Star formation occurs in , where dense, cold regions collapse under gravity to form protostars. These young stars accrete matter, develop accretion disks, and eventually ignite hydrogen fusion in their cores, becoming main sequence stars.
The process involves various structures like bright-rimmed clouds, - objects, and proplyds. Conditions such as high density, low temperature, and turbulence are crucial for stellar birth. Molecular clouds evolve through triggered star formation, supernova feedback, and cloud dispersal.
Star Formation in Molecular Clouds
Stages of star formation
Molecular clouds consist primarily of molecular hydrogen (H2) and contain denser, colder regions called clumps and cores that provide the ideal environment for star formation
Gravitational collapse occurs when clumps and cores become gravitationally unstable and collapse under their own gravity to form protostars
Protostellar phase involves protostars accreting matter from surrounding gas and dust, with their luminosity coming from gravitational energy released during accretion, while being highly obscured by surrounding dust cocoons
An forms around the , funneling material onto its surface and potentially giving rise to planetary systems
phase begins when the becomes visible as surrounding material disperses and the protostar contracts and heats up until its temperature reaches ∼10 million K, initiating hydrogen fusion and marking the birth of a main sequence star
During this phase, some stars may go through a T Tauri stage, characterized by strong stellar winds and variable brightness
Main sequence is reached when the star achieves hydrostatic equilibrium, with the inward gravitational force balanced by outward radiation pressure, and hydrogen fusion in the provides energy output for most of the star's lifetime (Sun, Sirius)
Structures in star-forming regions
Molecular clouds appear as dark, obscuring regions composed of gas and dust (, )
Bright-rimmed clouds are dense clumps and cores illuminated by nearby massive stars (, )
Herbig-Haro objects manifest as bright patches caused by jets and outflows from young stars colliding with surrounding material (, )
Proplyds, or protoplanetary disks around young stars, appear as dark silhouettes against the bright background nebula (, )
Young star clusters are groups of newly formed stars still embedded in the (, )
Conditions for stellar birth
High densities in molecular clouds (∼100-1000 particles/cm3) allow gravity to overcome internal gas pressure and initiate collapse, much higher than the average density of the (ISM)
Low temperatures around 10-20 K in molecular clouds, due to effective shielding from external radiation, reduce thermal pressure and make the cloud more susceptible to gravitational collapse
Turbulence, or supersonic turbulent motions within the cloud, creates local density enhancements (clumps and cores) that are more likely to become gravitationally unstable and collapse
Magnetic fields partially support the cloud against gravity, slowing down the collapse, and can also guide the formation of jets and outflows from young stars (HH 30, HH 111)
The , which depends on temperature and density, determines the minimum mass required for a cloud fragment to collapse under its own gravity
Evolution of molecular clouds
Triggered star formation occurs when newly formed massive stars emit strong UV radiation and stellar winds that compress nearby gas, triggering the formation of a new generation of stars (, )
Supernova feedback involves massive stars exploding as supernovae at the end of their lives, with shock waves compressing gas and triggering further star formation while injecting heavy elements into the cloud, enriching the gas for future generations of stars (, )
Cloud dispersal gradually erodes the molecular cloud due to UV radiation, stellar winds, and supernovae from massive stars, causing star formation to slow down and eventually stop
Recycling of gas occurs when some of the dispersed gas later cools and recondenses, forming new molecular clouds, continuing the cycle of cloud formation, star birth, and cloud dispersal throughout a galaxy's lifetime (Milky Way, Andromeda)
Star formation outcomes and processes
begins in the core of newly formed stars, fusing hydrogen into helium and producing energy that supports the star against gravity
The describes the distribution of stellar masses resulting from a star formation event, influencing the evolution and characteristics of stellar populations
Brown dwarfs, objects with masses below the hydrogen-burning limit, can form through similar processes as stars but fail to sustain hydrogen fusion in their cores
Key Terms to Review (37)
Accretion Disk: An accretion disk is a rotating disk of dense, accreting material surrounding a central object, such as a star, black hole, or neutron star. It is formed by the gravitational attraction and conservation of angular momentum of material falling towards the central object.
