The galaxies that trip the light fantastic in Spacetime are usually members of groups, clusters, and superclusters–with groups being considerably smaller than clusters, and superclusters being much larger and heavily populated than clusters. Our own barred-spiral Milky Way Galaxy is a member of the Local Group that hosts more than 54 galaxies, most of them relatively small galactic dwarfs. Our Milky Way and another immense spiral galaxy named Andromeda are the largest constituents of the Local Group, and the twirling pair resemble enormous starlit pin-wheels in Space.
According to recent studies, our Milky Way and Andromeda reside in what is called the green valley of the Galaxy Color-Magnitude Diagram. The green valley of this diagram refers to galaxies currently in the process of evolving from the blue cloud (galaxies actively giving birth to new baby stars) to the red sequence (galaxies that no longer give birth to new stars). Starbirth activity in green valley galaxies is in the midst of slowing down. This is because the galaxies are running out of star-birthing gas floating around in the interstellar medium. For simulated galaxies that sport similar attributes to our Milky Way and Andromeda, starbirth will normally be extinguished within approximately five billion years from the present–even with the predicted, brief increase in the rate of stellar formation resulting from the collision that will occur between the Milky Way and Andromeda about 5 billion years from now. Our Milky Way and Andromeda actually make up a binary system of giant spiral galaxies inhabiting the Local Group. Measurements of other galaxies similar to our own indicate that it is among the reddest and brightest spiral galaxies still able to give birth to sparkling, fiery new baby stars–and it is also only slightly bluer than the bluest red sequence galaxies.
The Local Group of galaxies is surrounded by the Local Void, and the entire galactic group is a constituent of the Virgo Supercluster. Encircling the Virgo Supercluster are a number of voids, which are barren regions that are almost, though not entirely, bereft of galaxies. The Microscopium Void is situated to the “north”, the Sculptor Void to the “left”, the Bootes Void to the “right” and the Canis-Major Void to the South. In addition, these black, cavernous voids are shape-changers. Over time, these sparsely populated regions create filamentous structures of galaxies. For example, the Virgo Supercluster is being gravitationally pulled towards the Great Attractor, which in turn is part of the immense Laniakea Supercluster.
A duo of smaller galaxies as well as a number of dwarf galaxies inhabiting the Local Group are in orbit around our Milky Way. The largest of these is the Large Magellanic Cloud (LMC) that sports a diameter of 14,000 light-years. The LMC has a smaller close companon, the Small Magellanic Cloud (SMC).
Ever since the first generation of stars were born in the Universe, the Milky Way has been growing in size as a result of both galaxy mergers (early in the Milky Way’s formation) and the accretion of gas directly from the Galactic halo. The Milky Way today is is accreting material from several small galaxies, including both the LMC and SMC. Indeed, certain attributes displayed by our Milky Way, such as angular momentum, stellar mass, and metallicity in its outer limits, indicate that it has experienced no mergers with large galaxies in the last 10 billion years. The term metallicity, as used by astronomers, refers to all atomic elements heavier than helium. The Big Bang produced only hydrogen, helium, and traces of lithium–but the stars are responsible for all atomic elements heavier than helium. The lack of recent major mergers is considered to be rather odd among similar spiral galaxies. In fact, the neighboring large spiral galaxy Andromeda appears to have experienced a more typical history. Andromeda has apparently been shaped by more recent mergers with relatively large galaxies than our Milky Way.
Recent calculations of the number of galaxies inhabiting the observable Universe range from 200 billion to a trillion–or more. The galaxies of the Universe host more stars than all of the grains of sand on Earth! Most of the galaxies are about 3000 to 300,000 light-years in diameter, and are separated by distances of millions of parsecs (megaparsecs). By comparison, our Galaxy sports a diameter of at least 100,000 light-years and is separated from the Andromeda Galaxy by 2.5 million light-years.
The space between galaxies inhabiting the visible Universe is brimming with a tenuous gas (the intergalactic medium). The intergalactic medium has an average density of less than one atom per cubic meter.
The galaxies of the Universe formed very long ago, and began their ancient dance in the primordial Cosmos less than a billion years after the Big Bang. The most widely accepted theory of galactic formation among astronomers is called the bottom-up theory. This theory basically proposes that that the large galaxies of the Universe–like our Milky Way and Andromeda–only slowly attained their vast sizes as the result of ancient mergers between much smaller, amorphous protogalactic blobs. The galaxies themselves trace out an enormous and mysterious web-like structure called the Cosmic Web. The Cosmic Web is the largest-scale structure in the Universe, and it is woven of filaments composed of the weird, invisible dark matter. The starlit galaxies that do their fantastic celestial ballet in Spacetime congregate together in groups and clusters, and their starry constituents outline with their stellar fires that which we are not able to see.
In the primeval Cosmos, these opague shapeless clouds of mostly hydrogen gas collided and merged together along the massive filaments woven of the mysterious, invisible dark matter. The ancient “seeds” from which today’s large galaxies eventually emerged were strung out along the transparent filaments like dewdrops collecting on the web of a hidden spider. 720p mkv movie download