Spectral line in the context of Spectral class

Helium (from Greek: ἥλιος, romanized: helios, lit. 'sun') is a chemical element; it has symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is the lowest among all the elements, and it does not have a melting point at standard pressures. It is the second-lightest and second-most abundant element in the observable universe, after hydrogen. It is present at about 24% of the total elemental mass, which is more than 12 times the mass of all the heavier elements combined. Its abundance is similar to this in both the Sun and Jupiter, because of the very high nuclear binding energy (per nucleon) of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of both nuclear fusion and radioactive decay. The most common isotope of helium in the universe is helium-4, the vast majority of which was formed during the Big Bang. Large amounts of new helium are created by nuclear fusion of hydrogen in stars.

Helium was first detected as an unknown, yellow spectral line signature in sunlight during a solar eclipse in 1868 by Georges Rayet, Captain C. T. Haig, Norman R. Pogson, and Lieutenant John Herschel, and was subsequently confirmed by French astronomer Jules Janssen. Janssen is often jointly credited with detecting the element, along with Norman Lockyer. Janssen recorded the helium spectral line during the solar eclipse of 1868, while Lockyer observed it from Britain. However, only Lockyer proposed that the line was due to a new element, which he named after the Sun. The formal discovery of the element was made in 1895 by chemists Sir William Ramsay, Per Teodor Cleve, and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite, which is now not regarded as a separate mineral species, but as a variety of uraninite. In 1903, large reserves of helium were found in natural gas fields in parts of the United States, by far the largest supplier of the gas today.

In physics, a redshift is an increase in the wavelength, or equivalently, a decrease in the frequency, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and increase in frequency and energy, is known as a blueshift.

Three forms of redshift occur in astronomy and cosmology: Doppler redshifts due to the relative motions of radiation sources, gravitational redshift as radiation escapes from gravitational potentials, and cosmological redshifts caused by the universe expanding. In astronomy, the value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is greater than 1 for redshifts and less than 1 for blueshifts). Automated astronomical redshift surveys are an important tool for learning about the large-scale structure of the universe. Redshift and blueshift can also be related to photon energy and, via Planck's law, to a corresponding blackbody temperature.

An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the irradiance of the light but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light or a closely derived physical quantity, such as the corresponding wavenumber or the photon energy, in units of measurement such as centimeters, reciprocal centimeters, or electron volts, respectively.

A spectrometer is used in spectroscopy for producing spectral lines and measuring their wavelengths and intensities. Spectrometers may operate over a wide range of non-optical wavelengths, from gamma rays and X-rays into the far infrared. If the instrument is designed to measure the spectrum on an absolute scale rather than a relative one, then it is typically called a spectrophotometer. The majority of spectrophotometers are used in spectral regions near the visible spectrum.

Recessional velocity is the rate at which an extragalactic astronomical object recedes (becomes more distant) from an observer as a result of the expansion of the universe. It can be measured by observing the wavelength shifts of spectral lines emitted by the object, known as the object's cosmological redshift.

Krypton (from Ancient Greek: κρυπτός, romanized: kryptos 'the hidden one') is a chemical element; it has symbol Kr and atomic number 36. It is a colorless, odorless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. Krypton is chemically inert.

Krypton, like the other noble gases, is used in lighting and photography. Krypton light has many spectral lines, and krypton plasma is useful in bright, high-powered gas lasers (krypton ion and excimer lasers), each of which resonates and amplifies a single spectral line. Krypton fluoride also makes a useful laser medium. From 1960 to 1983, the official definition of the metre was based on the wavelength of one spectral line of krypton-86, because of the high power and relative ease of operation of krypton discharge tubes.

A spectral color is a color that is evoked by monochromatic light, i.e. either a spectral line with a single wavelength or frequency of light in the visible spectrum, or a relatively narrow spectral band (e.g. lasers). Every wave of visible light is perceived as a spectral color; when viewed as a continuous spectrum, these colors are seen as the familiar rainbow.Non-spectral colors (or extra-spectral colors) are evoked by a combination of spectral colors.

In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the rainbow of colors interspersed with spectral lines. Each line indicates a particular chemical element or molecule, with the line strength indicating the abundance of that element. The strengths of the different spectral lines vary mainly due to the temperature of the photosphere, although in some cases there are true abundance differences. The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature.

Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O, B, A, F, G, K, and M, a sequence from the hottest (O type) to the coolest (M type). Each letter class is then subdivided using a numeric digit with 0 being hottest and 9 being coolest (e.g., A8, A9, F0, and F1 form a sequence from hotter to cooler). The sequence has been expanded with three classes for other stars that do not fit in the classical system: W, S and C. Some stellar remnants or objects of deviating mass have also been assigned letters: D for white dwarfs and L, T and Y for brown dwarfs (and exoplanets).

Positronium (Ps) is a system consisting of an electron and its anti-particle, a positron, bound together into an exotic atom, specifically an onium. Unlike hydrogen, the system has no protons. The system is unstable: the two particles annihilate each other to predominantly produce two or three gamma-rays, depending on the relative spin states. The energy levels of the two particles are similar to that of the hydrogen atom (which is a bound state of a proton and an electron). However, because of the reduced mass, the frequencies of the spectral lines are less than half of those for the corresponding hydrogen lines.

