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Milky Way

Milky Way a faint band of light crossing the sky, clearly visible on dark moonless nights and discovered by Galileo to be made up of vast numbers of faint stars. It corresponds to the plane of our Galaxy, in which most of its stars are located.

The Milky Way was sometimes named from famous pilgrimage routes; as Walsingham Way and the Way of St James (the road to Santiago de Compostela).

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Milky Way

Milky Way Faint band of light visible on clear dark nights encircling the sky along the line of the galactic equator. It is the combined light of an enormous number of stars, in places obscured by clouds of interstellar gas and dust. It is in fact the disc of our galaxy, viewed from our vantage point within it.

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Milky Way

Milk·y Way a faint band of light crossing the sky, made up of vast numbers of faint stars. It corresponds to the plane of our Galaxy, in which most of its stars are located. ∎  the galaxy in which our sun is located.

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Milky Way

Milky Way Candy bar developed by Frank Mars in Minneapolis‐St Paul, 1923.

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Milky Way

Milky Way, the galaxy of which the sun and solar system are a part, seen as a broad band of light arching across the night sky from horizon to horizon; if not blocked by the horizon, it would be seen as a circle around the entire sky. Although its motion is not readily apparent, the entire galaxy is rotating about the Milky Way's center. Relative to the universe, the galaxy is moving at a speed of c.370 mi per sec (c.590 km per sec) in the same direction that the constellation Leo lies relative to the earth; it is also moving at c.60 mi per sec (c.100 km per sec) relative to the center of mass of the Local Group of galaxies. The sun, traveling at a speed of c.150 mi per sec (c.240 km per sec) in a nearly circular orbit, takes some 230 million years to complete one revolution.

Visual Characteristics of the Milky Way

Among the constellations the Milky Way passes through are Carina, Crux (the Southern Cross), Sagittarius (where it is brightest), Scorpius, Aquila, Cygnus, Perseus, Cassiopeia, Auriga, and Gemini. In the direction of Cygnus is the Great Rift, a band of dark matter that lies along the Milky Way, dividing it into two forks. Another dark region is the Coalsack, in Crux. Once believed to be vast empty regions in space, these dark areas are now known to be clouds of dark matter blotting out the light behind them. Such nonluminous clouds of dust and gas, called dark nebulae, obscure many parts of the sky from sight; in the direction of the galactic center, the view is almost entirely obscured.

Size and Shape of the Milky Way

The Milky Way is a large barred spiral galaxy comprising an estimated 200 billion stars (some estimates range as high as 400 billion) arrayed in the form of a disk, with a central elliptical bulge (some 12,000 light-years in diameter) of closely packed stars lying in the direction of Sagittarius. It is surrounded by a flat disk marked by six spiral arms that project from a dense, elongated concentration of stars, or bar, that runs through the bulge—four major and two minor—which wind out from the nucleus like a giant pinwheel. Our sun is situated in one of the smaller arms, called the Local or Orion Arm, that connect the more substantial next inner arm and the next outer arm. The sun lies roughly 27,000 light-years from the center of the galaxy, and in the galactic plane. When we look in the plane of the disk we see the combined light of its stars as the Milky Way. The diameter of the disk is c.100,000 light-years; its average thickness is 10,000 light-years, increasing to 30,000 light-years at the nucleus.

Certain features of the region near the sun suggested that our galaxy resembles the Andromeda Galaxy. In 1951 a group led by William Morgan detected evidence of spiral arms in Orion and Perseus. Another bright arm stretches from Sagittarius to Carina in the southern sky. With the development of radio astronomy, scientists have extended a nearly complete map of the spiral structure of the galaxy by tracing regions of hydrogen that dominate the spiral arms. The development of telescopes that could be placed in orbit led by 2005 to confirmation that the Milky Way was a barred spiral galaxy, not a spiral one as had been believed.

