Information:  - An interstellar cloud is the generic name given to an accumulation of gas, plasma, and dust in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium. Depending on the density, size, and temperature of a given cloud, the hydrogen in it can be neutral (H I regions), ionized (H II regions) (i.e. a plasma), or molecular (molecular clouds). Neutral and ionized clouds are sometimes also called diffuse clouds, while molecular clouds are sometimes also referred to as dense clouds.  - The density, or more precisely, the volumetric mass density, of a substance is its mass per unit volume. The symbol most often used for density is  (the lower case Greek letter rho), although the Latin letter "D" can also be used. Mathematically, density is defined as mass divided by volume:  - Rats are various medium-sized, long-tailed rodents of the superfamily Muroidea. "True rats" are members of the genus "Rattus", the most important of which to humans are the black rat, "Rattus rattus", and the brown rat, "Rattus norvegicus". Many members of other rodent genera and families are also referred to as rats, and share many characteristics with true rats.  - Red is the color at the longer-wavelengths end of the spectrum of visible light next to orange, at the opposite end from violet. Red color has a predominant light wavelength of roughly 620740 nanometers. Light with a longer wavelength than red but shorter than terahertz radiation and microwave is called infrared.  - In physics, the wavelength of a sinusoidal wave is the spatial period of the wavethe distance over which the wave's shape repeats,  and thus the inverse of the spatial frequency. It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns.  Wavelength is commonly designated by the Greek letter "lambda" (). The concept can also be applied to periodic waves of non-sinusoidal shape.  The term "wavelength" is also sometimes applied to modulated waves, and to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids.  - 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.  - Infrared radiation, or simply infrared or IR, is electromagnetic radiation (EMR) with longer wavelengths than those of visible light, and is therefore invisible, although it is sometimes loosely called infrared light. It extends from the nominal red edge of the visible spectrum at 700 nanometers (frequency 430 THz), to 1000000 nm (300 GHz) (although people can see infrared up to at least 1050 nm in experiments). Most of the thermal radiation emitted by objects near room temperature is infrared. Like all EMR, IR carries radiant energy, and behaves both like a wave and like its quantum particle, the photon.   - Astrophysics is the branch of astronomy that employs the principles of physics and chemistry "to ascertain the nature of the heavenly bodies, rather than their positions or motions in space." Among the objects studied are the Sun, other stars, galaxies, extrasolar planets, the interstellar medium and the cosmic microwave background. Their emissions are examined across all parts of the electromagnetic spectrum, and the properties examined include luminosity, density, temperature, and chemical composition. Because astrophysics is a very broad subject, "astrophysicists" typically apply many disciplines of physics, including mechanics, electromagnetism, statistical mechanics, thermodynamics, quantum mechanics, relativity, nuclear and particle physics, and atomic and molecular physics.  - In physics, and in particular as measured by radiometry, radiant energy is the energy of electromagnetic and gravitational radiation. As energy, its SI unit is the joule (J). The quantity of radiant energy may be calculated by integrating radiant flux (or power) with respect to time. The symbol "Q" is often used throughout literature to denote radiant energy ("e" for "energetic", to avoid confusion with photometric quantities). In branches of physics other than radiometry, electromagnetic energy is referred to using "E" or "W". The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.  - In astronomy, the interstellar medium (ISM) is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar radiation field.  - 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.  - A pulsar (from "pulse" and "-ar" as in quasar) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. This radiation can be observed only when the beam of emission is pointing toward Earth (much the way a lighthouse can be seen only when the light is pointed in the direction of an observer), and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range from milliseconds to seconds for an individual pulsar. Pulsars are believed to be one of the candidates of high and ultra-high energy astroparticles (see also Centrifugal mechanism of acceleration).  - An astronomical object or celestial object is a naturally occurring physical entity, association, or structure that current science has demonstrated to exist in the observable universe.  - A quasar is a compact region surrounding a supermassive black hole. The region emits enormous amounts of electromagnetic energy as a result of mass from the core of a surrounding galaxy, under the influence of the black hole's gravity, falling onto the black hole's accretion disk. Although ranging in size from only 10 to 10,000 times the Schwarzschild radius of the enclosed black hole, quasars are the most energetic form of active galactic nucleus, emitting radio waves and visible light up to 100 times the luminosity of the entire Milky Way Galaxy.  - In physics, a wave is an oscillation accompanied by a transfer of energy that travels through a medium (space or mass). Frequency refers to the addition of time. "Wave motion" transfers energy from one point to another, which displace particles of the transmission mediumthat is, with little or no associated mass transport. Waves consist, instead, of oscillations or vibrations (of a physical quantity), around almost fixed locations.  - Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of , hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass. Non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed "protium" (name rarely used, symbol H), has one proton and no neutrons.  - The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. In older literature, the CMB is also variously known as cosmic microwave background radiation (CMBR) or "relic radiation". The CMB is a cosmic background radiation that is fundamental to observational cosmology because it is the oldest light in the universe, dating to the epoch of recombination. With a traditional optical telescope, the space between stars and galaxies (the "background") is completely dark. However, a sufficiently sensitive radio telescope shows a faint background glow, almost isotropic, that is not associated with any star, galaxy, or other object. This glow is strongest in the microwave region of the radio spectrum. The accidental discovery of the CMB in 1964 by American radio astronomers Arno Penzias and Robert Wilson was the culmination of work initiated in the 1940s, and earned the discoverers the 1978 Nobel Prize.  - A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit the formation of molecules, most commonly molecular hydrogen (H). This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas.  - Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. When the temperature of a body is greater than absolute zero, inter-atomic collisions cause the kinetic energy of the atoms or molecules to change. This results in charge-acceleration and/or dipole oscillation which produces electromagnetic radiation, and the wide spectrum of radiation reflects the wide spectrum of energies and accelerations that occur even at a single temperature.  - In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space carrying electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-, and gamma radiation.  - A photon is an elementary particle, the quantum of the electromagnetic field including electromagnetic radiation such as light, and the force carrier for the electromagnetic force (even when static via virtual photons). The photon has zero rest mass and is always moving at the speed of light. Like all elementary particles, photons are currently best explained by quantum mechanics and exhibit waveparticle duality, exhibiting properties of both waves and particles. For example, a single photon may be refracted by a lens and exhibit wave interference with itself, and it can behave as a particle with definite and finite measurable position and momentum. The photon's wave and quanta qualities are two observable aspects of a single phenomenon, and cannot be described by any mechanical model; a representation of this dual property of light, which assumes certain points on the wavefront to be the seat of the energy, is not possible. The quanta in a light wave cannot be spatially localized. Some defined physical parameters of a photon are listed.  - An HI region or H I region (read H one) is a cloud in the interstellar medium composed of neutral atomic hydrogen (HI), in addition to the local abundance of helium and other elements. (H is the chemical symbol for hydrogen, and "II" is the Roman numeral. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionisedHII is H in other sciencesIII for doubly-ionised, e.g. OIII is O, etc.) These regions do not emit detectable visible light (except in spectral lines from elements other than hydrogen) but are observed by the 21-cm (1,420 MHz) region spectral line. This line has a very low transition probability, so requires large amounts of hydrogen gas for it to be seen. At ionization fronts, where HI regions collide with expanding ionized gas (such as an H II region), the latter glows brighter than it otherwise would. The degree of ionization in an HI region is very small at around 10 (i.e. one particle in 10,000). At typical interstellar pressures in galaxies like the Milky Way, HI regions are most stable at temperatures of either below 100 K or above several thousand K; gas between these temperatures heats or cools very quickly to reach one of the stable temperature regimes. Within one of these phases, the gas is usually considered isothermal, except near an expanding H II region. Near an expanding H II region is a dense HI region, separated from the undisturbed HI region by a shock front and from the H II region by an ionization front.  - 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.  - Infrared astronomy is the branch of astronomy and astrophysics that studies astronomical objects visible in infrared (IR) radiation. The wavelength of infrared light ranges from 0.75 to 300 micrometers. Infrared falls in between visible radiation, which ranges from 380 to 750 nanometers, and submillimeter waves.  - A dark nebula or absorption nebula is a type of interstellar cloud that is so dense it obscures the light from objects behind it , such as background stars and emission or reflection nebulae . The extinction of the light is caused by interstellar dust grains located in the coldest , densest parts of larger molecular clouds . Clusters and large complexes of dark nebulae are associated with Giant Molecular Clouds . Isolated small dark nebulae are called Bok globules . Like other interstellar dust or material , things it obscures are only visible using radio waves in radio astronomy or infrared in infrared astronomy . Dark clouds appear so because of sub-micrometre - sized dust particles , coated with frozen carbon monoxide and nitrogen , which effectively block the passage of light at visible wavelengths . Also present are molecular hydrogen , atomic helium , C18O , CS , NH3 ( ammonia ) , H2CO ( formaldehyde ) , c - C3H2 ( cyclopropenylidene ) and a molecular ion N2H + ( diazenylium ) , all of which are relatively transparent . These clouds are the spawning grounds of stars and planets , and understanding their development is essential to understanding star formation . The form of such dark clouds is very irregular : they have no clearly defined outer boundaries and sometimes take on convoluted serpentine shapes . The largest dark nebulae are visible to the naked eye , appearing as dark patches against the brighter background of the Milky Way like the Coalsack Nebula and the Great rift . These naked - eye objects are sometimes known as dark cloud constellations and take on a variety of names . In the inner outer molecular regions of dark nebulae , important events take place , such as the formation of stars and masers .  - In physics, a quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction. The fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude of the physical property can take on only certain discrete values.  - An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. (H is the chemical symbol for hydrogen, and "II" is the Roman numeral for 2. It is customary in astronomy to use the Roman numeral I for neutral atoms, II for singly-ionisedH II is H in other sciencesIII for doubly-ionised, e.g. O III is O, etc. H II, or H, consists of free protons.) It is typically a cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic cm. The short-lived blue stars created in these regions emit copious amounts of ultraviolet light that ionize the surrounding gas. H II regionssometimes several hundred light-years acrossare often associated with giant molecular clouds. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.  - The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nm. In terms of frequency, this corresponds to a band in the vicinity of 430770 THz.  - Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of different sources of radio emission. These include stars and galaxies, as well as entirely new classes of objects, such as radio galaxies, quasars, pulsars, and masers. The discovery of the cosmic microwave background radiation, regarded as evidence for the Big Bang theory, was made through radio astronomy.  - A temperature is an objective comparative measurement of hot or cold. It is measured by a thermometer. Several scales and units exist for measuring temperature, the most common being Celsius (denoted °C; formerly called "centigrade"), Fahrenheit (denoted °F), and, especially in science, Kelvin (denoted K).    What is the relationship between 'dark nebula' and 'galaxy'?
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