Spiral Galaxies Facts and Info

Spiral Galaxies:-
                 
Spiral galaxies get their name by the fact that they have a spiral or whirl shape. The "arms" of the spiral are the areas where stars are formed. These arms appear as waves. The galaxy we live in, the Milky Way, is a spiral galaxy. Our planet and Sun lie in the arms of the Milky Way. If you can clearly see the spiral shape, the galaxy is called a "face-on spiral." If you instead see the galaxy from the side, it is called an "edge-on spiral." You can recognize edge-on spiral galaxies because you can see their bright central bulges. Face-on and edge-on spiral galaxies aren't really any different; they only look different because of the angle from which you see them.

galaxy that exhibits a central nucleus or barred structure from which extend curved arms of higher luminosity —called Spiral Galaxy. Spiral galaxies may remind you of a pin-wheel. They are rotating disks of mostly hydrogen gas, dust and stars. Through a telescope or binoculars, the bright nucleus of the galaxy may be visible but the spiral arms are dimmer and difficult to see. The range of masses for spiral galaxies is ~ 10^9 – 10^12 solar masses, with the typical mass being ~ 10^11 solar masses. Spiral galaxies are rich in gas and dust, which is often visible as lanes of dust when viewed from the "top" or "bottom", and as layers of dust when viewed from the side. In our own Milky Way the center of the galaxy is invisible from our vantage point because the interstellar dust between us and the center (in Sagittarius) is so thick.


Spirals have a bulge and a disk with a spiral structure. A big bulge with tightly wound arms corresponds to a Sa; a small bulge with loosely wound arms is an Sc – pay attention to the length and tightness of the spiral arms and to the relative size of the bulge. Sometimes you might even be able to determine intermediate cases; for example a Sbc is intermediate between a Sb and Sc. If there is a bar, you might have a SBb, or a SBc. Do not confuse spiral arms with tidal arms (a gravitational influence of a close neighbor).

In the more modern view, spiral galaxies are decomposed into a bulge, a “thin disk” and a “thick disk”. The thick disk may be composed of accreted material, or it may be an older thin disk population that has been “puffed up” by the action of accreted satellite galaxies. Still another view has the thick disk being the simple extension of the bulge population.

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