Chapter 15: Galaxies

The local group of galaxies surrounding the Milky Way

To understand the neighborhood around the Milky Way, let's take a reconnaissance journey, describing the nearby galaxies in order of distance. Our journey will span a distance of 1000 kpc or 3.25 million light years. The galaxies in this volume of space make up the Local Group. Our cosmic neighborhood teaches us a very important feature of the universe: galaxies are not randomly distributed through space; they tend to cluster. Most of the galaxies in the Local Group are clumped into two subgroups, those around the Milky Way and those around the Andromeda Galaxy. To visualize the distances, imagine the volume containing the Local Group to be a medium-sized room. On this scale, the Milky Way would be the size of a dinner plate. The Magellanic Clouds would be like crumpled balls of cotton 8 or 10 centimeters across within a meter of the plate. A dozen or so galaxies of various shapes, from 1 to 15 centimeters across, would be scattered across the room. Andromeda (or M31), the nearest galaxy resembling the Milky Way, would be another dinner plate 7 meters away.

Andromeda, our closest big neighbor galaxy

Seen from the southern skies, the Large and Small Magellanic Clouds (the LMC and SMC, respectively) are bright patches in the sky. 

Since prehistoric times, humans must have been aware of two glowing patches in the southern sky. Since there is no bright south polar star, the clouds helped navigators to mark the pole. Europeans heard them described during Magellan's around-the-world expedition in the early 16th century, so they came to be called the Magellanic Clouds. The clouds are in fact small galaxies moving in orbits around the Milky Way. The Large Magellanic Cloud is 50 kpc away, and the Small Magellanic Cloud 61 kpc, less than three times the distance to the far edge of our own galaxy. The light we see from these galaxies started its journey after humans had evolved but before they left Africa.

Central portion of the Tarantula Nebula

What kind of galaxies are the companions of ours, the Magellanic Clouds? They are several times smaller than the Milky Way, judging by star counts and measures of neutral hydrogen that show that they are only a few percent as massive as the Milky Way. The clouds do not show the Milky Way's beautiful spiral structure; they are irregular galaxies. Each contains a softly glowing, bar-like structure composed of stars. Somewhat off the end of the bar in the Large Magellanic Cloud is the spectacular Tarantula Nebula, also known as 30 Doradus. This luminous nebula can be seen with the naked eye. In fact, if it were moved to the distance of the Orion Nebula, it would fill the whole constellation of Orion and be bright enough to cast shadows on Earth! In its center is a cluster 60 pc in diameter containing thousands of massive, bluish supergiant stars.

Large Magellanic Cloud

We can best understand other galaxies and their evolution by considering their stellar populations. The Large Magellanic Cloud has a disk population similar to that found in our own spiral arm of the Milky Way. Faint-star photometry has also identified red giants and main-sequence stars. The bulk of the star formation in both clouds occurred 1 to 3 billion years ago, but it continues to the present day. There is little dust in the two clouds, except in the prominent young nebulae. The stellar populations are younger and more deficient in heavy elements than the solar neighborhood.

The Andromeda Galaxy

At 670 kpc, we encounter the first spiral galaxy truly comparable to the Milky Way, along with several of its smaller satellite galaxies. This galaxy must have been known since prehistoric times since it's visible to the naked eye as a hazy patch on a clear, dark night. It was first recorded in a star catalog by Arab astronomer al-Sufi in 964 A.D. Edwin Hubble used Cepheid variables in the Andromeda galaxy (M 31) to finally settle the debate over the nature of the so-called spiral nebulae. The Andromeda galaxy is slightly larger than the Milky Way and similar in stellar content. The naked eye sees it as a faint patch of light, but this is really only the brightest, innermost region, a few kiloparsecs across. Images made with large telescopes show that the spiral arms form a disk at least 30 kpc across. As with the Milky Way, there are globular clusters and a halo of HI gas reaching perhaps 100 kpc in diameter. The Andromeda Galaxy played an important role in the discovery of the two main stellar populations.

A parsec is the distance from the Sun to an astronomical object which has a parallax angle of one arcsecond (1 AU and 1 pc are not to scale (1 pc = 206265 AU)).

An imaginary voyage through the galactic neighborhood brings to mind the vast distances that separate galaxies. When astronomers gather information about the universe, they use electromagnetic radiation that travels at 300,000 km/s. This enormous number — the speed of light and all other forms of electromagnetic radiation — is nature's ultimate speed limit. Nothing can move faster. So far we have used parsecs and kiloparsecs to measure distances beyond the Solar System. The parsec is a unit based on the geometry of stellar parallax. We could equally well use the time it takes light to travel a particular distance. The natural unit is a light-year; the distance that light travels in a year. Remember that 1 parsec = 3.26 light-years (remember also that a light-year is a unit of distance, not time). The distance to Andromeda is therefore 670,000 × 3.26 = 2.8 x 10⁶ light-years. When you go out on a dark night and look at Andromeda through a small telescope, the light you are seeing has taken nearly 3 million years to reach Earth!