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[7.0] The Milky Way Galaxy

v3.3.0 / chapter 7 of 10 / 01 jan 18 / greg goebel

* Our Solar System is part of a huge "community" of stars, the Milky Way Galaxy. This chapter discusses the discovery of the nature of the Milky Way and its place in the larger cosmos.

the Milky Way


[7.1] THE DISCOVERY OF THE MILKY WAY GALAXY
[7.2] THE DISCOVERY OF THE ANDROMEDA GALAXY
[7.3] MAPPING THE MILKY WAY / GALACTIC CORE
[7.4] GLOBULAR CLUSTERS / STELLAR COLLISIONS
[7.5] MISSING MASS & GALACTIC DARK MATTER

[7.1] THE DISCOVERY OF THE MILKY WAY GALAXY

* The question of just how big our Universe is has been around since prehistory, but it wasn't until the development of the telescope and modern astronomy in the 17th century that astronomers decided to actually try to measured the size of the Universe. Before that time, early astronomers such as Hipparchus had of course made catalogs of the visible stars, with the count going up to a few thousand. However, the telescope made far more many stars visible, and the number kept multiplying every time a new, more powerful telescope was put into service.

There was a still a reason, a very logical one, for believing that there was a finite number of stars in the Universe. In 1826, a German astronomer named Heinrich Wilhelm Matthaeus Olbers (1758:1840) pointed out that if there were an infinite number of stars in the sky, no matter where we looked, we would see a star, and the sky would be uniformly bright. This became known as "Olbers' paradox". There was a fallacy in it, one which Olbers could be easily forgiven for not seeing, but an explanation has to be put off to later.

* If the number of stars in the Universe was finite, then the other question was to ask how these stars were arranged. The night sky gave a big hint, in the form of a lovely pale band of light that cut across the heavens like a river. The Greeks called it the "Galaxias Kyklos (Milky Circle)", and the Romans called it the "Via Lactea (Milky Way)"; the Greek term survived as the modern name "Galaxy". Even before the invention of the telescope some thinkers had wondered if it was made up of countless stars. This was basically confirmed by the telescope.

In 1784, the energetic William Herschel decided to characterize the distribution of stars in the Milky Way. Counting them all would have taken more than his lifetime, so he mapped out a set of 684 sampling regions over the sky, counted the stars in each one, and obtained the statistics on the samples. Herschel concluded that the density of stars was a maximum along the central plane of the Milky Way through the sky, and that it fell off gradually with the separation of stars from the plane, reaching a minimum at a right angle to that plane. However, the average density of stars along that plane, and any plane parallel to it, seemed constant.

Herschel concluded from these facts that the stars in the sky were arranged in a disk, with our Sun inside that disk, in fact roughly near the center of it. Based on the knowledge of stellar distances known at the time he even put together an estimate of the size of this disk, judging it to be about 8,000 light-years across, 1,500 light-years from top to bottom, and containing 300,000,000 stars.

Herschel was right about the disk, wrong about the Sun being near its center, and far too small in his estimate of its size. The size estimate was gradually increased into the early 20th century. In 1920, following exhaustive photographic sky surveys, the Dutch astronomer Jacobus Cornelius Kapteyn (1851:1922) estimated the Galaxy to be 55,000 light-years across and about 11,000 light-years from top to bottom. The idea that the Sun was at the center of the disk remained intact, but by that time the notion was just about to fall over.

* As discussed earlier, Charles Messier had published his list of Messier objects in 1781, and William Herschel studied them carefully. Among the Messier objects were spherical puffballs of light, the brightest of them being "M13", located in the constellation Hercules. There were similar but less spectacular spherical nebulas in the Messier list. Nobody was sure if they were just luminous clouds of gas or collections of stars.

Herschel, using the excellent telescopes he had built for himself, was able to get a better look at M13 and realized that it was in fact a dense collection of stars. Such objects became known as "globular clusters", as opposed to the more irregular clusters such as the Pleiades, which were eventually referred to as "open clusters". M13 became known as the "Great Hercules Cluster".

