Astronomy Picture of the Day
Explanation of Index Categories
Below is an explanation of the categories used for the APOD images.
Some categories are self-explanatory, but for others there are nuances.
The image is the primary determiner of the category. The text may
suggest a second category. Other categories may be added as
appropriate. Each image may fall into as many as five categories.
But there are some pages that don't fit into any category
(ex. October 4, 2011).
"--" indicates a label for a group of categories. Categories are in bold-face.
--Planets & Moons - Images of these objects. Note that images of moons are put with parent planet. This category does not include extra-solar planets (see instead Extra-solar Planets under Miscellaneous).
Solar System Views - Views of our solar system. This includes
sky views which contain more than two planets.
Earth - Includes atmospheric effects and aurora. Also used when
it plays a role in the background of a space station or shuttle
image, or as a noteworthy foreground location of a sky view image.
Earth's Moon - This includes moonscapes taken by the Apollo missions.
Mars - Includes Mars-scapes taken by Mars landers and rovers.
Pluto - Yes, we keep Pluto with the planets.
Other Planets? - Other planets or bodies in our solar system, e.g. trans Neptunian objects.
--Special Phenomena - These are also categorized with the parent body (e.g. a crater on Mars goes under Craters and Mars).
Meteors - This also includes meteorites
Craters - On Earth, Moon, Mars, etc., any solar system body.
Eclipses - Includes lunar and solar eclipses.
Volcanoes - On Earth, Moon, Mars, etc., any solar system body.
--Sky - These images include images of the night sky or large portions of the sky.
Binary Stars - This includes accretion disks.
--Nebulae - We include the image in all appropriate nebulae categories seen in the image.
Black Holes - Stellar mass and intermediate only. This may also appear in Binary Stars.
Clusters of Stars - This includes both open clusters and globular clusters.
Neutron Stars - This also includes pulsars, and may also appear in Binary Stars.
Nurseries - Star-forming region, newborn stars or nebulae forming newborn stars.
Sun - This links to the same page as Sun under Solar System.
Supernovae - These are recent supernovae, and does NOT include supernova remnants.
White Dwarfs - These may also appear in Binary Stars.
Dark Nebulae - These are usually dust clouds or molecular clouds.
Emission Nebulae - These are glowing clouds of gas which emit light on their own. They may be absorbing light from nearby stars, but they emit a significant portion on their own. This also includes HII regions.
Other Nebulae - None of the other types of nebulae.
Reflection Nebulae - These are usually dust clouds that reflect light from nearby stars (e.g. the faint emission surrounding the bright stars in the Pleiades). Optically these are often blue in color, because blue stars are bright enough to illuminate them.
Active Galaxies - This includes quasars, blazars, etc. Images referring to supermassive black holes are also included here.
Clusters of Galaxies - This also includes groups of galaxies.
Interacting Galaxies - Includes galaxies gravitationally interacting, or galaxies in the process of colliding.
Local Group - Members of the Local Group, including the
Magellanic Clouds, Andromeda galaxy, and its satellite galaxies.
These also go into their respective type (e.g. Andromeda also goes
into Spiral Galaxies).
Milky Way Galaxy - Includes Sky Views and Multi-wavelength images of the Milky Way. It also includes images of the galactic center.
Cosmology - This includes diagrams of early universe, large
scale structures in the universe, topics such as dark energy, and
results from COBE and WMAP.
Extrasolar Planets - Includes protoplanetary disks, but NOT Brown Dwarfs.
Gravitational Lenses - Objects (usually galaxies or clusters of galaxies) which act as a gravitational lens for background objects.
Interstellar Medium - NOT nebulae. Rather this category covers
what makes up the interstellar medium: dust and dust grains. This category also includes the interaction of shocks with interstellar medium.
SETI - This includes the search for, or messages to, alien civilizations.
Constellations - Recognizable constellations or larger portions thereof.
--Space Technology - These categories highlight telescopes, satellites, etc. used in astronomy.
Messier Objects - A special category for the Messier objects.
Multi-wavelength Sky - Large portions or full sky images in non-visible wave bands. These are often in galactic coordinates. May also include multiple images (or overlay images) of a single object in more than one wavelength. It does NOT include a single image of a particular object in a non-visible wave band.
Sky Views - These images usually include the horizon and
non-telescopic objects. These are optical images and are often in
other categories as well (e.g. constellations, a particular planet,
Earth's moon, Earth (for noteworthy locations), etc.).
Ground-based Observatories - These are images taken of a particular telescope observatory, etc., NOT images by them. The exception is an image which is a "first light" of a particular observatory/satellite, or an image that shows particular capabilities of the observatory/telescope.
Orbiting Observatories/Satellites - These are images taken of a particular satellite or orbiting observatory, NOT images by them. The exception is an image which is a "first light" of a particular observatory/satellite, or an image that shows particular capabilities of the observatory/satellite.
Planetary Missions - These images show a lunar or planetary rover, probe, spacecraft, etc.
Rockets/Launch Vehicles - These are rockets on the ground or during launch, but NOT Shuttle launches.
Space Shuttle - Any images of the Shuttle, including Shuttle launches.
Space Stations - Images of MIR, ISS, Skylab, etc.
Sub-orbital Observatories - Balloon missions, sounding rockets, SOFIA.
Technology - Images of instruments, satellite-borne detectors.
Can include images showing a specific instrumentation effect or
capability. NOT SETI (for which there is a separate category).
Astronauts - Pictures of astronauts
Astronomers - Pictures of astronomers.
Authors & editors:
(MTU) & Jerry Bonnell (USRA)
Specific rights apply.
A service of:
ASD at NASA/GSFC
& Michigan Tech. U.
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