For several centuries, people have wondered about the milky strip extending across the entire firmament. In the modern era, Galileo Galilei discovered that the Milky Way is formed of innumerable stars. Yet, it was only after the 20 th century that astronomers were successful in interpreting its form and its true nature.
Fire wheel: The Milky Way system, called galaxies, resembles a gigantic spiral with an estimated 200 billion stars. One of them is our Sun. (Image credit: Robert Hurt/SSC/Caltech/JPL/NASA)
My third observation relates to the nature of the Milky Way (...) No matter which part of it one targets with the telescope, one finds a huge number of stars, several of which are quite large and very striking; yet, the number of small stars is absolutely unfathomable.” In 1610, these words were written by a man who used his self-constructed telescope to study unknown lands that were not of this world. It was this study that earned him a spot in history: Galileo Galilei.
The land that he outlined is exactly out of this world, and the document is titled Sidereus Nuncius (“Starry Messenger”). In that document, the Italian mathematician and astronomer has presented his observations of the satellites of Jupiter, the Earth’s moon, and even the Milky Way. To that point, their nature had been a mystery, and had most importantly been the subject of mythology. In the 5th century BC, Democritus, the Greek natural philosopher, had already claimed that the diffusely glowing strip in the sky—known by the African !Kung bushmen as the “backbone of the night”—included innumerable weak stars.
Grindstone in the Firmament
However, after Galilei’s discovery, almost one and a half centuries had passed before this celestial structure again became the subject of scientific study. Thomas Wright of County Durham considered that stars were arranged in a flat region like a grindstone, which extended over the entire sky. He considered the Milky Way to be nothing other than the projection of this grindstone. The German philosopher Immanuel Kant took over this theory—and came quite close to revealing the truth.
In his General Natural History and Theory of the Heavens, published in 1755, he described the Milky Way as an extended and very diluted layer of stars. The Sun, the Earth, and all the other planets were part of this layer; however, they not at its center. Based on the line of sight, along the plane of the layer or vertically out of it, different numbers of stars can be observed.
However, what are the means by which astronomers can find out whether the obvious view of the Milky Way in the sky reflected its actual spatial structure? Stellar statistics developed by Friedrich Wilhelm Herschel at the end of the 18
th century seemed to be a promising solution: Herschel recorded the brightness and coordinates of all the stars that he could observe using his telescope.
However, the project was a failure: besides the unreliability of these measurements—for instance, though it could determine the apparent brightness of the stars, it was not possible to determine their absolute luminosity and eventually their distance—there was also a basic issue: the Milky Way is filled with gas, interstellar matter, and dust clouds that absorb the light from the stars. This hides the view of the central region and renders it impossible to observe the overarching structure. Hence, stellar statistics can never cover the system as a whole but only the area around the Sun up to a radius of nearly 10,000 light-years. The advancement was not achieved until the middle of the 20
th century, when astronomers learned to view the sky with different eyes with the help of radio telescopes.
A Look Through Curtains of Dust
The most common element in the universe is hydrogen. Neutral hydrogen (H1) is part of the interstellar matter and fills the space between the stars, and hence also fills the Milky Way. This suggests that the distribution of the hydrogen gas clouds trace the shape of the entire system, akin to the way in which the human body is shaped by bones.
However, in what way can these cosmic “bones” be rendered visible? The nanouniverse answers this question: in hydrogen’s ground state, the spin direction of the atomic nucleus and the electron orbiting around it are antiparallel. During the collision between two hydrogen atoms, the spin direction of the nucleus and the electron may be swapped to be parallel to each other—and after a specific time, they return to their fundamental antiparallel state.
Energy is released in this process and radiated as an electromagnetic wave. This line is in the radio range of the electromagnetic spectrum. In spite of the very low density of interstellar matter, atoms constantly collide with one another, making the H1 areas to glow in the light of this hydrogen line.
This radiation pierces through the dust curtains nearly unobstructed and can be detected by radio telescopes. This new door into the universe has enabled astronomers to unearth Milky Way’s spiral structure. Yet, in the 1970s, scientists discovered that hydrogen alone was not adequate as an indicator of the morphology of the galaxy since, for instance, it is less concentrated in the spiral arms than it was anticipated. The search started afresh.
Arms in Motion
Clouds of interstellar molecules turned out to be the most critical indicator. Radiation is emitted by these molecules in the light of carbon monoxide, or CO. The representation of the Milky Way can now be refined gradually. Consequently, the galaxy (derived from the Greek word gala: milk) is nothing but a bent wheel that has a diameter of 100,000 light years and a thickness of only 5,000 light years. A circular bulge of stars with an integrated cigar-shaped structure—a type of bar—surrounds the wheel hub with its black hole.
A ring extends about 15,000 light years from the center, consisting of stars as well as gas and dust clouds. A number arms of characterize the galaxy, with majority of them bearing the labels of the stellar constellations in which humans view them: These include the Norma and Scutum-Crux Arms, the Sagittarius and Perseus Arms, the Cygnus Arm, and the 3-Kiloparsec Arms.
The solar system is situated in the Orion Arm, 26,000 light-years from the center and practically on the main plane. The system includes nearly 200 billion suns and is surrounded by a spherical halo that contains thousands of globular star clusters and a spherical region comprising of very thin hydrogen plasma. The whole galaxy rotates, with objects very near to the center rotating faster, and those located farther from the center rotating more slowly. The curve of this differential rotation reveals irregularities that cannot be explained by only visible mass.
In this case, it is probable that invisible dark matter has a part to play. Moreover, the astronomers have faced one more problem: in spite of the rotation, the spiral arms do not unwind but retain their shape for billions of years. One interpretation for this is shockwaves that travel throughout the entire system and compact the matter in the spiral arms similar to a traffic jam on the motorway. Scientists still do not have a clear idea over what causes these density waves.