Surrounding the galaxy is a large spherical halo of globular star clusters that extends to a diameter of about 130,000 light-years; this is called the stellar halo. The galaxy also has a vast outer spherical region called the corona, or dark halo, which is as much as 600,000 light years in diameter and, in addition to dark matter which accounts for most of the Milky Way's mass, includes some distant globular clusters, the two nearby galaxies called the Magellanic clouds, and four smaller galaxies.

Stellar Populations and Galactic Evolution

The stars, gas, and dust that make up the Milky Way can be grouped into two broad stellar populations that suggest how the galaxy evolved. The spiral arms and central plane of the Milky Way contain the interstellar gas, cosmic dust, and bright young stars categorized as Population I. The halo, spaces between the spiral arms, and central core of the galaxy contain the older, less spectacular stars that are categorized as Population II. This distribution can be explained by an evolutionary model in which an enormous cloud of gas and dust began to condense to form what are now Population II stars. The remaining gas and dust then collapsed, either suddenly or in stages, into the relatively thin disk in which Population I stars were (and still are being) formed.

Like other galaxies, the Milky Way is growing by absorbing small satellite galaxies. It is currently merging with the Large and Small Magellanic Clouds, a process that will be completed in about 100 million years. In 2003 a previously unknown galaxy was found to be colliding with the Milky Way. Its distinctive red stars are slowly being pulled into the Milky Way, and the dwarf will soon lose all its structure. Called the Canis Major dwarf galaxy after the constellation in which it lies, it is about 25,000 light years away from the solar system and 42,000 light years from the center of the Milky Way. This is closer than the Sagittarius dwarf galaxy, discovered in 1994, which is also colliding with the Milky Way. Several other galaxies are also, apparently, on a collision course with the Milky Way. The biggest and most spectacular collision will be with the Andromeda Galaxy. In about 2 billion years, massive tidal gravitational effects will tear spiral arms apart and start to shred the pinwheels from the outside in. The result will be an elliptical rather than a spiral Milky Way.

Bibliography

See E. J. Alfaro and A. J. Delgado, ed., The Formation of the Milky Way (1995); G. L. Vogt, The Milky Way (2002).

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Milky Way

Milky Way ★★½ 1936

Loopy comedy about milkman who finds unhappiness after accidentally knocking out champion boxer. Adequate, but not equal to Lloyd's fine silent productions. 89m/B VHS, DVD . Harold Lloyd, Adolphe Menjou, Verree Teasdale, Helen Mack, William Gargan; D: Leo McCarey.

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Milky Way

Milky Way

History

Structure of the Milky Way

Formation of the Milky Way

Nucleus of the Milky Way

Resources

The Milky Way galaxy, sometimes simply called the galaxy, is a barred spiral galaxy that formed as part of the Local Group, which is an association of approximately 25 galaxies. The word milky came from the band of white light that early astronomers saw in the sky on clear nights. Greek philosopher Democritus (c. 450c. 370 BC) is generally considered the first astronomer to have explained that the Milky Way consisted of a band of stars. Based on research performed in 2005, the age of the Milky Way is estimated to be about 13.6 billion years.

On a clear, moonless night, away from the bright lights of the city, the Milky Way galaxy is visiblea fuzzy, milky band stretching across the sky. The Milky Way is a galaxy, a vast spinning carousel of a few hundred billion stars. The solar system is located about half way between the center and the edge of this 120,000-light-year diameter pancake shaped galactic disk. (A light-year is the distance that light travels within a vacuum in one year.) The visible Milky Way is simply the light from billions of faint stars blending into a fuzzy band across the sky.

In the northern hemisphere summer, the Milky Way passes through the constellations Scorpius and Sagittarius in the south and heads north through

Aquila and Cygnus. In winter, the Milky Way slips between the hunting dogs, Canis Major and Minor, over the head of Orion the hunter, and through the feet of Gemini, the twins. In spring and summer, the Milky Way passes through the constellations Cassiopeia and Perseus.

