Spiral galaxies are the beauties of the extragalactic universe. Their spiral arms of stars, gas, and dust stand out because of their many bright and young 0 and B stars. Most spiral galaxies have two arms. The Milky Way is a spiral galaxy, perhaps of type Sbc.
The basic structure of a spiral galaxy is a disk within a spheroid rotating around a dense core of stars and a massive central black hole. The spheroid is composed of old low-metallicity stars, meaning that the spheroid formed early in the life of the galaxy, before many supernovae seeded the interstellar medium with the products of nuclear fusion. The stars in the spheroid are referred to as population 2 stars. With time, dissipation of thermal energy within the gas of a galaxy lead to the collapse of this gas into a disk with an angular momentum determined by the angular momentum of the galaxy. The stars created from this gas are younger than the population 2 stars; they are the population 1 stars, and they have high metallicities and nearly circular orbits around the galactic center within the galactic disk. The orbital velocity of the population 1 stars in the galaxy rises with radius until a characteristic radius is reached at around 5 kpc, after which the orbital velocity is nearly constant with radius. The maximum orbital velocity of population 1 stars varies from galaxy to galaxy from 100 km s-1 to 350 km s-1. The timescale for a star to orbit a galaxy is of order 100 million years, which is short for stars the mass of the Sun—the Sun's expected lifespan is 9 billion years—but is long for massive blue stars, which have lifetimes as short as 2 million years.
The ratio of stars within the spheroid to stars within the disk, as implied by the relative luminosities, varies widely from galaxy to galaxy. This ratio is the principal factor that determines the characteristics of the spiral arms within the galactic disk. As the luminosity of the spheroid decreases relative to the luminosity of the disk, the spiral arms become more loosely wound and clumpy. The galaxies with the most strongly-defined spiral arms are the galaxies with the dimmest spheroidal components.
The spiral arms are regions of collapsing gas clouds and star formation. The arms are rich with newly-formed O and B stars, the most massive of stars. But spiral arms in the galactic plane are apparent in both the blue and the red energy bands. This means that the structure exists both in the young blue stars, which burn out before passing through a spiral arm, and in the old red stars, which have lives many times longer than it takes to orbit the galaxy. The spiral arms are therefore more than just regions of star formation. This suggests a collective mechanism for the spiral arms; the dominant theory for this is the density wave theory.
The surface brightness of a spiral galaxy with distance R from the galactic center is given by
where the characteristic radius Rd and the characteristic surface brightness I0 typically have the values of
|Rd||∼||3 h-1 kpc,|
|I0||∼||140 LsV pc-2,|
where LsV is the solar luminosity in the visible band. The parameter h is the Hubble parameter in units of 100 km s-1 Mpc-1.
The Solar system is about 8 kpc from the center of the Milky Way, which means that we are well away from the brightest portion of our own galaxy. From these equations, we can conclude that the brightness in our own solar neighborhood is 20% of the brightness at the galactic center (for h = 0.6).
The galactic core of a spiral galaxy has a very broad and generally random velocity distribution. With modern instruments, the velocities at the centers of nearby galaxies can be measured for areas as small as 8 parsecs in width. For our own galactic center, velocities can be measured at a spatial resolution of about 1 parsec.
If a massive object were sitting at the core of a galaxy, the stars orbiting this object would have Keplerian orbits,—meaning that the stars at the core have a negligible effect on the gravitational field—and the orbital velocity would depend on distance from the galactic center as R-1/2. This signature is expected to appear in the inner parsec of the core of a galaxy, so testing of this aspect of the theory is beyond current observational capabilities. Instead, velocity dispersions at the center of the galaxy are measured and tested against the theory.
Studies of the core of the Milky Way and of cores of several other spiral galaxies have found that the velocity dispersion of the stars at the very centers of these galaxies are higher than the velocity dispersion of the surrounding stars. For the Milky Way, the dispersion is consistent with a central object of 3×107 Ms. For galaxies like M31, the central velocity dispersion is consistent with a central mass of approximately 109 Ms. Radio observations of maser emission of the nearby galaxy NGC 4258 have found rotational velocities of ∼900 km s-1 within 0.1 parsecs of the galactic center, which implies a central object of 3.7×107 Ms.
Under our current understanding of the equation of state of nuclear matter and under general relativity, objects the size of the galactic center objects must be black holes. This is the origin of the many claims that black holes exist at the centers of galaxies; a more accurate statement is that the observations suggest that massive objects exist at the centers of galaxies, and if general relativity is the correct theory of gravity, these objects must be black holes.
An interesting problem that arose early in the study of spiral galaxies is the disparity of the distribution of mass implied by the velocities of the stars with that implied by the surface brightness of the galaxy. The velocity studies imply that there is many times more mass within a galaxy than is indicated by the light emitted by the galaxy, and the ratio of gravitational mass to luminous mass increases as one moves way from the galactic center. Under the assumption that Newtonian gravity is valid over lengths of tens of kiloparsec—an important assumption—the observations suggest that most of the mass in a galaxy emits no light, and so this mass is referred to as dark matter. The assumption is that this dark matter sits in a halo around the galaxy.
One of the primary questions in astronomy is the nature of dark matter. Particle physicists have spun theories that the dark matter is a weakly interacting fundamental particle. The one particle we know of that does interact weakly is the neutrino. Whether it has the mass and other properties that would enable it to constitute the bulk of the mass in a galaxy is an open question. Not surprisingly, particle theorists have proposed many other hypothetical particles as possible sources of the dark mass. Another possibility, at least in part, is that mass tied up in brown dwarfs and Jupiters, which produce very little observable radiation. This hypothesis is being tested in part by observing the rate distance stars become brighter because an object passes in front and acts as a lens through its gravitational influence on the light.