Looking up at the night sky, dotted with distant lights that slowly precess, one might be fooled into thinking that the universe is serene. In fact, it is racked with violent collisions and explosions that send shock waves of all scales throughout interstellar space. Those shock waves aren’t just cosmic waves in the ocean; they kick-start the formation of stars and set up surprising astrochemical laboratories.

NASA/ESA/CSA/STSCI/KLAUS PONTOPPIDAN (STSCI)/ALYSSA PAGAN (STSCI)

NASA/ESA/CSA/STSCI/KLAUS PONTOPPIDAN (STSCI)/ALYSSA PAGAN (STSCI)

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In general, a shock is created when fast-moving gas travels through a medium at a speed greater than the speed of sound. At large scales, galactic flyby encounters, catastrophic galaxy merging, and black holes at galactic centers all create interstellar shock waves that can travel across the galaxy for tens of parsecs and hit whatever is in their path. In the context of star formation, shocks are generated by gigantic collisions between molecular clouds, supernovae explosions, and stellar winds. Even events as common as star births will propel gas through the interstellar medium (ISM) and potentially trigger more star formation.

The space between stars is not an empty void. Rather, the ISM is filled by tenuous and cold gas and dust. The story that will eventually end up with the birth of a star and its planetary system starts inside gigantic ISM cold clouds of molecular hydrogen. Within the clouds are regions known as prestellar cores. They are regions of cold (10–20 K) gas that extend to several thousand astronomical units (AU). Their densities are about 105–106 cm−3, which correspond to extremely low pressures that are difficult to achieve in terrestrial laboratories (see box 1). That density is large enough to make the gravitational force prevail over any force, especially the outward pressure from the movement of atoms, or thermal pressure, that prevents the collapse.1,2 

Box 1.
The nurseries of stars

Molecular clouds contain the progenitors of planetary systems. They are huge in size, from a few to several tens of light-years in diameter, and contain hydrogen, the most abundant element in the universe, in the molecular form, H2. Despite their size, the clouds are tenuous with respect to terrestrial standards. Molecular clouds contain about 103–104 particles/cm3, equivalent to a pressure of about 10−13–10−12 torr. To put that in perspective, pressures of 10−13–10−9 torr are obtained with ultravacuum techniques in terrestrial laboratories. In other words, the clouds are regions almost void of matter.

NASA, ESA, CSA, STSCI, JOSEPH DEPASQUALE (STSCI), ANTON M. KOEKEMOER (STSCI), ALYSSA PAGAN (STSCI)

NASA, ESA, CSA, STSCI, JOSEPH DEPASQUALE (STSCI), ANTON M. KOEKEMOER (STSCI), ALYSSA PAGAN (STSCI)

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Although they are almost void, molecular clouds still contain large amounts of matter, from a few to millions of solar masses, more than enough to form stars and planets. In addition, molecular clouds have very low temperatures, about 10 K, which makes their pressure caused by the movement of atoms low. In regions with relatively larger densities (105–106 particles/cm3), called prestellar cores, the gravitational force will eventually dominate expansive forces. The collapse will lead to the formation of a star and its planetary system.1,2 

The picture to the right, taken in the IR by the James Webb Space Telescope, shows the region called the Pillars of Creation in the Eagle Nebula. The region contains many molecular clouds, where several star and planetary systems are in the process of forming. Because this image was taken using IR wavelengths, we can see through much of the dust to the young stars within.

The interplay between molecular clouds and interstellar shocks turns the study of shocks into a study of star formation. Understanding how shocks form and how they enrich the gas around them helps astronomers understand the evolution of our universe. The overdensity can be caused by the turbulence inside the molecular clouds, or sometimes by violent events that create interstellar shocks.

Most dying stars don’t extinguish quietly. They eject most of their interior material either in massive winds or explosions. In both cases, shock waves expand out in all directions, traveling through much of the galaxy. When the waves interact with a molecular cloud, they compress the gas, which increases its density. If the density becomes large enough, it will trigger a collapse of the molecular cloud (see box 1).

