Milky Way

Our Sun and solar system belong to the Milky Way galaxy, a large system of several hundred billion stars, gas, and dust. Beyond our galaxy are billions of other galaxies. The study of our galaxy adds to our knowledge of galaxies in general, while the study of other galaxies has revealed features later discovered in the Milky Way.

Overview

The Milky Way galaxy is a vast grouping of several hundred billion stars, gas, and dust to which the Sun and solar system belong. All the stars visible to the unaided eye in the night sky are a part of this same huge collection. Most of the stars, gas, and dust observed in our galaxy are contained in a thin disk more than 100,000 light-years in diameter, with a lens-shaped bulge at its center. This central bulge, also called the galactic nucleus, contains the greatest concentration of stars and, right at its center, a supermassive black hole. Within the disk are several spiral arms in which new stars form. Surrounding our galaxy's disk and central bulge is the roughly spherical galactic halo, a few hundred thousand light-years in diameter, sparsely populated with old stars and old star clusters but with virtually no gas or dust. The Sun and solar system are located in the disk, within one of the spiral arms or arm branches, about 26,000 light-years from the center. It takes the Sun and solar system more than 200 million years to complete one orbit around the galactic center.

This picture of our galaxy originated in the 1600s and developed in its modern form during the twentieth century. However, speculations and myths about the Milky Way date back to ancient times since it is such an obvious feature of the dark night sky. On any clear, moonless night away from city lights or other light pollution, the Milky Way is easily seen by the naked eye as a beautiful, pale white, diffuse band of light stretching across the sky. In ancient times, it was given the name Milky Way because of its pale, milky appearance. (The word galaxy comes from the Greek word gala, meaning milk.) In some mythologies, the Milky Way was the milk of the gods spilled on the sky. Some other cultures viewed it as the path by which departed souls ascended to the realm of the gods. In all, there were many imaginative mythological explanations for it.

In 1610, Galileo scanned the Milky Way with his small telescope and found it to be composed of lots of faint stars. By the 1700s, the arrangement of stars making up the Milky Way was described using terms such as "sheet," "disk," "millwheel," and "grindstone." In 1784, William Herschel developed a model of the Milky Way using a technique that he called "star gauging," in which he counted the number of stars he could see in his telescope’s field of view when he pointed it in different directions. Based on his star counts, he described the Milky Way as a layer of stars with crooked or jagged outer edges and the Sun near its center. During the following century, Herschel’s technique and model were refined, and by the early 1900s, the Milky Way galaxy was pictured as an oblate ellipsoid, about 60,000 light-years in diameter and 13,000 light-years from top to bottom, with the Sun not far from the center.

In 1918, Harlow Shapley of Harvard University concluded that the galaxy was much bigger than previously thought, and the Sun was not near its center. Based on his study of the distribution in space of globular clusters (large clusters of about 100,000 to 1,000,000 very old stars), he determined that the galactic center was located about 50,000 light-years from the Sun and solar system in the direction of the constellation Sagittarius. During the 1950s, the Milky Way’s disk was found to have a spiral structure. First, in the early 1950s, several spiral arm segments were traced optically using the locations of gaseous nebulae and young stars, objects concentrated in spiral arms. In the late 1950s, the first radio maps of the galaxy’s disk were produced, using radio radiation with a wavelength of twenty-one centimeters (and frequency of 1,420 megahertz), produced by hyperfine "spin-flip" transitions in neutral hydrogen atoms. The twenty-one-centimeter radio maps traced the distribution of neutral hydrogen concentrated in the spiral arms.

During the last half of the twentieth century, the Milky Way was observed over the entire electromagnetic spectrum, from radio waves to gamma rays, and this resulted in further refinements to our ideas about our galaxy. In modern models, the galactic disk has an overall diameter of 100,000 to 130,000 light-years, possibly with a warped outer edge. The Milky Way seems to be a multiarmed spiral, with at least four and possibly more spiral arms and smaller branches and spurs. There may be a central bar running through the galactic nucleus, with the spiral arms originating from the ends of it. Although visible light from the galactic nucleus is blocked by interstellar dust in the galactic disk between our location and the galactic center, the central region has been studied with radio waves, infrared, and gamma rays, which can penetrate the dust. The galactic nucleus is a packed concentration of stars, gas, and dust, with a supermassive black hole of several million solar masses at the center. The galactic nucleus and flat galactic disk are contained within a nearly spherical galactic halo of widely separated old stars and globular star clusters. An even more spread-out corona or outer halo extends beyond it, as far out as 150,000 to 300,000 light-years. The total mass of the galaxy is calculated to be between several hundred billion and two trillion solar masses, based on the observed motions of stars and gas within our galaxy and small satellite galaxies on the fringes of the galaxy.

