Milky Way

"It suggests a nebulous dim puff of star dust lost in the blaze of the Milky Way. Properly, the Jew ought hardly to be heard of; but he is heard of, has always been heard of. He is as prominent on the planet as any other people, and his importance is extravagantly out of proportion to the smallness of his bulk."

"The Milky Way is a spiral disk galaxy, similar to many others we see in the sky. This surprisingly beautiful shape is so common among galaxies that the universe almost seems to delight in building them. The end product is especially remarkable in the light of what is believed to be the starting point: nebulous blobs of gas."

"The gas blobs that evolved into the Milky Way consisted merely of hydrogen and helium (and a smattering of lithium), the elements that were created in the Big Bang. All the other elements were literally created by the stars. Unlike the medieval alchemists, the stars can actually transmute one element into another—they are prodigious chemical factories. Nevertheless, even today hydrogen and helium make up about 98 percent of the normal matter in the universe. It’s the distribution of the elements that make up the final 2 percent that makes all the difference to studies of galactic evolution."

"Although the development of new technologies has improved the quality of the observations and so refined the constraints on astronomers’ models, we are still far from a complete understanding of our galaxy’s evolution. Like our galaxy, the field itself is still evolving."

"As we arbitrarily divide the human body into a torso with a head and limbs, so we can conceptually separate the Galaxy into various components. The flying-saucer shape—consisting of the central bulge and the spiral disk—is only the most obvious part of the Galaxy. The spiral disk itself can be subdivided into a thin disk, which rises about 1,000 light-years above and below the galactic mid-plane, and a thick disk, which extends to about 3,500 light-years on either side of the plane. The relative flatness of our galaxy is evident when one considers that the galactic disk is generally thought to be about 120,000 light-years across. Our sun resides in the thin disk about 28,000 light-years from the galactic center... About 200 globular clusters are known, and they appear to be some of the oldest objects in the Galaxy. It took many decades of careful study to tease apart the various regions of the Milky Way, and the process of dissecting out fine-scale subregions continues even today.One of the reasons it’s so difficult is that we cannot measure the properties of all the stars in the Galaxy—they are simply too far away....the different orbits of the stars, their kinematic properties, provide a crucial distinction between stars that belong to different regions of the Galaxy"

"A star’s kinematic properties are one of the ways that astronomers can recognize an interloper from another part of the Galaxy. ...The most metal-poor star ever observed in our galaxy is located in the halo. It is old and has a metallicity [Fe/H] of about -4.0, or about 10,000 times less than the Sun!"

"There is another type of exploding supernova that also seeds the Galaxy with elements. This is the type Iasupernova. This explosion involves a binary system in which a white dwarf star and an intermediate-mass star (a red giant) orbit each other. The two stars are so close to each other that the white dwarf gradually pulls a considerable amount of material from the outer envelope of the expanding red giant. At a certain point the white dwarf will acquire so much mass that it collapses under its own weight and produces an explosion that blasts the bulk of its material into the interstellar medium—mostly in the form of iron, but also some sulfur, silicon and calcium. Such explosions contributed about 70 percent of the iron we see today in the Galaxy....three processes hold an important key to understanding the evolution of the Milky Way precisely because they occur on very different timescales. By measuring specific abundance ratios in stars from different parts of the Galaxy, astronomers can discover how fast the metal enrichment proceeded and the timescale over which the region was formed."

"The granddaddy of galactic-formation models was conceived in the early 1960s by three astronomers: Olin Eggen, Donald Lynden-Bell and Allan Sandage. Their 1962 publication played a seminal role in the field and is now simply referred to by the authors’ initials: ELS....According to ELS, the Milky Way began as a spherical cloud of gas—a protogalaxy—that was born collapsing toward its center.The original gas was poor in metals, and so stars formed as the cloud was collapsing would also be metal poor.These newly made stars maintained the kinematic properties of the gas in the collapsing cloud, and so followed eccentric orbits around the center of the Galaxy, forming the population II stars of the halo and the globular clusters. As the cloud contracted, some of its energy would have been lost to heat in a dissipative collapse;"

"In the decades that followed, a number of observations indicated that the Galaxy could not have formed in such a rapid collapse. The ELS model, as originally proposed,could not be right. One notable alternative was suggested by the American astronomers Leonard Searle and Robert Zinn in 1978… Instead of a single-cloud collapse, Searle and Zinn proposed that the halo of the Milky Way formed by the aggregation of many cloud fragments, each of which may have already formed stars and globular clusters"

"It appears, however, that our galaxy’s formation was neither smooth nor continuous."

"More observations are needed before we can refine our models of the Milky Way’s evolution. For one thing, we are still uncertain about the formation timescale For the thin disk outside the w:Local Interstellar Cloud}solar neighborhood because we lack precise observational constraint."

"So locker und leicht geschlagen, der schwimmt sogar in Milch."