Easily seen by eye as a narrow, winding, hazy-white band that follows a great circle on the sky, our own Milky Way Galaxy is as familiar to us as the stars and the planets. It is the object that we name all other galaxies after: the Greek word for milk is gala (ΓΑΛΑ). But what we see with our eyes doesn't greatly resemble the photographs of other galaxies. We are deep within the Galactic plane, which is a poor position to easily see the large-scale structure of our Galaxy. The milky band we see on the sky is the light from relatively nearby stars. Dust in the galactic plane obscures our sight. We are like a man in a low fog, able to see the stars overhead, but unable to see nearby buildings. We can see the other galaxies in the direction of the Galactic north and south poles, but we are unable to see at visible wavelengths much more than a kiloparsec along the Galactic Plane. Much of our knowledge of the Milky Way's structure comes at other wavelengths, particularly at radio, infrared, and x-ray wavelengths. The picture we have today for the Milky Way is of a very large barred spiral Galaxy.
The Milky Way Galaxy can be though of as three components: a bright bulge of stars at the center of the Galaxy orbiting a massive black hole, a disk of gas, dust, and bright, young stars that rotates around the core, and a large, nearly spherical halo of old stars, compact stellar clusters called globular clusters, and dark matter.
The Galactic bulge is an ellipsoid of stars with a radius of around 2 kpc. At the very center of the bulge is Sgr A*, a black hole candidate with a mass of 3.6 million solar masses. The stars within half a parsec of the black hole orbit the black hole as though no other object were around; the stellar orbits are closed elliptical orbits. Unlike the Solar System, the stars orbiting the central black hole do not orbit in a common plane. The orientation of each orbit is random. Beyond half a parsec of the black hole, the total mass of individual stars is sufficient to overwhelm the gravitational field of the black hole, so that the orbits of individual stars are no longer closed ellipses. The random orientation of the orbits within the Galactic bulge, however, remains.
Wrapped around the Galactic bulge is the Galactic disk. It is composed of gas, dust, and stars that precipitated from the gas. Stars are continually being born in the Galactic plane. Unlike the stellar orbits in the Galactic core, the orbits of stars and gas clouds in the Galactic disk are highly ordered. The principle motion of the plane is a rotation at a constant velocity around the galactic center, with the orbital velocity nearly independent of the distance from the Galactic center. This implies that the time to complete one rotation is proportional to the distance from the galactic center. The rotational velocity of the Galactic disk for the solar neighborhood is about 210 km s−1. Individual stars move at velocities that deviate from this average by small amounts. For instance, the Sun moves relative to the mean rotation of the Galactic disk by 16 km s-1.
The Galactic disk extends to about 15 kpc from the Galactic Center. The Sun is in this disk, 7.6 kpc from the Galactic center. The youngest and brightest stars form a thin disk that extends about 100 pc above the Galactic plane. Older stars constitute a thick Galactic disk that locally extends about 1 kpc above and below the galactic plane. This thick disk also contains the radio pulsars.
The spiral arms are in the thin Galactic disk. The spirals of most spiral galaxies are generally believed to be density waves that propagate in the galactic plane. Cool gas clouds and bright, young stars make the spirals in distant galaxies visible. Gas clouds in our own spirals are visible at radio wavelengths. The structure of our galaxy is that of a barred-spiral galaxy, meaning that the central part of the disk contains a bar of stars and gas, while the outer part of the disk contains spiral arms. The bar in our Galaxy extends about 3 kpc from the Galactic center. Two spirals waves extend outward from the tips of the bar, and two additional waves extend outward from 3 kpc in the space between the first-two spirals. The spirals arms of our Galaxy are tightly wrapped, so that they are nearly perpendicular to the radius from the galactic center. The Sun lies between two of these spiral arms, with each arm at least 1 kpc from the Sun.
The most mysterious component of the Galaxy is the Galactic halo. The galactic halo contains no gas, so we can easily see through it to the distant galaxies. The halo contains very old stars, stars that contain few “metals,” meaning that they contain very little carbon, nitrogen, and oxygen. They are the first generations of stars, containing only the elements created in the big bang. They are principally hydrogen and helium. The main-sequence stars in the Galactic halo are smaller than the Sun, since the larger stars long ago exhausted their nuclear fuel and collapsed to neutron stars and degenerate dwarfs. The halo stars can be seen locally. They move at high velocity relative to the stars of the Galactic plane. Halo stars far from the Galactic plane, however, are invisible, because they generate little light compared to the massive stars of the Galactic plane. Besides individual stars, the halo contains many massive and compact star clusters, which are called globular clusters. Each globular cluster contain from 100,000 to 1 million stars. The stars in each of these ancient clusters were born about the same time. The final constituent of the Galactic halo is a great unknown, something seen through its gravitational influence alone. This so-called “dark matter” (or “dark energy,” depending on how your biases run) may be something mundane, such as very low mass stars, brown dwarfs, and Jupiters, it may be something unusual, such as neutrinos with mass, or it may be something entirely exotic, such as one of the numerous particles and phenomena that mathematically-minded physicists love to invent.
The Milky Way Galaxy itself is very large. From its influence on the Magellanic Clouds, which are nearby dwarf galaxies, and from the motion of the most distant globular clusters, a mass of about 7.5×1011 times the mass of the Sun has been derived.
Melia, Fulvio, and Falcke, Heino. “The Supermassive Black Hole at the Galactic Center.” In Annual Reviews of Astronomy and Astrophysics, edited by Geoffrey Burbidge, Allan Sandage, and Frank H. Shu, vol. 39. Palo Alto, California: Annual Reviews, 2001.