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Long-Wavelength Observations of Water in Extraterrestrial Environments

Trent Dupuy & Sandrine Bottinelli
Based on the presentation by Jonathan Williams

1. Introduction

Life on Earth would not be possible without the presence of water. Living organisms require water because of the crucial role it plays in biochemical reactions that regulate and sustain metabolic processes. In its liquid phase, water acts as a universal solvent that allows many different dissolved ions and molecules to come together and react with one another. In space, however, water does not exist as a liquid but rather as a gas, like the water vapor in clouds, or in some cases water in space can exist in its solid phase as ice.

The space between the stars is filled with gas and grains of material called dust, and this mixture is referred to as the interstellar medium (ISM). In some regions of space, this cold material is particularly dense and forms a sort of cloud, which astronomers call a molecular cloud because all of the gas in the cloud is in the form of molecules just like the air we breathe and exhale on Earth, e.g., oxygen (O2) and carbon dioxide (CO2). Unlike the air on Earth (78% nitrogen, N2; 21% O2; 1% other gases), the gas in the ISM is composed mostly of hydrogen (90%) and helium (10%) with only trace amounts of heavier elements like oxygen (0.7%). Although the two are related, molecular clouds have very different characteristics from the clouds of water vapor on Earth. The typical temperature of a molecular cloud is 10 Kelvin (ten degrees above absolute zero), which is more than 200 degrees colder than the coldest places on Earth. Also, the density of the material in a molecular cloud is far lower than in clouds on Earth. In fact, the density of particles in the ISM, anywhere from 100 to one billion particles (atoms or molecules) per cubic centimeter, is much lower than the best vacuum that can be achieved in a laboratory on Earth. The density in a molecular cloud is so low that reactions that occur on Earth over short timescales instead take millions of years.

Even at such low densities, the sheer amount of gas and dust in a molecular cloud makes them completely opaque to optical (Figure 1, left) and ultraviolet radiation, which are located on the short-wavelength/high-energy end of the electromagnetic (EM) spectrum. This means that astronomers who are interested in the (potentially biologically significant) chemical reactions going on in these clouds cannot study them using EM radiation at visible wavelengths (what we refer to as light). Instead, they must use telescopes that collect and measure very long wavelength EM radiation since that radiation is not easily scattered nor absorbed by gas or dust grains. And as we will see later, the cold temperature of the cloud also implies that the only meaningful observations can be done at long wavelengths, from the mid-infrared to the radio.

Figure 1. Two images of a molecular cloud, the site of star and planet formation, at different wavelengths seen against a backdrop of distant stars. Left (visible EM radiation): The cloud is opaque to visible radiation and so appears black. Stars near the edge of the cloud appear extremely red due to the scattering of the bluest radiation by dust in the cloud, just as the setting Sun appears to be redder than usual. Right (infrared EM radiation): Radiation at longer (redder) wavelengths can more easily penetrate the gas and dust in the cloud. The cloud is completely transparent to EM radiation beyond the mid-infrared, including submillimeter, microwave, and radio waves. (Credit: European Southern Observatory,  

2. The Environment of Star Formation

Around five billion years ago, our solar system was nothing more than a cold, dense clump of gas and dust inside a molecular cloud just like those described above. The clump of material was special though because it was dense enough to collapse on itself under the weight of its own gravity. As the cloud clump collapsed, it grew denser, and collisions between gas and dust particles became more common. Many of the molecules in the gas, especially the polar molecules like water, were actually able to "freeze out" by colliding with and sticking to dust grains. Many molecules were frozen as ice on individual dust grains, and when molecules are present in large numbers in a small volume (such as the dust grain's surface), chemical reactions are much more likely to happen than when they are spread out, as in the gas phase. The accumulation of ices on dust grains was a crucial phase in the chemistry of the cloud from which the Sun and planets formed because it encouraged chemical reactions that would normally be impossible in the relatively low density and low temperature environment of the interstellar medium.