However, only a fraction of the mass has been observed. The mass that has not yet been observed is referred to as dark matter, but astronomers and cosmologists are not sure what it is. The distance of the Sun and solar system from the galactic center has been reduced to half of Shapley’s original value, about 26,000 light-years. The Sun and most of the stars, gas, and dust in the galactic disk have a nearly circular orbit around the galactic center. The orbital speeds of these solar-system bodies are between about 200 and 250 kilometers per second, and they take between about 200 million and 250 million years to complete one orbit around the galactic center. Some stars, particularly those in the galactic halo, have much more elongated orbits that carry them close to the center and then far out into the distant parts of the galaxy.

Knowledge Gained

Hydrogen, the most abundant element, exists in our galaxy as ionized and neutral single atoms, as well as diatomic molecules. Ionized atomic hydrogen is the most obvious form because it can be seen at visible wavelengths as glowing clouds called H II regions. Hydrogen atoms are ionized by ultraviolet radiation from nearby hot stars, and visible light is produced when electrons recombine with the bare hydrogen nuclei. Clouds of neutral atomic hydrogen, called H I regions, were first detected in our galaxy in 1951, at the radio wavelength of twenty-one centimeters (frequency of 1,420 megahertz) emitted by hyperfine (spin-flip) transitions in neutral hydrogen atoms. Astronomers had thought molecular hydrogen could not exist in interstellar space because ultraviolet radiation from hot stars would break the molecules apart into individual atoms. The discovery of interstellar molecules of carbon monoxide in 1970 spurred the search for other molecules. Since then, more than one hundred molecules, including molecular hydrogen, have been detected in large, dense, cold molecular clouds, some of them hundreds of light-years across. The molecules in these interstellar clouds withstand the effects of ultraviolet starlight because they are shielded by interstellar dust. Making up about half of the Milky Way’s interstellar gas, these clouds provide the cold, dense environment needed for the birth of new stars. Giant molecular clouds are concentrated around the central bulge and spiral arms.

The dust in our galaxy is confined, for the most part, to the central plane of the galactic disk. There, it effectively obscures the view of more distant objects at visible and shorter wavelengths. The Great Rift, the Coalsack, and the Horsehead Nebula are some prominent dark concentrations of dust. However, longer wavelengths can penetrate the dust; the longer the wavelength, the less the dust blocks the view. This provides a clue to the size of the dust grains. Small, solid particles are very effective at scattering electromagnetic radiation with wavelengths comparable to or shorter than the size of the grains. Therefore, the size of the grains is a bit larger than the longest wavelengths of visible light, or about 0.001 millimeters. Polarization studies suggest the grains probably are elongated, perhaps needle-like. The dust grains probably comprise iron alloys, silicate minerals, carbon compounds, various ices, or a combination.

Our galaxy appears to have at least four spiral arms, marked by clouds of gas and dust, as well as young stars that have recently formed in them. Objects near the galactic center revolve around it in less time than objects farther out. If spiral arms were permanent collections of objects, they would quickly "wind up" because the orbital period around the galactic center increases as the distance from the center increases. A solution to this "winding dilemma" is provided by the spiral density wave theory, which postulates that the spiral arms are produced by an underlying density wave pattern that propagates through the gas and dust of the galactic disk. The entire spiral pattern rotates with the same period around the galactic center. The stars, gas, and dust in the galactic disk move faster than the pattern, regularly overtaking the spiral-shaped density waves. Stars are not affected much by the slight increase in density, so they pass right through the density waves. Clouds of gas and dust, however, are slowed by the increase in density, so they pile up and are compressed, triggering star formation. The spiral arms are highlighted by young stars that have just recently formed and the gas clouds in which stars can form.

An intriguing feature of the Milky Way galaxy is its central region since it is hidden from visual observations by thick clouds of dust. Radio radiation from the galactic center was first detected in 1932 by Karl Jansky, an engineer at Bell Telephone Laboratories. He was studying long-range radio communication for Bell Labs when he discovered steady radio static emanating from the galactic center in the direction of the constellation of Sagittarius. Obscured by interstellar dust at visible wavelengths, the galactic center received little attention until the 1960s and 1970s, when advances in radio astronomy and infrared astronomy made it possible to study.

