Great stuff, as usual, from The Sky This Week.
This is a good week to try to make sense of the sheer scale of the cosmos as we have a variety of bright objects that we can look at and think of in terms of distance as well as brightness. The first thing we need to do is find a convenient “measuring stick”, and fortunately the laws of physics provide us one that’s good for measuring vast distances. Light, the essential “messenger” of all celestial objects, travels at a finite speed that amounts to about 300,000 kilometers (186,000 miles) per second, so if we can measure the time it takes light to reach us from an object we can gauge its distance.
When we look at the Moon, we see it as it appeared a just over a second ago. Light from the Sun takes eight minutes to get from there to here. The ruddy glow of Mars is five minutes old by the time it arrives here at earth. Giant Jupiter’s bright glow has taken 50 minutes to cross the gulf of space that separates us. … Our last bright planet is Saturn, and the view you’ll get of his wonderful rings tonight will take 77 minutes to get here. Far-flung Pluto, which marked the edge of the solar system when I was in school, is still relatively close-by, just under 4.5 hours at light speed.
Now let’s jump to the rising stars of the summer Triangle in the eastern part of the sky. Bright Vega, highest and brightest star of the trio, is located at a distance that takes light 27 years to cross! Altair, southernmost of the three stars, is even closer, a mere 16 “light years” away. Looking at the last, northernmost member of the triangle one would naturally assume that Deneb would be a similar distance from Earth, but here is where assumptions break down. Vega and Altair are relatively bright “normal” stars, but Deneb is a “blue supergiant” star that shines with the equivalent light of 100,000 suns! Its distance is some 100 times farther away than Altair, so the light you see from it tonight started its journey toward us when Constantine was Emperor of Rome. And we’ve barely ventured into the vast starfields of the Milky Way.
from The Sky This Week