Supernova explosions also cause shocks in the magnetized gas of the surrounding ISM and create cosmic rays, particles with teraelectron-volt energies that permeate the galaxy and interact with molecular clouds, triggering the formation of molecules like carbon monoxide. Radiation emitted in transitions between CO’s rotational states (so-called rotational lines) then cools the cloud to a mere 10–20 K, which reduces the thermal pressure and triggers a cloud collapse and subsequent stellar birth.

Interstellar shocks can additionally be triggered by star formation. Young, massive stars (more than 8 solar masses) emit large amounts of UV photons that ionize the surrounding gas. The resulting so-called HII regions expand into the ISM and create shocks at the interface of molecular clouds.

Less massive stars don’t emit enough UV radiation to create shock waves, but they do create protostellar jets when they are born. A star is formed in a prestellar core, which extends thousands of astronomical units but will eventually form a planetary system only a few tens of astronomical units across. Prestellar cores are embedded in the Milky Way, which rotates as a rigid body. When they collapse, they acquire an angular momentum that needs to be eliminated. The most important mechanism to accomplish that is the ejection of a large fraction of the collapsing gas by protostellar jets. Those supersonic jets produce molecular outflows emanating from the forming star. Eventually those jets will collide with surrounding material and create disruptive shocks.

How do astronomers study invisible shock waves? They are adept at analyzing a phenomenon by observing its effects on the surrounding environment. As an example, figure 1 captures NGC 1333, a region in the Perseus molecular cloud where several Sunlike stars are forming. Although the gas is too cold to be visible in the optical or IR spectrum, radio observations reveal the velocity of the molecular cloud. Expanding holes and moving filaments modify the morphology of the cloud on a large scale in reaction to the star formation. Shocks from protostellar jets emanating from several protostars alter the gas on a smaller scale. The current molecular cloud, however, doesn’t tell astronomers which kinds of shocks caused the change in morphology, nor which mechanism caused the shock. To learn that, they need to look at the molecular makeup and the magnetic field.

Figure 1.

The molecular cloud NGC 1333 is one of the closest stellar nurseries to Earth. It is too cold to be seen in visible light, and only a few of its protostars—SVS 13A and IRAS 4A—appear in the IR (left). Instead, radio observations (right) are used to see the molecular cloud and its movement in the line-of-sight direction. NGC 1333 is being shattered by interstellar shocks pushing different parts of the cloud at different speeds and in different directions. (Adapted from ref. 9.)

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Figure 1.

The molecular cloud NGC 1333 is one of the closest stellar nurseries to Earth. It is too cold to be seen in visible light, and only a few of its protostars—SVS 13A and IRAS 4A—appear in the IR (left). Instead, radio observations (right) are used to see the molecular cloud and its movement in the line-of-sight direction. NGC 1333 is being shattered by interstellar shocks pushing different parts of the cloud at different speeds and in different directions. (Adapted from ref. 9.)

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In the molecular gas, where the ionization is low (the relative electron abundances with respect to the hydrogen atoms are less than 10−4), there are two major types of shocks, as seen in figure 2: Jump, or J, shocks have an abrupt change in physical parameters; continuous, or C, shocks are mediated by the presence of a magnetic field. Those types are the two extremes of the range of possibilities regulated by the strength of the magnetic field with respect to the velocity of the shock.3 

Figure 2.

Two supersonic jets (left) emanating from a young protostar (here, HH 211, imaged in the IR) in opposite directions are shaped by various shocks erupting from the birth of the star. The jets flow away from the star, with the farthest gas being the first to escape. Shock types (right) exist on a spectrum that ranges from jump shocks (J type) to continuous shocks (C type); which type of shock depends on how gradually the velocity decreases over time. A smoother velocity change indicates the presence of a magnetic field.

ESA/WEBB, NASA, CSA, TOM RAY (DUBLIN)

Figure 2.

