Below is a glossary of all the terms highlighted in green throughout this website.
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The Abell Catalog of Planetary Nebula is a catalog of 86 objects, most of which are planetary nebula, compiled by astronomer George O. Abell between 1955 and 1966.



Ansa (plural is ansae) is the Latin word for handle. It is often used in astronomical terms to describe the appearance of an object with projections on either side.



Arp stands for the Atlas of Peculiar Galaxies, authored by astronomer Halton C. Arp. In it, Arp cataloged 338 galaxies of unusual shape, mainy of them interacting with one another in pairs, triplets or larger groups.


Barred Spiral

Barred spiral galaxies, like regular spiral glaxies, tend to be younger, have a small bright core of densely packed stars, and long arms of stars, gas and dust. The main difference is that while the arms of regular spirals emanate from the core, barred spirals have roughly rectangular projections on either side, which rotate along with the galaxy, from which the arms extend.


Black Hole

A black hole is the collapsed remnant core of an extremely massive star after it has ended its life in a supernova explosion. Its gravity is so strong, that not even light can escape, hence they cannot be directly observed. Their presence must be inferred from the effects of their intense gravity on nearby stars and gas.



A conjunction occurs when two solar system objects pass close to one another in the sky, as seen from our viewpoint here on Earth.


Dark Nebula

Dark nebula only appear dark in relation to a brighter background. Consisting of dust and gas that neither emits nor reflects light, they can only be seen when a bright nebula lies behind them.


Doubly Ionized Oxygen

Doubly ionized oxygen is an oxygen atom that has taken on three extra electrons, instead of the usual two as occurs on Earth. Doubly ionized oxygen emits light in the green and blue portions of the spectrum (unlike ionized hydrogen that emits red light), giving these nebulae their characteristic color.


Elliptical Galaxy

Elliptical galaxies are older, lens-shaped galaxies, that often result from the merger of two spiral galaxies. In the process, they lose their spiral arms, as all the stars are pulled into a more compact form. They contain mostly older red and yellow stars, and so appear yellowish in photographs.


Emission Nebula

Emission nebulae consist mostly of hydrogen gas. The gas absorbs energy from nearby stars, and when it radiates that energy away, it emits red light.


First Quarter Moon

First quarter moon occurs when the moon is ninety degrees from the sun in it's orbit around the Earth. This angle of sunlight makes crater rims and mountains stand out in stark contrast to the surrounding terrain.


Flocculent Galaxy

Flocculent galaxies are those galaxies whose spiral arms have fragmented into many pieces, giving the galaxy a "patchy" appearance.



Full-spectrum images use all the wavelengths of visible light, creating an image similar to what the eye actually would see if we had a telescope powerful enough, but often with less detail. Narrowband imaging uses very small portions of the entire visible spectrum, confined to narrow wavelengths of light given off by indivdual atoms, to create an image with false colors but more detail.


Globular Cluster

Globular clusters are spherical, dense conglomerations of stars. Hundreds of thousands of stars can be packed into a space only a few lightyears across.


Grand Design Galaxies

Grand Design Galaxies are those that have two major spiral arms that wrap around the core of the galaxy, sometimes multiple times. Several small stubby arms may be present. Examples include M51, M74, M81 ande M100.



"IC" stands for Index Catalog. It actually consists of two catalogs, compiled during 1888-1894, and from 1895-1907. They were supplements to the New General Catalogs published years earlier, containing many objects too faint to be seen visually (when the New General Catalogs were published, astrophotography did not yet exist).



Ionized hydrogen atoms have absorbed energy from some external source, and this excess energy ejects the atoms' electrons. When the electrons reunite with the rest of the atom, this excess energy radiates away as red light.



"LDN" stands for Lynd's Catalog of Dark Nebula. Published in 1962, it is a comprehensive catalog of dark nebula (those appearing black against a brighter background).



A lightyear is a measure of distance, not time. It is the distance that light, travelling at 186,000 miles per second (282,000 km per second), travels in one year. It is equal to approximately six trillion miles (nine trillion kilometers).


Local Group

Many galaxies occur in large clusters. Our own Local Group consists of our Milky Way galaxy, the Andromeda Galaxy (M31), the Pinwheel Galaxy (M33), the Large and Small Magellanic Clouds, and several other small dwarf galaxies.



Charles Messier (1730 - 1817) was a French comet hunter. Even though he discovered a number of comets, he is most remembered for his list of "things that look like comets but aren't". The quality of the telescopes in his day was such that most star clusters, nebula and galaxies appeared as nothing more than fuzzy blobs, much as a distant comet would look. His list of 110 objects "not to be confused with comets" remain a favorite source of deep-sky objects for both visual observers and imagers.



