Galaxies

Galaxies are large systems of stars and interstellar matter, typically containing several million to some trillion stars, of masses between several million and several trillion times that of our sun, of an extension of a few thousands to several 100000s light years. typically separated by millions of light years distance. They come in a variety of flavors: Spiral, lenticular, elliptical and irregular. Besides simple stars, they typically contain various types of star clusters and nebulae..

We live in a giant spiral galaxy, The Milky Way Galaxy, of 100000 light years diameter and a mass of roughly a trillion solar masses, our sun is one of  several 100 billions of stars of the Milky Way. The nearest dwarf galaxies, satellites of the Milky way, are only a few 100000 light years distant , while the nearest giant neighbor, the Andromeda Galaxy (M31), also a spiral, is about 2-3 million light years distant.

There are probably more than 170 billion galaxies in the observable universe most galaxies are 1000 to 100000 parsecs in diameter and are usually separated  by distances on the order of millions of parsecs Intergalactic space is filled with a tenuous gas of an average density  less than one atom per cubic meter. the majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form lager groups called super clusters. these large structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

How big is the Galaxy

Although it is not yet understood, dark matter appears to account for around 90% of the mass of most galaxies. observational data suggests that super massive black holes may exists at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The milky way galaxy appears to harbor at least one such object within its nucleus.

At present, most star information occurs in smaller galaxies where cool gas is not so depleted. Spiral galaxies, like the Milky way only produce new generations of stars as long as they have dense molecular clouds of interstellar hydrogen in their spiral arms. Elliptical galaxies are already largely devoid of this gas, and so form no new star forming material is finite: once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years. as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects; brown dwarfs, white dwarfs that are cooling  or cold,neutron stars, and black holes. Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.