Stellar molecules are

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...

s that exist or form in stars. Such formations can take place when the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

is low enough for molecules to form – typically around 6000 K or cooler. Otherwise the stellar

matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...

is restricted to

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

s (

chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements cannot be broken down into simpler sub ...

s) in the forms of

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

or – at very high temperatures – plasma.

Background

Matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which are made up of interacting subatomic part ...

is made up by

s (formed by protons and other subatomic particles). When the environment is right, atoms can join together and form molecules, which give rise to most materials studied in materials science. But certain environments, such as high temperatures, don't allow atoms to form molecules. Stars have very high temperatures, primarily in their interior, and therefore there are few molecules formed in stars. For this reason, a typical

chemist A chemist (from Greek ''chēm(ía)'' alchemy; replacing ''chymist'' from Medieval Latin ''alchemist'') is a scientist trained in the study of chemistry. Chemists study the composition of matter and its properties. Chemists carefully describe t ...

(who studies atoms and molecules) would not have much to study in a star, so stars are better explained by astrophysicists or astrochemists. However, low abundance of molecules in stars is not equated with no molecules at all. By the mid-18th century, scientists surmised that the source of the Sun's light was incandescence, rather than

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combus ...

Evidence and research

Although the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

is a star, its photosphere has a low enough temperature of , and therefore molecules can form.

Water Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...

has been found on the Sun, and there is evidence of H₂ in

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

stellar atmospheres. Cooler stars include absorption band spectra that are characteristic of molecules. Similar absorption bands are found in sun spots which are cooler areas on the Sun. Molecules found in the Sun include MgH, CaH, FeH, CrH, NaH, OH, SiH, VO, and TiO. Others include CN CH, MgF, NH, C₂, SrF, zirconium monoxide, YO, ScO, BH. Stars of most types can contain molecules, even the Ap category of A class stars. Only the hottest O, B and A class stars have no detectable molecules. Also carbon rich white dwarfs, even though very hot, have spectral lines of C₂ and CH.

Laboratory measurements

Measurements of simple molecules that may be found in stars are performed in laboratories to determine the wavelengths of the spectra lines. Also, it is important to measure the dissociation energy and

oscillator strength In spectroscopy, oscillator strength is a dimensionless quantity that expresses the probability of absorption or emission of electromagnetic radiation in transitions between energy levels of an atom or molecule. For example, if an emissive state ha ...

s (how strongly the molecule interacts with electromagnetic radiation). These measurements are inserted into formula that can calculate the spectrum under different conditions of pressure and temperature. However, man-made conditions are often different from those in stars, because it is hard to achieve the temperatures, and also local thermal equilibrium, as found in stars, is unlikely. Accuracy of oscillator strengths and actual measurement of dissociation energy is usually only approximate.

Model atmosphere

A numerical model of a star's atmosphere will calculate pressures and temperatures at different depths, and can predict the spectrum for different elemental concentrations.

Application

The molecules in stars can be used to determine some characteristics of the star. The isotopic composition can be determined if the lines in the molecular spectrum are observed. The different masses of different isotopes cause vibration and rotation frequencies to significantly vary. Secondly the temperature can be determined, as the temperature will change the numbers of molecules in the different vibrational and rotational states. Some molecules are sensitive to the ratio of elements, and so indicate elemental composition of the star. Different molecules are characteristic of different kinds of stars, and are used to classify them. Because there can be numerous spectral lines of different strength, conditions at different depths in the star can be determined. These conditions include temperature and speed towards or away from the observer. The spectrum of molecules has advantages over atomic spectral lines, as atomic lines are often very strong, and therefore only come from high in the atmosphere. Also the profile of the atomic spectral line can be distorted due to isotopes or overlaying of other spectral lines. The molecular spectrum is much more sensitive to temperature than atomic lines.

Detection

The following molecules have been detected in the atmospheres of stars:

References

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