176 



EARTH AND HEAVENLY BODIES 



How large is our galaxy of stars? The earth on 

 which you live is only a tiny body among billions of 

 heavenly bodies. As you look at the sky on a clear, 

 dark night, space seems endless. However, the billions 

 of stars visible through our modern telescopes do not 

 extend to infinite distances. These stars are a part 

 of a large group of stars that make up our galaxy. The 

 Milky Way is a part of pur galaxy. There may be more 

 than 10,000,000,000 stars in our galaxy, though the 



FIG. 290. NORTHERN CONSTELLATIONS 



total number is as yet uncertain. Our galaxy occupies 

 a disk-like or watch-shaped region whose thickness, 

 through its shortest diameter is estimated to be 5,000 

 light years, and whose long diameter is about 200,000 

 light years. Our sun with its family of planets is deep 

 in the interior, not far from the center of our galaxy. 



Outside our galaxy are globular clusters of thou- 

 sands of stars. Most of them, because of their great 

 distances from us, cannot be seen by the unaided eye. 

 Even the Great Hercules cluster (see Fig. 288), one 

 of the nearest and largest of the star clusters, appears 

 without the aid of a telescope to be only a faint star. 



Beyond our galaxy and the globular star clusters 

 modern telescopes have disclosed countless other gal- 

 axies similar in many respects to our own galaxy. 

 They are spread out in thin disks and contain hun- 

 dreds of millions of suns. These galaxies, when ob- 

 served with powerful telescopes, appear as great whirl- 

 ing masses called spiral nebulae (see Fig. 289). You 

 will need to ponder these statements a long while to 

 gain an appreciation of the vastness of space. 



How many constellations are there? When we ob- 

 serve the night sky we notice that the brighter stars 

 can be divided into groups which help us to locate and 

 identify individual stars. Thousands of years ago the 

 people of Persia, Egypt, Greece, and Rome noticed 



this, too, and they associated various groups of stars 

 with animals, heroes, and even their gods. The names 

 given centuries ago to these star groups or constella- 

 tions are still used. 



At the present time ninety constellations are recog- 

 nized. Some of these are in the southern heavens, and 

 cannot be seen by people in the United States be- 

 cause they are below the horizon. Most people find it 

 interesting to identify constellations and to learn the 

 names of the brightest stars. Star maps with direc- 

 tions telling how to use them are given with the 

 laboratory problems at the beginning of this topic. 

 See how many constellations you can become ac- 

 quainted with before the end of your course in general 



IN 16(19 CAI.IU-.l) WAS ,\<:CI MMK1) K\ OI.STIX 



.. rnxr.NS ot vr.Nur wiu.x IIF DF.MON 

 ins HUM TKI I..SCOPI H<OM nir. rowr.ii 01 .1 



Courtesy Bausch and Lomb Optical Company 



FIG. 291. GALILEO'S TELESCOPE 



science. If you are a Boy Scout or a Girl Scout, why 

 not obtain the Scout booklet on astronomy and see 

 what you must learn about the heavenly bodies in 

 order to pass the various tests? 



Exercise. Suggest the reason why the constellations 

 around Polaris, the North Star, never set in the North- 

 ern Hemisphere but are always above the northern hori- 

 zon. 



Exercise. Carefully study the star diagrams, Figure 

 285. Observe the position of the constellation Ursa 

 Major, the Great Bear, from month to month through- 



