The is a cosmic soup of gas and dust between stars. It's mostly in three forms: atomic, molecular, and ionized. Each type has unique properties and detection methods, shaping our understanding of galactic structure and star formation.
plays a crucial role in the life cycle of stars and galaxies. From the cold, dense where stars are born to the hot, ionized regions surrounding young stars, this gas is constantly recycled and transformed throughout the cosmos.
Interstellar Gas
Categories of interstellar gas
(HI)
Consists of neutral hydrogen atoms not bound to other atoms
Most abundant element in the (ISM) makes up majority of gas between stars
(H2)
Formed when two hydrogen atoms bond together through shared electrons
Forms in the densest, coldest regions of the ISM where gas clouds are shielded from radiation ()
(HII)
Hydrogen atoms stripped of their electrons by high-energy radiation
Found in regions surrounding hot, young stars that emit intense ultraviolet light (HII regions)
Detection methods for interstellar gas
Atomic hydrogen (HI)
Detected through the emission
Caused by a between two energy levels in the hydrogen atom's electron
Allows mapping of HI distribution throughout the galaxy (radio telescopes)
Molecular hydrogen (H2)
Cannot be directly observed in cold interstellar clouds as H2 lacks a permanent dipole moment
Presence inferred through the detection of other molecules, such as (CO)
CO emits radio waves when its rotational state changes, acting as a tracer for H2
Ionized hydrogen (HII)
Detected through the observation of characteristic emission lines in the visible spectrum
lines like (656.3 nm) and (486.1 nm)
Caused by electrons transitioning between energy levels in the hydrogen atom
Appear as bright in visible light images ()
Characteristics of interstellar gas types
Atomic hydrogen (HI)
Temperature ranges from 50-100 K, too warm for atoms to bind together
Density varies from 1-100 atoms per cm³, diffuse and spread out
Distributed throughout the Milky Way galaxy, fills much of the space between stars
Molecular hydrogen (H2)
Temperature remains low at 10-20 K, allowing molecules to form and survive
Density much higher at 10³-10⁶ molecules per cm³, gas is concentrated
Found in , the birthplaces of new stars and planets ()
Ionized hydrogen (HII)
Temperature reaches 10,000-20,000 K due to heating by UV radiation from hot stars
Density ranges from 1-10⁴ ions per cm³, varies with distance from ionizing stars
Found in HII regions surrounding hot, young stars (O and B-type)
Intense ultraviolet radiation from these stars ionizes the surrounding hydrogen gas ()
Other components of the interstellar medium
: Tiny solid particles mixed with the gas, affecting light transmission and absorption
: Streams of particles ejected from stars, contributing to the interstellar medium
: High-energy particles traveling through space, interacting with interstellar gas
: Expanding shells of gas and dust from exploded stars, enriching the interstellar medium with heavy elements
Key Terms to Review (49)
21-cm line: The 21-cm line is a spectral line emitted by neutral hydrogen atoms due to the hyperfine transition. It is a crucial tool in radio astronomy for mapping interstellar gas and studying galactic structures.
21-cm radio line: The 21-cm radio line, also known as the hydrogen line, is an important spectral line emitted by neutral hydrogen atoms in the interstellar medium. This line is produced by the hyperfine transition of the electron in the ground state of the hydrogen atom, which results in a small energy difference between the two possible spin configurations of the electron and proton.
Atomic Hydrogen: Atomic hydrogen refers to the simplest and most abundant form of hydrogen in the universe, consisting of a single proton and a single electron. It plays a crucial role in the study of interstellar gas and the composition of the universe.
Balmer Series: The Balmer series is a series of spectral lines in the visible and ultraviolet regions of the electromagnetic spectrum that are emitted by hydrogen atoms when electrons transition from higher energy levels to the second energy level. This series of spectral lines is named after the Swiss mathematician and physicist Johann Balmer, who discovered the mathematical formula that describes the wavelengths of these lines.
Bok Globules: Bok globules are small, dense clouds of molecular hydrogen and dust found in star-forming regions of the Milky Way galaxy. These dark, opaque globules are believed to be the earliest stage of stellar formation, where gravity causes the collapse of the cloud into a new star.
Carbon Monoxide: Carbon monoxide (CO) is a colorless, odorless, and highly toxic gas that is produced through the incomplete combustion of carbon-based fuels. It is an important component in the context of interstellar gas, as it is one of the most abundant molecules found in the interstellar medium.
Cosmic rays: Cosmic rays are highly energetic particles that originate from outer space and travel at nearly the speed of light. They consist mostly of protons, but also include heavier atomic nuclei and electrons.
