The stars of summer have returned! Step outside after dark and look east, about halfway up the sky. You’ll see three bright stars that form a triangle. If you’re not sure which three stars, hold out your hand at arm’s length, palm out (like you’re stopping traffic). The three bright stars we’re focusing on will fit neatly around your hand and fingers with room to spare. They are Deneb, Vega, and Altair, the stars of the Summer Triangle.
The Summer Triangle is an asterism — stars that create a pattern apparent to the human eye, but that are not an “officially” recognized constellation. (The International Astronomical Union, the body that decides such things, is not to be trifled with.) In the case of the Summer Triangle, the three stars are each members of different constellations. Vega, the brightest of the three, is in Lyra, the Lyre; Altair is in Aquila, the Eagle; and Deneb is in Cygnus, the Swan — my favorite constellation and one of the easiest to visualize. Let’s explore it this month.
Cygnus (swan in Latin) is a beautiful sight. To find it, first locate Deneb; of the Summer Triangle stars, it’s the one to the lower left, and it’s the dimmest of the three (though still brighter than most other stars).
Found it? Deneb is the tail of the swan. Next, just a bit to the south (to your right), look for a line of relatively bright stars, oriented up and down from your point of view. Those are the wings of the swan, spread wide as it soars across the summer sky.
Then look further south (to the right) and you’ll see the swan’s long graceful neck. It ends at a bright star called Albireo, which marks the swan’s head. Take a moment to trace the lines of Cygnus with your mind’s eye; few constellations look so much like what they’re named for.
I would be remiss if I did not mention that Cygnus is also known as the Northern Cross (there’s also a Southern Cross, visible from the Southern Hemisphere). But once you see it as a swan, I’m confident you’ll agree that Northern Cross is a woefully prosaic name for such a pretty constellation.
Besides its aesthetic appeal, Cygnus contains wonders invisible to the unaided eye. A telescope can show you the Veil Nebula, the North American Nebula, and a portion of the Great Rift. These are vast clouds of interstellar dust spanning thousands of light years. But the most mysterious denizen of Cygnus is invisible to amateur telescopes: Cygnus X-1, the first black hole to be discovered.
In 1964, astronomers detected a strong X-ray source in Cygnus that they could not correlate with any visible object; they named it Cygnus X-1. Subsequent observation and analysis led to only one possible explanation: a black hole, which until then had been predicted by theory but never observed, even indirectly.
Black holes like Cygnus X-1 (there are other types) are the remnants of large stars that have exploded as supernovae. They pack an incredible amount of mass into the small space they occupy. And the more massive an object, the stronger its gravity.
Black holes are so massive that as anything draws near, the escape velocity — the speed needed to overcome that object’s gravity and fly off somewhere else — eventually exceeds the speed of light.
Imagine traveling toward it in your own little spaceship. Since nothing can travel faster than light, once you cross this invisible line, that is, once you get too close, you cannot return. Not even light itself can escape. The name for this line is the event horizon. Events inside this line (actually a sphere around the black hole) cannot affect or be observed by those outside it.
So how did scientists find Cygnus X-1? Black holes are surrounded by disks of orbiting gas and dust. As this matter spirals in closer to the black hole, it accelerates and heats up; as it gets hotter, it begins to emit powerful X-ray radiation, which is still able to escape because the matter emitting it has not yet reached the event horizon. Astronomers can observe this radiation with various telescopes and detectors.
In other words, though we can’t see the black hole itself, we can see the death throes of the matter surrounding it, before that matter crosses the event horizon and — who knows?
We cannot yet describe what happens within a black hole; there, our laws of physics fail us. (For a fun and highly speculative jaunt across an event horizon, check out the movie Interstellar.)
As summer goes on, the Summer Triangle will rise earlier each night. By summer’s end, Cygnus will be high overhead at nightfall. If you’d like to look oblivion in the eye, begin at Albireo, the head of the swan, as described earlier. Move your eyes back along the swan’s neck about halfway towards its wings, then a bit north, above the neck. There. See it? That totally empty patch of space? That nothingness is Cygnus X-1.
When cruising the galaxy in your spaceship, always observe the minimum posted speed, and steer clear of event horizons. Clear skies!