Over the next several tens of thousands of years, the core of the collapsing cloud clump became hotter as gravitational potential energy was converted into thermal energy. The heat produced was enough to release the ices from the dust grains back into a gas phase. However, after undergoing many reactions on the dust grains, the ices consisted not only of the common molecules that originally froze onto the grains such as water (H2O), carbon monoxide (CO), and ammonia (NH3) but also the molecules that formed by grain-surface chemistry: methanol (CH3OH), formaldehyde (H2CO), and possibly formic acid (HCOOH). These molecules, released in a hot environment, underwent further reactions to produce even more complex (prebiotic) molecules such as alcohols and amino acids. As the cloud containing these molecules continued to collapse, it was forced to conserve angular momentum and collapse into a rotating disk around the protostar. High-velocity outflows from the protostar helped clear out the remnant envelope, i.e., gas and dust, which did not collapse into the disk or accrete onto the star, and these outflows left behind a disk of gas and dust surrounding a star, which was then hot enough to generate energy through hydrogen fusion. This entire process is illustrated in Figure 2.

Figure 2. The process of star formation as a large cloud of cold molecular gas and dust is transformed into a much smaller planetary system over millions of years. Water exists in the gas phase in the dark cloud cores and during gravitational collapse, though water and other molecules begin to freeze out onto dust grains in the dense center of the collapsing core. After 105-106 years, the protostar emerges, and water frozen onto dust is released along with the molecules that were formed by reactions on dust grain during the collapse phase. These molecules react in the hot gas at this stage to form prebiotic molecules such as amino acids. By the T Tauri phase, water is present either in the inner disk as vapor or as ice far from the star in the remnant envelope or the outer disk. (Courtesy NASA/JPL-Caltech)

Over the next few hundred million years, the planets and other bodies in the solar system formed from the disk, which still contained prebiotic molecules created earlier. The disk material closest to the star was so hot that all the molecules separated into their component atoms, but the colder disk material farther out, where asteroids and comets formed, did not suffer such destruction of complex molecules. In the farthest reaches of the remnant disk, many of these molecules, including water, existed as ices frozen onto the surface of dust grains.

To understand the processes involved in the creation of prebiotic molecules during the formation of the solar system, astronomers study other young stars as they form from clumps within their own parent molecular clouds. The chemical environment in these young star systems helps us understand how molecules necessary for life in the solar system formed. Water is probably the most important molecule necessary for life, so by studying water in other young star systems, we should be able to understand better where the water on Earth came from and what role it played in the beginnings of life in the solar system.

3. Observing Water Transition Lines

3.1. How do we detect molecules in space?
Water vapor in the ISM is very cold, about 10 K, which is much colder than we normally think of water vapor on Earth. It is possible for water to be this cold without being a solid because the very low pressure conditions in space (almost a vacuum) allow water to exist in its gas phase even at low temperatures. Any gas that has some heat, even if it is very little, will emit radiation at certain wavelengths that correspond to specific energies. The energy of the photons emitted is determined by the energy levels that electrons in the gas molecules or atoms can occupy. When a gas molecule absorbs some of the thermal energy around it, the electrons in that molecule have to move to a higher energy level in order to conserve energy. It doesn't take very long for that excited electron to spontaneously move back down to its previous lower energy level, and in the process, it emits a photon to conserve energy. The photon emitted will have energy exactly equal to the difference in energy between the electron's energy levels (Figure 3, left). The photons emitted by the gas molecules have an energy that is equal to or lower than the thermal energy in the gas, since it is that energy that excites the electrons in the first place. The thermal energy in a 10 K cloud corresponds to photons with very low energy, and so astronomers must observe the cloud at long wavelengths to observe the energy-level transitions in a cold gas like the ISM (Figure 3, right).

Figure 3. (Left) Emission process: an electron in an excited state with energy E2 will spontaneously drop to the ground state with energy E1 and emit a photon with energy E2 E1. (Image credit: adapted from (Right) Wavelength: distance between two consecutive peaks of a wave. If the energy is low, the number of peaks per unit distance will be small and therefore the distance between two peaks will be large; hence low energy corresponds to long wavelength. (Credit: Adapted from Image:Stimulatedemission.png and

The energy levels of electrons in a given molecule or atom are unique. This means energy level transitions within a molecule cause it to emit of photons with unique energies (and hence unique frequencies), and these photons are like the fingerprint of that molecule. It is by identifying photons at specific frequencies that astronomers can determine which and how much of certain molecules are in the molecular clouds where stars are forming. If one molecule is more abundant than another, then its emission line will be brighter because there will be more photons at the corresponding wavelength.

