Glossary

Astronomical Unit (AU):: The average distance from the Sun to the Earth, 150 million km (93 million miles).
Binary Star:: One of a pair of gravitationally bound stars orbiting about a common center of gravity.
Black Hole:: For an object to break free of the gravitional grip of a planet, star or other body it must reach the so-called escape velocity which depends on the mass and density of the body concerned. For Earth the escape velocity 11.2 km/s (25,000 mph) and for the Sun 618 km/s (1.38 million mph). The crushing force of gravity can create bodys that are so massive and dense that the escape velocity exceeds the speed of light 300,000 km/s (670 million mph). This is a black hole from which nothing, not even light, can escape. Black holes can be created from the supernova explosion of a massive star. Most galaxies host a supermassive black hole at their cores with masses of 100,000 to several billions of times the mass of the Sun.
Globular Cluster:: A gravitationally bound spherical cluster containing tens of thousands to millions of stars with the concentration of stars increasing toward the center. Globular clusters tend to inhabit the outskirts of galaxies and are some of their very oldest constituents. Our own Milky Way galaxy is home to around 150 globular clusters but other galaxies can have many more.
Hydrogen alpha (Hα) Emission:: Hydrogen is the most abundant substance the universe and forms vast gas clouds in interstellar space. In the presence of a suitable energy source, such as a very hot star, the hydrogen will emit light at specific wavelenths. The dominant visible one is a deep red color known hydrogen alpha (Hα). Special narrowband filters are designed to transmit only Hα light and block all other wavelengths. This enhances the visiblity of Hα regions and helps to reduce the impact of light pollution but distorts the colors of stars. Other elements in space also emit light of characteristic colors. For example oxygen glows with a blue-green hue known as Oxygen III (OIII).
Kuiper Belt:: The doughnut-shaped ring of icy bodies lying beyond the orbit of Neptune from 30 to 50 AU from the Sun. The dwarf planet Pluto is a Kuiper Belt object (KBO) and thousands of others have been discovered since 1992, It is believed that there may be 100,000 KBOs over 100 km (62 miles) in diameter. The Kuiper belt is the source of short-period comets with orbits close to the plane of the solar system.
Light Year:: The distance light travels in a year. The speed of light is 300,000 km per second (186,000 miles per second) meaning that in one year it travels 9.47 trillion km (5.87 trillion miles). For scale, the Earth lies around 150 milllion km (93 million miles) from the Sun and light takes 8.3 seconds to traverse the distance.
Magnitude:: The visual brightness of a star or planet is specified by its magnitude. The 19 brightest stars in the sky are first magnitude or brighter. A 1^st magnitude star is 2.512 (= 100^⅕) times brighter than a 2^nd magnitude star which is 2.512 times brighter than a 3^rd magnitude star and so on. A 6^th magnitude star is 100 times fainter than a 1^st magnitude star.
Under the dark skies of Eustis someone with good eyesight should be able to see stars of magnitude 6.5 and perhaps fainter. Long exposure photographs taken with a 5 inch telescope from Eustis Ridge routinely reach beyond 20^th which is 250,000 times fainter than the human eye can see.
Neutron Star:: When a massive star ends its life in a supernova explosion what remains is a neutron star. Unchecked by the outward pressure of nuclear fusion the remaining matter contracts under its own enormous gravity forcing the protons and electron in ordinary matter to combine to form neutrons giving rise to a massive ball neutrons. This object has a radius of around 10 km but contains from 1.4 up about 2.9 times the mass of the Sun (M_☉). This means that a teaspoonful of neuton star matter would have a mass of a few billion tonnes. The neutron star conserves its angular momentum from the rotation of the progenitor star and because of its reduced radius its spin rate greatly increases with the fastest known rotating 716 times per second. Some neutron stars can be detected as pulsars.
O-Type Star:: O-type stars are hot blue-white stars with searing surface temperatures of 32,000 to 50,000 Kelvin and range from 10,000 to 1,000,000 times the luminousity of the Sun. Their masses however do not exceed 200 solar masses. O-Type stars live short lives, of order a few million years or less, that end in a violent supernova. Although rare, they can be seen over vast distances with 4 out of the 71 brightest stars visible from Earth being of this type. O-type stars are typically found in regions of active star formation and are largely responsible for the bluish-white and pink color in the spiral arms of galaxies.
OB Association:: A loose cluster of stars of spectral type O and B. The Double Cluster in Perseus is an example of an OB association in our own Milky Way galaxy. Although large and hot, B-Type stars are the modest cousins of O-types with temperatures from 10,000 to 30,000 Kelvin and luminosities roughtly between 75 and 45,000 that of the Sun and masses up 18 solar masses. The star Regulus, α Leonis, is an example of the B spectral type.
Open Cluster:: A loose collection of tens to thousands of stars that form from the same giant molecular gas cloud. Stars in the cluster are loosely-bound gravitationally and gradually disperse generally on timescales of a few hundred million years although more massive examples may persist for a few billion.
