Meet the Milky Way.
Editor's Note: Sky & Telescope's coverage of astronomical news often includes discoveries involving minute parts of our galaxy. And while these tidbits are intrinsically interesting, it is easy to lose sight of the big picture they are helping to create. This article, the first in a four-part series aimed at painting that picture, presents an introduction and overview to the Milky Way. In a future issue this series will continue with an article by James Binney discussing our galaxy's formation and the ongoing dynamic processes affecting its development. Gillian Knapp will present the third installment, including the latest findings about the interstellar medium -- the gas and dust component of our galaxy. The series will conclude with Ray Jayawardhana exploring the Milky Way's heart, where a massive black hole may lurk.
FOR MODERN, URBANIZED STARGAZERS, THE Milky Way no longer rules summer skies. That it once did is clear from its importance both in Northern Hemisphere mythologies and, especially, in the dreamtime narratives of Australian aborigines. In fact, that we see it at all from Earth is a bit of good luck. Even from a dark site a typical patch of Milky Way is only a 50 percent enhancement over the general sky brightness, which comes mostly from zodiacal light and airglow. Brighten natural skies by a factor of 10 to 100 with city lights, and the band of light that is the disk of our galaxy subsides to an invisible 1 percent effect.
Buried in that dusty disk, we do not get a very good view of our galaxy under any conditions. Thus no human observer has ever seen the image on page 28. It is intended to show what an optical astronomer working on a planet in some galaxy in the direction of Leo or Bootes might see. Julian Baum's image bears a strong resemblance to some of the distant galaxies that we do see whole.
The visible light of spiral galaxies like ours comes largely from stars -- especially ones younger, brighter, bluer, and more massive than our Sun -- and from gas clouds near those stars and illuminated by them. Galaxies with a very high rate of current star formation, like the Large and Small Magellanic Clouds will look bluer and patchier than the Milky Way or other spirals. Those in which star formation tapered off long ago will look redder and smoother. Predictably, the former are rich and the latter deficient in the cool gas and dust from which new stars can form.
Why did some galaxies turn virtually all their gas into stars billions of years ago, while others have dragged the process out through the whole age of the universe? A hint comes from their shapes. Probably all galaxies have at least a bit of both disk (flat) and halo (spheroidal) components. But the larger the star-formation rate now, the more you see of the disk component. It is nearly invisible in the elliptical galaxies that have only old stars but practically all you see in irregular galaxies like the Magellanic Clouds.
THE GALAXY WE SEE
Our eyes are most sensitive to the yellowish wavelengths where the Sun puts out most Of its energy. This is clearly not a coincidence. Creatures with far-infrared eyes would see nearly uniform brightness everywhere, emitted by equally warm ground, sky, buildings, and family members. Those peering with X-ray or gamma-ray eyes would find only darkness upon the face of the Earth and an opaque atmosphere above them. Radio eyes would have to be so large that only committees could carry them around, and you know how hard it is to make a committee see anything.
The technology of the second half of the 20th century has, however, brought us telescopes to collect wavelengths other than visible light, detectors to sense the energy collected, and satellites to lift some of these above the Earth's obscuring atmosphere. As a result we now have maps of the Milky Way in every possible band of electromagnetic radiation. No two maps are the same. Each tells us about classes of objects and phenomena different from the bright stars and ionized gas that illuminate the spiral arms.
As we move from the optical to the shorter wavelengths of ultraviolet (UV), first dust and then gas becomes more opaque, and our horizons contract. In the most extreme case, just beyond the wavelength that ionizes hydrogen, we see not much more than the so-called local bubble, shown on the facing page. Curiously, strong UV sources include both the hottest and coolest stars. Bright emission from hot stars is just what you would expect, since the hotter the object the shorter the wavelength at which it radiates. Thus we see both bright, young, massive stars and hot white dwarfs newly formed by the deaths of smaller stars like the Sun. Cool stars, on the other hand, often have chromospheres and coronas with temperatures in the UV-emitting range, as indeed does the Sun.
