When the stars align ... well, one star with a hundred billion stars.

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Coincidences happen. Even in space.

Or, at least, when we look into space from Earth. The sky is big, and you can only see a few thousand stars by eye, so honestly without optical aid you don’t see many that are super close together.

But part of the main function of a telescope is to gather light, collecting it like rain in a bucket. This makes fainter objects look brighter, so when you use a telescope much dimmer stars become visible; even a small one allows you to see millions. When you use something like Hubble, that number can get substantially larger. That makes the sky more crowded.

And it’s not just stars. Only a handful of galaxies are visible to the eye, but with even a small ‘scope you can see hundreds. Hubble? Billions. Billions.

So if you look long enough, you’re bound to see some overlap.

And that brings us to HD 107146 ... but, before we get to the coincidence, let me explain some fun stuff.

The star is relatively nearby, about 90 light-years from us. It’s a “solar analog,” a star much like the Sun in mass, size and temperature. But there’s a big difference: It’s very young, just 100 million years old or so (the Sun is 45 times older than that).

Stars that young are sometimes surrounded by “debris disks,” material left over from their formation. And yup, HD 107146 has one as well. It was discovered in 2004, but the best image of it was made using Hubble in 2011:



The star is very bright, a million or more times brighter than the disk of material around it, so the astronomers who took the data (including my old friend Glenn Schneider) were clever: First they placed the star behind a metal occulting bar, literally a bar of metal inside the camera (my old camera, STIS!) that blocks the light from the core of the star. They also took multiple images, rotating the observatory between each observation; that helps reduce noise in the image. Finally, they also did exactly the same thing with another star that happens to be very similar to HD 107146 (but lacks the disk), and they subtracted those images from the ones of HD 107146, further reducing the light from the star.

That leaves an odd-looking image of the star, with the center blacked out from the occulting bar and lots of radial spikes from light scattered and diffracted by Hubble’s optics. But it blocks the vast majority of the starlight, allowing the far fainter disk to appear. That’s the wide band of light circling the star.

This all, by itself, is really interesting! Better yet, the material is not so much a disk as it is a ring; that annulus is real, with less light closer to the star. The ring is huge, starting about 7 billion kilometers from the star and stretching out to well over 20 billion km. Neptune’s orbit around the Sun is only about 5 billion km, so this ring is like our own Kuiper Belt, the region past Neptune populated by smaller icy bodies. (Pluto is likely the biggest of these objects).

Observations using the fantastic radio telescope array ALMA indicate this material may be small grains of dust, which you’d expect if Pluto-sized objects formed out there and started smashing into one another and any planets forming there. The ALMA data also indicate the presence of a gap in the ring, too narrow to show up in the Hubble images, about a billion km wide, which may have been carved out by the presence of a newly formed planet there. If so, it would have several times Earth’s mass.

But there’s more! And now we get to the fun coincidence.

See that blob to the lower right of the ring? That’s not a part of the ring, or even the star. That’s a galaxy, far, far in the background, likely hundreds of millions of light-years away. It was first seen in 2004, when the disk itself was discovered in a different set of Hubble observations:



By coincidence, the star happens to lie very nearly along our line of sight to the galaxy, so they appear very close in the sky despite their mind-crushingly different distances. Now take another look at the two images ...here, I’ll put them side by side to make it easier for you:



Notice anything? The galaxy is closer to the star in the 2011 image than it was in 2004! Or more accurately, the star is closer to the galaxy. That’s because the star is moving across the sky!

Sure, the stars rise and set, so there’s a daily (what astronomer’s call “diurnal”) motion. But stars also orbit the center of our galaxy at different speeds, and over time we can see that motion with powerful telescopes. They move relative to one another across the sky, which is “proper motion.” It’s small, but higher for nearby stars due to perspective (just as nearby trees seem to whiz by you as you drive past them, but distant buildings or mountains seem to crawl).

HD 107146 has a decent proper motion, and over the seven or so years between Hubble observations, it moved southwest (to the lower right), bringing it closer to the galaxy’s position.

Extrapolating, the galaxy is already behind the ring right now. In a few more years (around 2020) it’ll be fully behind the ring. The ring, itself, is “optically thin,” which means light can pass through it, so we’ll see the galaxy right through it. It’ll look like a really bright blob in the ring. It’s actually a good thing the galaxy was seen a few years ago; if the first observations were made in 2020, it could’ve been confused for something happening in the ring itself, like a planetary collision!

Given the star’s motion, the ring will be superposed on the galaxy for about a decade before the galaxy slips into the gap between the star and ring.

How cool is that? Answer: Pretty damn cool.

And useful. This is a unique geometry, with a galaxy behind a ring like this. While the ring is optically thin, it’s not transparent. It’ll block some of the galaxy’s light, and that can be measured. Because we have images of the galaxy when it was well outside the thickest part of the ring, measuring how much light is blocked will give some insight into the ring particles’ physical properties. In fact, Glenn and his team have follow-up Hubble observations planned for the next few years to take advantage of this serendipitous situation.

One final bit. What do we call this event? When the Sun is blocked by the Moon, we call that a solar eclipse. When the Moon is blocked by the Earth, we call that a lunar eclipse. So I suppose that this event could be called a “galactic eclipse”.

I love that! It has a Flash Gordon-esque feel to it ... but, unfortunately, it won’t work. In an eclipse, an object has its light source blocked by an intruding object, and the ring is lit by the star. The galaxy is literally millions of times of times farther away, so it can’t block the starlight!

Another type of  astronomical event like this is an occultation, when an object passes in front of another (it’s a more general term for eclipse, I suppose). But again, the ring is mostly transparent and isn’t blocking the light from the galaxy (at least not all of it), so this doesn’t fit, either.

I talked to Glenn about it, and he suggested calling it a transit, which is fair enough. That’s when one body passes in front of another, regardless of whether it’s opaque or not. That works, but dangit. I liked galactic eclipse. Oh, well. The Universe is under no compulsion to obey our etymological astronomical desires.

Still, whatever we call it, this is a fantastic chance to get a wholly new type of observation. I worked on observations of these kinds of disks using STIS back in the day, and I can remember when this field was brand spanking new. Something like this is amazing to me. It’s an actual event where the stars align!

Science! I love this stuff.