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NGC 1316
NGC 1316, also known as Fornax A, is a lenticular galaxy in Fornax about 62 to 75 million light-years away and one of the strongest radio sources in the sky.
NGC 1399
NGC 1399 is a giant elliptical galaxy at the center of the Fornax Cluster, known for its rich globular cluster system and hot gas halo.
NGC 2403
NGC 2403 is a low-mass barred spiral galaxy in the constellation Camelopardalis, an outlying member of the M81 group about 11 million light-years from Earth.
NGC 3227
NGC 3227 is a barred spiral galaxy with an active Seyfert nucleus in the constellation Leo, gravitationally interacting with its smaller elliptical companion NGC 3226.
NGC 3516
NGC 3516 is a nearby Seyfert 1 spiral galaxy, one of the original six in Carl Seyfert's class, with an active nucleus powered by a supermassive black hole.
NGC 3783
NGC 3783 is a bright barred spiral galaxy about 130 million light-years from Earth in the constellation Centaurus, hosting one of the best-studied active galactic nuclei in the southern sky.
NGC 4051
NGC 4051 is a barred spiral galaxy in Ursa Major, one of the six original Seyfert galaxies from Carl Seyfert's 1943 paper, known for its variable X-ray nucleus.
NGC 4151
NGC 4151 is a nearby barred spiral galaxy in the constellation Canes Venatici with a Seyfert 1.5 active nucleus, one of the nearest and brightest Seyfert galaxies.
NGC 4253
NGC 4253, also known as Markarian 766, is a Seyfert 1 barred spiral galaxy in the constellation Coma Berenices, one of the most frequently observed active galactic nuclei in X-ray astronomy.
NGC 4261
NGC 4261 is a giant elliptical galaxy in the Virgo Cluster region, best known for the spiral dust disk imaged by Hubble around its 1.6-billion-solar-mass black hole.
NGC 4388
NGC 4388 is a Seyfert 2 spiral galaxy in the Virgo Cluster, roughly 60 to 65 million light years away, and the brightest Seyfert 2 galaxy in hard X-rays.
NGC 4593
NGC 4593, also cataloged as Mrk 1330, is a barred spiral galaxy about 39 Mpc away with a Seyfert 1 active nucleus, discovered by William Herschel in 1784.
NGC 4636
NGC 4636 is a giant elliptical galaxy at the southern border of the Virgo cluster, about 47 to 50 million light years away, with a disturbed X-ray halo.
NGC 5506
NGC 5506 is a nearly edge-on spiral galaxy in Virgo whose Seyfert 2 nucleus was revealed in 2002 to be a dust-obscured narrow-line Seyfert 1.
NGC 5548
NGC 5548, also cataloged as Mrk 1509, is a nearby Seyfert 1 spiral galaxy whose nucleus hosts a rapidly accreting supermassive black hole about 79 megaparsecs away.
NGC 6946
NGC 6946, also called Caldwell 12 or the Fireworks Galaxy, is a face-on spiral galaxy about 22 to 25 million light-years away, discovered by William Herschel in 1798.
NGC 7469
NGC 7469 is a barred spiral Seyfert 1 galaxy in Pegasus about 220 million light-years away, one of the original six Seyfert galaxies, interacting with IC 5283.
Night sky
The night sky is the appearance of the heavens after dark: stars, the Moon and planets, the Milky Way, and faint glows seen against a never-black background.
NOAO Deep Wide-Field Survey
The NOAO Deep Wide-Field Survey (NDWFS) was a deep optical and near-infrared survey of 18 square degrees of sky, begun by NOAO in 1999 to study how large-scale structure evolves.
Nodding (astronomy)
Nodding is an infrared and submillimeter observing technique in which the telescope repeatedly moves between a source and nearby blank sky, subtracting the bright atmospheric and telescope background, usually paired with chopping.
Non-Gaussianity
Non-Gaussianity in cosmology is any deviation from Gaussian statistics in the primordial density fluctuations that seeded cosmic structure, probed through higher-order correlators such as the bispectrum, with Planck finding no primordial signal.
Northern Extended Millimeter Array
The Northern Extended Millimeter Array (NOEMA) is a millimeter radio interferometer of twelve 15-meter antennas on the Plateau de Bure in the French Alps, operated by IRAM since 1990.
Nucleosynthesis
Nucleosynthesis, also called nucleogenesis, is the set of nuclear processes that create atomic nuclei, from big bang hydrogen and helium to elements forged in stars.
Observable universe
The observable universe, also called the visible universe, is the sphere of space about 46 billion light-years in radius from which light has reached Earth.
OJ 287
OJ 287 is a blazar, a jet-pointed active galactic nucleus at redshift 0.306, and a leading binary black hole candidate whose optical outbursts recur about every 12 years.
Olbers' paradox
Olbers' paradox, also called the dark night sky paradox, asks why the night sky is dark if the universe is infinite, eternal, and full of stars.
Optimal extraction
Optimal extraction is an algorithm in astronomical spectroscopy, formulated by Keith Horne in 1986, that converts a two-dimensional detector image into a one-dimensional spectrum with the best possible signal-to-noise ratio.
Orbit determination
Orbit determination is the estimation of a satellite's or celestial body's orbital elements from observed positions, pioneered by Gauss in 1801 to recover Ceres and now central to space situational awareness.
Orbital decay
Orbital decay, or a decaying orbit, is the progressive loss of orbital energy that shrinks a satellite's orbit, driven mainly by atmospheric drag in low Earth orbit.
Outer space
Outer space is the near-vacuum region beyond the atmospheres of celestial bodies, extending from the conventional boundary above Earth through interplanetary, interstellar, and intergalactic space.