As telescopes and observations improved, more globular clusters were discovered. William Herschel's son John Herschel (1792:1871) inspected the map of globular clusters known at the time and found something puzzling: the globular clusters were heavily concentrated in the sky in the direction of the constellation Sagittarius, and were absent in the opposite direction. There had to be some significance to this distribution, but he didn't know what it might be.

It wasn't until the 1920s, with the discovery of the period-luminosity relationship of Cepheid variables, that the reason for this distribution of globular clusters was understood. The American astronomer Harlow Shapley (1885:1972) used the relationship to obtain distances to the globular clusters, and his analysis showed they were arranged in a sphere whose center was in the direction of the Sagittarius.

A simple understanding of the law of gravity suggested that the globular clusters were orbiting around the center of mass of the Milky Way. That meant that the Earth wasn't at the center of the Galaxy after all. Shapley's analysis showed that this center of mass was about 50,000 light-years away. As mentioned, the Cepheid yardstick had to be adjusted to compensate for reddening by the interstellar medium later, but that still gave a distance of about 28,000 light-years to the Galactic center. With this adjustment, the Milky Way appeared to be about 100,000 light-years in diameter, and was estimated to contain about 100,000 million stars.

Astronomers were puzzled because star counts of the Milky Way gave about the same density at any place along its central plane. The answer to the puzzle came from the Galaxy's dark molecular clouds, such as the Coal Sack. Their nature was not clearly understood at the time and they were simply known as "dark regions". In 1919, the American astronomer Edward Emerson Barnard (1857:1923) published a list of 182 such dark regions. Barnard and the German astronomer Max Wolf (1863:1932) correctly suspected that these regions were not empty of stars, they were actually cold dark clouds that blocked out the light of stars behind them. Given that idea, it didn't take too much imagination to realize that the presence of such "dark nebulas" in the plane of the Milky Way would create a curtain to hide the center of the Galaxy from view, at least in the visible region of the electromagnetic spectrum.

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[7.2] THE DISCOVERY OF THE ANDROMEDA GALAXY

* In the meantime, astronomers were discovering that the Milky Way was not alone. To be sure, the Milky Way had long been known to have small satellite galaxies, in the form of the two Magellanic Clouds visible from the Southern Hemisphere, but they were part of the Milky Way system and could not be regarded as "island universes" in themselves. However, the Messier list of nebulas also included a few odd-shaped objects that looked like lenses or spirals, the most spectacular being the "Andromeda Nebula" in the constellation Andromeda, which is so bright that its central region can be seen on a dark night with the naked eye. Of course, the question arose again: were they structures of luminous gas, or communities of stars?

In 1755, the German philosopher Immanuel Kant (1724:1804) published a work that suggested the Andromeda Nebula and its like were actually "island universes" of their own, or in other words they were other Milky Ways. The French astrophysicist Pierre Simon de Laplace (1749:1827) suspected that the Andromeda Nebula was a young planetary system in the process of formation. The matter was debated back and forth for a century until 1907, when a report was published that claimed a parallax measurement of the Andromeda Nebula showed it was only 19 light-years away, right next door in cosmic terms.

That might have ended the argument, except that in 1899 the first spectroscopic measurements were made of the Andromeda Nebula and showed it to have a continuous spectrum -- consistent with a star or mass of stars, not a cloud of glowing gas. Promoters of the "local" theory for the Andromeda Nebula suggested that it was reflecting light from nearby stars, and pointed out that no individual stars could be made out in the nebula.

The second observation wasn't entirely true. In 1885, a bright starlike object appeared in the central region of the Andromeda Nebula and then faded out. The notion of a nova was understood at the time, and it was plausible to believe that the object that had been observed, which was named "S Andromedae", was a nova. Nobody was exactly sure just how bright a nova was at the time, but in 1901 a nova was observed in the constellation Perseus. "Nova Persei", as it was logically named, was close enough for parallax measurement, and it turned out to be about 100 light-years away. An analysis was published in 1911 that assumed S Andromedae was about as bright as Nova Persei -- a reasonable enough assumption given the state of knowledge at the time -- giving a distance to the Andromeda nebula of about 1,600 light-years.

