A Light Year (LY) is the distance light can travel in a year. The velocity of light (usually symbolized with "c") in a vacuum is 299,792,458 m/s by definition. This means a Light Year is 9,460,528,401,200 km. The Parsec is often used in astronomy instead of the Light Year. A Parsec (Parallax Second or PC) is 3.2616 LY and is the distance of an object from the sun such that the triangle whose base is the distance from the earth to the sun (called the Astronomical Unit, or AU) makes an angle of one second of arc at the object. This means that a Parsec is equal to the cotangent of one arc second, in Astronomical Units = 206264.8 AU.
Alpha Centauri is the closest star, but it is too far south in the sky to be seen from the continental United States. It is a system of three stars. The dimmest of those, Alpha Centauri C, or Proxima Centauri, in orbit around the other two, is presently the closest to the sun.
The visibly brightest star in the sky.
The Local Bubble is a spherical void space, probably cleared by an ancient supernova, surrounded by denser dust and gas, within which the sun and other local stars move. Counting down no further than magnitude 8 in brightness, the Local Bubble contains no less than 5000 stars. Other voids within the Orion Arm of the Galaxy abut on the Local Bubble. The Local Fluff is an area where some of the surrounding cloud has been blow into the Bubble, probably by an adjacent supernova. The sun lies on the near edge of the Local Fluff.
Open Clusters are young stars that have recently blown away the dust and gas from the nebula in which they condensed. In time the stars scatter and become isolated, like the sun. The Hyades take up a large part of the constellation Taurus and are close enough that they do not look conspicuously cluster-like to the naked eye. The Hyades are also just within the radius of the Local Bubble.
Many kinds of non-stellar objects have Messier numbers, from a list compiled by Charles Messier (1730-1817), who wanted to identify objects that might be confused with comets. Messier's list of 103 objects was published in 1781.
The Great Orion Nebula, covering about one degree of arc, is part of one the principal areas of star formation in the Orion (or Local) Arm of the Galaxy. Diffuse nebulae are large bodies of dust and gas. Bright diffuse nebulae scatter blue starlight or are excited by starlight to glow red. Dark nebulae can be identified by the way they block light from background stars, or bright nebulae, behind them. New stars condense within these nebulae. Some nebulae, although invisible to the naked eye, cover relatively large parts of the sky. The North America Nebula stretches across 2.5 degrees of arc, which is five times the diameter of the sun or the moon.
k is the abreviation for the metric or SI (Système International) prefix kilo, which means 1000. M abreviates Mega for a million, and G, Giga for a billion. MLY is thus millions of light years. M$ is a Megabuck, G$ a Gigabuck.
The most distant of the bright stars visible to the naked eye.
In the Sagittarius Arm of the Galaxy, the next arm in from the Orion Arm.
The "Double Cluster" in Perseus. One of the first objects to be recognized as belonging to a distinct spiral arm of the Galaxy, in this case the Perseus Arm, the next one out from the Orion Arm. NGC numbers refer to the New General Catalogue of non-stellar objects. This was J.L.E. Dreyer's 1888 update of John Herschel's 1864 General Catalogue and contains 7840 objects.
The capacity of Campus Center 205, if we measure 2 meters high (represent the 2000 LY thickness of the galaxy in our area), would be 137.7 billion cubic millimeters. If there is a star in our part of the Galaxy for about every four cubic light years, which would be 4 cubic millimeters to scale, that would mean that Campus Center 205 would hold about 34 billion, 435 million stars.
Globular Clusters are great, permanent, compact balls of stars, much larger (they can contain a million stars or more), older, and denser than Open Clusters, and are in orbit around the Galaxy. The distribution of Globular Clusters in the sky was the first solid evidence that the sun was not at the center of the Milky Way Galaxy.
Last glacial advance of the Pleistocene Epoch.
The Magellanic Clouds, although they look like detached pieces of the Milky Way, are irregular galaxies that are the closest separate members of the Local Group of galaxies.
All present mitochondrial DNA in humans appears to decend from one human female who lived at this time. This means that we are related to the other females only through their male descendants.
Or the Great Spiral Nebula in Andromeda: a spiral galaxy that is the near twin and companion of the Milky Way in the Local Group. The bright nucleus of the galaxy is the most distant object that can be seen with the naked eye. The whole galaxy stretches across at least 3 degrees of arc in the sky, six times the apparent diameter of the sun or the moon.
The velocity of light can be used to measure radial distance in the universe because of the velocity the expansion of the universe gives to all distant objects. Here the expansion is up to a thousanth of c.
The conspicuous cluster of galaxies in the constellation Virgo, with the giant elliptical galaxy M 87 at the center, is the heart of the Local (or Virgo) Supercluster. The Coma-Sculptor Cloud, containing the Local Group, the Milky Way, etc., is gravitationally part of the Virgo Supercluster.
At this range clusters of galaxies start becoming more conspicuous than individual galaxies. Several are listed here with George Abell's catalogue numbers.
Everything up to this point, at a radius of 1% c/H (The Hubble Radius, see below), contains only a millionth of the volume of the universe.
The Red Shift from the Doppler Effect is how we actually know about the radial velocity of recession, and distance, of galaxies. The Red Shift (z) is the change in a wavelength of radiation () from the object divided by what the original wavelength was (/). This can be related to velocity (u) as: (z+1)=(c+u)/(c-u) or u/c=(z+2z)/(z+2z+2). Distance (s) then depends on Hubble's Law: u=sH, where the Hubble Constant (H) is taken to be 75 km/s/MPC (it is somewhere between 50 and 100 km/s/MPC). 1/H is the Hubble Time, which for the given H is 13.04 Gy. c/H, the Hubble Radius, is thus 13.04 GLY. At low values, z is virtually identical to u/c. This is the point where the two values begin to diverge. u/c cannot be larger than 1, but z can be any number up to infinity (at the speed of light).
Many nearby galaxies, including our own, are headed towards a concentration of mass equal to 100,000 Milky Ways, dubbed The Great Attractor. Abell 3627, which matches The Great Attractor in direction and distance, has recently been suggested as a candidate, though its visible mass is only about 10% of what it would need to be.
Everything up to this point, at a radius of 10% c/H, contains only a thousanth of the volume of the universe.
The oldest Earth rocks date the beginning of this aeon.
Quasars (Quasi-Stellar Objects, or QSO's) are among the most mysterious objects in the universe. Small, distant, and incredibly bright, their energy output (a mass the size of the moon is turned into energy every second) and their absence from nearby space are largely unexplained. Some quasars appear to be in the middle of galaxies, and it is theorized that they all are. Their strangeness leads some astronomers to argue that they are really not so distant and that their red shift is from some other effect, but that theory hasn't work out very well. 3C [the Third Cambridge Catalogue of Radio Sources] 273 is actually one of the intrinsically brightest Quasars, and it is relatively close, as close as many observable galaxies. If we were to bring it merely as close as the star Arcturus, it would appear as bright as the sun in the sky.
Begins with the origin of the Earth and the Moon.
Closed universes, which will collapse upon themselves, cannot be older than this.
The most distance visible galaxy, now observed as part of cluster.
Finite universes in their entirety and the observable parts of infinite universes exist within the Hubble Radius, as the Big Bang must have occurred within the Hubble Time.