Two old Suns

Next time you're observing the Turtle nebula in Hercules' belly, point your telescope slightly to the south and you'll bump into this lovely pair. STF2094 is a couple of greenish-yellow giant stars over 500 lightyears away. The term "giant" seems somewhat overrated at first sight since the biggest of the two's just 3,8 times the diameter of our Sun and contains only 1,3 solar masses. The giant classification in this case doesn't concern the star's mass or diameter, but rather its luminosity and the phase of stellar evolution. Originally these stars were not much unlike our Sun and they used to be much dimmer too. Then, suddenly, they ran out of hydrogen and it's exactly the change into a helium-fusing red giant that we're witnessing here, something that will also happen to our Sun in over 5 billion years. The transition into a cool, red giant isn't instantaneous and initially the star in question will not only brighten but also heat up slightly. The surface temperature of the two stars on my sketch measures almost 1.000°C hotter than our Sun. A famous example of a star in this phase of evolution is Capella, the brightest star of the winter constellation of Auriga and the sixth brightest star in our sky. When the star expands further, its surface will gradually cool down and the star becomes an old, red giant. 

The couple on my sketch is therefore quite interesting because they used to be so similar to our Sun and because they seem so close to one another. Well, you have to interpret the word "close" in astronomical terms because in reality these two stars are 171 times the distance between the Earth and the Sun apart, or almost six times the distance to Pluto! And yet, even from this distance, these stars would appear hundreds of times brighter than a full Moon to each other!

This double star represents a bit of a challenge for small to medium telescopes and you need a good-quality sky in order to separate them. Then again, that's precisely what double star fanatics are looking for or course. 

But... wait a minute! Before you go, focus on the upper-right corner of the field of view. Isn't there... something... ? Yes! The faint patch you may notice is a spiral galaxy (catalogued as UGC10525), quite similar to our Milky Way, but very distant. It lies 430 million lightyears away...