3.2. How do we detect water?

3.2.1. Emission in radio bands
There are very few energy-level transitions that can be excited by gas at 10 K, and the only such energy transition that has been observed for water vapor in cold molecular clouds is the 557 gigahertz (GHz) line. However, there is an inherent problem with trying to observe transition lines of water vapor from Earth: We are surrounded by a thick layer of water vapor at 300 K. Just as water molecules can emit photons of specific frequencies when an electron loses energy, they can also absorb photons at those same frequencies and use the energy from the photon to bump an electron up an energy level. In fact, molecules can absorb only photons that have the exact energy needed to move an electron between levels; slightly higher or lower energy photons are not absorbed.

This creates a problem for astronomers studying interstellar water from Earth, because the water vapor in the atmosphere absorbs all 557 GHz photons before they reach the telescope on the ground. In fact, it is the absorption due to the large amount of water in the atmosphere (Figure 4) that makes it very difficult to observe a wide range of infrared radiation from the ground. The same concept underlies the absorption of UV photons, which are harmful to most forms of life including humans, by ozone (O3) in the upper atmosphere.

Because of the difficulties introduced by water vapor in Earth's atmosphere, water transition lines can be directly observed only from space. Water can be indirectly observed from the ground, however. If the hydrogen (H) in water is replaced by its most common isotope deuterium (D), the result is so-called "heavy water" (D2O). Replacing only one hydrogen per molecule with a deuterium yields HDO, singly deuterated water, which has transition lines that are observable from the ground. The HDO transition lines occur in "atmospheric windows," frequencies at which radiation is not heavily absorbed by Earth's atmosphere. By observing molecules like HDO that are closely related to water but have transition lines that are not absorbed by the atmosphere, astronomers can bring powerful ground-based instruments to bear on the problem of studying water in interstellar environments.

The transitions of deuterated water (HDO) occur at around 470 GHz and 250 GHz, and normal water vapor has a transition at 557 GHz. These frequencies lie in the range of the EM spectrum between the far-infrared and radio, a range of wavelengths between roughly 200 microns (µm) and one millimeter astronomers call the submillimeter wavelength range.

Figure 4. This graph shows the percentage of radiation transmitted by the atmosphere across the frequency range of water transitions in the submillimeter. One of the strongest absorption features, at 557 GHz, is due to water vapor in the atmosphere. At other frequencies, transmission of 50% of the radiation is possible if the air is dry enough. The black curve indicates the percentage of transmission if the water vapor content of the atmosphere is as low as it is at Mauna Kea Observatories (altitude 4,200 m) on a good night. Telescopes sites at lower altitudes are condemned to high water-vapor content above them, so the atmospheric transmission is many times lower than at a high, dry site like Mauna Kea. The two deuterated water transitions occur in relatively high-transmission "atmospheric windows" where the water vapor in the atmosphere does not absorb too much radiation. (Credit: Mark Gurwell,

3.2.2. Absorption in the mid-infrared band
Water ice has the advantage of having transitions in the mid-infrared (5-25 m), where Earth's atmosphere does not absorb as much of the radiation. At these wavelengths, the signatures are often seen in absorption: the cold envelope of gas and dust absorbs radiation from the warm regions surrounding the hot young stellar object (YSO), as illustrated in Figure 5.
Figure 5. Schematic view of infrared absorption toward an embedded source (not to scale). The dust close to the YSO is heated by the latter to 300-1000 K (red shell) and emits a certain amount of radiation in the infrared (red curve on the lower left diagram). The cooler material (light-blue region) absorbs some of this radiation, which shows as the blue troughs on the diagram. The wavelengths at which absorption features occur are characteristic of different molecules, and the depth of the trough gives information on the amount of a given molecule. (Credit: van Dishoeck et al. 2003)

3.3. What telescopes do we use to observe water?
To observe radiation at submillimeter wavelengths from the ground, special telescopes are needed. The largest ones currently in operation are the Caltech Submillimeter Observatory (CSO) and the James Clerk Maxwell Telescope (JCMT), both located on Mauna Kea on the island of Hawaii (Figure 6, top). These telescopes look somewhat like large radio dishes (10 m and 15 m in diameter, respectively), but they detect higher frequency radiation. These two telescopes achieve relatively poor angular resolution, i.e. the images they generate are not very sharp and cannot distinguish small features. However, their large collecting area makes them quite sensitive, and they are also equipped with receivers that have very high spectral resolution. High spectral resolution is needed to resolve the intensity of photons at one wavelength compared to another, which allows astronomers to resolve the width of transition lines that reveal information about the velocity of the gas they originate in.