Some common elements, including carbon and oxygen, have their strongest spectral lines in this wavelength region, so the UV Milky Way reveals more about element abundances in stars and gas clouds than can be learned from visible-light data alone. For instance, I once participated in a UV study of the Crab Nebula that showed that the nebula is not enriched in carbon and so must have come from a parent star not more than 8 to 10 times the mass of the Sun. This may not strike you as terribly exciting, but astronomers have to learn things like that to become associate professors. UV is the ideal band in which to probe astronomical objects at about 100,000 [degrees] Kelvin.
Shorter wavelengths, coming from still hotter gases and stars, have more penetrating power, so X-ray astronomers can survey the entire galaxy. The X-ray galaxy is not one in which normal stars are conspicuous. Rather, the brightest sources are binary stars in which a main-sequence or red-giant star is transferring material to a neutron star or black hole remnant of a previously more massive companion. And the brightest X-ray gas clouds are not the ionized-hydrogen (H II) regions illuminated by young stars but the remnants of supernova explosions whose ejecta were blown out so vigorously that collisions with interstellar gas heat both to a million degrees or more, temperatures where X-rays are the brightest radiation. The galactic disk is home to most X-ray binaries and supernova remnants, so the X-ray sky shows a familiar Milky-Way-like outline.
Only when large satellites began to see fainter sources did normal stars turn up in X-ray inventories, mostly coronal emission. The sky then be look more like the optical one. Binaries with white-dwarf accretors, including novae and their cousins, are also among the fainter X-ray stars.
Some galaxies have bright X-ray sources at their centers. The Milky Way does not. Rather, there is a confusing cluster of a half dozen or more emitters. Probably all are normal X-ray binaries, at most one of which, and a faint one at that, can be associated with the true center of the galaxy. This is one of the stronger arguments against thinking that we might have a massive black hole there, as will be discussed in an upcoming article by Ray Jayawardhana.
Looking between the bright sources you will see diffuse X-ray emission. Most of it comes from far outside the Way, probably from quasars and other active galaxies that do have central black holes. But there is also a component emitted by the hottest interstellar gas,including that inside the local bubble. One expects a similar diffuse UV background from slightly cooler gas studded through the Milky Way, as well as from UV starlight scattered by dust grains. Authorities disagree about whether either of these has actually been seen.
The most energetic photons of all are the gamma rays. Two satellites, COS B and the Compton Gamma Ray Observatory, have mapped the gamma-ray sky and found that the Milky Way is a strong diffuse emitter. The main source is cosmic rays -- protons and other particles accelerated to speeds near that of light by shocks and magnetic fields These crash into interstellar gas, showering gamma rays, like sparks, about them. The brightness variations from place to place, therefore, tell us both where the gas is densest and where there are the most cosmic rays -- typically in the spiral arms.
Some point sources of gamma rays are familiar from other wavelengths, including supernova remnants like the Crab Nebula and the more vigorous X-ray binaries. Others were a complete surprise. The third-brightest source in the sky, called Geminga, has only in recent years revealed itself as a rotating, magnetized neutron star. It is therefore a pulsar, but a radio-silent one, and very faint in all other wavebands. Famous pulsars, including those in the Crab and Vela supernova remnants, have also been seen and were more or less expected. The unexpected part is that the older the pulsar, the larger the fraction of its total luminosity that goes into gamma rays. This enables us to detect Geminga and a couple of others whose progenitor stars must have exploded more than 100,000 years ago, compared to only 1,000 years for the Crab.
Moving away from optical astronomy in the opposite direction, we enter the realm of the infrared, emitted by objects with temperatures between about 20[degrees] and 4,000[degrees] K. Near-infrared (shorter wavelength, higher temperature) sources are mostly stars cooler than the Sun. These include both old, evolved ones on their way to becoming supernovae and white dwarfs and the youngest ones that have not yet settled down to respectable main-sequence lives as hydrogen burners. The near-infrared sky, therefore, looks familiar but rather bland.
Interstellar dust is less opaque to infrared than to visible light. We can again see the whole galaxy, including a cluster of bright but partly obscured stars very near the galactic center called IRS 16. Winds from these stars set a lower limit to the amount of gas available for a hypothetical central black hole to accrete, and so put an upper limit on its mass. The Milky Way is not a secret quasar! Just to make our problem more difficult, infrared spectral lines can be used to map gas velocities close to the galactic center. These are so large that 1 to 3 million solar masses of material must be there in some combination of dense star duster and central black hole!
