Chapter 14: The Milky Way

The Milky Way galaxy as seen in Earth's sky. Photos covering the complete celestial sphere have been stitched and transformed to a panoramic image with the Milky Way as central line.

One of the limitations of astronomy is the fact that we are trapped on the Earth. Even our best space probes have only covered a tiny fraction of the distance to the nearest star. Much of astronomy deals with overcoming this limitation through remote sensing and clever inference based on limited data from remote sources of light. We cannot roam through space and discover the size and shape of the Milky Way galaxy directly. However, we can understand the size and shape of the Milky Way, and our place in it, by counting stars.

This dazzling infrared image from NASA's Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view.

We are "lost" in the Milky Way galaxy. If there were equal numbers of stars in every direction, we would have no clue where the edge of the Milky Way is. There would be a limit to the number of stars seen in any direction, but that could just be due to limited light gathering power and the stars being too faint to detect. We might be tempted to believe that we were at the center of the galaxy. In fact, the distribution of stars in the sky is anisotropic, and this anisotropy helps us measure our galactic environment. For simplicity, we can ignore the obscuring effects of dust.

European Southern Observatory's VISTA Telescope's view of VVV CL002 Globular Cluster .

Suppose we lived in a spherical distribution of stars. At the center of the distribution, we would see roughly equal numbers of stars in every direction. We would conclude either that we lived in the center of the universe or that the universe extended so far that we could not see its edge. But if we are offset from the center of the sphere, we would see more stars in all directions toward the center of the sphere and fewer stars in all directions away from the center. Each of these examples shows how we can use anisotropy in the counts of stars to deduce the size and shape of our galactic environment. In the real sky, hot young stars or open clusters trace the shape of the galactic disk and globular clusters trace the shape of the spherical galactic halo.

S103-E-5037 (21 December 1999)--- Astronauts aboard the Space Shuttle Discovery recorded this rarely seen phenomenon of the full Moon partially obscured by the atmosphere of Earth. The image was recorded with an electronic still camera at 15:15:15 GMT, Dec. 21, 1999.

The same slab-like geometry can be used to explain why stars dim when they set in the night sky. Suppose that the slab is the Earth's atmosphere, and the dots are dust particles in the atmosphere (we ignore Earth's curvature and treat the atmosphere as a flat slab). The dust will dim and redden the light from all stars; recall that the opacity and amount of extinction just depend on the number of particles encountered or the path length through the particles. The minimum amount of extinction occurs looking directly overhead. This is the distance X, and an elevation angle of 90°. Stars overhead typically have their red light dimmed by 10% and their blue light dimmed by 30% — the extra loss of blue light shows the effect of reddening. A star at an elevation angle of 45° is dimmed by an extra 40% since Y = X / cos 45 = 1.4 X. A star at an elevation angle of 70° (only 20° above the horizon) is seen through an even larger amount of atmosphere, Y = X / cos 70 = 2.9 X. It is therefore dimmed by 65% compared to a star that is overhead.

Meridian-International; 1=Circle of latitude, 2=Meridian (geography)

Astronomers prefer to observe targets that are high in the sky and now we can understand why. As a star sets toward the horizon, its light is rapidly dimmed and reddened by dust particles in Earth's atmosphere. Chasing a star as it sets in the western sky leads to diminishing returns, as its light is steadily extinguished by an increasing path through the atmosphere. Astronomers try to plan their observations to catch stars as they transit the meridian. In the case of a circumpolar star, it can be observed all night.