Galaxies were initially discovered telescopically and were known as spiral nebulae. Both the Local Group and the Virgo Supercluster are contained in a much larger cosmic structure named Laniakea. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The group is part of the Virgo Supercluster. The Milky Way is part of the Local Group, which it dominates along with Andromeda Galaxy. Most galaxies are gravitationally organized into groups, clusters and superclusters. The space between galaxies is filled with a tenuous gas (the intergalactic medium) with an average density of less than one atom per cubic meter. For comparison, the Milky Way has a diameter of at least 26,800 parsecs (87,400 ly) and is separated from the Andromeda Galaxy (with diameter of about 152,000 ly), its nearest large neighbor, by 780,000 parsecs (2.5 million ly.) Most galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3,000 to 300,000 light years) and are separated by distances on the order of millions of parsecs (or megaparsecs). It is estimated that there are roughly 200 billion galaxies ( 2 ×10 11) in the observable universe. It has a comoving distance of 32 billion light-years from Earth, and is seen as it existed just 400 million years after the Big Bang. As of March 2016, GN-z11 is the oldest and most distant galaxy observed. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. Many are thought to have supermassive black holes at their centers. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Supermassive black holes are a common feature at the centres of galaxies.
Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. The word is derived from the Greek galaxias ( γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System.
Dwarf galaxy pairs hubble space telescope images Patch#
However, it seems that our nearby patch of the Universe may not contain any galaxies that are currently undergoing their first burst of star formation.Ī version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Nick Rose.NGC 4414, a typical spiral galaxy in the constellation Coma Berenices, is about 55,000 light-years in diameter and approximately 60 million light-years from Earth.Ī galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. Dwarf galaxies which contain very few of the heavier elements formed from having several generations of stars, like SBS 1415+437, remain some of the best places to study star-forming processes similar to those thought to occur in the early Universe. Dwarf galaxies like this are thought to have formed early in the Universe, producing some of the very first stars before merging together to create more massive galaxies. SBS 1415+437 is an interesting target for another reason. It is possible that dwarf galaxies undergo a star formation cycle, with bursts occurring repeatedly over time. However, it is still unclear whether all dwarf galaxies experience starbursts as part of their evolution. They have been seen in gas-rich disc galaxies, and in some lower-mass dwarfs. Starbursts are an area of ongoing research for astronomers - short-lived and intense periods of star formation, during which huge amounts of gas within a galaxy are hungrily used up to form newborn stars. Astronomers initially thought that SBS 1415+437 was a very young galaxy currently undergoing its very first burst of star formation, but more recent studies have suggested that the galaxy is in fact a little older, containing stars over 1.3 billion years old. This particular dwarf is well studied and has an interesting star formation history. It is type of galaxy known as a blue compact dwarf. The bright streak of glowing gas and stars in this NASA/ESA Hubble Space Telescope image is known as PGC 51017, or SBSG 1415+437.
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