History

In the minds of the ancient Greeks, the Milky Way clashed with the perfection expected in the heavens, and as such they thought it had to be an atmospheric phenomenon. The first hint of the true nature of the Milky Way came in 1610 when Italian astronomer and physicist Galileo Galilei (15641642) examined it with his telescope, and realized that the Milky Way was composed of an uncountable number of faint, individual stars.

In 1785, astronomer, German-born English astronomer William Herschel (17381822) pioneered the technique of star counts in an attempt to deduce the structure of the Milky Way. Herschel pointed his telescope in various directions in the sky and counted the number of stars he could see in a standard size field of view. If he saw more stars in a given direction, Herschel assumed that the Milky Way extended farther in that direction. Herschel correctly concluded that the Milky Way was a disk shape, but he mistakenly concluded that Earth was at the center of that disk. The star counting technique he used was misleading. It did not work because the galaxy is so vast that the interstellar dustdust between the stars blocks out light from the more distant stars. While Earth is at the center of the small part of the galaxy mapable by the star counting method, the galaxy extends beyond the region Herschel could map. Consequently, astronomers must find beacons enough brighter than individual stars to be seen from the edges of the galaxy.

In 1917, American astronomer Harlow Shapley (18851972) mapped the extent of the galaxy by counting globular clusters rather than stars. Globular clusters are collections of roughly 100,000 stars. They can be seen from the distant reaches of the Milky Way. Just as Polish astronomer Nicolaus Copernicus (1473 1543) before him concluded that Earth is not the center of the solar system, Shapley proved that the solar system was not at the center of the galaxy. Since Shapleys time, astronomers have refined his technique and discovered new ways to deduce the size, structure, and contents of the Milky Way.

Structure of the Milky Way

The problem astronomers face trying to deduce the structure of the Milky Way from their location within it is analogous to the problem faced by the Amazon Indians trying to map the rain forest while confined to its boundaries: it is simply too vast. To map the rainforest today mappers can simply fly over the rain forest in a plane; yet, astronauts cannot yet fly out of the galaxy in a spaceship to map its structure. Astronomers must find other methods.

Clues to the structure of the Milky Way galaxy can be found by examining other galaxies similar to it. The Milky Way has a disk structure with two spiral arms winding out from the center in the plane of the disk. The center, or nucleus, contains a small bulge. Surrounding this disk shape is a spherical halo composed of globular clusters similar to those used by Shapley to deduce Earths location within the Milky Way.

Astronomers map the spiral arm structure of the Milky Way using ordinary optical light and looking for objects commonly found in spiral arms of other galaxies. To map the largest region possible astronomers use bright objects. These objects, known as spiral arm tracers, include O and B spectral class stars, O and B associations, HII regions, and Cepheid variable stars.

Astronomers put stars into different spectral classes. The O and B spectral classes are the two with the brightest and most massive stars. O and B associations are loose clumps of roughly a few dozen or so O and B stars. HII regions are clouds of ionized hydrogen surrounding very recently formed O or B stars. Cepheid variable stars vary in brightness in a particular way. The first member of this class was discovered in the constellation Cepheus, hence the name. These stars are one of the fundamental yardsticks used by astronomers to measure distances in the universe, so they can be used to find the distance to the spiral arm containing them. These spiral arm tracers allow us to map the spiral arm structure of the Milky Way. Astronomers can only map a small part, however, as interstellar dust blocks the optical light from the more distant parts of the galaxy.

Astronomers use radio waves to map the far reaches of the Milky Way because the interstellar dust does not block radio waves as much as optical light. Spiral arms also contain interstellar gas composed mostly of hydrogen atoms. This interstellar gas is so thin (on average slightly less than one hydrogen atom per cubic centimeter of space) that it would be an excellent vacuum on Earth, but interstellar space is so vast that the interstellar gas still adds up to many hydrogen atoms. These hydrogen atoms emit radio waves with a wavelength of 8 in (21 cm) that can penetrate the interstellar dust. The 21-cm radio waves allow astronomers to map the spiral arm structure of even the distant parts of the Milky Way.

