The Milky Way and Beyond (5 page)

The least-understood component of the Galaxy is the giant massive halo that is exterior to the entire visible part. The existence of the massive halo is demonstrated by its effect on the outer rotation curve of the Galaxy. All that can be said with any certainty is that the halo extends considerably beyond a distance of 100,000 light-years from the centre and that its mass is several times greater than the mass of the rest of the Galaxy taken together. It is not known what its shape is, what its constituents are, or how far into intergalactic space it extends.

It was once thought that the spiral structure of galaxies might be controlled by a strong magnetic field. However, when the general magnetic field was detected by radio techniques, it was found to be too weak to have large-scale effects on galactic structure. The strength of the galactic field is only about 0.000001 times the strength of Earth's field at its surface, a value that is much too low to have dynamical effects on the interstellar gas that could account for the order represented by the spiral-arm structure. This is, however, sufficient strength to cause a general alignment of the dust grains in interstellar space, a feature that is detected by measurements of the polarization of starlight.

In the prevailing model of interstellar dust grains, the particles are shown to be rapidly spinning and to contain small amounts of metal (probably iron), though the primary constituents are ice and carbon. The magnetic field of the Galaxy can gradually act on the dust particles and cause their rotational axes to line up in such a way that their short axes are parallel to the direction of the field. The field
itself is aligned along the Milky Way band, so that the short axes of the particles also become aligned along the galactic plane. Polarization measurements of stars at low galactic latitudes confirm this pattern.

The motions of stars in the local stellar neighbourhood can be understood in terms of a general population of stars that have circular orbits of rotation around the distant galactic nucleus, with an admixture of stars that have more highly elliptical orbits and that appear to be high-velocity stars to a terrestrial observer as Earth moves with the Sun in its circular orbit. The general rotation of the disk stars was first detected through studies made in the 1920s, notably those of the Swedish astronomer Bertil Lindblad, who correctly interpreted the apparent asymmetries in stellar motions as the result of this multiple nature of stellar orbital characteristics.

The disk component of the Galaxy rotates around the nucleus in a manner similar to the pattern for the planets of the solar system, which have nearly circular orbits around the Sun. Because the rotation rate is different at different distances from the centre of the Galaxy, the measured velocities of disk stars in different directions along the Milky Way exhibit different patterns. The Dutch astronomer Jan H. Oort first interpreted this effect in terms of galactic rotation motions, employing the radial velocities and proper motions of stars. He demonstrated that differential rotation leads to a systematic variation of the radial velocities of stars with galactic longitude following the mathematical expression:

radial velocity =
Ar
sin 2
l
,

where
A
is called Oort's constant and is approximately 15 km/sec/kiloparsec (1 kiloparsec is 3,260 light-years),
r
is the distance to the star, and
l
is the galactic longitude.

A similar expression can be derived for measured proper motions of stars. The agreement of observed data with Oort's formulas was a landmark demonstration of the correctness of Lindblad's ideas about stellar motions. It led to the modern understanding of the Galaxy as consisting of a giant rotating disk with other more spherical, and more slowly rotating, components superimposed.

The total mass of the Galaxy, which had seemed reasonably well-established during the 1960s, has become a matter of considerable uncertainty. Measuring the mass out to the distance of the farthest large hydrogen clouds is a relatively straightforward procedure. The measurements required are the velocities and positions of neutral hydrogen gas, combined with the approximation that the gas is rotating in nearly circular orbits around the centre of the Galaxy. A rotation curve, which relates the circular
velocity of the gas to its distance from the galactic centre, is constructed. The shape of this curve and its values are determined by the amount of gravitational pull that the Galaxy exerts on the gas. Velocities are low in the central parts of the system because not much mass is interior to the orbit of the gas; most of the Galaxy is exterior to it and does not exert an inward gravitational pull. Velocities are high at intermediate distances because most of the mass in that case is inside the orbit of the gas clouds and the gravitational pull inward is at a maximum. At the farthest distances, the velocities decrease because nearly all the mass is interior to the clouds.

