X-stuff and Planet Nine

The most intense X-ray source these nights is in Cygnus the Swan low in the west, a black hole labeled Cygnus X-1. (NASA/Wendy Stenzel)

Coming up is the only holiday with an X in its name, which makes us now explore that letter’s link to the universe – and how X-stuff includes totally amazing objects.

This X-thread is easy to follow. We sometimes spell Christmas with an X because it used to be considered sacrilegious to write out Christ’s name, or even the word God. The letter X then came to denote any deep mystery. On maps, it represented treasure in a hidden location. In math or in the sky, it depicted an unknown.


That’s why “Planet X” was the long-sought world beyond Neptune. When Pluto was found in 1930, some shifted the hunt to a new Planet X imagined to lie farther still – although recently the fashion has changed and the surmised unseen out-there world is now usually called “Planet Nine.”

Everyone’s aware of X-rays, named by their discoverer, Wilhelm Roentgen, in 1895. It’s an unnecessarily mysterious term for a section of the electromagnetic spectrum. X-rays are just a form of light that we cannot see, for a couple of interesting reasons: First, the sun emits very few X-rays. Second, our atmosphere blocks whatever X-rays do come our way, so that none occur naturally here on Earth. Third, our eyes were naturally designed to see photons bouncing off various surfaces around us. Since no X-rays are bouncing around, it would have been pointless for vision to be able to perceive them. And fourth, that’s just fine, because their high energy, zipping completely through soft ocular tissues and out the other side, would be extremely challenging to detect anyway.

Astronomers use the letter X followed by a constellation name to represent objects emitting X-rays – a rarity in the cosmos. The most intense X-ray source these nights is in Cygnus the Swan low in the west, a black hole labeled Cygnus X-1. In the spring sky, an even-more-intense X-ray emitter is the awesome supermassive black hole weighing the same as 10 billion Suns, sitting at the center of the weirdly massive galaxy M87. That one is called Virgo X-1.

Back to the sky: Each year’s Christmas (or Xmas or Holiday) “star” is often considered to be whatever star or planet is brightest. This year it’s Venus, low in the southwest as soon as night falls. It gives off a strangely steady light that doesn’t twinkle like normal stars. That’s because, although it looks pointlike to the naked eye, it has got a little size to it, which gives it immunity from having its light bent one way and then another by different-temperature air layers. So, it’s steady and bright.

As to why the Evening Star always seem so enchanting: It’s a curious X-factor that bonds us with skygazers through the ages.

Note: I recently mentioned that this Night Sky column has been continuously running for 47 years. It’s actually 44 or 45 years. Not sure, because no one seems able to confirm whether it started in 1974 or 1975.

Want to know more? To read Bob’s previous columns, click here. Check out Bob’s podcast, Astounding Universe, co-hosted by Pulse of the Planet’s Jim Metzner.