The night's spine

June 12, 2013

For millenia, people of all cultures have been fascinated by the Milky Way. But what's behind the diffusely gleaming band of light that stretches across the sky on clear nights? Only in recent times, astronomers were able to decipher the mystery surrounding the nature and shape of the celestial phenomenon. And not until the 1920s, did we discover that the Milky Way is an independent galaxy, which is drifting in the far reaches of space with hundreds of billions of others.

Text: Helmut Hornung

Catherine wheel in space: the galaxy resembles a spiral with many arms. Astronomers discovered the bulge in the center, which is about 27,000 light years away, only recently. Our Sun is one of approx. 100 billion stars.

"My third observation concerns 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.” The man who wrote this in March 1610 was using a self-constructed telescope to advance into unknown regions, for which he would go down in history: Galileo Galilei. He discovered what was literally out of this world – hence the book title "Sidereus nuncius" (“The Starry Messenger”). In it, the Italian mathematician and astronomer described his observations of Jupiter’s satellites, the Earth’s moon and the Milky Way, who was regarded as enigmatic and above all, a subject of mythology.

But already in the 5th century BC, Greek philosopher Democritus expressed his belief that this “Milky Way” – called "The night's spine" by !Kung bushmen - consisted of countless faint stars. Almost 150 years passed before a scientist again concerned himself with this celestial structure. Thomas Wright of Durham believed that the stars were arranged as a flat object similar to a grindstone that stretched across the entire sky, and that the Milky Way was nothing but the projection of this grindstone. German philosopher Immanuel Kant seized on this theory – and came very close to the truth.

The search for form

In his General Natural History and Theory of the Heavens, published in 1755, he explained 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, but not at its centre. Depending on the line of sight, along the plane of the layer or vertically out of it, we would see different numbers of stars. But how were the astronomers to find out whether the apparent view of the Milky Way in the sky reflected its actual spatial structure?

Stellar statistics devised at the end of the 18th century by Friedrich Wilhelm Herschel promised a solution: Herschel recorded the coordinates and brightness of all stars visible through his telescope. The undertaking failed: apart from the uncertainty of these measurements – for example, although it was possible to determine the apparent brightness of the stars, it was impossible to determine their absolute luminosity and hence their distance – there was also a fundamental problem: the Milky Way is filled with gas and dust clouds that absorb the light from the stars. This obscures the view, particularly toward the central region, and prevents the overarching structure from being recognized.

Therefore, the stellar census will never be complete, except for the region around the Sun up to a radius of about 10,000 light-years. The breakthrough did not come until the middle of the 20th century, when astronomers had learned to look at the sky with different eyes: with radio telescopes.

Curtains of dust

Hydrogen is the most common element in the universe. As part of interstellar matter, neutral hydrogen (H1) fills the space between the stars, and thus fills the Milky Way. This means that the distribution of clouds of hydrogen gas trace the shape of the whole system, similar to the way in which bones shape the human body. But how can these cosmic “bones” be made visible? The nanouniverse provides the answer: in the ground state of hydrogen, the spin directions of the atomic nucleus and the electron that orbits around it are antiparallel. If two hydrogen atoms collide, the spin directions of the nucleus and the electron may be flipped to end up parallel to each other – and after a certain time, they return to the antiparallel ground state. This process releases energy, which is radiated as an electromagnetic wave. Its wavelength is 21.049 centimetres (frequency: 1420.4 MHz), and therefore lies in the radio range of the electromagnetic spectrum. Despite the extremely low density of interstellar matter, atoms will always collide frequently enough to cause the H1 areas to glow in the light of the 21-centimeter-line. This radiation penetrates the dust curtains almost unobstructed and can be picked up by radio telescopes. The Doppler effect from the galactic rotation makes it possible to determine, from the measured wavelength, the distance of the source of radiation along the line of sight. This new window on the universe allowed astronomers to discover the spiral structure of the Milky Way. However, in the 1970s, researchers found that hydrogen alone was not sufficient as an indicator for the galaxy’s morphology because, for example, it is less concentrated in the spiral arms than expected. The search began anew.

Arms in motion

The most important indicator turned out to be clouds of interstellar molecules; they emit radiation in the characteristic light of carbon monoxide (CO) at a wavelength of 2.6 millimetres. Now it was gradually becoming possible to refine the portrait of the Milky Way. Accordingly, the galaxy (from the Greek word gala: milk) is a slightly bent wheel, 100,000 light years in diameter and 5,000 light years in thickness. The centre with its supermassive black hole is surrounded by a spherical bulge of stars with an embedded, elongated, cigar-shaped structure – a kind of bar – approximately 15,000 to 25,000 light-years in length.

Several spiral arms make up the characteristic feature of the galaxy. Most of them bear the names of the heavenly constellations in which we observe them: the Sagittarius and Perseus Arms, the Norma and Scutum-Crux Arms, the 3-Kiloparsec Arms and the Cygnus Arm. Our solar system is located in the Orion Arm, 26,000 light-years from the galactic centre and 50 light-years north of the main galactic plane. The system, which contains around 150 billion suns, is surrounded by a spherical halo containing thousands of globular star clusters and consisting of a thin hydrogen plasma.

The entire galaxy rotates; objects closer to the centre rotate faster, and those further from the centre, more slowly. The curve of this differential rotation with a galactocentric radius becomes constant at relatively small galactocentric distances; this cannot be explained by visible mass alone, and is the main argument for the existence of invisible dark matter. And the astronomers face yet another problem: despite the rotation, the spiral arms do not unwind, but maintain their shape for billions of years. One explanation for this is shockwaves that compact the matter in the spiral arms and that propagate throughout the whole system like a traffic jam on the highway. Researchers are still puzzling over what causes these density waves.

At the heart is a black hole

For several years now astronomers have been focusing their attentions on the centre of our galaxy, which is to be found in the Sagittarius constellation when viewed from Earth. Here as well, the interior is hidden from view by dark clouds of dust and gas. Only in the last 60 or so years have the scientists been able to gain an accurate idea of the heart of the Milky Way by means of observations in the radio wave, infrared light and X-ray wavelength ranges. And at the beginning of the 21st century precision measurements revealed some surprising facts.

At the centre of the Milky Way is the bright radio source Sagittarius A*. It is a strong concentration of mass around which stars orbit; the nearest one – known as S2 – takes only 15.2 years to orbit the galactic heart at a distance of no less than 17 light hours. From the motions of this and the other stars the astronomers conclude that a mass of approximately 4.3 million solar masses must accumulate within a central region measuring 15.4 million kilometres in diameter. The only plausible explanation: a gigantic black hole.

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