Parts of the far-infrared sky look like colonies of spiders gone mad. The fine structure seen there is called cirrus, by analogy with filamentary clouds on Earth. Its emission, as well as that from more extended bright regions, comes from dust heated to between 10 and 100 [degrees] K by starlight. The hottest, brightest, young stars are the most efficient heaters and so, somewhat counterintuitively, far-infrared emission is an excellent tracer of current star-formation rates The infrared emission from our Milky Way is only 50 percent or so that of visible light. But some very distant, massive galaxies, presumably in the first throes of star formation, put out 99 percent or more of their energy as dust-reradiated infrared.
Radio waves are excellent tracers of interstellar gas, as described in Gillian Knapp's article later in this series. Emission at some wavelengths reveals cold molecular gas. Other wavelengths, particularly 21-centimeter, come from neutral atomic-hydrogen gas. This is the most widely distributed phase of hydrogen and has been well mapped for the entire galaxy. It is strongly concentrated toward the galactic plane, but, because it can be shoved around by expanding supernova remnants, stellar winds, and so on, it also displays looplike features and filaments. The 21-cm emission is especially valuable because its wavelength is known precisely, so we can measure accurate Doppler velocities for any gas, to find out where it is going as well as where it is now. As in infrared and X-ray, radio observations generally can see the entire galaxy.
Two additional components of radio emission reveal more specialized gas features. First, H II regions like the Orion Nebula (a site of current star formation) and the Ring Nebula in Lyra (a planetary nebula left by a stellar death 10,000 or so years ago) emit bremsstrahlung radiation when their high-speed electrons are slowed down by passing protons. Even the winds of normal stars can be seen this way, if you look hard enough. Second, still-higher-speed (relativistic) electrons moving through the galaxy's ubiquitous magnetic field put out a form of radiation called synchrotron, recognizable because it is polarized. Such electrons make up about 1 percent of the cosmic rays mentioned earlier in connection with diffuse gamma-ray emission. The whole galactic plane thus shines in synchrotron radiation. Denser concentrations of relativistic electrons and magnetic fields occur in supernova remnants, which are therefore bright radio sources as well as optical and X-ray emitters. And, at long last, in radio synchrotron we see a source that is unambiguously associated with the physical center of the Milky Way, Called Sagittarius A*, it is negligible compared to the central sources in many quasars and radio galaxies, but we should not feel discriminated against. The one in M31, the Andromeda Galaxy that is our near neighbor and fraternal twin, is even fainter.
THE GALAXY WE INFER -- GLOBAL PROPERTIES
Equipped with the full range of wavelengths of electromagnetic radiation, together with telescopes, spectrometers, and both classical and modern physics, the 20th-century astronomer may seem a bit overloaded (and this is probably why we now take five to seven years to complete the doctoral studies that occupied only four years in past generations). But the combination also permits asking, and sometimes answering, an enormous range of questions about masses, ages, brightnesses, chemical compositions, and the formation and evolution of the Milky Way and objects in it.
Adding up all the wavelengths we find that the galaxy emits more than 35 billion times the luminosity of our own Sun. Dwarf galaxies can be as faint as "only" a million Suns, while some of the bright infrared ones radiate the equivalent of a million million Suns or more. Most of our galaxy's luminosity comes from the few brightest stars. The much more numerous faint stars are the repository of most of the mass in our galactic disk.
The total mass of our galaxy is surprisingly large, and it gets larger as you measure it farther away from the galactic center. For the part of the galaxy closer to the center than we are, rotation speeds of gas clouds imply a mass of about 100 billion Suns. Thus the mass-to-light ratio for the inner galaxy is roughly four solar masses per solar luminosity -- about what you would expect for a mix of big and little stars, gas, and dust. But make your measurements farther out, using the motions of stars in the halo or the interaction of the whole galaxy with Andromeda and other neighbors, and you get more like 1,000 billion Suns. The implied mass-to-light ratio of between 20 and 30 to I cannot be accounted for by stars -- even countless tiny, faint ones.