Using these and other techniques, astronomers have deduced the structure, size, and content of the Milky Way. The Milky Way consists of a fairly flat disk about 120,000 light years in diameter and 1,000 light years thick. (A light year is about 6 trillion miles.) The edge of the galaxy has a fuzzy rather than a sharp boundary, so the size estimates depend on what one calls the edge.

The flat disk consists of a complex spiral pattern, rather than the two graceful arms found in some galaxies. In addition to the spiral arm tracers mentioned previously, the disk and spiral arms contain young stars of all spectral classes, galactic clusters composed of several hundred young stars, and interstellar clouds of gas, molecules, and dust where new stars form. There is some recent evidence that the spiral arms do not begin at the center of the galaxy but at either end of a central bar structure like those found in barred spiral galaxies. The sun and solar system are located on a spiral arm about 25,000 light years from the center.

The nucleus of the galaxy is surrounded by a nuclear bulge that is 12,000 light-years in diameter and 10,000 light-years thick. Surrounding this disk is a spherical halo, composed primarily of globular clusters. The halo may be as much as 300,000 light-years in diameter and contains a considerable amount of unseen dark matter, perhaps as much as several times the amount of mass that astronomers can see. The extent of the dark matter is difficult to determine because it is not clearly defined or measurable. Some astronomers suggest that the dark matter portion of the halo may extend as far as half the distance to the Andromeda galaxy.

In 1997, astronomers discovered an astounding sighta fountain of hot gas and antimatter shooting some 3,500 light-years out of the nucleus of the Milky Way perpendicular to the disk. The discovery was made by the Compton Gamma Ray Observatory, which registered the massive flow of gamma rays emitted by the antimatter. Astronomers were unclear

whether the antimatter jets are continuous or whether they are merely observing an antimatter cloud slowly separating from the rest of the galaxy.

The fountain appears to primarily contain positrons, rather than more massive antimatter particles or atoms. Positrons can be created by heated gas spiraling into a black hole, or by the explosions of supernovas and white dwarf stars, leading astronomers to speculate that the fountain is caused by massive star formation near the black hole that is thought to exist at the Milky Ways center, or the explosion of young massive stars. In 2005, a black hole was discovered to be traveling at twice the escape velocity of the galaxy as it exited the Milky Way. Scientists think that such a black hole may help to support the theory that a black hole exists in the center of the Milky Way galaxy.

How many stars are there in the Milky Way? The spinning provides clues to the answer. The Milky Way spins because the individual stars in the galaxy are orbiting the center of the galaxy. Just as the suns gravity causes the planets to orbit the sun, the cumulative gravitational effect of the stars in the Milky Way cause the stars farther out to orbit the center of the Milky Way. The amount of gravitational force and hence the orbital properties depend on the mass.

Astronomers can therefore study the orbital motions of the outer stars in the Milky Way to find the mass of the Milky Way. As of 2005, the estimated mass of the Milky Way within a diameter of 120,000 light-years is about 600 hundred billion to 3 trillion times the mass of the sun. Because the sun is a fairly average star, the Milky Way contains roughly two to four hundred billion stars. The orbital motions of the sun are such that it moves around the center of the galaxy at about 220 km/s and takes about 250 million years to orbit the galactic nucleus once.

Formation of the Milky Way

The nucleus and halo of our galaxy contain older stars from the first batch to form. The globular clusters in the halo are anywhere from 10 to 17 billion years old and are among the oldest objects in the galaxy. These older stars are called population II stars. The disk and spiral arms consist of younger, second to third generation stars (population I) as well as interstellar gas and dust. This difference in location between the older and younger stars in the Milky Way suggests something about the origin and evolution of the Milky Way. The older population II stars are distributed spherically in the halo, suggesting that when the galaxy first formed it had a spherical shape. The youngest stars are found in the flat disk, suggesting that the Milky Way has gradually flattened into a disk shape. Why? It is spinning. As the galaxy spins around, it flattens out.