This threw the parallax measurement of distance to the Andromeda Nebula into doubt, and of course now we know it was wildly wrong. However, 1,600 light-years still put the Andromeda Nebula well inside our Milky Way -- but there was plenty of good cause to doubt that distance as well. The assumption behind the calculation that gave 1,600 light-years was that Nova Persei and S Andromedae were roughly as bright, and in fact it was an assumption that S Andromedae was actually in the Andromeda Nebula. For all anyone knew, S Andromedae could have been much closer than the Andromeda Nebula and just happened to be on the same line of sight.

The analysis was still a step in the right direction. The sciences, like any other activity grounded in the real world, work by steps, using available knowledge to acquire improved insights that can then be validated or discarded, like searching through a maze by checking out and discarding branches that come to dead ends.

The way to resolve such questions is to acquire more and better data. The American astronomer Heber Doust Curtis (1872:1942) decided to see if he could find more novas in the Andromeda Nebula and managed to discover about a hundred from archival photographic plates and observations. The observations covered a very short period of time, a few decades, and novas are a fairly rare phenomenon. The finding of so many novas meant that the probability that they weren't associated with the Andromeda Nebula and just happened to lie in the line of site was vanishingly small; novas simply didn't happen at that rate over the entire sky. In fact, novas were rare enough to suggest to Curtis that the Andromeda Nebula was not only composed of stars, it had to composed of a lot of stars to account for such a rate of nova explosions.

Furthermore, the novas that Curtis found in the Andromeda Nebula were much fainter than S Andromedae. In 1918, Curtis suggested that the novas he had found were in a league with Nova Persei, and that if S Andromedae had actually occurred in the Andromeda Nebula, it was much brighter than an ordinary nova. Many astronomers, including Harlow Shapley, were skeptical. Since so little was known about novas, any assumptions about their actual properties one way or another were hard to take seriously. Then Edwin Hubble figured out an unarguable way to break the deadlock.

A groundbreaking new telescope had come on line on Mount Wilson in southern California in 1917. The "Hooker telescope", funded by donations by a Mr. John D. Hooker, was a reflector with a mirror 2.54 meters (100 inches) in diameter, making it the most powerful telescope in the world until after World War II. Hubble used the Hooker telescope to resolve stars in the Andromeda Nebula, proving that it was not a cloud of gas, and in 1923 he spotted a Cepheid variable that allowed him to estimate its distance as 800,000 light-years. It actually turned out that he had spotted an RR Lyrae variable, and it wasn't until well after World War II that astronomers realized that they had their own, brighter, period-luminosity curve. That led to readjusting the distance to the Andromeda Nebula to 2,500,000 light-years.

However, even the short value of 800,000 light-years made the Andromeda Nebula an "island universe" in its own right, much like our own Milky Way Galaxy. Hubble wanted to call it an "extragalactic nebula", but Harlow Shapley, who had come around with almost everyone else, thought this was inadequate. It was something much like the Milky Way, so it might as well be called the "Andromeda Galaxy".

* Of course, many other similar objects had been cataloged to that time, with such objects making up a good proportion of the Messier list, for example "M51", the "Whirlpool Nebula". By the time of the discovery that the Andromeda Nebula was another galaxy, tens of thousands of such objects had been cataloged. About 75% were spiral in form, with some of these seen edge-on and appearing as spindles, while 20% were spherical or elliptical, and the other 5% were irregular in shape. Now it was realized that most of these objects were actually galaxies as well. It must have been a mind-boggling revelation, showing that the Milky Way was not the entire Universe, but just one island universe among vast numbers of them.

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[7.3] MAPPING THE MILKY WAY / GALACTIC CORE

* For years astronomers had no clear idea of the structure of the Milky Way Galaxy, and many thought nobody ever would. It was like living inside a box and trying to figure out what the box looked like from the outside. Fortunately, there turned out to be a way to see through the walls of the box.

