Galaxies

                PTR SCIENCE

                   ★ Galaxies

Millions of star together make a galaxy and several million galaxies make up a universe. The solar million including planet earth, also belongs to a galaxy called the Milky Way or Dugdh Mekhla. 

Galaxies are sprawling systems of dust, gas, dark matter and anywhere from a million to a trillion stars that are cohered by gravity. Proximately all sizably voluminous galaxies are thought to withal contain supermassive ebony apertures ta their centers. In our own galaxy, the Milky Way, the sun is just one of about 100 to 400 billion stars that spin around Sagittarius A*, a supermassive ebony aperture that contains as much mass as four million suns. 

★ What kinds of Galaxies Are There?

Astronomers relegate galaxies into three major categories: elliptical, spiral and Irregular. These galaxies span a wide range of sizes, from dwarf galaxies containing as few as 100 million stars to giant galaxies with more than a trillion stars.
Elliptical, which account for about one-third of all galaxies vary from proximately circular to very elongated. They possess comparatively little gas and dust, contain older star are not actively composing stars anymore. The most immensely colossal and most infrequent of these, called giant ellipticals, are about 30,00,000 light-years across. Astronomers theorize that these are composed by the mergers of more minute galaxies. Much more mundane are dwarf ellipticals, which are only a few thousand light-years wide.
Spiral galaxies appear as flat, blue-white risks of stars, gas and dust with yellowish bulges in their centers. These galaxies are divided into two groups: mundane spirals, the bar of stars run through the central bulge. The arms of barred spirals conventionally start at the terminus of the bar in lieu of from the bulge. Spirals are actively composing stars and comprise an astronomically immense fraction of all the galaxies in the local macrocosm.

Anomalous galaxies, which have very little dust, are neither disk like nor elliptical. Astronomers often visually perceive aberrant galaxies as they peer deeply into the macrocosm, which is identically tantamount to looking back in time. These galaxies are abundant in the early macrocosm, afore spirals and elliptical developed.
Aside from these three classic categories, astronomers have withal identified many eccentrically shaped galaxies that seem to be in a transitory phase of colliding or interacting, and those with active nuclei ejecting jets of gas.

★Type of Galaxies 

Having established the subsistence of other galaxies, Hubble and others commenced to visually examine them more closely-noting thier shapes, their contents, and as many other properties os they could quantity. This was a daunting task in the 1920s when obtaining a single photograph or spectrum of a galaxy could take a full night of tireless optically canvassing. Today, more sizably voluminous telescopes and electronic detectors have mode this task less arduous, albeit optically canvassing the most distant galaxies (those that show us the macrocosm in its earliest phases) still requires gargantuan effort.
Virtually all current systems of galaxy relegation are outgrowths of the initial scheme proposed by the American astronomers Edwin Hubble in 1926. In Hubble's scheme, which is predicated on the optical appearance of galaxy images on photographic plates, galaxies are divided into three general classes ellipticals, spirals, and irregulars. Hubble subdivided these three classes into finer groups.