Complementary to the large single-dish telescopes is the Submillimeter Array (SMA), an arrangement of eight smaller dishes on Mauna Kea designed to produce optimal angular resolution. The concept is similar to that used at the Very Large Array (VLA) of radio dishes in New Mexico: The farther spread apart the dishes are, the higher angular resolution that can be obtained and the higher the image sharpness. This is because two small dishes placed, for example, 36 km apart (or 22 miles, the largest distance between two antennae at the VLA) will yield the same angular resolution as a single dish with a 36 km diameter, but such a large telescope is currently impossible to build. While single-dish telescopes are useful for studying the large-scale end of star formation such as dense cores and envelopes, the SMA is able to resolve smaller scale structures such as circumstellar disks (see Figure 2). In the next decade, the Atacama Large Millimeter Array (ALMA) will produce even sharper submillimeter images (Figure 6, bottom).

Ground-based instruments can observe only in very limited atmospheric windows in the submillimeter, so to observe at all wavelengths, and particularly to directly detect emission lines from water in molecular clouds, space telescopes must be used. The only telescope designed to perform submillimeter observations currently in space is NASA's Submillimeter Wave Astronomy Spacecraft (SWAS). It was launched in 1998 and is still flying, although it is nearing the end of its mission. Despite providing crucial information on water in space, its goals were very specific and capabilities limited. The next generation of submillimeter space telescope will revolutionize this area of astronomy. Indeed, in February 2007, the European Space Agency's will launch the Herschel Space Observatory (HSO), which will be able to observe from 60 to 670 µm with high spectral resolution.


Figure 6. Pictures of ground-based instruments available to study submillimeter radiation. All these telescopes are operated at high, dry sites, where atmospheric absorption due to water vapor is least severe. The CSO (top left), JCMT (top right), and SMA (bottom left) are located on Mauna Kea on the island of Hawaii. ALMA (bottom right) will be built high on a plateau in the Atacama Desert in Chile. (Credits, in order mentioned: Caltech Submillimeter Observatory, Joint Astronomy Centre, Trent Dupuy, European Southern Observatory)

Mid-infrared observations of absorption features due to water ice (e.g., around 3 and 6 m), were first obtained with the Kuiper Airborne Observatory (KAO), an aircraft with an onboard infrared telescope operating between 1 and 500 m that flew from 1975 to 1996, and with the United Kingdom Infrared Telescope (UKIRT) and NASA's Infrared Telescope Facility (IRTF), both located on top of Mauna Kea. The Infrared Space Observatory (ISO, 1995-98) allowed astronomers to make large advances in this area (thanks to its ability to perform in the far-infrared as well), but it could only observe very massive YSOs. Currently, large ground-based telescopes (8-10 m in diameter) are providing the next big step with the observation of low-mass protostars. These include the two Keck and the Subaru telescopes (Mauna Kea) and the Very Large Telescope (VLT) in Chile.

4. Radio and Far-Infrared Observations of Water in the Gas Phase: Science Results

4.1. Observed properties in clouds and dense clumps
ISO observations with the Long Wavelength Spectrometer (45-190 µm) showed that the gas-phase abundance of H2O could be larger than 10-5. However, observations with the KAO showed that in cold clouds (regions with temperatures of 10-100 K where stars form), this abundance is low, ~10-8-10-7. These large variations can be explained by the freeze-out of water at low temperatures (hence it cannot be observed in the gas phase), ice evaporation at warmer temperatures (>90 K) and gas-phase reactions resulting in production of water at high temperatures (>230 K).