In 1931, an American radio engineer named Karl Jansky (1905:1950) of Bell Telephone Laboratories was investigating natural sources of radio interference. He managed to determine that some interference was coming from the sky, and that the interference was particularly strong in the direction of the constellation Sagittarius, in the direction of the core of the Milky Way. Jansky's efforts were focused on improving radio communications, not performing astronomy, so he reported his findings and went on about his work. Most astronomers didn't notice the report, but another American radio engineer named Grote Reber (1911:2002) decided to perform his own amateur investigation of cosmic radio sources. He built a crude radio telescope in his mother's backyard in 1937 and located a number of sources, publishing his results in 1940.

World War II was in progress at the time. Most formal astronomy work went onto the back burner for the duration, but the war had a profound effect on the history of radio astronomy. New radio technologies were developed, mostly for radar, that could be used in more peaceful times to probe the radio sky, and there was also more need to understand natural sources of radio interference.

Some astronomical work was done during the war that would also have a major influence on the future of radio astronomy. In 1944, a Dutch astronomer named Hendrik Christoffel van de Hulst (1918:2000), living in the Nazi-occupied Netherlands, decided for want of anything better to do under the circumstances to perform some analyses in theoretical physics. He got to tinkering with the characteristics of cold monatomic hydrogen, the most common constituent of the interstellar medium, and found that it could occupy two energetic states. It could drop from the more energetic state by emitting a photon of 21 centimeter (1.4 gigahertz) electromagnetic radiation, in the microwave band of the spectrum, or jump up to the more energetic state by absorbing such a photon.

Such events were rare in the life of any one hydrogen atom, maybe taking place once every 11 million years or so, but the Milky Way contains a lot of hydrogen atoms. Dense regions of cold hydrogen, known as "H-I regions" in contrast to the much less common "H-II regions" of hot ionized hydrogen, could be expected to be radiate strongly enough at 21 centimeters. They would be singing the "song of hydrogen", as it was called, which could be picked up by sufficiently sensitive receivers. The 21-centimeter radiation could also penetrate through the Milky Way's dark clouds with relatively little attenuation, allowing astronomers to create a map of the distribution of cold hydrogen clouds.

Astronomers had been trying to build such maps at optical wavelengths, but with the development of workable radio telescopes in the 1950s, observations were finally able to cut through the murk and create a useful map. The Milky Way had been thought to be a spiral galaxy, and the radio map of H-I regions showed that it seemed to have three spiral arms.

The cold hydrogen observations were somewhat ambiguous and provided relatively limited detail. However, beginning in the early 1960s, astronomers began to find that interstellar space contains a wide range of molecules, such as carbon monoxide, ammonia, acetylene, and formaldehyde, which have their own distinctive spectral signatures. These interstellar molecules were interesting in themselves, but they also helped provide a better map of the Galaxy. Although monatomic hydrogen emitted radio waves on the 21-centimeter wavelength, the diatomic hydrogen molecule, or "H2", which can form in the opaque molecular clouds, has no strong radio signature. However, carbon monoxide does, and it is associated with H2 clouds, providing a "tracer" that helps to map out the location of the large molecular clouds.

About half the interstellar gas in our Galaxy resides in such molecular clouds, with giant clouds of this type are concentrated along the Milky Way's spiral arms. The carbon monoxide in these clouds makes them perfectly and distinctly visible in the radio sky.

* The radio mappings revealed the structure of the Milky Way. It is disk-shaped, about 100,000 light-years in diameter, with the Sun about 28,000 light-years from the core and performing an orbit around the core about once every 250 million years. There is a concentration of stars and mass at the center of the disk, in the direction of the constellation Sagittarius, with about 150 globular clusters in wide orbits around the galactic core.