This portion of the Galaxy is said to have Keplerian orbits, since the material should move in the same manner that the German astronomer Johannes Kepler discovered the planets to move within the solar system, where virtually all the mass is concentrated inside the orbits of the orbiting bodies. The total mass of the Galaxy is then found by constructing mathematical models of the system with different amounts of material distributed in various ways and by comparing the resulting velocity curves with the observed one. As applied in the 1960s, this procedure indicated that the total mass of the Galaxy was approximately 200 billion times the mass of the Sun.

During the 1980s, however, refinements in the determination of the velocity curve began to cast doubts on the earlier results. The downward trend to lower velocities in the outer parts of the Galaxy was found to have been in error. Instead, the curve remained almost constant, indicating that there continue to be substantial amounts of matter exterior to the measured hydrogen gas. This in turn indicates that there must be some undetected material out there that is completely unexpected. It must extend considerably beyond the previously accepted positions of the edge of the Galaxy, and it must be dark at virtually all wavelengths, as it remains undetected even when searched for with radio, X-ray, ultraviolet, infrared, and optical telescopes. Until the dark matter is identified and its distribution determined, it will be impossible to measure the total mass of the Galaxy, and so all that can be said is that the mass is several times larger than thought earlier.

The nature of the dark matter in the Galaxy remains one of the major questions of galactic astronomy. Many other galaxies also appear to have such undetected matter. The possible kinds of material that are consistent with the nondetections are few in number. Planets and rocks would be impossible to detect, but it is extremely difficult to understand how they could materialize in sufficient numbers, especially in the outer parts of galaxies where there are no stars or even interstellar gas and dust from which they could be formed. Low-luminosity stars, called brown dwarfs, are so faint that only a few have been detected directly. In the 1990s, astronomers carried out exhaustive lensing experiments involving the study of millions of stars in the galactic central areas and in the Magellanic
Clouds to search for dark objects whose masses would cause lensed brightenings of background stars. Some lensing events were detected, but the number of dark objects inferred is not enough to explain completely the dark matter in galaxies and galaxy clusters. It appears likely that there is more than one form of dark matter, with the most important being hypothetical types of objects, such as WIMPs (weakly interacting massive particles).

The Milky Way Galaxy is made up of about one hundred billion stars. Stars come in many different masses, from a few percent that of the Sun to a hundred times greater. Stars also appear in different colours, from a dim, cool red to an incandescent blue. Despite their different properties, stars can be divided into populations. The differences between the populations can also be seen in how stars are distributed and how they move.

The concept of different populations of stars has undergone considerable change over the last several decades. Before the 1940s, astronomers had been aware of differences between stars and had largely accounted for most of them in terms of different masses, luminosities, and orbital characteristics around the Galaxy. Understanding of evolutionary differences, however, had not yet been achieved, and, although differences in the chemical abundances in the stars were known, their significance was not comprehended. At this juncture, chemical differences seemed exceptional and erratic and remained uncorrelated with other stellar properties. There was still no systematic division of stars even into different kinematic families, in spite of the advances in theoretical work on the dynamics of the Galaxy.

In 1944 the German-born astronomer Walter Baade announced the successful resolution into stars of the centre of the Andromeda Galaxy, M31, and its two elliptical companions, M32 and NGC 205. He found that the central parts of Andromeda and the accompanying galaxies were resolved at very much fainter magnitudes than were the outer spiral arm areas of M31. Furthermore, by using plates of different spectral sensitivity and coloured filters, he discovered that the two ellipticals and the centre of the spiral had red giants as their brightest stars rather than blue main-sequence stars, as in the case of the spiral arms.