More recently, SWAS observation by Bergin & Snell (2002) confirmed the lack of water vapor in cold (temperatures <20K) molecular clouds. Toward two such clouds, they found that the water vapor abundance was smaller than 0.8-3 x 10-8. They also compared their observations with a chemical model that includes both gas-phase and grain-surface chemistry and found that the predictions did not conflict with the observations

Ice mantles surrounding dust grains have been observed in the dense, prestellar cores. The abundances of polar molecules such as H2O were found to be much lower than what gas phase reactions predict. This is because polar molecules "freeze out" or "deplete" (i.e., stick) onto the grain mantles in the central, densest (density larger than a few times 104 per cm3) parts of these cores.

To probe the central region, astronomers carry out observations of molecules such as NH3 (see Figure 7) that do not deplete at the above high densities because their chemical properties are such that they do not have a tendency to stick to grain mantles, unlike other molecules such as CS that do stick more easily on grain surfaces at high densities and are therefore not observed towards the central regions of prestellar core, as in Figure 7.

Figure 7 CCS (thioxoethenylidene, green contours for the low-resolution data, grayscale for the high-resolution observations), CS (carbon monosulfide, red contours), and NH3 (blue contours) observations of the prestellar core L1498 showing a chemically differentiated structure with CS and CCS more abundant in the atomic-carbon-rich, low-density outer part, and NH3 peaking in the high-density inner region. (Credit: T. B. H. Kuiper,
A characteristic of the regions that will form low-mass (Sun-like) stars is that they show enhanced deuteration. Deuterium is a form of hydrogen that was 105 times less abundant than hydrogen after the Big Bang (so called cosmic abundance). When a deuterium atom is substituted for a hydrogen atom in a molecule (for example HDO instead of H2O), the molecule is said to be deuterated. In dense cores, deuterated species are only 10 to 100 times less abundant than their non-deuterated counterparts, i.e. deuteration is 100 to 1,000 times more than cosmic abundance. This enhancement in deuterated molecules comes from the fact that most of the substitution is generally thought to occur through a reaction involving the ion H2D+, and the main destruction mechanism of this ion involves CO. However, as mentioned, most of the CO is frozen onto the grains in these cores, hence there are less destruction paths for H2D+ and therefore more deuterated molecules can be produced.

4.2. Hot cores

The hot core stage occurs shortly after a protostar is born. It consists of a small region (<150 AU for low-mass protostars and <20,000 AU for massive protostars) surrounding the protostar and has the following characteristics:
  • high temperatures: >100 K, compared to about 30 K in the outer envelope
  • high densities: 107 per cm3
  • a rich chemical inventory: presence of complex, prebiotic molecules

The high temperatures allow the grain mantles to evaporate, liberating molecules formed by grain surface chemistry during the collapse phase (the phase leading from the dense core to an embedded protostar). The liberated molecules then react to form the observed complex molecules. Because of their small sizes, the hot cores of low-mass, solar-type protostars (the so-called "hot corinos") can be imaged only with interferometers such as the SMA. Such images show that, like massive hot cores, these small regions contain complex organic molecules. These results are of major importance for understanding the formation and evolution of prebiotic molecules in the environment of the proto-Sun.

4.3. Disks
Planetary systems form from the disks surrounding protostars (Figure 8). Therefore, it is important to understand the chemical processes in these environments in order to follow the evolution of the prebiotic molecules observed in hot cores. Indeed, gas and dust in disks may undergo a complex chemistry before being incorporated in the protoplanets, and a better knowledge of the chemistry in disks would provide information not only on the chemical but also on the dynamical conditions.

Deuterated molecules can be used as a probe of the temperature history of the gas. As mentioned in section 4.1, deuterated molecules in cold cores are more abundant than expected. Unfortunately, there are so far only a few measurements of deuterated molecules in disks, but recent studies show that disks are highly deuterated. In fact, measurements show that abundances of deuterated species in disks are similar to those found in cold cores. This provides evidence for the low temperatures in disks and for the fact that freezing out (which has been observed in cold cores and is necessary for a high deuteration) also occurs in disks.