Within the galactic plane, there are vast numbers of stars. Old stars shed planetary nebulas into the galactic medium, while supernovas scatter out tortured masses of gas in a more spectacular fashion. Stars form in dark clouds of dust and gas, emerging into galactic clusters.

Radio observations of the molecular clouds revealed the arm structure of the Galaxy. In the direction of the Galactic core lie the Sagittarius arm and, closer to the core, the Carina arm, which merge into a single larger arm about 6,000 light-years from our Sun that then curves around the Milky Way about two-thirds of its distance. Interesting sights in the Sagittarius arm include:

Trifid Nebula

Our own Sun lies on the "Orion Arm", named after one of its prominent features, the Orion Nebula, about 1,600 light-years from the Sun. Other prominent sights in the Orion Arm include:

The Sun sits near the center of a roughly cylindrical region of space known as the "Local Bubble" where the interstellar medium is relatively thin. The Local Bubble is about 300 light-years across and was apparently formed by a supernova in the distant past.

Orion Nebula

The relatively loose "Perseus Arm" lies about 6,000 light-years from our Sun in the direction of the edge of the Milky Way. Interesting attractions in the Perseus arm include:


Rosette Nebula

* Observations of 21-centimeter radiation also showed that the Milky Way is actually embedded in a much larger spherical "halo" of cold hydrogen, thinly populated by very ancient stars and star clusters, possibly 160,000 light-years across.

It is believed that the Milky Way Galaxy was born after the Big Bang, about 13.7 billion years ago, beginning life as a spinning sphere of hydrogen and helium that collapsed under the influence of its own gravity. Since the sphere was spinning, it formed into a spiral disk that now contains, as mentioned, about 100,000 million stars. It was traditionally thought that the Milky Way was a simple spiral galaxy, but in the mid-1970s radio telescope observations began to suggest that it was a "barred spiral", with arms extending from the core to link to the spiral arms. In 2005, observations from the NASA Spitzer infrared space telescope provided conclusive evidence that this was the case.

Five to ten percent of the mass of the Milky still persists as gas, with a fraction of a percent in the form of microscopic dust grains of silicate and graphite. The gas is actually being replenished, at least to a degree. In the 1950s, clumps of unusually dense gas were discovered in the galactic halo, which were falling in toward the galactic plane without sharing in the rotational motion of the galaxy itself. These "high velocity clouds (HVCs)" represented fresh material being added to the Milky Way, but astronomers were puzzled as to where they could have come from. Did they represent accumulations of gas from the days of the origin of the Galaxy that are only now being assimilated? Were they clumps of gas that the halo had swept in from intergalactic space? Were they stripped off from small galaxies torn apart by a close passage with the Milky Way? Maybe they weren't really fresh at all, maybe they had been thrown up from the Galaxy from some sort of "fountain" out of the galactic plane.

Spectral analysis of the HVCs showed that they weren't all of a kind, and so all three of the first processes were plausible. The fourth, the "fountain" concept, was not, at least for HVCs, since they were deficient in the "metals" associated with gas from the galactic plane. However, "intermediate velocity clouds (IVCs)" were known that did have spectral signatures like those of galactic gas, and were very likely blasted out of the galactic plane by supernova explosions. In any case, the impression that after its formation, the Milky Way has simply rolled down the path of its evolution in a direct fashion is clearly wrong.

* The center of the Milky Way is extremely interesting. Radio observations show that the central bulge of the Milky Way is surrounded by what appears as a broad ring of molecular gas about 1,000 light-years in diameter, which may consist of a number of spiral segments seen edge-on. The ring may have been blasted outward by some event in the Galactic core that took place a few tens of millions of years ago, or it may be part of the normal structure of the Milky Way.

The core is surrounded by an "arc", a twisted set of filaments of gas, a few hundred light-years across. The core itself can be seen at radio, infrared, and high-energy wavelengths, and it is spectacular, with millions of stars crammed into a space only about 30 light-years across, and filaments of gas formed by magnetic fields. The core is very energetic, radiating at about the rate of ten million Suns. High-resolution radio maps showed the central source to be only about half the diameter of the Earth's orbit around our Sun. The compact object was designated "Sagittarius A*" or "(Sgr) A*", pronounced "sadge a-star".

