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A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. [1] [2] The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System.
Comparison of the night sky with the night sky of a hypothetical planet within the Milky Way 10 billion years ago, at an age of about 3.6 billion years and 5 billion years before the Sun formed. [261] Globular clusters are among the oldest objects in the Milky Way, which thus set a lower limit on the age of the Milky Way.
The listed objects currently include most objects in the asteroid belt and moons of the giant planets in this size range, but many newly discovered objects in the outer Solar System are missing, such as those included in the following reference. [58]
The observable universe contains as many as an estimated 2 trillion galaxies [36] [37] [38] and, overall, as many as an estimated 10 24 stars [39] [40] – more stars (and, potentially, Earth-like planets) than all the grains of beach sand on planet Earth. [41] [42] [43] Other estimates are in the hundreds of billions rather than trillions.
Compared to many extrasolar systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. [ 70 ] [ 71 ] The known Solar System lacks super-Earths , planets between one and ten times as massive as the Earth, [ 70 ] although the hypothetical Planet Nine , if it does exist, could be a super-Earth orbiting in the edge of ...
The following well-known stars are listed for the purpose of comparison. Antares (α Scorpii A) 680 [66] AD Fourteenth brightest star in the night sky. [67] Widely recognised as being among the largest known stars. [19] Betelgeuse (α Orionis) 640, [68] 764 +116 −62, [69] 782 ± 55 [70] AD & SEIS Tenth brightest star in the night sky. [67]
Second-brightest star in the night sky with a multiplanetary system after 7 Canis Majoris. All exoplanets orbit around star A in the binary system. 47 Ursae Majoris: Ursa Major: 10 h 59 m 27.97 s +40° 25′ 48.9″ 5.10: 46: G0V: 1.029: 5892: 7.434: 3: Planet Taphao Thong was discovered in 1996 and was one of the first exoplanets to be ...
The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology. [8] The virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at 1 trillion solar masses (2.0 × 10 42 ...