However, the history of the Milky Way may not be so simple. Detailed studies of the globular clusters in the halo and the different ages of stars in the halo and disk reveal some anomalies. For example, the contents of the halo do not always orbit in the same direction that the disk does. In addition, portions of the halo have very different ages. From this evidence, astronomers have concluded that the Milky Way may have formed as the result of a merger of smaller systems such as globular clusters or dwarf elliptical galaxies.

Nucleus of the Milky Way

What is in the nucleus of the Milky Way? If astronomers look with optical telescopes, they see nothing. The interstellar dust obscures the optical light. The center of the Milky Way does, however contain very strong sources of radio waves, infrared light, and x rays. One such source, called Sagittarius A*, appears to lie at the precise center of the galaxy, the point about which the entire system rotates.

The vast energy omitted by Sagittarius A* comes from a region that is less than one light-day in diameter (about the size of the solar system) compared to over 120,000 light-years for the entire galaxy. There is more energy produced in a very small volume of space than astronomers can easily explain. There is certainly not enough room in this volume to contain enough stars to explain the energy production. What produces so much energy in such a small region of space? Most astronomers think that there is a supermassive black hole with the mass of a million suns, in the core of the Milky Way. Black holes are so highly compressed that a supermassive black hole capable of explaining the energy output of the Milky Ways core would still have a small volume.

Quasars and other active galaxies also emit far more energy than can easily be explained from a small region in their nuclei. An active galaxy is a galaxy with at least 100 times the energy output of the Milky Way. Quasars are among the most energetic and distant types of active galaxy. These galaxies are also thought to contain supermassive black holes in the nucleus, even more energetic than the one in the nucleus of the Milky Way. The nucleus of the Milky Way may be a quieter version of the nucleus of an active galaxy or a quasar.

There are many mysteries concerning the Milky Way galaxy, including the antimatter fountains, the nature of the energetic activity at its core, the

KEY TERMS

Cepheid variable star A type of star that varies in brightness as the star pulsates in size. Cephied variables are important distance yardsticks in establishing the distance to nearby galaxies.

Disk The flat disk-shaped part of the Milky Way galaxy that contains the spiral arms.

Galactic cluster A cluster of roughly a few hundred young stars in a loose distribution. Also called an open cluster.

Galaxy A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

Globular cluster A cluster of roughly 100,000 older stars in a compact spherical distribution.

Halo A spherical distribution of older stars and clusters of stars surrounding the nucleus and disk of our galaxy.

Light-year The distance light travels in one year, roughly 9.5 trillion kilometers or 6 trillion miles.

Milky Way The galaxy in which Earth is located.

Nucleus The central core of a galaxy.

Spiral arms The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy A galaxy in which spiral arms wind outward from the nucleus.

unknown composition of the dark matter in the halo, and the uncertain process by which it formed.

Resources

BOOKS

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Begelman, Mitchell. Turn Right at Orion. Cambridge, MA: Perseus Publishing, 2001.

Belkora, Leila. Minding the Heavens: The Story of Our Discovery of the Milky Way. Bristol, UK: and Philadelphia, PA: Institute of Physics, 2003.

Inglis, Mike. Astronomy of the Milky Way. London, UK, and New York: Springer, 2004.

Matteucci, Francesca. The Chemical Evolution of the Galaxy. Dordrecht, Netherlands, and Boston, MA: Kluwer Academic Publishers, 2001.

Paul A. Heckert

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Milky Way

Milky Way

On a clear, moonless night, away from the bright lights of the city, the Milky Way is visible—a fuzzy, milky band stretching across the sky. The Milky Way is the plane of our galaxy , a vast spinning carousel of a few hundred billion stars. Our solar system is located about half way between the center and the edge of this 120,000-light-year diameter pancake shaped galactic disk. The "visible" Milky Way is simply the light from billions of faint stars blending into a fuzzy band across the sky.

