Context: A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giant stars late in their lives. The word "nebula" is Latin for mist or cloud, and the term "planetary nebula" is a misnomer that originated in the 1780s with astronomer William Herschel because when viewed through his telescope, these objects resemble the rounded shapes of planets. Herschel's name for these objects was popularly adopted and has not been changed. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years., A supernova remnant (SNR) is the structure resulting from the explosion of a star in a supernova. The supernova remnant is bounded by an expanding shock wave, and consists of ejected material expanding from the explosion, and the interstellar material it sweeps up and shocks along the way., Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more paths due to localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of reflected radiation from the angle predicted by the law of reflection. Reflections that undergo scattering are often called "diffuse reflections" and unscattered reflections are called "specular" (mirror-like) reflections. , Astronomy is a natural science that studies celestial objects and phenomena. It applies mathematics, physics, and chemistry, in an effort to explain the origin of those objects and phenomena and their evolution. Objects of interest include planets, moons, stars, galaxies, and comets; while the phenomena include supernovae explosions, gamma ray bursts, and cosmic microwave background radiation. More generally, all astronomical phenomena that originate outside Earth's atmosphere are within the purview of astronomy. A related but distinct subject, physical cosmology, is concerned with the study of the Universe as a whole., Orion's Belt or the Belt of Orion, also known as the Three Kings or Three Sisters, is an asterism in the constellation Orion. It consists of the three bright stars Alnitak, Alnilam and Mintaka., A star is a luminous sphere of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth. Historically, the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the stars in the Universe, including all stars outside our galaxy, the Milky Way, are invisible to the naked eye from Earth. Indeed, most are invisible from Earth even through the most powerful telescopes., Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", fuse to form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function., An emission nebula is a nebula formed of ionized gases that emit light of various colors. The most common source of ionization is high-energy photons emitted from a nearby hot star. Among the several different types of emission nebulae are H II regions, in which star formation is taking place and young, massive stars are the source of the ionizing photons; and planetary nebulae, in which a dying star has thrown off its outer layers, with the exposed hot core then ionizing them., The Sun is the star at the center of the Solar System. It is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process.<ref name="doi10.1146/annurev-astro-081913-040012"></ref> It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99.86% of the total mass of the Solar System.
About three quarters of the Sun's mass consists of hydrogen (~73%); the rest is mostly helium (~25%), with much smaller quantities of heavier elements, including oxygen, carbon, neon, and iron., Stewart Sharpless (March 29, 1926  January 19, 2013) was an American astronomer who carried out fundamental work on the structure of the Milky Way galaxy., The Sharpless catalog is a list of 313 HII regions (emission nebulae), intended to be comprehensive north of declination -27°. (It does include some nebulae south of that declination as well.) The first edition was published in 1953 with 142 objects (Sh1), and the second and final version was published by US astronomer Stewart Sharpless in 1959 with 312 objects. Sharpless also includes some planetary nebulae and supernova remnants, in addition to HII regions., Earth, otherwise known as the world, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets., A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union (IAU). The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names., A nebula (Latin for "cloud"; pl. nebulae, nebulæ, or nebulas) is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, "nebula" was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. The Andromeda Galaxy, for instance, was once referred to as the "Andromeda Nebula" (and spiral galaxies in general as "spiral nebulae") before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others., NGC 1973/5/7 is a reflection nebula 1/2 degree northeast of the Orion Nebula . The three NGC objects are divided by darker regions . It is also called The Running Man Nebula and Sharpless Catalog 279 . This object was named ' The Running Man Nebula ' by Texas Astronomical Society member Jason Ware . Approximately 20 years ago his down stairs neighbor looked at the object and said it looked like a running man . He brought this up a TAS club meeting and the name stuck . Now widely accepted as ' The Running Man ' ., The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light years across. It has a mass of about 2000 times the mass of the Sun. Older texts frequently refer to the Orion Nebula as the Great Nebula in Orion or the Great Orion Nebula., Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked". In astronomy, the naked eye may be used to observe events that can be viewed without equipment, such as an astronomical conjunction, the passage of a comet, or a meteor shower. Sky lore and various tests demonstrate an impressive wealth of phenomena that can be seen with the unaided eye., An ion is an atom or a molecule in which the total number of electrons is not equal to the total number of protons, giving the atom or molecule a net positive or negative electrical charge. Ions can be created, by either chemical or physical means, via ionization., The Milky Way is the galaxy that contains our Solar System. The descriptive "milky" is derived from the appearance from Earth of the galaxy  a band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term "Milky Way" is a translation of the Latin ', from the Greek (', "milky circle"). From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies., In astronomy, reflection nebulae are clouds of interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to ionize the gas of the nebula to create an emission nebula, but is enough to give sufficient scattering to make the dust visible. Thus, the frequency spectrum shown by reflection nebulae is similar to that of the illuminating stars. Among the microscopic particles responsible for the scattering are carbon compounds (e. g. diamond dust) and compounds of other elements such as iron and nickel. The latter two are often aligned with the galactic magnetic field and cause the scattered light to be slightly polarized., Subject: ngc 1973, Relation: instance_of, Options: (A) addition (B) asterism (C) astronomical object (D) atmosphere (E) belt (F) catalog (G) charge (H) chemical (I) comet (J) constellation (K) debate (L) diamond (M) disk (N) distance (O) emission nebula (P) energy (Q) equipment (R) formation (S) four (T) galaxy (U) gas (V) giant (W) interstellar cloud (X) ion (Y) january (Z) magnetic field ([) march (\) material (]) may (^) meteor (_) meteor shower (`) microscope (a) mirror (b) molecule (c) motion (d) name (e) natural science (f) nebula (g) negative (h) perception (i) phenomenon (j) physics (k) planet (l) planetary nebula (m) practice (n) process (o) science (p) single (q) sound (r) sphere (s) star (t) supernova (u) supernova remnant (v) system (w) telescope (x) theory (y) three (z) vision
Output: galaxy