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“Follow The Arc To Arcturus . . .”

The above phrase is one of the most often quoted chestnuts of astronomical navigation and applies to evening observers from April through July. Actually that phrase is only half of the full saying; I’ll tell you the second half a little later. Look at the curve formed by the “handle” stars of The Big Dipper. Mentally extend this curve until you encounter a very brilliant reddish star Arcturus, one of the brightest stars in the night sky.

Yes, stars can indeed have color! For example, our Sun (the closest star to Earth) is classified by astronomers as a yellow dwarf. This does not mean it is truly yellow, but that its light has more yellow in it than any other color. Though it is 1,300,000 times larger than the earth, the sun is relatively small compared to a star such as Arcturus. That is the reason for its humbling “dwarf” status. If Arcturus were suddenly shifted to the center of the solar system where the Sun resides, the orbits of Mercury, Venus, Earth and Mars would be swallowed up beneath its surface! Needless to say the Earth and we would not be here.

Figure 14 shows you how to follow the arc to Arcturus in the kite shaped constellation of Boötes (pronounced Boh oh teez). Boötes was The Bear Driver of ancient mythology, forever chasing Ursa Major and Ursa Minor around the sky. Arcturus is the brightest star in Boötes. It also has a more formal scientific name: Alpha Boötis. The brightest star in any constellation is always given the designation Alpha which is the name of the first letter in the Greek alphabet. The second brightest star is named Beta, that is, the second letter in the Greek alphabet. I guess you see where this is going? Ever fainter stars are given the appropriate Greek letter. On star charts you will not see Alpha, Beta, Gamma, etc. spelled out in English; instead you will see the actual Greek letter α, β, γ, etc. The fact that the constellation’s spelling ends with the letters ‘is’ in the star name rather than ‘es’ may seem strange to you - I’ll explain this later.

Why does Arcturus appear so small to us while the sun seems so large? It is because Arcturus is much farther away from us than the Sun is. Notice how much brighter Arcturus appears compared to Polaris or the stars in the Big Dipper. Now is a good time to introduce you to the brightness scale used by astronomers. Each level of brightness in the scale is called a magnitude. Most stars visible to the naked eye lie in a brightness range from zero to six. Zero magnitude stars are very bright while stars of magnitude six are the very faintest stars visible to a normal naked eye at a totally dark site. There are even some extremely bright stars that sport negative number magnitudes! In Figure 14 (as in all of the star charts in this book) the bigger the dot representing a star, the brighter the star. See how large the dot representing Arcturus is? Also stars within the same constellation will have solid lines drawn between them. Should you see dashed lines, they will indicate a line that points to an object of interest.

Arcturus is a magnitude 0 star, while the brightest stars in the Big Dipper are of magnitude 2. Want a good example of a magnitude 1 star? Well ...