Instead we invoke the infamous dark matter, also known to make up most of the mass of other galaxies (both spiral and elliptical), clusters of galaxies, and the universe. What is it? Your guess may not be quite as good as mine, but almost. Several dozen theories exist, ranging from substellar brown dwarfs to minuscule or massive black holes, weird particles that neither emit nor absorb any kind of electromagnetic radiation, and an arbitrary constant in Einstein's equations of general relativity. Remarkably much of the structure and evolution of the Milky Way and other galaxies can be understood without knowing just what the dark matter is, precisely because it does not emit or absorb light or otherwise much interact with the rest of us.
The age of the Milky Way sets a lower limit to the age of the universe. This is so obvious you may wonder why I bother to mention it. But there is a catch. Studies of the evolution of stars in the galactic halo, especially ones in clusters, give a lower age limit of 15 billion years (give or take 2 billion) according to several independent investigators. And it can hardly be pulled below 11 or 12 billion years, even if you put all the tow ropes on one side. This is still awkwardly large.
The universe as a whole is trying to tell us its age from how fast it is expanding now and what that expansion was like in the past. The Hubble parameter, [H.sub.0], describes the present expansion and a second parameter, [q.sub.0] the change. Different astronomers find different values for them and will defend these against each other with considerable vigor. But all agree that, unless both parameters have just about the smallest possible values, the age of the universe is not more than 15 billion years. Widely popular values of [H.sub.0] (85 kilometers per second per mega-parsec) and [q.sub.0] (such that the universe just barely expands forever) yield an age of 8 billion years. And 15 into 8 won't go -- unless you are prepared to live with that arbitrary constant in Einstein's equations.
Finally, the chemical composition of the galaxy turns out to be very similar to that of the Sun -- roughly 75 percent of the stuff is hydrogen, 25 percent helium, and only one or two percent everything else, including carbon, iron, oxygen, and silicon (all fairly common), as well as gold, silver, uranium, and gadolinium (all fairly rare).
The issues of the age of the galaxy, the amount of dark matter, whether most of it has to be weird stuff, the value of Einstein's arbitrary constant, and the amount of helium in old stars are all closely coupled -- in a way that belongs in an article on cosmology, not the Milky Way.
THE GALAXY WE INFER -- STELLAR POPULATIONS AND CHEMICAL EVOLUTION
Despite the wide range of types of stars and nebulae revealed by observations at many wavelengths, the stellar citizens of our galaxy settled fairly well into a handful of populations differing in age, chemical composition, location, and orbital motion through the galaxy. The population characteristics help to sketch a picture of how the galaxy formed.
Starting at the outside we find the roughly spherical halo, more than 300,000 light-years across. Its most conspicuous residents are the globular clusters, about 150 of them, each with 10 to 100 thousand stars. Not all are quite the same age, but all are older than the rest of the galaxy. The oldest set the 15-billion-year floor to the age of the galaxy. Less conspicuous but more numerous are a billion or so halo stars not now in clusters.
In addition to their age, halo stars share a chemical composition that is 99.9 percent or more hydrogen and helium, and less than 0.1 percent of all the heavier elements (frequently called metals, though oxygen and silicon are among the commonest). This spheroidal population does not share the rotation of the galactic disk. As many halo stars go in one direction as the other. Massive, bright blue stars belonging to the halo population died long ago, so the brightest remaining stars are red giants. The globular clusters have far more than their fair share of neutron stars in binary pairs -- both X-ray emitters and binary pulsars. These come somehow from interactions among single and double stars in the clusters' dense cores.
The halo shades inward gradually to merge with the bulge. The populations are probably not really distinct, but the bulge gets a separate name because it shows up in pictures of other galaxies, where the rest of the halo is too hint to find without special searches. Bulge stars are relatively old, rich in metals, and red. Planetary nebulae and some kinds of variable stars are common there, as well as X-ray binaries and other evolved stages of low-mass stars. Star formation ceased long ago in the halo and bulge of our galaxy. Some gas drifts among the stars (hot in the halo, mostly cold in the bulge), but it is only visiting and really belongs to the flat disk.