Latin for sea, maria is the name given to the darker areas on the Moon. Once thought to be ancient sea beds, we now know these to be areas of massive ancient lava flows following impacts of asteroids large enough to crack through the surface.



Narrowband imaging uses very small portions of the entire visible spectrum, confined to narrow wavelengths of light given off by indivdual atoms, to create an image more detailed than one using all the wavelengths of light. Common wavelengths used are red light given off by hydrogen and sulfur atoms and greenish-blue light given off by oxygen atoms. Full-spectrum images use all the wavelengths of visible light, creating an image similar to what the eye actually would see if we had a telescope powerful enough.



"NGC" stands for New General Catalog. The original General Catalog, published in 1864, was a list of about 5000 nebulae, galaxies and star clusters, compiled by William Herschel (the discoverer of the planet Uranus) and his son, John Herschel. In 1888, the catalog was expanded by almost 3000 more objects from various discoverers, and renamed the New General Catalog. The NGC remains one of the most fertile resources for amateur astronomers seeking objects to view or photograph.


Open Cluster

Open clusters are loose aggregations of stars, weakly bound by their mutual gravity. Open clusters are relatively young structures, and actually disperse into individual stars after millions of years.



Opposition is when a planet farther from the sun than the earth lies on a straight line extending from the sun, through the earth, to that planet. At that time, the planet is at its closest point to earth in its orbit, and so appears its largest and brightest.


Planetary Nebula

Planetary nebulae actually have nothing to do with planets. They are spherical shells of gas given off by stars like our sun as they near the end of their lives. In the crude telescopes of the 18th century, these objects appeared as small, featureless disks, much like the outer planets of our solar system. Astronomers of the day thought they were newly discovered planets. Modern telescopes reveal the true, gaseous nature of these objects, as well the central stars from which the gas comes.



PLN stands for planetary nebula, and the numbers that follow define the position of the object with respect to the galactic plane (galactic coordinates), instead of using right ascension and declination, which are extensions of terrestrial longitude and latitude.


Reflection Nebula

Unlike emission nebula that glow with red light from ionized hydrogen gas, reflection nebula are only seen by the light they reflect from nearby stars. Just as our sky appears blue from scattered light off of dust in the atmosphere, these nebula appear blue because dust scatters blue light more than it does the other colors.


Sh2, Sharpless 2

"Sh2" stands for the second catalog published by the astronomer Stewart Sharpless. These catalogs contain 313 HII regions, which are nebulae that glow red from the light of ionized hydrogen. Many are very faint, and require very long exposures to image them.



The light spectrum is the familiar row of colors seen in a rainbow, or when using a prism to separate "white" light into all its colors. From longest to shortest wavelength the colors are red, orange, yellow, green, blue, indigo and violet.


Spiral Galaxy

Spiral galaxies tend to be younger, have a small bright core of densely packed stars, and long arms of stars, gas and dust trailing from the core. Rotation of the galaxy causes the arms to be pulled into a spiral.


Stellar Wind

The stellar wind is an intense stream of atomic and sub-atomic particles continuously emanating from the surface of all stars. Extremely energetic stars have a wind so strong that it can actually push aside the gas near these stars.



Mars' polar ice caps consist of frozen carbon dioxide, not water ice. Upon warming, frozen carbon dioxide "sublimates", which means passing directly from a solid to a gas.



When stars much more massive than our Sun run out of fuel, they rapidly collapse. The resulting intense gravitational pressure causes the collapsing star to heat up to millions of degrees, and the star literally explodes as a brilliant but short-lived supernova. A single supernova can easily outshine all the stars of its entire parent galaxy for days to weeks before fading.



The terminator is the border between the bright, daytime side of the moon and the dark, nighttime side. The glancing angle of sunlight along the terminator provides contrast that allows visualization of the lunar surface in fine detail.



A transit occurs when a moon of one of the larger planets passes in front of the planet during its orbit. In a telescope, one often sees only the dark shadow moving slowly across the planet's face, as there is not enough contrast between the moon and the planet itself to see visually. With electronic imaging, the moon is also visible.


Virgo Galaxy Cluster

Galaxies do not exist in isolation: they group together in large clusters with vast areas of empty space between clusters. The closest cluster to us, consisting of over 2500 galaxies, appears in our sky in the direction of the constellation Virgo.


Wolf-Rayet Star

Wolf-Rayet stars are extremely large, very powerful stars that are nearing the end of their lives. They have intense stellar winds which interact with the gas surrounding them, usually ejected from the star during earlier periods of its life. Many will end their lives in a supernova explosion.