Barnard 68: Barnard 68 is a dark molecular cloud located in the constellation Ophiuchus. It is a prime example of a starless core, a dense region within a molecular cloud that has not yet begun the process of star formation.
Bright-Rimmed Cloud: A bright-rimmed cloud is a dense, opaque cloud of gas and dust surrounding a newly formed star or a young stellar object. These clouds are illuminated from the inside, creating a distinctive bright rim around the edge of the cloud.
Brown Dwarf: A brown dwarf is a substellar object that is too massive to be considered a planet, but not massive enough to sustain hydrogen fusion in its core like a star. These objects occupy the mass range between the heaviest gas giant planets and the lightest stars.
Carina Nebula: The Carina Nebula is a large, complex star-forming region located in the southern constellation of Carina. It is known for its stunning visual appearance and its significance in the study of stellar formation and the structure of our Milky Way galaxy.
Clump: A clump is a small, dense grouping of matter, such as stars or gas and dust, that forms within a larger structure or environment. Clumps are an important concept in the study of star formation, as they represent the initial stages of the process where gravity causes material to coalesce and eventually collapse to form new stars.
Core: The core is the innermost layer of a planet, primarily composed of metal. It plays a crucial role in generating the planet's magnetic field.
Core: The core refers to the central, innermost region of a planet, star, or other celestial body. It is typically the densest and most massive part of the structure, often composed of highly compressed materials like metals and heavy elements.
Crab Nebula: The Crab Nebula is a supernova remnant, the expanding debris field from the explosion of a massive star. It is located in the constellation of Taurus and is one of the most studied and well-known objects in the night sky, providing insights into the aftermath of a star's death and the formation of neutron stars.
Haro: Haro objects are small patches of nebulosity associated with newly born stars, often found in star-forming regions. They are characterized by their emission lines and are indicative of energetic processes in young stellar objects.
Herbig: Herbig-Haro objects (HH objects) are small, bright patches of nebulosity associated with newly born stars. They form when jets of gas ejected by young stars collide with nearby gas and dust at high speeds.
Herbig-Haro (HH) object: Herbig-Haro (HH) objects are small patches of nebulosity associated with newly born stars. They form when jets of ionized gas ejected by young stars collide with nearby clouds of gas and dust.
Herbig-Haro Object: A Herbig-Haro object is a small, bright nebula that is formed when jets of material ejected from a young star collide with the surrounding interstellar medium. These objects provide important insights into the early stages of star formation.
HH 34: HH 34 is a young stellar object, a protostar in the early stages of star formation. It is located in the Orion Nebula, a star-forming region within the Orion Molecular Cloud Complex, and serves as an important example for understanding the process of star birth.
HH 47: HH 47 is a young stellar object (YSO) located in the Vela Molecular Cloud, a star-forming region in the southern constellation of Vela. It is a protostar, a young, still-forming star, that is actively accreting material from its surrounding circumstellar disk, a process that is crucial for the star's formation and evolution.
Horsehead Nebula: The Horsehead Nebula is a dark nebula in the constellation Orion, located just south of the bright star Alnitak, which is the easternmost star in Orion's Belt. It is a prominent feature of the Orion Molecular Cloud Complex, a star-forming region, and is known for its distinctive shape that resembles the head of a horse.
HST-10: HST-10 is a key term related to star formation, specifically referring to a type of infrared telescope that has been used to study the early stages of star and planetary system development. This space-based observatory has provided invaluable data and insights into the complex processes involved in the birth of new stars and the formation of planetary systems around them.
IC 1396: IC 1396 is an emission nebula located in the constellation Cepheus. It is a star-forming region where new stars are actively being born from the dense clouds of gas and dust that make up the nebula.
Initial Mass Function: The initial mass function (IMF) is a statistical distribution that describes the relative number of stars formed with different masses in a given star-forming region. It is a fundamental concept in the study of star formation and the evolution of stellar populations.
