25.1 The Architecture of the Galaxy
3 min read•june 12, 2024
The , our cosmic home, is a vast and complex structure. Its study has evolved from early star-counting efforts to modern multi-wavelength observations, revealing a rich tapestry of stars, gas, and dust arranged in a spiral pattern.
Mapping our galaxy is challenging due to our position within it and the obscuring effects of . Despite these obstacles, astronomers have identified key components: the , bulge, and , each playing a unique role in the 's structure and evolution.
The Milky Way Galaxy
Historical reasoning for Herschels' model
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- William Herschel and sister Caroline conducted "" survey in late 18th century counted stars in 600+ sky regions to estimate Milky Way shape
- Assumed stars had similar intrinsic brightness, so fainter stars were farther away led to model of flattened disk with Sun near center
- Disk extended farther in Milky Way plane than perpendicular explained appearance of Milky Way band in night sky
- Model limited by assumption of uniform stellar brightness and lack of knowledge about absorption
Challenges of galactic mapping
- Earth located within Milky Way disk makes observing galaxy structure challenging like mapping a forest while standing inside it
- Interstellar dust absorbs and scatters visible light, obscuring distant galaxy parts causes dark bands in Milky Way () limits observing distant stars and structures in visible spectrum
- Vast distances within galaxy make determining precise locations and motions of celestial objects difficult parallax measurements only effective for nearby stars due to angular resolution limitations
- Sun's location within provides edge-on view of galaxy structure complicates determining true shape and size
- helps astronomers describe positions of objects relative to the Milky Way's center and plane
Primary components of Milky Way
- Galactic disk: flat, rotating component where most stars, gas, and dust reside
- : contains younger stars (Sun), ~1,000 light-years thick
- : contains older stars, ~3,000 light-years thick
- : higher density gas, dust, and young star regions winding outward from center (, )
- Exhibits , with stars and gas orbiting the at different speeds
- : central, spheroidal component
- Contains mostly older stars, ~10,000 light-years diameter
- Galactic center: innermost bulge region with supermassive black hole ()
- May have a bar-like structure, classifying the Milky Way as a
- : spherical component surrounding disk and bulge
- Extends up to 150,000 light-years from galactic center
- Contains ancient stars, (), and small amount of hot gas
- Home to different , reflecting the galaxy's formation history
- : extended, spherical component of non-baryonic matter
- Extends well beyond visible halo, responsible for galaxy's
- Plays crucial role in Milky Way formation and evolution
Galactic environment and structure
- permeates the disk and halo, influencing cosmic ray propagation and star formation
- Milky Way is part of the , a collection of galaxies bound by gravity, including Andromeda and other smaller galaxies
Key Terms to Review (40)
Barred spiral galaxy: A barred spiral galaxy is a type of spiral galaxy with a central bar-shaped structure composed of stars. The Milky Way is an example of a barred spiral galaxy.
Barred Spiral Galaxy: A barred spiral galaxy is a type of spiral galaxy characterized by the presence of a central bar-shaped structure of stars. This bar-like feature is a distinct component that extends from the galactic center and connects to the spiral arms, giving the galaxy a distinctive appearance.
Central bulge: The central bulge is a densely packed group of stars found in the center of spiral galaxies, including the Milky Way. It contains a mix of older stars and has a higher concentration of stars than the galaxy's disk.
Coal Sack nebula: The Coal Sack nebula, also known as the Coalsack Nebula, is a large dark nebula located in the constellation Crux, near the Southern Cross. It is one of the most prominent dark nebulae in the night sky, appearing as a conspicuous dark patch against the Milky Way background.
Dark matter halo: Dark matter halo is a theoretical, spherical region surrounding galaxies, including the Milky Way, composed mostly of dark matter. It extends beyond the visible components of the galaxy and contains most of its mass.
Dark Matter Halo: A dark matter halo is a hypothetical distribution of dark matter that surrounds and extends well beyond the visible component of a galaxy. Dark matter is a mysterious substance that does not emit, reflect, or absorb light, yet it makes up a significant portion of the universe's total mass and plays a crucial role in the formation and evolution of galaxies.