Two supersonic jets (left) emanating from a young protostar (here, HH 211, imaged in the IR) in opposite directions are shaped by various shocks erupting from the birth of the star. The jets flow away from the star, with the farthest gas being the first to escape. Shock types (right) exist on a spectrum that ranges from jump shocks (J type) to continuous shocks (C type); which type of shock depends on how gradually the velocity decreases over time. A smoother velocity change indicates the presence of a magnetic field.

ESA/WEBB, NASA, CSA, TOM RAY (DUBLIN)

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The most important consequences of the passage of the shock wave are gas compression and heating. But shocks also are crucial in enriching the gas. As the shock wave passes through, the interstellar grains of dust are sputtered and shattered, and molecular species that were previously frozen into icy mantles that envelop the grains are released (see box 2). Although the gas remains compressed, its temperature decreases because of H2O molecules injected from the icy mantles, which emit photons that cool the gas.

Box 2.
Interstellar dust grains

Interstellar dust grains are submicron solid agglomerates of silicates and carbonaceous material that permeate most of the interstellar medium. Although the dust particles represent only about a hundredth of the mass of the interstellar medium, they have a huge role in molecular cloud formation and evolution.

In molecular clouds, dust grains are composed of refractory silicates and carbonaceous cores, encased by volatile mantles. The mantles contain water, carbon dioxide, carbon monoxide, and other less abundant but more complex molecules, such as formaldehyde and methanol. Most of them do not have enough energy to return to a gaseous state but instead remain frozen on the dust-grain surface.

Interstellar shocks can send those frozen molecules back into a gas phase through two processes: mantle and grain sputtering and shattering. In the former, ions and atoms propelled by shocks hit the grains and eject molecules and atoms from the mantles and refractory cores into the gas. In the latter, the shocks are so violent that the grains collide and fragment, causing the species in both the mantle and the refractory core to be injected into the gas.

As a result, several molecules are abundant in the shocked gas, regardless of shock type. Because some of those molecules are almost exclusively present in shocked regions, it is easier for astronomers to find and study them. The molecule used most often to trace interstellar shocks is silicon monoxide, primarily because more than 99% of it is trapped in interstellar dust grains and only a tiny fraction is gaseous. In a molecular gas, the few silicon atoms not locked into dust grains form SiO, a linear molecule whose rotational transitions can be observed with ground-based telescopes operating between radio and millimeter wavelengths.

The measured abundance of gaseous SiO with respect to H2 in molecular clouds is 10−12 or less. The sputtering and shattering from molecular shocks, however, makes gaseous SiO a million times more abundant. Once in the gas phase, SiO can be observed at high spectral and spatial resolutions, thus allowing astronomers to study the physical structure and properties of the triggering shock.

Water is another tracer of shocks, even though it is not as exclusive as SiO. Water molecules are mostly formed on the grain surfaces by the successive addition of hydrogen atoms to frozen atomic oxygen. That means gaseous water is scarce in molecular clouds and becomes abundant in shocked regions.

Both tracers are used to observe shocks around young protostars. Figure 3 shows the protostar HH 212-mm traced by SiO, and figure 4 shows the protostar L 1157-mm traced by water.4,5 Both protostars possess jets emanating outward for hundreds of astronomical units that create shocks, traced by SiO and water.

Figure 3.

The shocked gas in the jet and outflow emanating from the young protostar HH 212-mm, shown in continuum light, is traced by silicon monoxide, abundant because the shocks caused by the jet sputter and shatter silicate grains in the interstellar medium. The silicon liberated from the grains is quickly oxidized into SiO, which can be easily observed with ground-based radio telescopes. (Adapted from ref. 4.)

Figure 3.

The shocked gas in the jet and outflow emanating from the young protostar HH 212-mm, shown in continuum light, is traced by silicon monoxide, abundant because the shocks caused by the jet sputter and shatter silicate grains in the interstellar medium. The silicon liberated from the grains is quickly oxidized into SiO, which can be easily observed with ground-based radio telescopes. (Adapted from ref. 4.)