Sgr A* object may be a supermassive black hole, with a mass of about 4 million Suns, that binds rapidly rotating clouds of gas and a set of fast-moving stars into orbits the core. It may swallow up a star every 10,000 years or so, resulting in a violent flareup. If it is a black hole, it seems to be quiet for now, though there is a faint outburst every now and then, and traces of faint "jets" above and below the core. Other galaxies seem to have oversized black holes at their cores, and it is tempting, maybe too much so, to see the same pattern in the Milky Way.

Galactic core

However, Sgr A* is not the only thing in the core. There are also a number of bright and distinct infrared sources. They may be dense and energetic clusters of stars, providing their own major contributions to the core's energy output. Recent observations suggest that though the core may be a place where stars are torn apart, it is also a site of star formation, with the dense clumps forming giant stars that live hard and die young in supernova explosions -- if they aren't swallowed up by the core first. The central light-month of Sgr A* is crammed with such stars, which by all evidence are no more than about 6 million years old -- too young for them to have been pulled into the core region from elsewhere.

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[7.4] GLOBULAR CLUSTERS / STELLAR COLLISIONS

* The 150 or so globular clusters that orbit through the halo of the Milky Way might seem like forgettable fuzzballs of stars, but they are very interesting places as well. For starters, they consist of a 100,000 to a million stars crammed into a volume of space a few dozen light-years across. The night sky of a planet orbiting a star in a globular cluster would be spectacular, carpeted with brilliant diamonds, far more impressive than the faint patterns of stars in our own night sky.

globular cluster NGC2808

Such high stellar density has a number of implications, one of them being that nice neat planetary systems are rare or unknown in globular clusters. The interactions between stars would tear any planetary system apart, scattering the planets into the void. Stars themselves are often thrown out of a cluster through such interactions, and stars may also collide, creating what appear to be young stars in an environment where all their neighbors are clearly old.

Observations with the NASA Hubble Space Telescope and Chandra X-ray Observatory have observed globular clusters in detail, even seeing into their cores, where the density of stars is a maximum. The observations have opened up a can of puzzles. For example, the relatively nearby globular cluster 47 Tucanae seems to be full of the spun-up millisecond pulsars, while the Milky Way's biggest globular cluster, Omega Centauri, with over 3 million stars, doesn't seem to have any millisecond pulsars at all.

Computer simulations have also been constructed to investigate the dynamics of stellar interactions in globular clusters. The simulations show, to no great surprise, that the high density of globular clusters influences the evolution of the stars in the clusters in many ways. The most significant factor in a cluster's evolution turns out to be binary stars.

Binary star systems are common in globular clusters, since the high stellar densities leads to the gravitational capture of one star by another. Incidentally, this is why globular clusters have about a tenth of the number of known X-ray sources in the Milky Way, even though they contain only about a ten-thousandth the number of stars. Gravitational capture allows a white dwarf, neutron star, or black hole to pair up with a normal star. If the two objects are close enough, the normal star will lose mass to the superdense object, with the violent descent of the mass generating X-rays.

Anyway, the mutual orbits of binary star systems contain a great deal of energy that can be released through near flybys of other stars or star systems, with the energy "heating up" the entire cluster. The concept of "heating up" a globular cluster tends to create images of interactions in a ball of gas, and those involved in the studies say that the analogy is actually fairly close.

Encounters between binary star systems and other stars or star systems tend to increase sharply as the globular cluster inevitably shrinks under its own gravitational attraction. A passing star may rob a binary system of kinetic energy, causing the two stars in the binary system to fall closer together, increasing the speed of their mutual orbit. This process is known as "binary burning." The visiting star may also end up staying, casting one of the original stars off to the remote regions of the cluster and taking its place. Such exchanges tend to make the cluster expand again, and so a globular cluster with plenty of binary star systems tends to resist collapsing in on itself.

