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Celestial sleuthing around a spectacular spiral galaxy
In the late 18th century, comet hunter Charles Messier was becoming irritated. He scanned the skies with his telescope looking for the fuzzy visitors, but kept getting fooled by other fuzzy objects that didn’t appear to move from night to night. Frustrated, he created a list of these distractions so that in the future he wouldn’t mistake them for comets.
Messier may not have invented irony, but he sure gave us a great example of it. The Messier Catalog, as we now know it, compiles 110 of the most glorious deep-sky objects you can see through the telescope and is a staple of amateur astronomers the world 'round. Star clusters, supernovae remnants, nebulae and quite a few spectacular galaxies make up the catalog.
The 77th object on his list is a spiral galaxy that is located just under 50 million light-years from us, and it’s gorgeous. Don’t believe me? Well then, see for yourself:
[VLT image of the spiral galaxy M77. Credit: ESO. Click here to embiggen.]
Oh, yeah. Now that, there, is a spiral.
That image was taken using Antu, one of the four monster 8-meter telescopes making up the Very Large Telescope in Chile. It’s comprised of images taken in four filters: blue, yellow, red, and another called H-alpha, which brings out the light from hydrogen gas (in order, they’re displayed as blue, green, orange, and red).
The red marks the location of vast clouds of hydrogen gas, glowing due to the energy of massive, hot stars newly born inside them. The blue glow of these stars can be seen throughout the galaxy, too.
The core of this galaxy is odd. Can you see how, at the very center, it’s quite bright? That’s because M77 is an active galaxy. Every big galaxy we see has a supermassive black hole at its heart, formed along with the galaxy itself. Our Milky Way has one, 4 million times the mass of the Sun. But ours is quiescent, calm. It’s not feeding on any material falling into it.
M77’s central black hole, on the other hand, is quite eagerly consuming material. As this stuff falls into the black hole, it heats up tremendously, and glows furiously before it takes that last step from which there is no return, into the maw of the black hole. This powerful glow from the material is what makes the center of M77 so bright.
As I gazed at this image in awe, I noticed something else odd. There’s a very bright star to the left of the galactic center; that’s actually a star in our own galaxy, shining brightly in the image because it’s much closer to us than M77. But look just to its left and little below: There’s a multi-colored streak there, going from blue to green to red.
What’s that?
As soon as I saw it, I had an idea: It must be a satellite, a human-made object orbiting our own planet. These sometimes move across astronomical images, leaving streaks during the long exposures. In this case, the observation of M77 was taken using different filters, so during each exposure as the satellite moved it left a different color streak (it’s missing one color, so perhaps that observation was taken at a different time).
But even then, I was baffled for a moment. Why are the streaks so short? Usually exposure times are long, so the streaks are, too. Even an exposure time of a minute should result in a streak going clear across the image!
What kind of satellite leaves a short streak? Well, I reasoned, one that doesn’t move quickly. Aha! This must be a geosynchronous satellite, one that orbits over the Earth’s equator about 40,000 km up. A satellite in that orbit takes 24 hours to orbit the Earth once, the same time the Earth spins once. From their point of view, they see the same face of the planet, making them useful watchdogs on weather, or for communications. From our point of view on Earth, they appear to stay in one point in the sky, moving only a little bit if their orbit is tilted a bit to the equator or is not perfectly circular.
Wondering if I was right, I checked the sky coordinates of M77. To my delight, I found it to be at a declination (a celestial version of Earthly latitude) of almost exactly 0°: precisely above the Earth’s equator!
Aha again! That’s exactly where you’d expect to see a geosynch satellite, so I’m pretty sure my guess is correct.
[Hubble has a higher resolution than VLT, so you can see more detail in this image, but it also has a smaller field of view, so you don't get the same context. Credit: NASA, ESA & A. van der Hoeven. Click here to galactinate.
I’ll admit I’m pretty pleased with myself and my sleuthing. But that’s what science is all about, right? You see something in your observations, you wonder what it is, and you come up with an explanation. You can then ask what predictions you can make, and then test them to see if they pan out.
And mine did!
But could I be wrong? Sure! Another idea is that the streaks are from an asteroid, a smallish rock orbiting the Sun. It, too, would leave short multi-colored streaks in the image. Asteroids tend to stick to the plane of the solar system, though, and M77 is about 15° off that plane. Not hugely far, and it’s entirely possible to find an asteroid out there. But the exposure time for these images is pretty short, and an asteroid wouldn’t move much in that time.
It’s not impossible that this is an asteroid, but I lean more toward it being a satellite. Most likely I’ll never know, which is OK in this case; it’s not of cosmic importance. But the point is, it’s fun to investigate, to examine these images closely, and to wonder what’s in them. There is beauty and art to science, as this image makes obvious.
But also? It’s so much fun.
[Update (July 7, 2017 at 17:15 UTC): The plot thickens! BA reader Shounak Mitra noted that during the exposure, the telescope would have guided on the galaxy, which means the satellite would have moved relative to the telescope. I don't know how long the exposures were, but that could mean the streak is too short for a geosynch sat. Morever, via email Rick Johnson mentioned that I forgot something: Parallax! From Chile, the geosynch sat position in the sky would actually be a few degrees north of the celestial equator due to the southern latitude of the observatory. He's right; I didn't think of that. So it seems more likely this could be an asteroid and not a satellite. I feel obligated to add that this is delightful; rather than fret about being wrong, I'm actually pretty tickled that there's even more detail going on here than I suspected, and it's important. Science!]
[Update 2 (July 7, 2017 at 22:00 UTC): My friend and master astrophotographer Adam Block put the nail in the coffin on this one: He told me a geosynch sat would move really far in even a few seconds of exposure, making it certain this is not a satellite. There would also be a bigger gap between color streaks, given the time it takes to switch filters between exposures. So this must be an asteroid. Or at least not a satellite. If I had the date and time of these observations it would be possible to figure out what asteroid it is, too. I'll see what I can find, but no promises!]