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Figure 4.

Formamide (NH2CHO) is an astrobiologically important molecule that is also formed in interstellar shocks. The molecular outflow in different regions of the protostar L 1157-mm (left) is traced by water. The B1 region (right) is abundant with deuterated formaldehyde (HDCO), indicated by the white contours. The fact that the contours of HDCO lag behind those of NH2CHO, indicated by black contours, means that the former was created after the passage of an older shock. (Left image adapted from ref. 5; right from ref. 8.)

Figure 4.

Formamide (NH2CHO) is an astrobiologically important molecule that is also formed in interstellar shocks. The molecular outflow in different regions of the protostar L 1157-mm (left) is traced by water. The B1 region (right) is abundant with deuterated formaldehyde (HDCO), indicated by the white contours. The fact that the contours of HDCO lag behind those of NH2CHO, indicated by black contours, means that the former was created after the passage of an older shock. (Left image adapted from ref. 5; right from ref. 8.)

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In addition to releasing SiO and water into the gas phase, shocks also liberate whichever other molecules are frozen into dust-grain mantles (see box 2). That makes shocks a rich laboratory for astrochemistry. In those regions, astronomers are able to glean crucial information about the chemical composition of nascent planetary systems.

Molecules that previously were frozen into grain mantles can be observed in their gaseous state via their rotational lines by powerful ground-based telescopes, which are capable of detecting small relative abundances down to approximately 10−13 with respect to H2. To put that in perspective, frozen SiO and water molecules can be observed in IR wavelengths by less powerful telescopes that can detect only abundances greater than 10−7.

Two properties make shocks unique for astrochemistry studies. The first is that shocks liberate the components of the grain mantles into the gas. The second is that astronomers can pinpoint almost exactly when the shock passed, providing one of the most elusive pieces of information in astronomy—a clock. The two aspects make it possible to study the abundance evolution of the molecules, whose release into the gas phase and formation are triggered by the interstellar shocks. There are almost no other objects for which that kind of study can be done.

One molecule that can be studied in shocks is formamide (NH2CHO). The small, abiotic molecule may be at the origin of large prebiotic genetic and metabolic compounds. Raffaele Saladino and colleagues argue that the presence of a small drop of formamide in the early Earth may have been the starting point of life—it would solve the question of whether metabolism appeared before genetics or after.6 Astronomers know that formamide is relatively abundant in regions that will eventually form planetary systems similar to our solar system, but they do not know how or when the molecule is formed.7 The answers may lie in closer study of molecular shocks caused by protostellar outflows.

For example, formamide rotational lines were observed in the shocked region L 1157-B1 of the protostar L 1157-mm. Combining the measurements of formamide abundance and the age of the shock, we and our colleagues were able to identify what caused the shock: a chemical reaction that can occur in every region that forms a solar-type planetary system.8 If formamide is a key molecule for the emergence of life, as Saladino and colleagues purport, then life may be rather common. As unlikely as it may seem, studying interstellar shocks may provide astronomers with that information.

Interstellar shocks profoundly shape the morphology and evolution of galaxies by directly and indirectly triggering the formation of stars. Shocks of all velocities, sizes, and properties can cause the collapse of molecular clouds. Amid the dramatic interactions, they can provide astronomers with an almost unique set of information about the regions that form solar-type planetary systems and give clues about our own origins. One could say that interstellar shocks are the behind-the-scenes, mysterious Drosselmeyer of the galactic Nutcracker ballet: They distribute the toys and control the plot: the life and death of stars and of the galaxy itself.

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Cecilia Ceccarelli is a professor at the Institute for Planetary Sciences and Astrophysics, Grenoble at Grenoble Alpes University in France. Claudio Codella is a director of research at the Arcetri Astrophysical Observatory, part of the National Institute for Astrophysics, in Florence, Italy.