Interstellar medium: Interstellar medium (ISM) is the matter that exists in the space between star systems within a galaxy. It consists of gas (both ionized and neutral) and dust, playing a crucial role in the life cycle of cosmic material.
Interstellar Medium: The interstellar medium refers to the vast expanse of gas and dust that fills the space between stars within a galaxy. It is the material that exists in the space between solar systems and plays a crucial role in the formation and evolution of stars, as well as the overall structure and dynamics of galaxies.
Jeans Mass: Jeans mass is a critical concept in the study of star formation, representing the minimum mass required for a cloud of gas and dust to gravitationally collapse and form a new star. It is named after the British astrophysicist Sir James Jeans, who first derived the mathematical expression for this mass threshold in the early 20th century.
Molecular Cloud: A molecular cloud is a type of interstellar cloud composed primarily of molecular hydrogen and other molecules. These dense regions of gas and dust serve as the birthplace for new stars, as the gravitational collapse of the cloud material leads to the formation of stars and planetary systems.
Molecular clouds: Molecular clouds are dense regions of gas and dust in interstellar space where molecules, particularly hydrogen, can form. They are crucial sites for star formation as they provide the raw material needed to create new stars.
NGC 3324: NGC 3324 is an open star cluster located in the constellation Carina. It is part of the Carina Nebula, a vast star-forming region in the Milky Way galaxy, and is an important object for understanding the process of star formation.
NGC 602: NGC 602 is an open star cluster located in the Small Magellanic Cloud, a satellite galaxy of the Milky Way. It is a region of active star formation, providing insights into the processes that govern the birth and early evolution of stars.
NGC 6357: NGC 6357 is a star-forming region located in the constellation Scorpius. It is a giant molecular cloud complex that is actively producing new stars, making it an important site for the study of star formation processes.
Orion Proplyds: Orion Proplyds are young, circumstellar disks of gas and dust surrounding newly formed stars within the Orion Nebula. These disks are the precursors to planetary systems and provide important insights into the early stages of star and planet formation.
Pre-Main Sequence: The pre-main sequence refers to the early stages of a star's life cycle, before it reaches the main sequence phase of stable hydrogen fusion in its core. This critical period involves the gravitational contraction and gradual heating of a protostar as it accumulates mass and prepares to transition into a mature, main sequence star.
Proplyd: A proplyd, or proplyds, is a young, newly formed star surrounded by a circumstellar disk of dense gas and dust. These disks are the birthplaces of planets and are an important feature in the process of star formation.
Protostar: A protostar is an early stage in the formation of a star, where gas and dust collapse under gravity. It is characterized by the accumulation of material from a surrounding molecular cloud.
Protostar: A protostar is an early stage of stellar evolution, where a dense cloud of gas and dust begins to collapse under its own gravitational force, marking the initial formation of a star. This process is a crucial step in the life cycle of stars, as it sets the stage for the subsequent stages of star development.
Stellar Nucleosynthesis: Stellar nucleosynthesis is the process by which new atomic nuclei are created inside stars through nuclear fusion reactions. This process is responsible for the creation and distribution of the elements that make up the universe, from the lightest elements like hydrogen and helium to the heavier elements like carbon, oxygen, and iron.
T Tauri star: A T Tauri star is a young, pre-main-sequence star that is less than 10 million years old. These stars are characterized by their variability and strong stellar winds.
T Tauri Star: A T Tauri star is a young, pre-main sequence star that is still in the process of forming and contracting towards the main sequence. These stars are characterized by their variable luminosity, strong stellar winds, and active chromospheres, all of which are indicative of the star's early stage of evolution.
Trapezium Cluster: The Trapezium Cluster is a small, dense grouping of young, hot, and massive stars located at the heart of the Orion Nebula. It is a prime example of a star-forming region and provides valuable insights into the process of star formation.
Vela Supernova Remnant: The Vela Supernova Remnant is the expanding debris field from a massive star that exploded as a supernova around 11,000-12,000 years ago. This remnant is located in the southern constellation of Vela and is one of the closest known supernova remnants to Earth, providing a unique opportunity to study the aftermath of a stellar explosion in detail.