Differential galactic rotation: Differential galactic rotation is the phenomenon where different parts of a galaxy rotate at different speeds. In spiral galaxies, such as the Milky Way, stars closer to the center rotate faster than those in the outer regions.
Disk: The disk of the Milky Way Galaxy is a flattened region that contains most of the galaxy's stars, gas, and dust. It is characterized by its spiral structure and significant star formation activity.
Flat Rotation Curve: A flat rotation curve is a characteristic feature of the observed rotation of stars and gas in the outer regions of spiral galaxies. It describes the observation that the orbital velocities of objects around the galactic center remain nearly constant, rather than decreasing with increasing distance from the center as would be expected from the gravitational effects of the visible matter alone.
Galactic Bulge: The galactic bulge is a prominent, spherical-shaped component of many spiral galaxies, including our own Milky Way. It is a densely packed region located at the center of the galaxy, containing mostly older stars with a variety of ages and compositions.
Galactic Center: The galactic center is the rotational center of the Milky Way galaxy, located approximately 25,000 to 28,000 light-years from Earth in the direction of the constellation Sagittarius. It is the region at the heart of our galaxy where a supermassive black hole, known as Sagittarius A*, is believed to reside.
Galactic Coordinate System: The galactic coordinate system is a celestial coordinate system that uses the plane of the Milky Way galaxy as the fundamental plane. It provides a standardized way to locate and describe the positions of objects within our galaxy, allowing astronomers to study the structure and properties of the Milky Way in a systematic manner.
Galactic Disk: The galactic disk is a flattened, disk-like component of a galaxy that contains the majority of the galaxy's stars, gas, and dust. It is the primary location where most of a galaxy's star formation and stellar activity occurs, making it a crucial feature in understanding the architecture, stellar populations, and formation of a galaxy.
Galactic Halo: The galactic halo is a spherical or ellipsoidal region surrounding the disk and bulge of a galaxy, consisting of diffuse gas, stars, and other matter not concentrated in the galactic plane. It is a crucial component in understanding the architecture, mass, stellar populations, formation, and evolution of galaxies.
Galactic Magnetic Field: The galactic magnetic field refers to the pervasive magnetic field that pervades the Milky Way galaxy, extending from the galactic center to the outer regions. This magnetic field is a crucial component of the overall structure and dynamics of the galaxy, influencing the behavior of charged particles and shaping the interstellar medium.
Galactic Mapping: Galactic mapping refers to the process of charting and visualizing the structure and contents of the Milky Way galaxy. It involves the systematic collection and analysis of data to create comprehensive representations of the galaxy's components, such as stars, gas, dust, and other celestial objects.
Galactic Rotation: Galactic rotation refers to the circular motion of stars and other celestial bodies within a galaxy around the galaxy's center of mass. This rotation is a fundamental feature of the architecture and structure of galaxies, including the Milky Way in which our solar system resides.
Globular clusters: Globular clusters are tightly bound groups of stars, typically containing hundreds of thousands to millions of members. They orbit the galactic core and are among the oldest objects in the universe.
Globular Clusters: Globular clusters are dense, spherical collections of tens of thousands to millions of old stars gravitationally bound together. They are found in the outer regions of galaxies, including the Milky Way, and provide insights into the formation and evolution of galaxies.
Halo: A halo is a roughly spherical region surrounding the Milky Way Galaxy that contains older stars, globular clusters, and dark matter. It is not as dense as other parts of the galaxy and extends beyond the visible disk.
Herschel's Model: Herschel's model, also known as the Milky Way model, is a representation of the structure and architecture of our galaxy, the Milky Way, proposed by the astronomer William Herschel in the late 18th century. This model was a significant step forward in understanding the shape and size of the galaxy we inhabit.
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.
Local Group: The Local Group is a small cluster of galaxies that includes the Milky Way Galaxy and the Andromeda Galaxy, along with about 50 other smaller galaxies. It is the closest major concentration of galaxies to the Milky Way and provides important insights into the structure and evolution of galaxies on a larger scale.