Figure 8. NASA's Hubble Space Telescope has given astronomers their first views of a dust ring around the star HR 4796A and a dark gap dividing an immense dust disk around the star HD 141569. ( Credit: left, Alycia Weinberger, and Eric Becklin, UCLA; Glenn Schneider, University of Arizona; and NASA; right, Brad Smith, University of Hawaii; Glenn Schneider, University of Arizona; and NASA)


Moreover, the analysis of cometary jets, which occur when a comet enters the inner solar system and expels the most pristine material located below its surface, indicates high deuteration, only a factor of 10 lower than that of cold cores and disks. This result can be interpreted in two ways: either the gas spends most of its lifetime at low temperatures and is incorporated into the disk before the protostar heats up and dissipates the envelope, or the deuterated species is enhanced by low-temperature gas-phase chemistry (similar to that of cold cores) in the disk

4.4. Envelopes surrounding massive young stellar objects
Envelopes surrounding massive YSOs can be studied in the mid-infrared by looking at the absorption of radiation from the embedded YSO by the envelope (section 3.2.2). This type of observation yields a spectrum, a graph showing the flux (a measure of brightness) as a function of wavelength (Figure9). ISO observations provided evidence that water ice is the main component of grain mantles. These observations also revealed the presence of other types of ices, such as CO2, CH4 (methane), and CH3OH (methanol), with abundances ranging from <1% to 30% of that of water ice.

Ground-based infrared observations made with the VLT also showed no absorption feature at the wavelength corresponding to HDO ice. The derived abundance of HDO is at most only 0.2 to 1% that of H2O. This provides a measure of deuteration that is lower than the values measured in cold cores and disks. However, it is consistent with that found in the coma of comets, the cloud of gas and dust that surrounds the comet's core includes gases evaporating from the comet's upper layers. This indicates that, unlike the deep layers mentioned in the previous section, the surface layers of comets have undergone processing in the protoplanetary nebula. Ice processing is illustrated in Figure 10 (left).


Figure 9. ISO spectrum of the deeply embedded massive YSO W33A. Various absorption features due to silicate grain cores and icy mantles are indicated. Regions that cannot be observed from the ground are in dark blue. (Credit: Gibb et al. 2000; van Dishoeck et al. 2003)
Figure 10. (Left) These diagrams represent the temperature-dependent processes involved in ice processing: freeze-out of CO (black) on water ice (top), diffusion (middle), and desorption, or removal (bottom), of the upper layer of CO. (Right) Modeling of the corresponding processes on the left: The green curve matches very well the observations shown by the black curve. The other colors represent the components of the model. (Credit: van Dishoeck et al. 2003)

4.5. The case of NGC 1333
A study of the star-forming region called NGC 1333 was carried out with SWAS (Bergin et al. 2003) to examine the production of water in quiescent and shocked molecular gas in this star formation region. Three distinct types of water were found:
  • Emission associated with outflows, characterized by broad features: this emission is large and could be due to the outflow producing a shock in the gas it encounters on its path through the envelope. The energy released from the shock would heat up the dust grains and release the water molecules that froze out during the collapse phase.
  • Narrow emission associated with the quiescent gas: in the low-density quiescent gas, small abundances of water were measured.
  • Absorption by the foreground quiescent gas against the outflow emission.

This and other studies showed that water is the most important molecule in oxygen chemistry and that it plays a dominant role in the energy balance and chemical evolution during star formation. Water is found in both solid and gas phase and in a large range of abundances, depending on the environment. Many advances have recently been made thanks to the presence of several ground- and space-based observatories, but more work needs to be done to understand the detailed role of water in all the discussed processes.



Bergin, E. A., & Snell, R. L. 2002. Sensitive Limits on the Water Abundance in Cold Low-Mass Molecular Cores. Astrophysical Journal (ApJ) Letters, 581, 105-108.

Bergin, E. A., Kaufman, M. J., Melnick, G. J., Snell, R. L., & Howe, J. E. 2003 A Survey of 557 GHz Water Vapor Emission in the NGC 1333 Molecular Cloud. ApJ, 582, 830-845.

Gibb, E. L., & 10 colleagues. 2000. An Inventory of Interstellar Ices toward the Embedded Protostar W33A. Astrophysical Journal (ApJ), 536, 347-356.

