Speaker: Shravya Bhandary
Date: 5th October'2024
On October 5th, Shrava Bhandary, a core member of the Antriksh Club of VI, led an insightful internal talk on H1 Line Emission, focusing on its foundational role in radio astronomy and its significance in understanding the Milky Way’s structure. This presentation delved into the physics behind the 21 cm wavelength, the behavior of neutral hydrogen, and how these elements collectively aid our exploration of the universe’s mysteries.
The discussion began with the concept of light across the electromagnetic spectrum as an investigative tool. Light, in its many wavelengths—from visible to radio waves—enables astronomers to observe various aspects of the cosmos. Radio waves, and in particular the 21 cm wavelength (or H1 line), have a special ability to penetrate cosmic dust, allowing us to observe regions that are otherwise obscured in visible light. This property makes the 21 cm wavelength vital for deep-space exploration and for gaining insights into regions of the universe that would otherwise remain hidden.
The 21 cm wavelength, a defining feature of the H1 line, is produced by neutral hydrogen atoms and corresponds to a frequency of 1.42 GHz. It’s unique because it can traverse the interstellar medium with minimal interference or distortion, providing a clear view of distant regions in the galaxy. This emission originates when the spin orientation of an electron in a neutral hydrogen atom flips, releasing a photon with a precise 21 cm wavelength. This process is relatively rare but has immense value in mapping large-scale structures in the universe.
One of the most significant applications of the 21 cm line is in studying the structure of the Milky Way. By measuring the H1 radiation emitted by neutral hydrogen clouds, astronomers can map various galactic features, including spiral arms, gas clouds, and star-forming regions. This mapping offers critical insights into the Milky Way’s structure, composition, and evolution, revealing a picture that visible light alone cannot provide.
Hydrogen’s simplicity—it consists of one proton and one electron—makes it an ideal subject for astronomical observation, especially in its neutral state, where the electron is bound to the proton. This neutral hydrogen atom’s characteristic 21 cm line has become a central element in radio astronomy, as it helps identify not only the composition but also the movement and distribution of hydrogen clouds across the galaxy.
Another key concept explored during the talk was the Doppler Effect, which is instrumental in astronomy for analyzing the motion of celestial bodies. By observing the Doppler shift in the frequency of H1 emissions, astronomers can create a “galactic rotation curve.” This curve, which plots gas cloud velocities at various distances from the galactic center, has unveiled unexpected findings. Specifically, rather than slowing at large distances from the center, as predicted by Keplerian motion, the rotational velocities remain relatively constant. This discrepancy has led to the hypothesis of dark matter, a form of matter exerting gravitational influence that cannot be directly observed.
Shrava also highlighted the importance of barycentric correction, a method used to adjust for Earth's movement relative to the solar system’s barycenter. This correction ensures that observed velocities—such as Doppler shifts in the hydrogen line—accurately reflect the velocity of the hydrogen clouds in relation to the galaxy, rather than incorporating errors due to Earth’s own motion.
In understanding the Milky Way’s structure, Kepler’s Laws—typically applied to planetary orbits—also come into play. Although these laws accurately describe planetary motion, the unexpected behavior observed in the galactic rotation curve (i.e., the constant velocity of gas clouds at large distances from the center) suggests the need for additional forces beyond visible matter. The inability of Kepler’s laws to fully explain galactic rotation at large scales further supports the dark matter hypothesis, which postulates an unseen mass affecting galactic dynamics far beyond visible stars and gas.
Shrava’s talk concluded by emphasizing how the 21 cm line continues to drive discoveries in our understanding of galactic structure, celestial motion, and dark matter. The insights gained from H1 line emissions not only illuminate the layout of the Milky Way but also hint at the presence of forces and matter that remain unseen, challenging our understanding of the universe.