In the northern hemisphere summer, the Milky Way passes through the constellations Scorpius and Sagittarius in the south and heads north through Aquila and Cygnus. In winter, the Milky Way slips between the hunting dogs, Canis Major and Minor, over the head of Orion the hunter, and through the feet of Gemini, the twins. In spring and summer, the Milky Way passes through the constellations Cassiopeia and Perseus.


History

In the minds of the ancient Greeks, the Milky Way clashed with the perfection expected in the heavens, and as such they thought it had to be an atmospheric phenomenon. The first hint of the true nature of the Milky Way came in 1610 when Galileo examined it with his telescope , and realized that the Milky Way was composed of an uncountable number of faint, individual stars.

In 1785, the musician turned astronomer, William Herschel pioneered the technique of star counts in an attempt to deduce the structure of the Milky Way. Herschel pointed his telescope in various directions in the sky and counted the number of stars he could see in a standard size field of view. If he saw more stars in a given direction, Herschel assumed that the Milky Way extended farther in that direction. Herschel correctly concluded that the Milky Way was a disk shape, but he mistakenly concluded that we were at the center of that disk. The star counting technique he used was misleading. It didn't work because the galaxy is so vast that the interstellar dust—dust between the stars—blocks out light from the more distant stars, while we are at the center of the small part of the galaxy mapable by the star counting method, the galaxy extends beyond the region we can map . We must find beacons enough brighter than individual stars to be seen from the edges of the galaxy.

In 1917 Harlow Shapley mapped the extent of the galaxy by counting globular clusters rather than stars. Globular clusters are collections of roughly 100,000 stars. They can be seen from the distant reaches of the Milky Way. Just as Copernicus before him concluded that Earth is not the center of the solar system, Shapley proved that the solar system was not at the center of the galaxy. Since Shapley's time, astronomers have refined his technique and discovered new ways to deduce the size, structure, and contents of the Milky Way.


Structure of the Milky Way

The problem we face trying to deduce the structure of the Milky Way from our location within it is analogous to the problem faced by the Amazon Indians trying to map the rain forest while confined to its boundaries: it is simply too vast. To map the rainforest today we can simply fly over it in a plane; we cannot yet fly out of the galaxy in a spaceship to map its structure. We must find other methods.

Clues to the structure of our galaxy can be found by examining other galaxies similar to our own. The Milky Way has a disk structure with two spiral arms winding out from the center in the plane of the disk. The center, or nucleus, contains a small bulge. Surrounding this disk shape is a spherical halo composed of globular clusters similar to those used by Shapley to deduce our location within the Milky Way. "Similar Galaxies" have some overall structure.

We map the spiral arm structure of the Milky Way using ordinary optical light and looking for objects commonly found in spiral arms of other galaxies. To map the largest region possible we use bright objects. These objects, known as spiral arm tracers, include O and B spectral class stars, O and B associations, HII regions, and Cepheid variable stars .

Astronomers put stars into different spectral classes. The O and B spectral classes are the two with the brightest and most massive stars. O and B associations are loose clumps of roughly a few dozen or so O and B stars. HII regions are clouds of ionized hydrogen surrounding very recently formed O or B stars. Cepheid variable stars vary in brightness in a particular way. The first member of this class was discovered in the constellation Cepheus, hence the name. These stars are one of the fundamental yardsticks used by astronomers to measure distances in the universe, so they can be used to find the distance to the spiral arm containing them. These spiral arm tracers allow us to map the spiral arm structure of the Milky Way. We can only map a small part, however, as interstellar dust blocks the optical light from the more distant parts of the galaxy.

Astronomers use radio waves to map the far reaches of the Milky Way because the interstellar dust does not block radio waves as much as optical light. Spiral arms also contain interstellar gas composed mostly of hydrogen atoms . This interstellar gas is so thin (on average slightly less than one hydrogen atom per cubic centimeter of space ) that it would be an excellent vacuum on Earth, but interstellar space is so vast that the interstellar gas still adds up to a lot of hydrogen atoms. These hydrogen atoms emit radio waves with a wavelength of 8 in (21 cm) that can penetrate the interstellar dust. The 21-cm radio waves allow us to map the spiral arm structure of even the distant parts of the Milky Way.