Kuiper, T. B. H., Langer, W. D., & Velusamy, T. 1996. Evolutionary Status of the Pre-Protostellar Core L1498. ApJ, 468, 761-773.

van Dishoeck, E. F. 2004. ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks. Annual Review of Astronomy and Astrophysics (ARAA), 42, 119-167.

van Dishoeck, E. F., & Blake, G. A. 1998. Chemical Evolution of Star-Forming Regions. ARAA, 36, 317-368.

van Dishoeck, E. F., & 8 colleagues. 2003. Origin and Evolution of Ices in Star-Forming Regions. The Messenger, no. 113, 49-55.

Weisskopf, V. F. 1983. Knowledge and Wonder: The Natural World As Man Knows It. ( Cambridge, MA: MIT Press).

For More Information:



absolute zero: A bsolute zero (0 K) corresponds to −273.15° C or −459.67° F.

absorption: A process by which the energy of a photon is transferred to an atom, a molecule, or a dust particle. The photon is destroyed in the process. The atom or molecule enters an excited state (an electron inside the atom/molecule jumps to a higher energy level), and the temperature of the dust particle increases.

absorption line: The lack of radiation due to the presence of cold gas between the observer and a heat source, producing a trough below the background radiation. Absorption lines occur at specific wavelengths, depending on the absorbing atom/molecules that constitute the cold gas.

abundance (dimensionless) : The ratio of the density of a molecule with respect to the density of molecular hydrogen (H2). It gives the number of other molecules present for one molecule of H2.

amino acids: Large molecules that are the building blocks for proteins. Twenty of them are also present in the genetic code.

angular momentum: The momentum a body has due to the motion around a given point of origin. For example, Earth has two angular momenta, one from its rotation on its axis, and one from its motion around the Sun.

angular resolution: The ability for a telescope to distinguish between features that are close to each other, but located at a large distance from the observer, just as the two headlights of a car look like one from far away.

atmospheric window: A range of frequencies for which most of the radiation reaching the top of the atmosphere also reaches an observer on the ground without too much radiation being absorbed.

circumstellar disk: A disk of gas and dust surrounding a YSO in the later stage of its evolution. Planets can eventually form from this disk.

coma: A cloud of water vapor, carbon dioxide, and other gases that are boiled off the comet as it gets close to the Sun. At 1 AU, the coma can be as larger as 10-100 times the diameter of the Earth.

dense core: High-density region of a molecular cloud that can collapse under its own weight to lead to the formation of a protostar.

desorption: In astronomy, this is the removal of an atom or molecule (by, for example, evaporation) from the surface of a grain mantle.

diffusion: In the case of diffusion of CO in a water ice mantle, this is the penetration and spreading of CO molecules into the layer of water ice surrounding a dust grain.

dust: Generic term for grains of conglomerate interstellar material with silicate (SiO2) or carbonaceous (containing carbon) cores. The grains tend to be "sticky" when colliding with polar molecules. Dust is primarily responsible for the preferential scattering of blue radiation, which is the reason why molecular clouds are only transparent to red wavelength radiation.

electromagnetic (EM) radiation: Wave carrying energy and momentum and characterized by a wavelength or energy. EM radiation whose wavelength is between 400 and 700 nm (nanometer; 10-9m) is referred to as (visible) light.

electromagnetic (EM) spectrum : Ensemble of EM radiation at all possible wavelengths or energies, from the high-energy/short-wavelength gamma rays to the low-energy/long wavelength radio waves.

emission: A process in which an electron (in an atom or a molecule) goes from a high energy level to a lower one. This loss of energy results in the creation of a photon, which carries an energy corresponding to the difference in energy of the two levels. For further details, see Figure 3 and text, or Weisskopf (1983).

emission line: The excess of radiation coming from a hot gas that produces a peak above the background radiation. Emission lines occur at specific wavelengths, depending on the emitting atom/molecules that constitute the gas.

far-infrared: The range of EM radiation between 25 and 200 µm.

freezing out: The process by which gas molecules collide with dust grains through random thermal motion and stick to them. Over the volume of a molecular cloud, this process can transfer a significant number of molecules onto the surfaces of dust grains, where, technically speaking, they are in the solid phase. Dust grains mixed with cold gas tend to develop a layer, or mantle, of ices on their surfaces due to this process.

gigahertz: One billion hertz (Hz). For a light wave, the number of hertz is the number of peaks occurring in a second.