Cosmic Rays: Cosmic rays are high-energy particles, primarily composed of protons and atomic nuclei, that originate from various sources in the universe and travel through space at nearly the speed of light. These particles play a crucial role in shaping the interstellar medium, interstellar gas, and the evolution of massive stars, while also providing important insights into the cosmic context for life.
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.
Dark Nebulae: Dark nebulae are dense clouds of interstellar dust and gas that appear as dark silhouettes against a brighter background of stars or glowing gas. They are an integral part of the interstellar medium, playing a crucial role in the formation of new stars and the cycling of matter within galaxies.
Eagle Nebula: The Eagle Nebula, also known as Messier 16, is a giant molecular cloud located in the constellation Serpens. It is famous for its iconic 'Pillars of Creation' feature, which showcases dramatic columns of gas and dust where new stars are actively forming.
Ewen: Ewen is often associated with Edward Mills Purcell, who alongside Harold Ewen, discovered the 21 cm hydrogen line. This discovery marked a significant advancement in radio astronomy, helping astronomers study interstellar gas.
Five-hundred-meter Aperture Spherical radio Telescope (FAST): The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the world's largest filled-aperture radio telescope, located in Guizhou, China. It is used for a variety of astronomical observations including pulsar discovery and interstellar communication research.
Formaldehyde: Formaldehyde is a simple organic compound with the formula CH2O, found in interstellar space. It is significant in astrochemistry due to its role in the formation of complex organic molecules.
Giant molecular clouds: Giant molecular clouds are vast regions of gas and dust in space, primarily composed of molecular hydrogen. They are the primary sites for star formation within galaxies.
H II region: An H II region is a cloud of gas that has been ionized by ultraviolet light from nearby young, hot stars. These regions emit bright visible light, often appearing red due to the strong emission line of hydrogen.
H II regions: H II regions are vast clouds of ionized hydrogen gas in which new stars are actively forming. They are some of the most luminous features in galaxies and are closely associated with the birth of massive, hot, and young stars.
Hydrogen: Hydrogen is the simplest and most abundant element in the universe, consisting of a single proton and electron. It is a key component in the formation and composition of many astronomical objects and phenomena, playing a crucial role in the study of the very small, the formation of spectral lines, the atmospheres of the giant planets, the spectra of stars, the interstellar medium, and the fundamental makeup of the universe.
Hyperfine Transition: A hyperfine transition is a type of atomic transition that occurs between the hyperfine energy levels of an atom or molecule. These hyperfine levels arise due to the interaction between the magnetic moments of the nucleus and the orbiting electrons, resulting in a slight splitting of the atomic energy levels.
Hα: Hα, or hydrogen-alpha, is a specific wavelength of light emitted by hydrogen atoms when they undergo a specific electronic transition. It is a crucial indicator of various astrophysical processes and phenomena related to the interstellar gas in our universe.
Hβ: Hβ, or the Hydrogen beta line, is a specific wavelength of light emitted by hydrogen atoms when they undergo a transition from the n=4 energy level to the n=2 energy level. This transition is one of the characteristic spectral lines of hydrogen and is commonly observed in the study of interstellar gas.
Interstellar dust: Interstellar dust consists of tiny solid particles found in the space between stars, within galaxies. It absorbs and scatters starlight, affecting astronomical observations.
Interstellar Dust: Interstellar dust refers to the tiny solid particles that are found throughout the space between stars in a galaxy. These microscopic grains of dust are composed of various elements and compounds, and they play a crucial role in the structure and evolution of the interstellar medium, as well as the formation of new stars and planets.
Interstellar gas: Interstellar gas is the collection of gas located in the space between stars within a galaxy. It primarily consists of hydrogen and helium, with trace amounts of heavier elements.
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.
Interstellar molecules: Interstellar molecules are complex chemical compounds found in the space between stars, within interstellar gas and dust clouds. These molecules can range from simple diatomic species to more complex organic compounds.
Ionization: Ionization is the process in which an atom or molecule loses or gains electrons, resulting in the formation of ions. This often occurs due to high energy photons interacting with atoms or molecules.
Ionization: Ionization is the process by which an atom or molecule loses or gains one or more electrons, resulting in the formation of an ion. This process is fundamental to understanding the formation of spectral lines, the spectra of stars and brown dwarfs, the composition of interstellar gas, the behavior of cosmic rays, and the nature of interstellar matter around the Sun.