Parsec (pc):: A parsec is a unit of distance commonly used by professional astronomers. It is 3.261 light years or 30.9 trillion km (19.2 trillion miles). A parsec is given the symbol pc. A megaparsec is a million parsecs and is denoted Mpc.
Planetary Nebula:: A glowing shell of gas cast off by a dying star going through its red giant phase before shrinking down to become a white dwarf. They are named for theirs appearance through a telescope which resembles the disk of a planet.
Pulsar:: Neutron stars exhibit extremely strong magnetic fields that create a hot spot at the magnetic poles that emit lighthouse-like beams of electromagnetic radiation. The beams rotate with the neutron star and should they sweep over if Earth we observe regular pulses across the full electromagnetic spectrum. The first pulsar was discovered in 1967 by its regular radio pulse every 1.337 seconds.
Quasar:: Short for quasi-stellar object. These appear like stars but are actually the bright centers of distant galaxies. Most galaxies are believed to harbor a black hole at their center. As gas and other matter is drawn in toward the black hole it is compressed and heated by friction to millions of degrees. The glow from the heated gas is far outshines all the stars of the galaxy and across the vast void of space we see a quasar.
Red Giant:: Young stars shine by converting hydrogen (H) in their core to helium (He) via thermonuclear fusion at extreme temperatures and pressures. This process procduces excess energy. For example, Sun's core temperature is around 15 million K (27 million °F). As its supply of hydrogen is used up the star's core temperature and pressure rise causing the remaining hydrogen to fuse at an evermore furious rate. The resulting increase in energy output causes the star's outer layers to expand and cool producing a red giant. In around 5 billion years this will be the fate of our own Sun which will expand to consume the inner planets, Mercury and Venus, and possibly the Earth itself. At the end of its red giant phase the Sun will contract to become a white dwarf.
Redshift:: The wavelength of sound waves emitted by an object moving away from an observer are stretched out due to the Doppler effect and are heard at a lower pitch than they were emitted at. A similar effect occurs in the light rays emitted by an object moving away from us. This increases the observed light's wavelength which shifts its color toward the red end of the spectrum, hence the name redshift. As the universe is uniformly expanding, most galaxies are receding from us at a speed that is proportional to their distance. The constant of proportionality is known as the Hubble Constant, H₀. The amount by which a galaxy's light is redshifted therefore provides a measure of its distance. (This is a somewhat simplistic view of the origin of redshift but us adequate for a general understanding.)
Reflection Nebula:: A reflection nebula is an interstellar cloud of dust and gas illuminated by star light that is not intense enough to ionize the gas but is sufficient for the light scattered by the dust to be visible. It's color "reflects" the color of the starlight providing the illumination.
Retrograde:: The outer planets, Mars and beyond, are generally seen moving west to east against the background fixed stars. However since the the Earth moves more rapidly in its orbit than the outer planets it will overtake them. As this happens they appear to reverse direction briefly before resuming there normal motion. While its apparent motion is reversed the planet is said to be in retrograde.
Right Ascension and Declination:: Right ascension (R.A.) and Declination are coordinates used to specify the location of an astronomical object in the sky with right ascension corresponding to longitude and declination to latitude.
Supernova: A supernova is a monumental explosion of a star caused by one of two possible mechanisms:
Type I: A white dwarf may accreate hydrogen from a binary companion star onto its surface or merge with another star thereby increasing its mass to a point where it ignites in runaway nuclear fusion. This completely disrupts the star.
Type II: These events are produced at the end of the lives of stars with a mass greater than eight times that of the Sun 8 times that of the Sun (8M_☉). Stars shine by combining elements in their cores to create progressively heavier elements. This fusion process liberates vast amounts energy generating an outward pressure that counters the force of gravity directed toward the star's center. For massive stars the fusion process evidentually produces iron that represents a dead end in that it cannot be fused to produce additional energy. The now-unrestrainted gravitational force causes the star to collapse onto its core from where it rebounds in a massive explosion. The remaining material then contracts under its own gravity to form a neutron star. If the mass of the mass of the neutron star exceeds about 2.5M_☉ the contraction continues to form a black hole.
White Dwarf:: After a typical star exhausts its available fuel, it can no longer sustain itself against against the force of gravity and it shrinks to a tiny stellar remnant only slightly larger than the Earth. This is known as a white dwarf. A white dwarf is extrememly hot with a temperature of 100,000K compared to the Sun's surface temperature of 5,800K. The typical density of white dwarf matter is such that a teaspoonful weighs about 5 tons.
Wolf-Rayet Star:: Wolf-Rayet stars are evolved, massive stars with strong stellar winds that have lost their outer layer of Hydrogen and are fusing heavier elements in their cores. They have very high surface temperatures in excess of 20,000 K which is hotter than most other kinds of stars. Consequently, Wolf-Rayet stars are many thousands of times more luminous than the Sun. One of the most massive stars known, R136a1 in the Tarancula Nebula (30 Doradus) in the Large Megellanic Cloud, is a Wolf-Rayet star. With a surface temperature of 46,000 K it is 4.7 million times as luminous as the Sun and 200 times as massive.