An interstellar cloud is generally an accumulation 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 ...

, plasma, and

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...

in our and other galaxies. Put differently, an interstellar cloud is a denser-than-average region of the interstellar medium, the

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 ...

and radiation that exists in the

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually cons ...

between the star systems in a galaxy. Depending on the

density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...

size Size in general is the magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions ( length, width, height, diameter, perimeter), area, or volume. Size can also be m ...

, and

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 ...

of a given cloud, its

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

can be neutral, making an H I region; ionized, or plasma making it an

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

; or molecular, which are referred to simply as

molecular cloud 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 absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

s, or sometime dense clouds. Neutral and

ionized Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule ...

clouds are sometimes also called ''diffuse clouds''. An interstellar cloud is formed by the gas and dust particles from a red giant in its later life.

Chemical compositions

The chemical composition of interstellar clouds is determined by studying

electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) li ...

that they emanate, and we receive – from radio waves through

visible light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...

, to

gamma rays A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

on the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging fro ...

– that we receive from them. Large radio telescopes scan the intensity in the sky of particular frequencies of electromagnetic radiation, which are characteristic of certain

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' spectra. Some interstellar clouds are cold and tend to give out electromagnetic radiation of large

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

s. A map of the abundance of these molecules can be made, enabling an understanding of the varying composition of the clouds. In hot clouds, there are often ions of many elements, whose spectra can be seen in visible and

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

. Radio telescopes can also scan over the frequencies from one point in the map, recording the intensities of each type of molecule. Peaks of frequencies mean that an abundance of that molecule or atom is present in the cloud. The height of the peak is proportional to the relative percentage that it makes up.

Unexpected chemicals detected in interstellar clouds

Until recently, the rates of reactions in interstellar clouds were expected to be very slow, with minimal products being produced due to the low temperature and density of the clouds. However,

organic molecule In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. Th ...

s were observed in the spectra that

scientist A scientist is a person who conducts scientific research to advance knowledge in an area of the natural sciences. In classical antiquity, there was no real ancient analog of a modern scientist. Instead, philosophers engaged in the philosoph ...

s would not have expected to find under these conditions, such as

formaldehyde Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...

, methanol, and

vinyl alcohol Vinyl alcohol, also called ethenol (IUPAC name; not ethanol), is the simplest enol. With the formula , it is a labile compound that converts to acetaldehyde. It is not a precursor to polyvinyl alcohol. Synthesis Vinyl alcohol can be formed by th ...

. The reactions needed to create such substances are familiar to scientists only at the much higher temperatures and pressures of earth and earth-based laboratories. The fact that they were found indicates that these

chemical reaction A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...

s in interstellar clouds take place faster than suspected, likely in gas-phase reactions unfamiliar to organic chemistry as observed on earth. These reactions are studied in the CRESU experiment. Interstellar clouds also provide a medium to study the presence and proportions of metals in space. The presence and ratios of these elements may help develop theories on the means of their production, especially when their proportions are inconsistent with those expected to arise from stars as a result of

fusion Fusion, or synthesis, is the process of combining two or more distinct entities into a new whole. Fusion may also refer to: Science and technology Physics *Nuclear fusion, multiple atomic nuclei combining to form one or more different atomic nucl ...

and thereby suggest alternate means, such as

cosmic ray spallation Cosmic ray spallation, also known as the x-process, is a set of naturally occurring nuclear reactions causing nucleosynthesis; it refers to the formation of chemical elements from the impact of cosmic rays on an object. Cosmic rays are highly ener ...

High-velocity cloud

These interstellar clouds possess a velocity higher than can be explained by the rotation of the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

. By definition, these clouds must have a v_lsr greater than 90 km s⁻¹, where v_lsr is the local standard rest velocity. They are detected primarily in the 21 cm line of neutral

, and typically have a lower portion of heavy elements than is normal for interstellar clouds in the Milky Way. Theories intended to explain these unusual clouds include materials left over from the formation of the galaxy, or tidally-displaced

drawn away from other galaxies or members of the Local Group. An example of the latter is the

Magellanic Stream The Magellanic Stream is a stream of high-velocity clouds of gas extending from the Large and Small Magellanic Clouds over 100° through the Galactic south pole of the Milky Way. The stream contains a gaseous feature dubbed the ''leading arm''. ...

. To narrow down the origin of these clouds, a better understanding of their distances and metallicity is needed. High-velocity clouds are identified with an HVC prefix, as with HVC 127-41-330.

References

External links

High Velocity Cloud
— The Swinburne Astronomy Online (SAO) encyclopedia. {{DEFAULTSORT:Interstellar Cloud

Cloud In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...

Nebulae Cosmic dust Articles containing video clips

Chemical compositions

Unexpected chemicals detected in interstellar clouds

High-velocity cloud

See also

References

External links