Ionized Hydrogen: Ionized hydrogen, also known as protons or H+, is the fundamental building block of the interstellar medium. It is a highly reactive and energetic state of hydrogen where the electron has been stripped away, leaving behind a bare proton. This ionized state of hydrogen is crucial in understanding the properties and behavior of the interstellar gas found between stars.
Light-year: A light-year is the distance that light travels in one year in a vacuum, approximately 5.88 trillion miles (9.46 trillion kilometers). It is commonly used to measure distances between stars and other celestial objects.
Light-Year: A light-year is a unit of distance used in astronomy, defined as the distance light travels in one year in a vacuum. It is a useful measure for expressing the vast distances between celestial objects in the universe.
Metallicity: Metallicity is a measure of the amount of elements heavier than hydrogen and helium present in a star, galaxy, or other astronomical object. It is an important parameter that provides insights into the chemical composition and evolution of these celestial bodies.
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.
Molecular Clouds: Molecular clouds are vast, dense regions of the interstellar medium composed primarily of molecular hydrogen and other molecules. These clouds serve as the birthplace for new stars and play a crucial role in the life cycle of cosmic material throughout the universe.
Molecular Hydrogen: Molecular hydrogen, also known as dihydrogen, is the simplest and most abundant molecule in the universe. It consists of two hydrogen atoms bonded together and plays a crucial role in the interstellar gas found between stars.
Nebulae: Nebulae are vast clouds of gas and dust in space, often serving as the birthplaces of new stars. These interstellar structures are crucial in understanding the formation of spectral lines, the composition of interstellar gas, and the discovery of galaxies beyond our own Milky Way.
Opacity: Opacity is a measure of the degree to which a material or substance obstructs the transmission of light or other forms of electromagnetic radiation. It is a critical concept in various fields, including astrophysics, where it plays a vital role in understanding the behavior and properties of celestial bodies and their environments.
Orion Molecular Cloud: The Orion Molecular Cloud is a giant molecular cloud located in the Orion constellation, known for its active star formation and the presence of the famous Orion Nebula. It is one of the closest and most well-studied star-forming regions in our galaxy, providing valuable insights into the processes of stellar birth and early stellar evolution.
Orion Nebula: The Orion Nebula is a vast, luminous cloud of gas and dust located in the Milky Way galaxy, situated in the constellation of Orion. It is one of the most well-known and extensively studied star-forming regions in our galaxy, providing valuable insights into the processes of stellar birth and early stellar evolution.
Parsec: A parsec is a unit of distance used in astronomy, equivalent to about 3.26 light-years or 31 trillion kilometers. It represents the distance at which one astronomical unit subtends an angle of one arcsecond.
Parsec: A parsec is a fundamental unit of distance used in astronomy, specifically to measure the distances between stars and other celestial objects within our galaxy and beyond. It is a derived unit that represents the distance at which a star would appear to shift by one arcsecond (1/3600th of a degree) in its position when viewed from Earth over the course of a year.
Planetary nebulae: A planetary nebula is an expanding shell of ionized gas ejected from red giant stars late in their life cycles. These nebulae glow due to the ultraviolet radiation from the hot central star remnant.
Purcell: Purcell is a scientist who made significant contributions to the understanding of interstellar gas. His work includes the discovery of the 21 cm hydrogen line, which is crucial for studying interstellar matter.
Radio Telescope: A radio telescope is a specialized astronomical instrument designed to detect and analyze radio waves emitted by celestial objects. These telescopes are used to study a wide range of phenomena in the universe, from the structure of galaxies to the formation of stars and planets.
Recombination: Recombination is the process by which free electrons and protons in a plasma (ionized gas) combine to form neutral atoms, releasing energy in the form of photons. This process is a crucial aspect of the evolution of the early universe and the formation of spectral lines in various astrophysical contexts.
Spectroscopy: Spectroscopy is the study of the interaction between matter and electromagnetic radiation, which provides valuable information about the composition, temperature, and motion of celestial objects. This technique is widely used in astronomy to analyze the properties of stars, galaxies, and other cosmic phenomena.
Stellar Winds: Stellar winds are streams of charged particles and gases that are ejected from the outer layers of stars, particularly from the upper atmospheres of red giants, supergiants, and other evolved stars. These winds play a crucial role in shaping the interstellar medium, contributing to the life cycle of cosmic material, and influencing the death of low-mass stars.
Supernova Remnants: Supernova remnants are the expanding shells of gas and dust left behind after a massive star has exploded in a supernova. These remnants play a crucial role in the interstellar medium, interstellar gas, cosmic rays, and the overall life cycle of cosmic material.