290 PHYSICAL SCIENCE 



Around our system lies a great abyss of 

 space. The nearest known star — a faint speck 

 called Proxima Centauri — is 24 million million 

 miles or 4.1 light years away, more than four 

 thousand times the stretch of Neptune's orbit. 

 Then come three other stars before we reach 

 Sirius at 8.6 light years. 



A good eye unaided may see upwards of 

 5000 stars, while a large modern telescope (100- 

 inch reflector) reveals a number estimated at 

 100 million. The number does not increase in 

 proportion to the power of the telescope ; hence 

 we may conclude that our stellar universe is not 

 infinite. The total number of stars is thought 

 to be somewhere about 1 500 million. 



Some of this colossal number of stars are 

 perhaps twenty thousand times as far away as 

 Sirius, at a distance of some 170,000 light years. 

 As we probe these appalling depths, we find 

 gigantic spiral and spheroidal nebulae, and 

 globular star-clusters. One of these clusters 

 is distant from us about 200,000 light years, 

 while another is so remote that the light by 

 which we see it probably started a million 

 years ago. 



The milky way which stretches across the 

 sky shows that the apparent distribution of 

 stars is not uniform ; the milky way contains 

 more than we see in other directions. . The 

 stellar system seems to be roughly circular in 

 one plane and flattened like a double convex 

 lens with a diameter of at least 300,000 light 

 years. Our sun lies somewhat to the north 

 of the median plane, and about 60,000 light 

 years from its centre. When looking at the 