The Fraunhofer lines are a set of spectral absorption lines. They are dark absorption lines, seen in the optical spectrum of the Sun, and are formed when atoms in the solar atmosphere absorb light being emitted by the solar photosphere. The lines are named after German physicist Joseph von Fraunhofer, who observed them in 1814.

An O-type star is a hot, blue star of spectral type O in the Yerkes classification system employed by astronomers. They have surface temperatures in excess of 30,000 kelvins (K). Stars of this type have strong absorption lines of ionised helium, strong lines of other ionised elements, and hydrogen and neutral helium lines weaker than spectral type B.

Stars of this type are very rare, but because they are very bright, they can be seen at great distances; out of the 90 brightest stars as seen from Earth, 4 are type O. Due to their high mass, O-type stars end their lives rather quickly in violent supernova explosions, resulting in black holes or neutron stars. Most of these stars are young massive main sequence, giant, or supergiant stars, but also some central stars of planetary nebulae, old low-mass stars near the end of their lives, which typically have O-like spectra.

Atomic emission spectroscopy (AES) is a method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum gives the identity of the element while the intensity of the emitted light is proportional to the number of atoms of the element. The sample may be excited by various methods.

Atomic Emission Spectroscopy allows us to measure interactions between electromagnetic radiation and physical atoms and molecules. This interaction is measured in the form of electromagnetic waves representing the changes in energy between atomic energy levels. When elements are burned by a flame, they emit electromagnetic radiation that can be recorded in the form of spectral lines.  Each element has its own unique spectral line because each element has a different atomic arrangement, so this method is an important tool for identifying the makeup of materials. Robert Bunsen and Gustav Kirchhoff were the first to establish atomic emission spectroscopy as a tool in chemistry.

The hydrogen line, 21 centimeter line, or H I line is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms. It is produced by a spin-flip transition, which means the direction of the electron's spin is reversed relative to the spin of the proton. This is a quantum state change between the two hyperfine levels of the hydrogen 1 s ground state. The electromagnetic radiation producing this line has a frequency of 1420.405751768(2) MHz (1.42 GHz), which is equivalent to a wavelength of 21.106114054160(30) cm in a vacuum. According to the Planck–Einstein relation E = hν, the photon emitted by this transition has an energy of 5.8743261841116(81) μeV [9.411708152678(13)×10 J]. The constant of proportionality, h, is known as the Planck constant.

The hydrogen line frequency lies in the L band, which is located in the lower end of the microwave region of the electromagnetic spectrum. It is frequently observed in radio astronomy because those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light. The existence of this line was predicted by Dutch astronomer H. van de Hulst in 1944, then directly observed by E. M. Purcell and his student H. I. Ewen in 1951. Observations of the hydrogen line have been used to reveal the spiral shape of the Milky Way, to calculate the mass and dynamics of individual galaxies, and to test for changes to the fine-structure constant over time. It is of particular importance to cosmology because it can be used to study the early Universe. Due to its fundamental properties, this line is of interest in the search for extraterrestrial intelligence. This line is the theoretical basis of the hydrogen maser.

In astronomy, a redshift survey is a survey of a section of the sky to measure the redshift of astronomical objects: usually galaxies, but sometimes other objects such as galaxy clusters or quasars. Using Hubble's law, the redshift can be used to estimate the distance of an object from Earth. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure detailed statistical properties of the large-scale structure of the universe. In conjunction with observations of early structure in the cosmic microwave background, these results can place strong constraints on cosmological parameters such as the average matter density and the Hubble constant.

Generally the construction of a redshift survey involves two phases: first the selected area of the sky is imaged with a wide-field telescope, then galaxies brighter than a defined limit are selected from the resulting images as non-pointlike objects; optionally, colour selection may also be used to assist discrimination between stars and galaxies. Secondly, the selected galaxies are observed by spectroscopy, most commonly at visible wavelengths, to measure the wavelengths of prominent spectral lines; comparing observed and laboratory wavelengths then gives the redshift for each galaxy.

Mössbauer spectroscopy is a spectroscopic technique based on the Mössbauer effect. This effect, discovered by Rudolf Mössbauer (sometimes written "Moessbauer", German: "Mößbauer") in 1958, consists of the nearly recoil-free emission and absorption of nuclear gamma rays in solids. The consequent nuclear spectroscopy method is exquisitely sensitive to small changes in the chemical environment of certain nuclei.

Typically, three types of nuclear interactions may be observed: the isomer shift due to differences in nearby electron densities (also called the chemical shift in older literature), quadrupole splitting due to atomic-scale electric field gradients; and magnetic splitting due to non-nuclear magnetic fields. Due to the high energy and extremely narrow line widths of nuclear gamma rays, Mössbauer spectroscopy is a highly sensitive technique in terms of energy (and hence frequency) resolution, capable of detecting changes of just a few parts in 10.