The disk is the rotating part of the Milky Way, complete with spiral arms to show the direction of rotation even without velocity data. Its stars are younger than those in the bulge and both younger and richer in heavy elements than those in the halo. On average the older the stars, the thicker the disk they inhabit, the larger their speeds relative to the average of galactic rotation, and the smaller the metal content. Whether we are seeing continuous gradations in all these properties or several discrete populations formed in different ways is under vigorous debate (and I have friends in both camps and so am not voting).
The thicker, older disk component is home to many planetary nebulae, white dwarfs, and binary systems that can lead to nova explosions. Observations of other galaxies say that one kind of supernova (called Type I and lacking lines of hydrogen in the spectra) can also explode here, though their remnants will be hard to see where there isn't much gas to push against. The densest gas clouds lie in the thin disk, which is also marked by the most massive, rapidly evolving stars including Cepheid variables and the stars that make the other kind of supernova (Type II, with strong spectral features of hydrogen). The brighter supernova remnants and the youngest pulsars also reside in the thin disk.
The correlations of stellar composition with age and location suggest the gradual accumulation of heavy elements -- produced by stellar nuclear reactions and blown out by supernova explosions -- at the same time that the galaxy was contracting, flattening, and spinning up. This picture has been with us for more than 30 years. Curiously, you can also account for many of the correlations in a very different picture (as discussed in a future article by James Binney) in which the Milky Way gradually put itself together out of a number of smaller protogalaxies accompanied by sequential bursts of star formation.
What would we still like to know about the Milky Way? Everything, of course. Scientists are like that. Several items surfaced in this article. Is there a central black hole, and, if so, how big is it? Just how opaque are galactic disks at different wavelengths? (This comes up when you try to figure out how much dark matter there really is.) Did the galaxy form from a single contracting gas cloud or by the mergers of many subunits? Are the disk populations of stars discrete or a continuum? Is there a real conflict between the ages of the oldest stars and the measured rate of the expansion of the universe? And two more not mentioned before -- just what is the underlying physics that makes spiral arms in the Milky Way and elsewhere, and why are there so few stars in the solar neighborhood with less than about 10 percent of the solar metal abundance? The latter, dignified by the name "the G-dwarf problem," continues to be a major intruder in all pictures of galactic evolution -- a bit like having to include a polar bear in what you thought was going to be a tropical landscape.
Galactic astronomy belongs almost entirely to the 20th century. Olin J. Eggen, Donald Lynden-Bell, and Allan Sandage, who first put together the properties of stellar populations into an evolutionary scenario, are all still actively engaged in astronomical research. The first detailed study of how galaxies change in brightness and color as their compositions and star-formation rates changed was the 1967 doctoral dissertation of Beatrice M. Tinsley. The very concept of stellar populations, due largely to Walter Baade, had not even been formulated when she and I were born, and we learned college astronomy out of books that never mentioned galactic evolution as a possibly interesting topic for investigation.
Stepping back a generation, Vesto Melvin Slipher exposed the very first spectrogram of a spiral nebula to show line features the year my father was born. The nebula was, of course. M31. The first features to show up were absorption lines, not emission ones. And another decade passed before Edwin Hubble's recognition of Cepheid variables in Andromeda showed that the spiral nebulae were truly separate galaxies, not just rotating gas clouds in our own.
The wary reader may legitimately wonder how many more such revolutions lie ahead of us, and therefore whether the articles in this series need to be read at all. The answer, I think, is that galactic astronomy is now on a firm footing -- but this became possible only with the recognition that the Milky Way is one among many, just as the ancient question of how the solar system formed has been subsumed, and partly answered, within the general topic of star formation. The Milky Way is a common sort of galaxy, just as the Sun is a common sort of star. Thus the processes that shaped them must be common ones. Cosmology, in the sense of the study of the origin and evolution of the universe as a whole, comes with no such guarantee.
Virginia Trimble, celebrated for her penetrating overviews of astronomical topics, divides her time between the University of California, Irvine, and the University of Maryland. Samantha Parker is associate editor of Sky & Telescope.