M13: M13, also known as the Great Globular Cluster in Hercules, is a massive, spherical collection of stars located in the northern constellation of Hercules. It is one of the brightest and most prominent globular clusters visible from Earth, making it a captivating object for astronomers and stargazers alike.
Milky Way: The Milky Way is the galaxy in which our solar system is located, comprising hundreds of billions of stars and vast amounts of gas and dust. It is a spiral galaxy, with a central bulge and a rotating disk of stars, gas, and dust. The Milky Way is an essential component in understanding the structure, formation, and evolution of the universe, as it provides a window into the larger cosmic landscape.
Milky Way Galaxy: The Milky Way Galaxy is the spiral galaxy that includes our Solar System, characterized by its barred structure and multiple spiral arms. It is one of billions of galaxies in the universe and contains over 200 billion stars.
Nuclear bulge: The nuclear bulge is the central, densely packed region of stars in a spiral galaxy, including the Milky Way. It has a higher concentration of older stars and may contain a supermassive black hole.
Orion Arm: The Orion Arm is a minor spiral arm of the Milky Way galaxy, situated between two major spiral arms known as the Perseus Arm and the Sagittarius Arm. It is the solar system's location within the Milky Way, and it plays a crucial role in understanding the structure and evolution of our galaxy.
Perseus Arm: The Perseus Arm is a minor spiral arm of the Milky Way galaxy, located between the Orion Arm and the Outer Arm. It is named after the constellation Perseus, which is situated near the arm's location. The Perseus Arm is an important feature in understanding the overall architecture and stellar populations of the Milky Way galaxy.
Sagittarius A: Sagittarius A* (Sgr A*) is a supermassive black hole located at the center of the Milky Way Galaxy. It is approximately 4 million times the mass of the Sun and plays a crucial role in the dynamics of our galaxy.
Sagittarius A*: Sagittarius A* (Sgr A*) is a supermassive black hole located at the center of the Milky Way galaxy. It is a key feature in understanding the architecture, dynamics, and evolution of our galaxy, as well as the nature of black holes and their role in the universe.
Shapley: Shapley refers to Harlow Shapley, an American astronomer who significantly contributed to our understanding of the Milky Way Galaxy's structure. He determined the Sun's position in the Galaxy and studied the distribution of globular clusters.
Spiral arms: Spiral arms are regions of a spiral galaxy that extend from the center and wind outward in a spiral pattern. They are sites of high star formation activity and contain young, hot stars, gas, and dust.
Spiral Arms: Spiral arms are the prominent, curving structures of stars, gas, and dust that extend outward from the center of a spiral galaxy. They are a defining feature of the Milky Way and many other galaxies, and play a crucial role in the architecture and evolution of these galactic systems.
Star Gauging: Star gauging is the process of measuring the brightness, position, and other properties of stars in the night sky. It is a fundamental technique used by astronomers to understand the structure and composition of the Milky Way galaxy, as well as the universe beyond.
Stellar Populations: Stellar populations refer to the different groups or categories of stars that make up the various components of a galaxy. These populations are distinguished by their age, chemical composition, and kinematics, which provide insights into the formation and evolution of galaxies.
Thick Disk: The thick disk is a structural component of the Milky Way galaxy, consisting of a population of older stars that are distributed in a thicker, more extended layer above and below the plane of the galaxy's thin disk. It represents an intermediate stage between the central bulge and the thin disk, and plays an important role in understanding the formation and evolution of our galaxy.
Thin Disk: The thin disk is a component of the Milky Way galaxy that consists of young, relatively metal-rich stars. It is a flattened, disk-like structure that makes up the majority of the visible stars in our galaxy and is the location where most star formation currently takes place.
Unbarred spiral galaxy: An unbarred spiral galaxy is a type of spiral galaxy that lacks a central bar structure. It features well-defined spiral arms emanating directly from the galactic core.