Using these and other techniques, astronomers have deduced the structure, size, and content of the Milky Way. The Milky Way consists of a fairly flat disk about 120,000 light years in diameter and 1,000 light years thick. (A light year is the distance light travels in one year, about six trillion miles.) The edge of the galaxy has a fuzzy rather than a sharp boundary, so the size estimates depend on what one calls the edge.

The flat disk consists of a complex spiral pattern, rather than the two graceful arms found in some galaxies. In addition to the spiral arm tracers mentioned previously, the disk and spiral arms contain young stars of all spectral classes, galactic clusters composed of several hundred young stars, and interstellar clouds of gas, molecules and dust where new stars form. There is some recent evidence that the spiral arms do not begin at the center of the galaxy but at either end of a central bar structure like those found in barred spiral galaxies. The Sun and solar system are located on a spiral arm about 25,000 light years from the center.

The nucleus of the galaxy is surrounded by a nuclear bulge that is 12,000 light years in diameter and 10,000 light years thick. Surrounding this disk is a spherical halo, composed primarily of globular clusters. The halo may be as much as 300,000 light years in diameter and contains a considerable amount of unseen dark matter , perhaps as much as several times the amount of mass that we can see. The extent of the dark matter is difficult to determine because it is not clearly defined and not yet measurable. Some astronomers suggest that the dark matter portion of the halo may extend as far as half the distance to the Andromeda galaxy.

In 1997, astronomers discovered an astounding sight—a fountain of hot gas and antimatter shooting some 3,500 light years out of the nucleus of the Milky Way perpendicular to the disk. The discovery was made by the Compton Gamma Ray Observatory, which registered the massive flow of gamma rays emitted by the antimatter. Astronomers were unclear whether the antimatter jets are continuous or whether they are merely observing an antimatter cloud slowly separating from the rest of the galaxy.

The fountain appears to primarily contain positrons, rather than more massive antimatter particles or atoms. Positrons can be created by heated gas spiraling into a black hole , or by the explosions of supernovas and white dwarf stars, leading astronomers to speculate that the fountain is caused by massive star formation near the black hole at the Milky Way's center, or the explosion of young massive stars.

How many stars are there in the Milky Way? The spinning provides clues to the answer. The Milky Way spins because the individual stars in the galaxy are orbiting the center of the galaxy. Just as the Sun's gravity causes the planets to orbit the sun, the cumulative gravitational effect of the stars in the Milky Way cause the stars farther out to orbit the center of the Milky Way. The amount of gravitational force and hence the orbital properties depend on the mass.

We can therefore study the orbital motions of the outer stars in the Milky Way to find the mass of the Milky Way. The mass of the Milky Way within a diameter of 120,000 light years is about 3 hundred billion times the mass of the Sun. Because the Sun is a fairly average star the Milky Way contains roughly 2 to 3 hundred billion stars. The orbital motions of the Sun are such that it moves around the center of the galaxy at about 220 km/s and takes about 250 million years to orbit the galactic nucleus once.

Formation of the Milky Way

The nucleus and halo of our galaxy contain older stars from the first batch to form. The globular clusters in the halo are anywhere from 10 to 17 billion years old and are among the oldest objects in the galaxy. These older stars are called population II stars. The disk and spiral arms consist of younger, second to third generation stars (population I) as well as interstellar gas and dust. This difference in location between the older and younger stars in the Milky Way suggests something about the origin and evolution of the Milky Way. The older population II stars are distributed spherically in the halo, suggesting that when the galaxy first formed it had a spherical shape. The youngest stars are found in the flat disk, suggesting that the Milky Way has gradually flattened into a disk shape. Why? It is spinning. As the galaxy spins around, it flattens out.

