Back in January we saw how to find Orion and some winter constellations. In April we found Leo and the spring constellations. I guess, now that it’s summer, we should look at what’s in the summer sky. Most amateur astronomers would suggest the way to do this is to start by recognizing the three bright first magnitude stars of the “summer triangle”.
Before seeing how to find these stars in the sky it might be interesting to explain the rather odd system astronomers use to classify the brightness of stars. The second century BCE Greek astronomer Hipparchus made the first extensive catalog of stars. He named the 20 brightest stars as being of the first magnitude. (From magnus, meaning greatest, think magnificent) By 100 CE, Ptolemy had expanded the system to include all the visible stars. 2nd magnitude stars were dimmer than first magnitude, and the dimmest naked eye stars were 6th magnitude. We now know that the dimmest stars we can see are actually 100 times dimmer than the brightest stars, so each magnitude difference is 2.51 times brighter (or dimmer). (Why? 6th magnitude is 5 magnitudes dimmer than 1st magnitude. Try multiplying 2.51 times itself 5 times)
Now, back to the task of finding the “summer triangle” stars in the sky. The easiest star to find is Vega in the constellation Lyra, the Harp. Look high in the northeastern sky about 9 PM. It is the brightest star in the summer sky at magnitude 0.04. (The smaller the magnitude number, the brighter the “apparent” magnitude) Vega is 25 light-years away and is 60 times more luminous than our sun. (Luminosity is also called the “absolute” magnitude). The relative nearness and luminosity make Vega the 4th brightest star in the sky.
To find the second star of the summer triangle look lower and a bit east from Vega. The next bright star in this direction is Altair in the constellation Aquilla, the Eagle. Altair is somewhat closer than Vega at 17 light-years. However, it looks a little dimmer than Vega because it is only 10 times more luminous than our Sun. Altair is the 12th brightest star in our sky. Altair is also interesting because it is spinning so fast that it is not spherical in shape. It is oblong. You can’t see this with your eye, but we can tell this is true by analyzing the light with an instrument called a spectroscope. (Spectroscopy is a fascinating topic to take up in another episode)
The third star of the summer triangle is Deneb in the constellation Cygnus, the Swan. To find it from Vega, look down and to the left (northeast). Deneb is the dimmest appearing of the three summer triangle stars at magnitude 1.3, but in fact, it is the most luminous of the three. It is 170,000 times more luminous than our Sun. It just appears dim because it is 200 times farther away at 2,500 light-years. A star’s apparent brightness depends on both its luminosity and its distance. Deneb is 200 times the diameter and 20 times the mass of our Sun. It is also young and has a planet-forming disk. More on recognizing the summer constellations next time.
Questions or comments: James Hill, Mississippi NASA/JPL Solar System Ambassador. jhill6333@gmail.com
