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.
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The Milky Way galaxy rotates with a period of approximately 225-250 million years, completing one full rotation around its center every few hundred million years.
The rotational velocity of stars in the Milky Way decreases with increasing distance from the galactic center, a phenomenon known as differential rotation.
Galactic rotation plays a crucial role in the formation and maintenance of the spiral structure of galaxies, as the differential rotation causes the winding of the spiral arms over time.
The rotational velocity of the Milky Way is estimated to be around 220-240 kilometers per second at the location of the Sun, which is about 25,000 to 28,000 light-years from the galactic center.
Observations of the rotational velocities of stars in galaxies have provided important evidence for the existence of dark matter, as the observed rotation curves cannot be explained solely by the visible matter in the galaxy.
Review Questions
Explain how galactic rotation is related to the architecture of a galaxy.
Galactic rotation is a fundamental feature of the architecture of a galaxy, as it governs the overall structure and organization of the stars, gas, and other matter within the galaxy. The rotation of a galaxy around its center of mass, along with the phenomenon of differential rotation where the inner regions rotate faster than the outer regions, is responsible for the formation and maintenance of the spiral structure observed in many galaxies, including the Milky Way. The rotational velocities of stars and other objects within a galaxy also provide important clues about the distribution of matter, including the presence of dark matter, within the galaxy.
Describe how galactic rotation is related to the spiral structure of a galaxy.
The spiral structure of galaxies is intimately linked to the phenomenon of galactic rotation. As a galaxy rotates, the differential rotation between the inner and outer regions causes the spiral arms to wind up over time. The faster rotation of the inner regions compared to the outer regions creates a shearing effect that shapes the spiral pattern. Additionally, the gravitational interactions between the rotating disk of a galaxy and the surrounding dark matter halo can also contribute to the formation and maintenance of the spiral structure. The observed spiral patterns in galaxies are a direct consequence of the underlying galactic rotation.
Analyze how observations of galactic rotation have provided evidence for the existence of dark matter.
Observations of the rotational velocities of stars and other objects within galaxies have been crucial in providing evidence for the existence of dark matter. The observed rotation curves, which plot the rotational velocity of a galaxy as a function of distance from the galactic center, often show that the outer regions of a galaxy are rotating much faster than would be expected based solely on the visible matter (stars, gas, and dust) in the galaxy. This discrepancy between the observed rotation curves and the expected rotation based on the visible matter can only be explained by the presence of a significant amount of unseen, or 'dark,' matter that exerts a gravitational influence on the visible matter within the galaxy. The study of galactic rotation has thus been instrumental in the discovery and understanding of the ubiquitous presence of dark matter in the universe.
The supermassive black hole located at the gravitational center of the Milky Way galaxy, around which the entire galaxy rotates.
Galactic Plane: The plane that contains the disk of a galaxy, where most of the stars and interstellar matter are concentrated, and around which the galaxy rotates.
The phenomenon where different parts of a galaxy rotate at different angular velocities, with the inner regions rotating faster than the outer regions.