21cm H1 Line.

Radio astronomy is the study of celestial objects that give off radio frequencies. When charged particles such as electrons and protons accelerated by changing their speed or direction, they emit electromagnetic radiation. Radio waves are a form of electromagnetic radiation with longer wavelength lower frequencies and less energy than visible light. Radio Astronomy has changed the way we used to study the celestial phenomenon. 

 

Hydrogen is the most abundant element in the cosmos; it makes up 80% of the universe's mass. No surprise that one of the most significant spectral lines in radio astronomy is the 21-cm hydrogen line. Optical observations are limited due to the interstellar dust, which does not allow the penetration of light waves. However, this problem does not arise when making radio measurements of the H1 region. The 21 cm H1 line can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light. Radiation from this region can be detected anywhere in our Galaxy.

 

The 21 cm H1 line refers to the electromagnetic radiation spectral line emitted by the Hyperfine transitions in the energy levels of neutral hydrogen. This electromagnetic radiation is at the precise frequency of 1420405751.7667 ± 0.0009 Hz, equivalent to the vacuum wavelength of 21.1061140542 cm in free space. 

 

Fig 1: Hyperfine Transition

 

The ground state of neutral hydrogen consists of an electron bound to a proton. Both the electron and the proton have intrinsic magnetic dipole moments ascribed to their spin, whose interaction results in a slight increase in energy when the spins are parallel (F=1), and a decrease when anti-parallel (F=0). The transition has an energy difference of 5.87433 μeV that, when applied in the Planck equation, gives λ = 21.106cm.

 

Measurements of the H1 region of the Galaxy can be used in various calculations. For example, observations of the 21-cm line can be used to create the rotation curve for our Milky Way Galaxy. Suppose hydrogen atoms are distributed uniformly throughout the Galaxy. A 21-cm line will be detected from all points along the line of sight of our telescope. The only difference will be that all of these spectra will have different Doppler shifts. Once the Galaxy's rotation curve is known, it can be used to find the distances to various objects. By knowing the Doppler shift of a body, its angular velocity can be calculated. Combining this angular velocity and the plot of the rotation curve, the distance to a particular object can be inferred. Using measurements of the H1 region, the mass of the Galaxy can also be determined. Hydrogen line

observations can also be used indirectly to calculate the mass of galaxies and limit any changes over time of the value of the universal gravitational constant. The line is also of great importance to big-bang cosmology. It is the only known probe into the dark ages, from recombination to reionization.

 

So, do you want to read more about how this 21cm H1 line can be detected? Tell us in the comment section, and we will talk about that in our upcoming blogs. Till then, keep looking up.

• ©Abhijeet Parkhi