gravitational potential energy: In a dense cloud clump, this is the energy stored by a particle because of its distance from the central, densest part; the closer the particle, the smaller the energy. As the cloud collapses, the particles get closer and need less energy, so they convert excess energy into heat.

kelvin (K): Temperature scale for which the zero is the absolute zero. The temperature in Kelvin, TK, is related to the temperature in degrees Celsius, TC, by TK = TC + 273.15, and to the temperature in degree Fahrenheit, TF, by: TK = (TF + 459.67)/1.8

hot corino: The small, dense, warm, and chemically rich region surrounding a low-mass protostar.

interferometer: An interferometer combines the signal from several telescopes, which can be separated by a few meters or many kilometers (for example, the Very Large Array). An interferometer can achieve very high angular resolution (the angular resolution that a single-dish would have if it had a diameter equal to the largest separation between any two telescopes of the interferometer), thereby probing very small regions of the interstellar medium.

interstellar medium (ISM): The term astronomers to describe all gas and dust in the plane of the galaxy that exists "between the stars." The ISM can be ionized if it is diffuse and near a radiation source (such as a star), but the denser more isolated regions of the ISM typically consist entirely of molecules both in the gas phase and frozen onto dust grains.

isotope: Atoms are made of protons, electrons, and neutrons (positively charged, negatively charged, and neutral particles, respectively). The isotope of a given atom contains the same number of protons and electrons, but not of neutrons. In the case of hydrogen (1 proton, 1 electron), its isotope deuterium contains an extra neutron, making its weight greater than that of hydrogen, hence its name of "heavy hydrogen."

light-year: The distance light travels in one year, 9.46 trillion (1012) km or 5.88 trillion miles.

mantle: A layer of ice on the surface of a dust grain.

micron (µm): Micrometer, that is, one millionth of a meter.

mid-infrared: The range of EM radiation arbitrarily classified by astronomers as between 5 and 25 µm.

molecular cloud: A cold (10 K), dense (102 to 109 per cm3) region of the ISM opaque to visible and UV radiation where all of the gas is in molecular form. Most of the gas is hydrogen (H2), but there is also carbon monoxide (CO) and other molecules.

momentum: A quantity relating to the mass and velocity of an object. The heavier and faster an object is, the more momentum it has.

NGC: An item listed in the New General Catalog of Nebulae and Clusters of Stars, which was published in 1888 and contains 7,840 galaxies, nebulae, and star clusters.

outflows: As a protostar grows by accreting material from its surrounding envelope, it spins faster and faster, and needs to get rid of some energy in order to slow down the spinning rate. It does so by shooting out jets (or outflows) along its spin axis.

parsec (pc) : A distance equivalent to 3.09 x 1016 m, 1.92 x 1013(19.2 trillion) miles, or 3.26 light-years.

photon : EM radiation can behave as a wave or as an ensemble of particles called photons. Short wavelength radiation (equivalent to high-frequency radiation) consists of photons of high energy, whereas long-wavelength radiation (equivalent to low-frequency radiation) consists of photons of low energy.

polar molecule : A molecule in which the positive and negative charges are not uniformly distributed and that acts like a magnet.

prebiotic molecule : A molecule that is used to synthesize large molecules (such as amino acids) that are necessary for the development of life.

protostar: An object that has condensed from the collapse of a molecular cloud and emits radiation only at very long wavelengths (submillimeter to millimeter).

radio: The range of EM radiation with wavelength greater than 1 millimeter (mm).

scattering: The deflection of a photon by dust.

shock: The collision of a fast-moving gas into a slow-moving one that produces high temperatures and enhanced densities, which allow the formation of certain molecules.

spectral resolution: The ability of a telescope's receiver to distinguish between the signatures of two molecules that have very close frequencies. This is like having an FM radio with a tuner sensitive enough to be able to tune in a radio station that is close in frequency to another without interference from the other station.

thermal energy: The energy due to the random motion and vibration of microscopic particles such as molecules and atoms. Heat is in fact the transfer of thermal energy.

young stellar object (YSO) : A generic term to describe the early phases of the evolution of a star, from the protostar to the point right before hydrogen fusion begins (which defines an "adult" star).