For thousands of years, people lead hold been puzzling over the milky strip that extends across the entire firmament. In the modern era, Galileo Galilei discovered that this Galaxy consists of countless stars. However, it was non until the 20th century that astronomers succeeded inward deciphering its cast in addition to its truthful nature.
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| Fire wheel: The Galaxy system, called galaxis, resembles a gigantic spiral amongst an estimated 200 billion stars. One of them is our Sun [Credit: Robert Hurt/SSC/Caltech/JPL/NASA Robert Hurt] |
The terra firma that he described is literally out of this world, in addition to the document bears the championship Sidereus Nuncius ("Starry Messenger"). In it, the Italian mathematician in addition to astronomer presents his observations of the satellites of Jupiter, the Earth's Luna in addition to besides the Milky Way. Until then, their nature had remained a mystery, in addition to had to a higher house all been the dependent land of mythology. The Greek natural philosopher Democritus had already claimed inward the fifth century BC that the diffusely glowing strip inward the heaven – known past times the African !Kung bushmen every bit the "backbone of the night" – consisted of countless weak stars.
Grindstone inward the firmament
After the uncovering made past times Galilei, however, nearly 150 years would transcend earlier this celestial construction would ane time to a greater extent than became the dependent land of scientific study. Thomas Wright of County Durham believed that stars were arranged inward a apartment share similar to a grindstone, which extended over the entire sky. For him, the Galaxy was aught other than the projection of this grindstone. The High German philosopher Immanuel Kant seized on this theory – in addition to came real unopen to discovering the truth.
In his General Natural History in addition to Theory of the Heavens, published inward 1755, he explained the Galaxy every bit an extended in addition to real diluted layer of stars. The sun, the globe in addition to all the other planets were business office of this layer—but non at its centre. Depending on the line of piece of occupation of sight, along the airplane of the layer or vertically out of it, nosotros would come across unlike numbers of stars.
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| Curved: from the side, the galaxy looks similar a slightly bent wheel. It has a diameter of nigh 100,000 in addition to a thickness of only 5,000 calorie-free years. Around the centre at that spot is a bright, spherical bulge [Credit: Helmut Rohrer] |
However, the task failed: apart from the unreliability of these measurements – for example, although it was possible to decide the apparent brightness of the stars, it was impossible to decide their absolute luminosity in addition to thence their distance – at that spot was besides a fundamental problem: the Galaxy is filled amongst interstellar matter, gas in addition to dust clouds that absorb the calorie-free from the stars. This obscures the persuasion of the fundamental share in addition to makes it impossible to come across the overarching structure. For this reason, stellar statistics tin never embrace the organization every bit a whole, but only the share around the Sun upwards to a radius of nigh 10,000 light-years. The breakthrough did non come upwards until the middle of the 20th century, when astronomers learned to expression at the heaven amongst unlike eyes using radio telescopes.
A expression through curtains of dust
Hydrogen is the most mutual chemical constituent inward the universe. As business office of interstellar matter, neutral hydrogen (H1) fills the infinite betwixt the stars, in addition to thus besides fills the Milky Way. This way that the distribution of clouds of hydrogen gas draw the shape of the whole system, similar to the way inward which bones shape the human body.
But how tin these cosmic "bones" hold upwards made visible? The respond is provided past times the nanouniverse: inward the Earth nation of hydrogen, the direction of spin of the atomic nucleus in addition to the electron that orbits around it are antiparallel. If ii hydrogen atoms collide, the direction of spin of the nucleus in addition to the electron may hold upwards flipped to halt upwards parallel to each other – in addition to afterwards a surely time, they render to their basic antiparallel state.
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| Close view: this icon of the fundamental business office of the Galaxy shows a share of 1000 x 500 calorie-free years in addition to was taken amongst the MeerKAT telescope stationed inward South Africa, a organization consisting of 64 radio antennas [Credit: SARAO] |
This radiations penetrates the dust curtains almost unobstructed in addition to tin hold upwards picked upwards past times radio telescopes. Thanks to this novel window into the universe, astronomers lead hold been able to regain the spiral construction of the Milky Way. However, inward the 1970s, researchers constitute that hydrogen solitary was non sufficient every bit an indicator of the galaxy's morphology because, for example, it is less concentrated inward the spiral arms than expected. The search began anew.
Arms inward motion
The most of import indicator turned out to hold upwards clouds of interstellar molecules; they emit radiations inward the calorie-free of carbon monoxide (CO). Now it was gradually becoming possible to refine the portrait of the Milky Way. Accordingly, the galaxy (from the Greek give-and-take gala: milk) is a bent wheel, 100,000 calorie-free years inward diameter in addition to amongst a thickness of but 5,000 calorie-free years. The bike hub amongst its dark hole is surrounded past times a spherical bulge of stars amongst an embedded cigar-shaped construction – a sort of bar.
Around 15,000 calorie-free years from the centre, a band extends that besides consists of dust in addition to gas clouds, every bit good every bit stars. The galaxy is characterised past times several arms. Most of them comport the names of the stellar constellations inward which nosotros detect them: the Sagittarius in addition to Perseus Arms, the Norma in addition to Scutum-Crux Arms, the 3-Kiloparsec Arms in addition to the Cygnus Arm.
Our solar organization is located inward the Orion Arm, 26,000 light-years from the centre in addition to almost on the original plane. The system, which contains around 200 billion suns, is surrounded past times a spherical halo containing thousands of globular star clusters in addition to a spherical share consisting of real sparse hydrogen plasma. The entire galaxy rotates, amongst objects closer to the centre rotating faster, in addition to those farther from the centre rotating to a greater extent than slowly. The flexure of this differential rotation shows irregularities that cannot hold upwards explained past times visible majority alone.
Here, it is probable that invisible dark affair plays a role. And the astronomers human face upwards yet some other problem: despite the rotation, the spiral arms exercise non unwind, but lead hold maintained their shape for billions of years. One explanation for this is shockwaves that propagate throughout the whole organization in addition to compact the affair inward the spiral arms similar a traffic jam on the motorway. Researchers are soundless puzzling over what causes these density waves.
Author: Helmut Hornung | Source: Max-Planck-Gesellschaft [July 27, 2018]
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