Gas-Phase Models
Gas-phase models are conceptual frameworks used in astrochemistry to explain the processes of star formation primarily through interactions among gaseous elements. Within the universe, the interstellar medium (ISM) consists of gas and dust that forms molecular clouds, often referred to as "stellar nurseries," where stars are born. The majority of these clouds are composed of hydrogen gas at extremely low temperatures, which inhibits chemical reactions but allows for ion-molecule interactions. The formation of molecular hydrogen is crucial in the process, as it occurs when hydrogen atoms share electrons.
As gas in these molecular clouds becomes denser, it eventually collapses under its own gravity, leading to the birth of protostars, the early stages of star development. Not only gas but also fine dust in the clouds contribute to star formation by influencing temperature and facilitating chemical reactions. The gas-grain model acknowledges the role of dust, suggesting that gas-phase molecules can freeze onto dust grains, enhancing the complexity of chemistry within molecular clouds.
Challenges in studying these models stem from the obscured nature of star formation processes, which occur within the clouds, and the difficulty in detecting molecular hydrogen. Instead, researchers often rely on observing carbon monoxide as a proxy to identify areas rich in hydrogen. By utilizing specialized telescopes and laboratory simulations, scientists aim to deepen their understanding of star formation, which is essential for grasping the universe's evolution and the origins of life on Earth.
Gas-Phase Models
FIELDS OF STUDY: Astrochemistry; Stellar Astronomy
ABSTRACT: Elements in a gaseous state make up the vast majority of the dense molecular clouds where stars are formed. Scientists have identified several ways to calculate or model the chemical reactions between these gases and other items around them that are responsible for the formation of stars. The gas-phase model is important to understanding the origins of stars and the role that they played in the formation of the universe.
Models of Star Formation
Space is often envisioned as groups of stars or planets surrounded by vast stretches of empty space. However, the area between star systems in a galaxy is filled with matter called interstellar medium (ISM). This mixture of gas and dust combines to form interstellar clouds of varying density, temperature, and composition. Areas with denser collections of ISM are known as molecular clouds. This is because most of the gases within them are sharing electrons in a molecular state.
Molecular clouds are sometimes called "stellar nurseries" because most, if not all, stars are born within such clouds. Star formation occurs when something reacts with the gas in a molecular cloud, causing it to become denser and collapse in on itself. This gaseous mass then breaks away from the cloud to form a protostar, the core or seed of a new star. The study of these and other chemical reactions in space is called astrochemistry. When all of the components of the reaction that creates the star are gaseous, astrochemists refer to it as a "gas-phase model."
Scientists believe that the very fine dust in a molecular cloud can also play a role in star formation, even though it makes up a very small portion of the cloud’s matter. Sometimes these tiny grains of dust interact with the gas in a way that changes its temperature and starts the reaction that increases its density. This model of star formation is known as the "gas-grain model."
The Chemistry of Star Formation
Molecular clouds are estimated to be about 99 percent gas and 1 percent dust. The majority of the gas is hydrogen gas, though some helium and other elements are also present. These clouds are very cold, with temperatures of about 10 to 20 kelvins, or just above absolute zero.
In this extreme cold, it is generally not possible for a chemical reaction to take place among the various gaseous elements. Instead, most of the chemistry in the gas phase takes place via ion-molecule reactions. Ultraviolet light, often emitted by young stars nearby, can provide the heat and energy needed to make molecules. Molecular hydrogen is formed when two hydrogen atoms share their electrons. This molecular hydrogen is essential to the formation of new stars.
The cold temperature causes the gas molecules to form clumps. These clumps grow denser until they collapse under their own gravity. The collapse causes pressure to build, increasing the temperature in the newly formed core until fusion becomes possible and the star is created.
In many cases, clusters of young stars are found close to other clusters of young stars. This is often caused by a supernova explosion of a massive star. The explosion creates shock waves that cause the molecules of hydrogen to compress and form new stars.
Researchers have also come to appreciate the importance of dust in star formation. Scientists came to understand that many of the types of molecules they could identify in the molecular clouds could not be created solely by gas-phase elements. It is now believed that some stars are formed by the molecular gas interacting with dust particles. In the cold conditions in the cloud, the gas-phase molecules freeze into an icy mantle around the dust. The hydrogen molecules later return to the gas phase. This gas-grain model of star formation helps account for the richness of the chemistry in molecular clouds, which cannot be accounted for in models that only allow for gas-phase interactions.
Challenges of Studying Star-Formation Models
Astrochemists encounter several difficulties in trying to study any model of star formation. All of the processes that lead to the birth of stars occur within the molecular cloud, obscured from the view of even the most powerful telescopes. In addition, molecular hydrogen is extremely difficult to detect. These processes may also take tens of thousands of years to move from stage to stage. Therefore, it is virtually impossible to follow the development of any particular star.
Embryonic and newborn stars do not give off their own visible light. However, they and the processes that create them do emit light in the nonvisible spectrum, such as infrared. Specially equipped telescopes can detect this light, allowing researchers to observe the process. They can find the areas of molecular hydrogen ripe for star formation by looking for carbon monoxide. Scientists have determined that carbon monoxide is found in space at a ratio of one carbon monoxide molecule for every ten thousand hydrogen molecules. Carbon monoxide is easier to detect in space, so searching for those molecules allows researchers to find areas of hydrogen molecules as well. In addition, scientists can determine the age of a star based on the light it emits and the elements it is burning to produce it, among other factors. So although studying one star through its entire lifetime is not possible, scientists can form theories about the future of young stars by comparing them to other, older stars.
Ongoing Efforts to Understand the Gas-Phase Model
Astrochemists study the gas-phase reactions of star formation using satellites and specially equipped telescopes. They also seek to recreate some of the reactions and processes that are part of star formation in laboratories on Earth. Understanding the processes that lead to the formation of stars is important to understanding how the universe was created, how it is changing, and what the future might have in store.
Studying the origin of stars is important for another reason. The same matter that makes up stars makes up everything on Earth, including people. In laboratory conditions, scientists have been able to observe organic molecules that are the precursors of life in ice that has been treated to replicate the conditions in star-formation areas. Through continued study of stars, scientists in a range of disciplines from astrophysics and astrochemistry to biology hope to gain a greater understanding of how the formation of the universe led to life on Earth.
PRINCIPAL TERMS:
- astrochemistry: the study of chemical elements in the universe and their interactions with each other and with radiation.
- gas-grain model: the theory that grains of space dust interact with gas to form stars.
- interstellar medium: a mix of gas, dust, and cosmic rays that fills the space between star systems in a galaxy.
- protostars: masses of gas formed from the collapse of a giant molecular cloud that will eventually become stars.
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