* However, binary systems that have been contracted by an interaction with another star or star system have lost energy, which means they have less energy to contribute to future interactions. This is apparently why the process is known as "burning", since it "burns away" energy. In addition, if a star acquires enough energy in such interactions, it may be cast out of the cluster entirely, a process known as "evaporation". Over long periods of time, evaporation will cause a cluster to slowly fade away.

Globular clusters also tend to be disrupted by the tidal forces set up across their width by the Galaxy. The gravitational attraction of the Galaxy is stronger on the near side of the cluster than on the far side, which sets up a net force across the cluster. Outside of a certain "tidal radius", stars can be stripped out of a cluster, being strung out along the cluster's orbit in both the forward and backward directions as faint "tidal streams".

The tidal streams grow when the globular cluster passes through the plane of the Galaxy during its long orbit. After enough passes, the cluster breaks up, if it doesn't evaporate first. Astronomers have observed a small cluster, known as "Pal 5", that is clearly breaking up into tidal streams, with dense "clots" of stars in the cluster's tidal streams that correspond to each time the cluster passed through the Galactic plane.

About 20 other globular clusters of our Galaxy have been observed to have tidal streams. The outer halo of the Galaxy is laced with streams of globular clusters that disappeared a long time ago, and it is clear that the 150 or so globular clusters that still survive are only a fraction of the number that once existed.

Interestingly, the Galaxy may also acquire new globular clusters. The Milky Way has swallowed up and absorbed neighboring dwarf galaxies in the past, but the tightly-bound globular clusters of the victim tend to survive the process and become satellites of the Milky Way itself. The oversized Omega Centauri globular cluster appears to be the core of one of these long-gone dwarf galaxies. In fact, it appears that the collisions between galaxies will generate new globular clusters, which are easily identified because they include plenty of clearly young stars. The globular clusters are not all the antiques they have been traditionally thought to be.

* A globular cluster begins dying the moment it is created. However, in the absence of external disruptive forces, a globular cluster tends to maintain a stable internal structure -- as long as 5% to 10% of the total number of stars are in binary systems. The binaries act as "springs", as described above providing energy in stellar encounters that resists the tendency of the cluster to fall in on itself. If there aren't enough binaries, the cluster undergoes a process known as "core collapse", with central stellar densities about 10,000 times greater than that of a normal globular star cluster.

Nobody is exactly sure what the long-term consequences of core collapse are. One school of thought believes that the collection of stars in the core is so "hot", with such an enormous rate of interactions, that it will scatter the stars back out to the edges of the cluster, effectively causing the cluster to "rebound" and then begin to collapse again in a cycle. Researchers regard this scenario as more plausible speculation than rigorous theory. They do know that under core collapse conditions, the number of collisions between stars will increase by a factor of up to a billion. Even under such conditions, space is vast, stars relatively small, and so the probability of stars simply running into each other like marbles is not great. However, the rate of formation of binary systems increases greatly. The objects in a close binary system will tend to distort each other, which uses up orbital energy. Eventually the two will collide.

For the entire collection of 150 known globular clusters associated with the Milky Way, a collision occurs maybe once every 10,000 years. This is a long time by historical standards, but as rapid as popcorn popping by cosmic standards; it only happens once in a billion years or so in the rest of the Galaxy. Exactly what happens in such collisions depends on the type of objects involved, their relative velocities, and whether the object sideswipe each other or hit head-on. For example, in a collision between a white dwarf and a Sunlike star, the white dwarf is barely affected, being heated up somewhat, while the Sunlike star is obliterated, in somewhat the same way that a powerful bullet blows apart a watermelon.

If both stars are Sunlike, the two bodies simply distort towards each other, trailing mass, and then coalesce in a few hours. The process is relatively nonviolent, though it might be safest to observe it from a long distance away. The new star will be of course bigger and much brighter than either of its two ancestors, and will have a much shorter future lifetime. Our Sun has an expected lifetime of 10 billion years, while a star twice as massive is ten times as bright and will have a lifetime of only 800 million years. Such "coalesced" stars are very easy to spot inside of globular clusters. Globular clusters are old and depleted of gas, and so they should only be populated by old and small stars. A coalesced star will be bigger, brighter, and seemingly much younger than its brethren.

