Gas-Grain Models
Gas-grain models are a crucial framework in understanding star formation, particularly within the complex environments of molecular clouds, often referred to as stellar nurseries. These clouds primarily consist of gases, with hydrogen being the most abundant, but they also contain a small percentage of solid particles known as interstellar dust or dust grains. Although these dust grains represent only about 1% of the cloud's mass, they play a significant role in star formation processes. The gas-grain model highlights how interactions between gas and dust can lead to increased density in the molecular cloud, facilitating the collapse necessary for star formation.
In the cold conditions of molecular clouds, chemical reactions predominantly occur through ion-molecule processes, but the presence of dust grains enhances the chemistry by providing surfaces for gas-phase molecules to freeze and form icy mantles. These mantles can attract and hold atoms, creating complex organic compounds that would not form solely through gas interactions. Additionally, dust grains absorb ultraviolet radiation, mitigating ionization effects that would otherwise hinder the gravitational collapse crucial for star creation.
Research in this area continues to evolve, utilizing advanced telescopes and laboratory experiments to deepen our understanding of the interactions between gas and dust, the formation of various elemental species, and the implications for both star formation and the potential for life in the universe. The gas-grain model thus serves as a vital tool for scientists studying the origins of stars and the broader processes shaping the cosmos.
Gas-Grain Models
FIELDS OF STUDY: Astrochemistry; Stellar Astronomy
ABSTRACT: Information gathered by specially equipped telescopes and in laboratories has shown that the relatively small amount of fine dust in molecular clouds has a key role in interacting with gas molecules. Such interactions initiate the chemical reactions that lead to the formation of stars. This gas-grain model of star formation is important because it advances the understanding of the formation of the universe and possibly the development of life.
Star Formation Models
The area between stars is filled with a blend of material that scientists call interstellar medium (ISM). This mixture of gas and dust combines in areas with thicker or denser collections of ISM known as molecular clouds. The bulk of the gases within them are sharing electrons in a molecular state.
These clouds are often called "stellar nurseries," because the conditions within them foster the birth of most stars. Star formation occurs when areas of the gas in the cloud become dense, causing the cloud to collapse in on itself to form the core or seed of a new star. This happens when something changes or reacts with the gas to increase its density. Astrochemistry focuses on the study of these and other kinds of chemical reactions in space. When all the aspects of the reaction that creates the star are gaseous, astrochemists refer to it as a gas-phase model.
Scientists have discovered that the fine dust grains in a molecular cloud can also play a role in star formation. They are important even though they make up a very small percentage of the cloud’s matter. Sometimes these tiny grains of dust interact with the gas to change their temperature and start the reaction that makes the gas become denser. This is known as the gas-grain model of star formation.
The Chemistry of Star Formation
Molecular clouds are estimated to be about 99 percent gas. The other 1 percent consists of solid particles called interstellar dust or dust grains. Most of the gas is hydrogen gas created at the same time as the universe. These clouds are quite cold. Temperatures stay at about 10 to 20 kelvins, or just above absolute zero.
This extreme cold makes chemical reaction between the gases impossible. Instead, most of the chemistry in the gas phase occurs by ion-molecule reactions. Ultraviolet light emitted by young stars provides the heat and energy needed to make molecular hydrogen. This compound is formed when two hydrogen atoms share their electrons. This molecular hydrogen is crucial to the formation of new stars.
Researchers have also recognized the importance of dust grains to the formation of stars. Dust grains make up a much smaller percentage of the stellar nurseries than the gaseous elements. However, scientists determined that many of the species or types of molecules they could identify in the molecular clouds could not be formed just by gas-phase elements. It is now believed that many stars are formed by the molecular gas interacting with dust grains. In the cold state of the cloud, the gas-phase molecules freeze into an icy mantle around the dust. This gas-grain model of star formation helps explain the richness of the chemistry in molecular clouds. This richness cannot be accounted for in models that only allow for gas-phase interactions.
Dust and Gas Interactions
The dust in molecular clouds is very fine and is the result of material cast off from nearby stars as they die. It is largely made up of carbon and silicate and picks up atoms of other elements, such as hydrogen, oxygen, and nitrogen. These interactions then result in an outer coating of ice. This ice mantle can also include methane, ammonia, and carbon monoxide. As this mantle reacts with ultraviolet light, the outer molecules can form more complex organic compounds.
This icy dust plays key roles in the formation of stars. The "sticky" outer surface of the grains attracts atoms and holds them. This fosters the formation of molecules that might not take place otherwise in the conditions of the cloud. For instance, two hydrogen atoms colliding with the dust are likely to share electrons and react because of the dust. The dust also absorbs ultraviolet radiation, protecting molecules from the radiation’s effects while reducing the ionization level of the cloud. This is important because excessive ionization would limit the ability of gravity to collapse the gas into a star core. Later, this radiation will be released as infrared energy, a cooler form of energy.
The interaction of the dust and gas causes the gas to freeze into a new state and come in contact with elements in ways that it could not in a gaseous state. The grain of dust becomes the nucleus for the new star. It collects hydrogen molecules and other atoms until the clump is dense enough for its gravity to make it collapse into the core of a new star.
This gas-grain model is also thought to explain the development of many of the more than one hundred different element species scientists have identified in molecular clouds. Many of these cannot be accounted for in a pure gas-phase model.
Research Continues
Scientists continue to probe deep into molecular clouds, looking for more details in the star formation process. Using infrared and radio telescopes, researchers conduct ongoing studies of far-away stellar nurseries to learn more about the stars and other elements found there. Information from the Herschel Space Observatory and Planck satellite telescopes has been crucial in advancing knowledge about star formation. This research has allowed scientists to observe and catalog more of the species of elements found in distant stellar nurseries. The Hubble Telescope’s infrared capabilities have given scientists many insights into the birth of stars, even though they are covered in clouds of dust. Infrared light can be detected through dust via the use of these special telescopes that can reveal the light’s electromagnetic spectrum for study.
The discoveries continue in labs on Earth as well. For example, scientists at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, performed experiments in a specially designed ultra-high vacuum surface lab. These experiments were designed to test the effects of ultraviolet light, x-rays, and electrons on icy surfaces similar to those found in star formation areas. This provides scientists with ways to understand and even predict how those icy mantles behave during star formation.
Some of these research efforts, such as the Spitzer Space Telescope mission from the National Aeronautics and Space Administration (NASA), have identified chemical combinations on distant stars that resemble the amino acids that are vital to life on earth. These discoveries stress the importance of continued study of the gas-grain model of star formation and its potential implications for the formation of the universe and the development of life on Earth.
Principal Terms
- astrochemistry: the scientific study of chemical elements in space and how they interact with each other and with radiation.
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