But the history of the Milky Way may not be so simple. Detailed studies of the globular clusters in the halo and the different ages of stars in the halo and disk reveal some anomalies. For example, the contents of the halo do not always orbit in the same direction that the disk does. In addition, portions of the halo have very different ages. From this evidence, astronomers have concluded that the Milky Way may have formed as the result of a merger of smaller systems such as globular clusters or dwarf elliptical galaxies.


Nucleus of the Milky Way

What is in the nucleus of the Milky Way? If we look with optical telescopes, we see nothing. The interstellar dust obscures the optical light. The center of the Milky Way does, however contain very strong sources of radio waves, infrared light, and x rays . One such source, called Sagittarius A*, appears to lie at the precise center of the galaxy, the point about which the entire system rotates.

The vast energy omitted by Sagittarius A* comes from a region that is less than one light day in diameter (about the size of the solar system) compared to over 120,000 light years for the entire galaxy. There is more energy produced in a very small volume of space than we can easily explain. There is certainly not enough room in this volume to contain enough stars to explain the energy production. What produces so much energy in such a small region of space? Most astronomers think that there is a supermassive black hole with the mass of a million suns, in the core of the Milky Way. Black holes are so highly compressed that a supermassive black hole capable of explaining the energy output of the Milky Way's core would still have a small volume.

Quasars and other active galaxies also emit far more energy than can easily be explained from a small region in their nuclei. An active galaxy is a galaxy with at least 100 times the energy output of the Milky Way. Quasars are among the most energetic and distant types of active galaxy. These galaxies are also thought to contain supermassive black holes in the nucleus, even more energetic than the one in the nucleus of the Milky Way. The nucleus of the Milky Way may be a quieter version of the nucleus of an active galaxy or a quasar .

There are many mysteries concerning the Milky Way, including the antimatter fountains, the nature of the energetic activity at its core, the unknown composition of the dark matter in the halo, and the uncertain process by which it formed.


Resources

books

Bacon, Dennis Henry, and Percy Seymour. A Mechanical History of the Universe. London: Philip Wilson Publishing, Ltd., 2003.

Bok, Bart J. and Priscilla F. Bok. The Milky Way. Cambridge: Harvard University Press, 1981.

periodicals

Binney, James. "The Evolution of Our Galaxy." Sky & Telescope 89 (March 1995): 20-26.

Trimble, Virginia and Samantha Parker. "Meet the Milky Way." Sky & Telescope 89 (January 1995): 26-33.

Van den Berg, Sidney, and James E. Hesser. "How the Milky Way Formed." Scientific American (January 1993): 72-78.

Verschuur, Gerrit L. "In the Beginning." Astronomy (October 1993): 41-45.

Verschuur, Gerrit L. "Journey into the Galaxy." Astronomy (January 1993): 33-39.


Paul A. Heckert

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Cepheid variable star

—A type of star that varies in brightness as the star pulsates in size. Cephied variables are important distance yardsticks in establishing the distance to nearby galaxies.

Disk

—The flat disk-shaped part of the Milky Way galaxy that contains the spiral arms.

Galactic cluster

—A cluster of roughly a few hundred young stars in a loose distribution. Also called an open cluster.

Galaxy

—A large collection of stars and clusters of stars, containing anywhere from a few million to a few trillion stars.

Globular cluster

—A cluster of roughly 100,000 older stars in a compact spherical distribution.

Halo

—A spherical distribution of older stars and clusters of stars surrounding the nucleus and disk of our galaxy.

Light year

—The distance light travels in one year, roughly 9.5 trillion kilometers or 6 trillion miles.

Milky Way

—The galaxy in which we are located.

Nucleus

—The central core of a galaxy.

Spiral arms

—The regions where stars are concentrated that spiral out from the center of a spiral galaxy.

Spiral galaxy

—A galaxy in which spiral arms wind outward from the nucleus.

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"Milky Way." The Gale Encyclopedia of Science. . Encyclopedia.com. 18 Sep. 2018 <http://www.encyclopedia.com>.

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"Milky Way." The Gale Encyclopedia of Science. . Retrieved September 18, 2018 from Encyclopedia.com: http://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/milky-way

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