This finding led Baade to suggest that these galaxies, and also the Milky Way Galaxy, are made of two populations of stars that are distinct in their physical properties as well as their locations. He applied the term Population I to the stars
that constitute the spiral arms of Andromeda and to most of the stars that are visible in the Milky Way system in the neighbourhood of the Sun. He found that these Population I objects were limited to the flat disk of the spirals and suggested that they were absent from the centres of such galaxies and from the ellipticals entirely. Baade designated as Population II the bright red giant stars that he discovered in the ellipticals and in the nucleus of Andromeda. Other objects that seemed to contain the brightest stars of this class were the globular clusters of the Galaxy. Baade further suggested that the high-velocity stars near the Sun were Population II objects that happened to be passing through the disk.

As a result of Baade's pioneering work on other galaxies in the Local Group (the cluster of star systems to which the Milky Way Galaxy belongs), astronomers immediately applied the notion of two stellar populations to the Galaxy. It is possible to segregate various components of the Galaxy into the two population types by applying both the idea of kinematics of different populations suggested by their position in the Andromeda system and the dynamical theories that relate galactic orbital properties with
z
distances (the distances above the plane of the Galaxy) for different stars. For many of these objects, the kinematic data on velocities are the prime source of population classification. The Population I component of the Galaxy, highly limited to the flat plane of the system, contains such objects as open star clusters, O and B stars, Cepheid variables, emission nebulae, and neutral hydrogen. Its Population II component, spread over a more nearly spherical volume of space, includes globular clusters, RR Lyrae variables, high-velocity stars, and certain other rarer objects.

As time progressed, it was possible for astronomers to subdivide the different populations in the Galaxy further. These subdivisions ranged from the nearly spherical “halo Population II” system to the very thin “extreme Population I” system. Each subdivision was found to contain (though not exclusively) characteristic types of stars, and it was even possible to divide some of the variable-star types into subgroups according to their population subdivision. The RR Lyrae variables of type ab, for example, could be separated into different groups by their spectral classifications and their mean periods. Those with mean periods longer than 0.4 days were classified as halo Population II, while those with periods less than 0.4 days were placed in the “disk population.” Similarly, long-period variables were divided into different subgroups, such that those with periods of less than 250 days and of relatively early spectral type (earlier than M5e) were considered “intermediate Population II,” whereas the longer period variables fell into the “older Population I” category. As dynamical properties were more thoroughly investigated, many astronomers divided the Galaxy's stellar
populations into a “thin disk,” a “thick disk,” and a “halo.”

An understanding of the physical differences in the stellar populations became increasingly clearer during the 1950s with improved calculations of stellar evolution. Evolving-star models showed that giants and supergiants were evolved objects recently derived from the main sequence (a distinctive, primary band of stars) after the exhaustion of hydrogen in the stellar core. As this became better understood, it was found that the luminosity of such giants was not only a function of the masses of the initial main-sequence stars from which they evolved, but was also dependent on the chemical composition of the stellar atmosphere. Therefore, not only was the existence of giants in the different stellar populations understood, but differences between the giants with relation to the main sequence of star groups came to be understood in terms of the chemistry of the stars.

At the same time, progress was made in determining the abundances of stars of the different population types by means of high-dispersion spectra obtained with large reflecting telescopes having a coudé focus arrangement. A curve of growth analysis demonstrated beyond a doubt that the two population types exhibited very different chemistries. In 1959 H. Lawrence Helfer, George Wallerstein, and Jesse L. Greenstein of the United States showed that the giant stars in globular clusters have chemical abundances quite different from those of Population I stars such as typified by the Sun. Population II stars have considerably lower abundances of the heavy elements—by amounts ranging from a factor of 5 or 10 up to a factor of several hundred. The total abundance of heavy elements,
Z
, for typical Population I stars is 0.04 (given in terms of the mass percent for all elements with atomic weights heavier than helium, a common practice in calculating stellar models). The values of
Z
for halo population globular clusters, on the other hand, were typically as small as 0.003.