Stellar collision theory helps explain the existence of "blue stragglers" in globular clusters. Blue stragglers are hot, blue, bright stars that seem too young to be found in globular clusters. It cannot be proven that stellar collisions are the only possible explanation for blue stragglers, but it is difficult to think up a more plausible mechanism.

* Simulations show that it will take thousands of years following a stellar collision for the resulting swirling, rapidly spinning ball of gas to settle down to begin nuclear fusion and become bright again. The object is spinning so fast after its creation that it nearly flies apart, and the simulations haven't been able to show how it slows down. A reality check would help, and observations of about 100 blue stragglers obtained by the Hubble Space Telescope have been carefully checked for clues.

Of course, given the density of stars implied by a core collapse, it's plausible that such a process might result in a black hole with a mass of thousands of Suns at the center of the globular cluster. Such an object would be intermediate in mass between the black hole that results from a supernova explosion, and the enormous black holes believed to' reside in the center of galaxies. Observations have suggested that some globular clusters may in fact have an "intermediate mass black hole" at the core, but nobody's confirmed it yet.

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[7.5] MISSING MASS & GALACTIC DARK MATTER

* As mentioned, the Solar System orbits around the Milky Way Galaxy about once in 250 million years. The spiral arms are of course not fixed, and the rate of rotation of arms is slower at the edges of the spiral than it is nearer the center. From the 1960s, astronomers have believed the spiral arms to be the transient manifestations of gravitational "density waves" that rotate around the Galaxy, creating molecular clouds and star-forming regions in their wake.

One of the puzzles of the Milky Way is that stars seem to be orbiting around its center much faster than can be accounted for from current estimates of the Galaxy's mass. Analyses show that maybe 90% of the Milky Way's mass is in the form of this unseen or "dark" matter. Fritz Zwicky had observed the effects of such dark matter in observations of nearby clusters of galaxies in the 1930s, but as mentioned Zwicky's abrasive reputation made his colleagues quick to dismiss his ideas, and so he was ignored -- though once again, he turned out to be dead right. The anomalous orbital behavior of the Milky Way was observed by astronomer Vera Rubin (1928:2016) in the early 1960s, but it wasn't until 1978 that she and her colleagues were able to build up enough of a brief from observations to convince the astronomical community that there was really something going on.

While this "missing mass" might be in the form of previously unknown heavy elementary particles or "weakly interacting massive particles (WIMPs)", most astronomers believe that galactic dark matter is made up of fairly ordinary objects in the halos of galaxies, such as dim stars, brown dwarfs, giant planets, and the like, collectively known as "massive compact halo objects (MACHOs)".

Since these objects are by definition dim or dark, they are hard to spot, but astronomers have been trying to see if MACHOs can be detected by their effects. If a MACHO passes through the line of sight from the Earth to a star or other bright object, the intensity of the bright object should increase for a short time through gravitational "microlensing". A statistical analyses of microlensing events by MACHOs would help set limits on the contribution of MACHOs to galactic dark matter. Surveys have been conducted with automated telescopes to detect gravitational microlensing events. An automated telescope is computer-controlled, with an electronic imager to feed observations to the computer and servomechanisms to allow the computer to steer the telescope. Such a robot telescope can perform sky surveys with unprecedented speed and accuracy.

The gravitational microlensing searches have been disappointing in terms of their original goal of tracking down galactic dark matter. All the observations have shown is that MACHOs may account for from 8% to 50% of the Galaxy's dark matter, which is a pretty broad range. The microlensing searches have still proven interesting for other reasons, and the automated telescope searches developed to conduct them are now being used for a wide variety of other studies, so the exercise has been useful. However, the galactic dark matter mystery remains open for the time being.

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