The galaxies

Galaxies  are sprawling system of dust ,gas ,dark matter ,and anywhere from a million to a trillion stars that are held together by gravity .Nearly all large galaxies are thought to also contain supermassive

black holes at their centers .In our own galaxies ,the milky way ,the sun is just one of about 100 to 400 billion stars that spin around Sagittarius a supermassive black hole that contains as much mass as four million suns .The deeper we look into the cosmos , the more galaxies we see .One 2016 study estimated that the observable universe contains two 

Continuation

Trillion or two million million galaxies. Some of those distant systems are similar to our own Milky way galaxy. Some of those distant system are similar to our own Milky way galaxy, while others are quite diffrent.

TYPES OF GALAXY

Before the 20th century, we didn't know that galaxies other than the Milky Way existed; earlier astronomers had classified them as as “nebulae,” since they looked like fuzzy clouds. But in the 1920s, astronomer Edwin Hubble showed that the Andromeda “nebula” was a galaxy in its own right. Since it is so far from us, it takes light from Andromeda more than 2.5 million years to bridge the gap. Despite the immense distance, Andromeda is the closest large galaxy to our Milky Way, and it's bright enough in the night sky that it's visible to the naked eye in the Northern Hemisphere.

GALACTIC  CLUSTERS AND MERGERS

Some galaxies occur alone or in pairs, but they are more often parts of larger associations known as groups, clusters, and superclusters. Our Milky Way, for instance, is in the , a galaxy group about 10 million light-years across that also includes the Andromeda galaxy and its satellites. The Local Group and its neighbor galaxy cluster, , both lie within the larger , a concentration of galaxies that stretches .The Virgo Supercluster, in turn, is a limb of Laniakea, an even bigger supercluster of 100,000 galaxies that astronomers defined in 2014.Galaxies in clusters often interact and even merge together in a dynamic cosmic dance of interacting gravity .