and by selectively removing the blue component, changing the color to a deeper yellow. Appropriately, near the center of the picture, is a region known as Baade's Window after the astronomer who selected this part of the Milky Way for special study     

The Sagittarius Dwarf galaxy is the nearest galaxy to the Milky Way Galaxy. This small galaxy is so close, it is being swallowed up by the Milky Way. It lies 80,000 light years from the Sun and 52,000 light years from the centre of the Milky Way. The next nearest galaxy is the Large Magellanic Cloud, 170,000 light years away, which was thought to be the nearest galaxy until 1994, when the Sagittarius Dwarf was discovered.
Originally it would have been a sphere of stars about 1000 light years across. But now it has been distorted and stretched by the gravitational pull of the Milky Way and is about 10,000 light years long. The few million stars that belong to the Sagittarius Dwarf are scattered over a patch of sky in the constellation Sagittarius. Just by looking it is impossible to tell them from stars in our own Galaxy.  


Future of Universe

The universe was born in a gigantic explosion called the big bang and is still expanding today. Its ultimate fate depends on how much matter it contains. Gravitation the pull of each piece of matter on every other--is slowing the expansion. If there is enough matter in the universe (more than the so-called "critical density"), the expansion will eventually halt and then reverse. Everything in the universe will fall together and be crushed, the reverse of the big bang. In these circumstances, the universe is said to be closed. It is not possible to say how far in the future the big crunch would be.If the universe is of less than the critical density, it is said to be open, and it will carry on expanding forever. About a million years from now, all star-making material will have been used up, and from then on galaxies will start to fade as stars die and are not recycled. Some stars will end up as black holes, others as cold balls of matter, in which, over enormous periods of time of 1033 years or more--even the protons may decay into radiation and positrons (the positive counterparts to electrons). Neutrons also decay, into electrons and protons, so that ultimately all of this matter will have been converted into radiation and electrons and positrons, which will annihilate one another to leave more radiation. Black holes also "evaporate" eventually, emitting radiation as they do so. Nothing would be left in an open universe but radiation.

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