The stars and star clusters that can be observed belong to closed systems called galaxies. These are large units in the universe, which may be found in small groups or large clusters. Within the galaxies are stars arranged singly, in pairs, in clusters, with planets, and embedded in clouds of dust and gas.
Best estimates indicate that there may be more than 200 billion galaxies in our universe. They have shapes that are classified as normal spirals, barred spirals, elliptical and irregular. The number of stars found in any given galaxy can vary from as few as a few hundred million to perhaps a trillion. The average distance between galaxies in our neighborhood seems to be about 2 million light years, but this may be different in other regions of space. These great distances, however, do not prevent the collision of galaxies. Several sites have been found were collisions have occurred or are in the process of occurring. Such collisions often produce radio waves that can be detected on Earth.
The galaxy in which we are located is called the Milky Way galaxy. It is part of a small cluster of galaxies (21) known as the Local Group. Nearest the Milky Way are two irregular galaxies called the Large and Small Magellanic Clouds which are 160,000 and 175,000 light years away respectively. At a distance of 2 1/2 million light years is M31 or the Andromeda Galaxy. The Milky Way and the Andromeda Galaxies are the largest members of the Local Group.
All of the stars we see in the sky, all the planets and their moons, all the asteroids and comets, all these objects are members of the Milky Way Galaxy. What we see as the Milky Way in our sky is narrow band that represents the concentration of stars within the galactic which will be discussed later. When we look toward the constellations Scorpius and Sagittarius in the summer skies, we are looking toward the center of our galaxy. During the winter, however, when we look toward Orion and Taurus we are looking out toward the edge. If we could see our galaxy from a distance of a few hundred thousand light years away, we would see that it is a huge pinwheel shape called a spiral galaxy. There are over 300 billion stars in this galaxy, one of which is the Sun.
The Milky Way consists of three main parts: a disk about 25 kiloparsec (80,000 light years) in diameter, a surrounding halo, and a bulge where the central part of the disk thickens. Within the disk, numerous bright, young stars cluster into spiral arms that wind outward from near the center. Our Solar System lies between two arms about 8.5 kiloparsecs (28,000 light years) from the center, with the orbits of the planets tilted by about 60o with respect to the plane of the galactic disk. Mingled with the stars of the Milky Way are huge clouds of gas and dust that amount to nearly 15% of the system's known mass. Some of these gas clouds (nebulae) can be seen because they emit visible light, whereas others reveal themselves by blocking the light of background stars. These nebulae are the material from which stars are born, and it is the repository of the gases blown off dying stars. This dust and gas also hides one of the more interesting parts of our galaxy - the nucleus, a dense swarm of stars and gas in which a massive black hole may reside. So much dust lies between us and the nucleus that we can "see" it only with radio and infrared telescopes.
The flattened shape of the disk implies that the Milky Way rotates. Astronomers have measured this rotation for the Sun and neighboring stars and found that they move around the center of the Milky Way at a speed of 220 kilometers per second (140 miles per second), which means that near us the disk makes one turn around the nucleus every 240 million years. Near the Sun, and throughout most of the galactic disk, stars are typically several light years apart. The Sun's nearest neighbor is 4.2 light years away, a separation equivalent to the heads of two pins by a distance of 15 miles. In the nucleus, the stars are packed far more densely, with a separation roughly 1500 times less than that near the Sun, equivalent to two pinheads at either end of a football field. Near the edge of the disk, stars are spread out very thinly, thus the Milky Way has no sharply defined outer edge.
Some of the stars of the Milky Way are bound together gravitationally into groups called star clusters. Within a star cluster, each star moves along its own orbit about the center of mass of the cluster. The most obvious star cluster is the Pleiades, or Seven Sisters. This cluster of 6 to 12 stars which can be seen with the unaided eye, appears as a spot on the shoulder of the constellation Taurus, the Bull. A pair of binoculars or a small telescope will reveal up to 60 stars. Many other clusters dimmer than the Pleiades can also be seen using binoculars or a small telescope.
There are two types of star clusters: open clusters and globular clusters. Open cluster, like the Pleiades, contain up to a few hundred members in a volume of space with a diameter of 15 to 40 light years. They are called "open" because their stars are scattered loosely. Sometimes open clusters may be referred to as "galactic clusters" because they are found in the disk of the Milky Way. Globular clusters contain far more stars than open clusters, from a few hundred thousand to several million per cluster. These large groupings cover a volume of space with diameters that range from 80 to 300 light years. The larger number of stars creates a stronger gravitational attraction and pulls the stars into a denser ball. Globular clusters make up the halo of the galaxy.
We do not know the true limits of the universe, however, the suggestion of Albert Einstein that this time-space system is closed has attracted a great deal of interest. The universe contains millions of galaxies, and by studying them, it may be possible to form a better concept of the size and origin of our universe.
1. What is meant by the term Milky Way?
2. Describe three types of galaxies and give an example of each.
3. Define galaxy, star, nebula, and constellation.
4. Name some constellations visible at your latitude and give the name of the brightest star in each.
5. Name a group of stars normally visible only in the Northern Hemisphere. Name the same for the Southern Hemisphere.
6. What is the closest galaxy to the Milky Way?
7. Distinguish between galactic and globular clusters.
