In the Serpens constellation, about 1,400 light years from Earth, a fledgling star flaps its wings.
OK, I’m being a little poetic there. It is more like a planet in formation gravitationally deformed from its circumstellar disk into a quadrupole shape, so that the shadow cast by the star on the nearby nebula looks like swinging wings.
OK, I’m being a little too much prosaic there. How about we commit to a picture and video of this strange and extremely cool object from the Hubble Space Telescope?
This image, taken in near infrared light, shows a star-forming region in Serpens. Many of the stars you see here are quite young, some are only a couple of million years old. You can also see a lot of gas and dust, common where stars are born (after all, that’s what they use as raw material).
In the upper right corner of the center is a bright star called EC 82, located in the center of the brightest area of the nebula. In fact, the star is likely to be massive (2.5 to 3 times the mass of the Sun) and quite bright (it exploits 30 times the energy of the Sun), and is the source of illumination in that part of the nebula, emitting a cold blue .
If you look carefully, you will also see that it is in the center of two diverging cones of darkness, one in the upper left corner and the other in the lower right corner. Those, it turns out, are shadows! Around the young star there is a disk of gas and dust, spinning around it and probably forming planets. The disk is thick enough to block light from the star … so You are actually casting a shadow over the material surrounding the star further!
That’s ingenious, but we have things like that before. What makes this particular case so cool is that two observations taken using Hubble a little over a year apart show the shadow moving!
That video switches between two images taken 404 days apart, repeating itself many times so you can see the movement. It is not often to see such rapid and obvious changes in the sky. And it really looks like a bird or a bat flapping its wings; The two sides of the shadow move up and down together. It turns out that that is important.
What we are seeing is a change in the inner disk around the star, something that happens to change the shadows that are cast. The best assumption from the astronomers who took the data is that the disk of material around EC 82 is not flat, but deformed. That can happen on galaxy disks, where one side is bent up and the other is down.
But that does not explain this motion; you would expect the two sides to move in opposite directions in that case. Instead, astronomers envision the disk having four fins, two folded down and two up, giving it a saddle shape (or, if you prefer, like a Pringles chip).
While it is unclear exactly how this shape would emerge, they speculate that a planet orbiting the star at a slight angle could gravitationally pull the disk due to the trick. The disc itself moves up and down, changing the shadows cast in space. It’s like an anti-beacon!
Coooool
They also point out that since a shape like this has never been seen before, there could be a different cause. One idea is that the star is actually a binary, and the two stars orbit each other perpendicularly to a relatively flat disk. Also, one of the stars is much brighter than the other, making it the light source for shadows. As the star rises and falls relative to the disk, the shadow changes. This also explains what we see here, but such a binary orbit is unlikely: Physics makes the orbit much more likely to be in the plane of the disk. Also, only one star is detected there, so they think the deformed disk is the best explanation.
If there is a planet warping the disk, it probably orbits EC 82 about once every 180 days or so, and would be about the same distance from the star as Earth from the Sun (EC 82 is more massive than the Sun, then the planet would orbit faster).
The scale of this thing is quite large. The best-defined part of the shadow stretches for about 45 light days (more than a trillion kilometers!) On either side, although they can trace the shadow to about twice that distance. This means that the finite speed of light makes a difference! If, for example, the star were to briefly light up, getting much brighter, you’d see that bright flash moving across the shadow (probably looking like a glow at the top and bottom like the Jacob’s Ladder), taking 45 days to move through it. The fact that the two sides of the shadow move together means that any movement is slow compared to that time, which is how they got that 180-day period from the planet (if there are four deformations on the disk, then the shortest period for the planet it is 4 x 45 days = 180 days).
Unfortunately, there are no other observations available to pinpoint the period of this, or to see what other changes might be occurring on shorter time scales. Hubble is too busy to revisit this object often enough. That’s too bad. The shadow magnifies the disc in a certain sense; a bump on the disk too small to see directly would cast a shadow large enough to be detected, so this would be a way of seeing a small-scale structure on it, and See them change over time! What an opportunity!
Ah good. We’ll just have to launch more space telescopes to look at it. However, that may take a while. Hopefully, in the meantime, this quirky item will continue to flap its wings, going nowhere.
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