Infrared images of the central region revealed giant molecular clouds and lots of stars. Near the center, the average distance between stars is on the order of a few light-weeks (compared to a few light-years in the neighborhood around the Sun). A number of radio sources have been found in the central region, including several supernova remnants. The most prominent radio emitter there is an extended region named Sagittarius A (Sgr A*). At the center of Sgr A* there is an extremely strong, very compact radio source named Sgr A* (pronounced “Sagittarius A star”). The orbital motion of several individual stars around Sgr A* has been recorded with high-resolution infrared imaging. These stars are at distances of only a few hundred to a few thousand astronomical units from Sgr A* and are moving at speeds of a few hundred to a few thousand kilometers per second, which indicates they are orbiting an object with several million solar masses. The only thing this massive that could fit inside the orbits of these stars is a supermassive black hole, which presumably is powering the Sgr A* radio source.

By the late 1970s, objects in the outer parts of the galactic disk were discovered to be revolving around the galaxy at about the same speed as objects in the inner part of the disk, implying the existence of a substantial mass distributed throughout the galaxy out to large distances. However, the mass that actually can be observed using any part of the electromagnetic spectrum falls far short of accounting for the needed mass. Originally referred to as missing mass, the unobserved mass is called dark matter in the twenty-first century since it is not really missing (at least if gravity behaves as physicists think it does). The most likely location for the dark matter is the galactic halo, which could contain considerable mass spread very thinly throughout its immense volume. Astronomers have yet to determine what dark matter is, but possible candidates include faint low-mass stars, dead stars, planets, rocks, black holes, neutrinos with mass, weakly interacting massive particles (WIMPs), massive compact halo objects (MACHOs), or some other form of matter.

Context

The Milky Way galaxy is only one of the billions of observable galaxies with various sizes and shapes. The Milky Way is a reasonably large spiral galaxy. Galaxies also may have elliptical or irregular shapes, ranging from giant galaxies with at least several trillion solar masses down to dwarf galaxies with as little as several million solar masses. The distribution of galaxies in space shows a hierarchy. Galaxies tend to occur in groups called clusters of galaxies. In turn, clusters of galaxies tend to form larger groups called superclusters. The Milky Way galaxy is one of the two largest galaxies in a small cluster called the Local Group, consisting of about forty known member galaxies spread out over about three million light-years. The Local Group is part of a supercluster centered on the rich Virgo cluster of galaxies.

The continuing study of the Milky Way has advanced our knowledge of galaxies in general. A case in point concerns the origin and development of galaxies. Galaxies, including the Milky Way, are thought to have condensed from large clouds of gas, probably starting between one hundred million and one billion years after the creation of the universe in the Big Bang about thirteen to fourteen billion years ago. This is supported by the ages of the oldest surviving stars in our galaxy, about twelve billion years old. Each protogalactic cloud was composed of hydrogen and helium since those were the only elements produced in significant amounts by the Big Bang. As the protogalactic cloud for the Milky Way contracted gravitationally, some parts were denser than others, and in these denser regions, the first stars and star clusters formed. Since these stars and star clusters formed from material falling inward toward the center of the cloud, they developed elongated orbits that would carry them close to the center and then back out into the outer fringes, where they spend most of the time. This became the halo of our galaxy. As the contraction proceeded, the protogalactic cloud began to spin faster to conserve angular momentum. The rapid rotation caused the protogalactic cloud to flatten, forming an equatorial disk around a central bulge of material. Thus, the galactic disk and nucleus were born. The supermassive black hole at the galactic center probably developed very early because of the growing density as the galactic bulge or nucleus contracted. Massive stars synthesized chemical elements heavier than helium as they generated energy through nuclear fusion reactions in their interiors. When the massive stars exploded as supernovas, they dispersed the heavier elements and enriched the gas from which new generations of stars and planets would form. The Sun and solar system formed about 4.5 billion years ago, at which time about 2 percent by mass of the gas cloud from which it formed consisted of elements heavier than helium. Star formation continues to the present time in the disk of our galaxy, as it does in other spiral galaxies.

A variation of this basic scenario is that galaxies formed not from a single protogalactic cloud for each galaxy but rather through the merger long ago of many smaller "seed galaxies." This version shows small seed galaxies formed first after the Big Bang. After the first generation of stars formed in them, they began to merge, forming larger galaxies. There is growing evidence that our galaxy has "devoured" other smaller galaxies through a process known as galactic cannibalism, and it continues to do so. Other galaxies show evidence of galactic mergers (two comparable-sized galaxies joining together) and galactic cannibalism (a larger galaxy devouring a smaller one).

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