Mapping the Milky Way
This article briefly explains how the structure of the milky way galaxy was found and what physical processes and observational techniques enable us to calculate distances and speeds at an astronomical scale.
We live in a rotating barred-spiral galaxy known as the Milky Way. Any casual Google search will reveal that our sun is located in the Orion Arm between the Sagittarius and Perseus Arms. Our galaxy is fairly planar and is about 1,00,000 light-years across and around 1000 light-years thick.
The simple fact that all these numbers are so readily available makes us forget how astonishing it is that we can find these distances here on earth. While the naked eye shows us nothing more than stars - point sources of light seemingly at the same distance, being able to map them along with the non-luminous gas and dust clouds and structure them into various spiral arms sprouting from a galactic center is one of the greatest accomplishments of astronomy. And what is more astonishing is that this kind of mapping at the galactic scale was made possible by processes that occur at the sub-atomic scale.
When a luminous celestial object is rotating in our reference frame, the wavelength of its light reaching us must vary periodically due to doppler shifting. As it comes closer to us, we would observe blue shifting, and as it moves away, we would see redshifting. By comparing our observations with the theoretical value, we can calculate the rate of rotation of these objects.
By the 1920s, through optical measurements of stars around us, astronomers could determine that our galaxy must be rotating in some manner. However, to be able to measure its actual rotation rate, we needed to observe the other side of the galaxy. This was not possible by optical means as visible light is blocked by the gas and dust clouds and hence, cannot reach us from the other side. However, radio waves, whose wavelengths are much longer than these dust particles, can traverse this distance and reach us.
Hence, the search began for a source of radio waves present all throughout our galaxy which could be measured and whose theoretical value could be determined to a high degree of accuracy.
Since hydrogen is the major constituent of our galaxy, it was logical to focus on any radio waves generated by electronic transitions in a hydrogen atom. While the ‘regular’ transitions between the different energy levels such as n=2 to n=1 release radiation of energy 10.2 eV, which lies in the visible range (and cannot be used), there is another kind of energy level splitting that occurs known as hyperfine splitting.
While we assume that the ground state of an electron in a hydrogen atom is an absolute state with a fixed energy, that of -13.6 eV, that is not entirely true. The electron and the proton both possess an intrinsic spin angular momentum. Their spins can be aligned either in a parallel or an anti-parallel manner. The magnetic interaction between these spins in different orientations further splits the ground state level of the neutral hydrogen atom into two different energy levels. The energy difference created by this effect is about 5.884 x10-6 eV, negligible compared to 10.2 eV. However, and most importantly in this context, the wavelength of this emitted radiation is 21.11 cm, which falls well in the radio part of the spectrum.
The Dutch astronomers Jan Oort and Henk van de Hulst were the first to realise that this 21 cm line can be used to find the structure of our galaxy. Just as an interesting side note, the probability of an electron in the upper hyperfine level spontaneously jumping down is extremely less. In fact, it is estimated that an electron will make this jump once every 10 million years! However, since the sheer magnitude of atomic hydrogen present in our galaxy is so large, the Milky Way is indeed glowing with the 21 cm radiation line.
Jan Oort, along with Henk van de Hulst and Mueller, used radio astronomy to detect this alongside E.M Purcell and Harold Ewen from Harvard, who were independently working towards the same goal. They found both emission lines where the electron jumped from a higher energy level to a lower energy level, as well as absorption lines where the electron in a lower level absorbed energy and transitioned to an upper level. Since these gas clouds are moving, both of these lines will be shifted due to the Doppler effect.
While there is a lot of random motion to account for due to shockwaves caused by supernova explosions and other processes, systematic observations enable us to map the distribution of atomic hydrogen gas in our galaxy. The Dutch astronomers soon published a map of the neutral hydrogen showing it to be distributed in spiral arms, just like we see in other galaxies.
With stronger telescopes, not only could we get a sharper picture of our Milky Way, but we could also look at other galaxies and find out their rates of rotation. As it generally goes, answering one question raises many more questions. We found out the structure of our galaxy, which allowed us to know more about its mass distribution, temperature, and density. It also revealed the existence of dark matter, which remains a big mystery.
Divyansh is a second-year student enrolled in the BS-MS program at IISER Kolkata. He is interested in physics and astronomy. Apart from that, his interests mainly lie in gaming and reading-particularly fiction.
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