What is the definition of a Galaxy?
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| Majestic spiral Galaxy NGC 4414 |
A galaxy is a huge mass of stars, nebulae, and interstellar material. The smallest galaxies contain about 100,000 stars, while the largest contain up to 3000 billion stars.
There are three main types of galaxies classified according to their shape; elliptical, which are oval-shaped, spiral, which have arms spiraling outwards from a central bulge (those whose arms spiral from a bar-shaped bulge are called spirals) and irregular,
which has no obvious shape?
Sometimes, the shape of a galaxy is distorted by a collision with another galaxy. Quasars are thought to be galactic nuclei but are so far away that their exact nature is still uncertain. They are compact, highly luminous objects in the outer reaches of the known universe: while the furthest known ordinary galaxies are about 12 billion light-years away, the furthest known Quasar is about 13 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 material falling into the central black holes.
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Credits: NASA, Hubble Team
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