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Last Thursday I gave y’all an overview of 3I/ATLAS, the third object confirmed to be passing through our solar system from interstellar space. I actually had a different post ready to go for that day, but the news about ATLAS broke so I had to hastily put together an article about it^*.

As I mentioned in that article, the object was discovered on July 1, and astronomers quickly realized this wasn’t your typical space rock. Its orbit is hyperbolic, meaning it isn’t gravitationally bound to the Sun. That means it either came from interstellar space (its origin is likely another star, though it could possibly have been born on its own in a molecular cloud and ejected by a close pass to a star), or it got close enough to a giant planet in our solar system and got a helluva gravitational assist. However, it didn’t come anywhere near any planets, so it must be from deep space. 

In that article I also noted that it came in pretty much in the plane of the solar system, the plane that most of the planets orbit the Sun in. Some folks wondered if that gave more credence to it having a solar system origin, but it occurred to me that this is still unlikely; it’s moving retrograde, in the opposite direction planets orbit the Sun. Given that it’s screaming past us at 60 kilometers per second — that’s damned fast, way faster than any solar system object — it’s almost certainly not from here. If it were an icy body out past Neptune that orbited in the system plane, it would very likely orbit prograde, in the direction of the planets. So getting it to orbit backwards and at such high speeds strikes me as pretty unlikely.

Speaking of which, a paper came out on Friday (just two days after ATLAS was discovered!) with some fun updates. It came from the direction of Sagittarius in the sky, more or less from the galactic center. That doesn’t mean it traveled all that way — 26,000 light-years — because it could have come from any number of stars near us that happen to lie in that direction. Getting the best candidate star is tough! But it’s possible to narrow the list down, and I fully expect to see a paper about that soon.

Also, I’ve been careful not to call it a comet or asteroid, because last week we really didn’t know. But now several observatories are reporting seeing weak cometary activity, meaning ice on the surface of the object is warming up from sunlight and turning directly into a gas. This surrounds the solid part (called the nucleus) and gets pushed back by sunlight, creating a stubby tail. So now we know it’s a comet!

One thing we’d like to measure is its light curve, the graph of how it changes brightness over time. This can tell us how rapidly it rotates, for one thing, and also how much it changes brightness can tell us about the shape. 1I/’Oumuamua had big swings in its brightness, indicating it was eerily elongated. It’s not clear how that happened, though there are several hypotheses about it. Anyway, ATLAS isn’t changing brightness much. It’s either a very slow rotator, or very spherical, or (most likely) the cometary activity is hiding the brightness changes like a fog hides objects in the distance.

It reaches perihelion in autumn, but on the other side of the Sun from us (though close enough to Mars that some folks are looking into observing it with orbiters there). However, it’ll reappear in November, so we have plenty of time to get a good, long look at it. I am quite sure it’ll be observed by Hubble and JWST (its location in the sky is ideal for them to take a peek, though it’s still pretty far from us) so stay tuned for updates from them.

And, as always, I’ll post updates here when I hear them.

^* So hastily, in fact, that it had a few typos, including getting the designations wrong for 1I/’Oumuamua and 2I/Borisov. Worse, I wrote “aphelion” when I meant “perihelion” in the text, which confused a few folks (in my defense Thursday July 3 was when Earth reached aphelion, so it was on my mind). I’m sorry about that! Note that when I have typos in the email I send out, I usually hear back from folks, so I correct them in the archive. If something doesn’t seem right you can always check there to see if I updated the article.

A tradition I’ve had here on the Bad Astronomy Newsletter for some time is to refresh the banner image on occasion. I usually do it at the new year or when the issue number is divisible by 100. I missed the opportunity to put it in the 900^th issue (see above), but hey, 901 is a fine number as well. So here we are!

At the top of this page (and for at least the next 33 weeks or so of issues) is the new one. For the cosmic object I picked the exquisite Trifid Nebula, a personal favorite. I couldn’t resist the timing; I realized I needed a new banner only a few days ago, and the Vera Rubin Observatory just released an incredible shot of the galactic center that included the Trifid. Kismet!

The Trifid is a star forming region, a gas cloud roughly 35 light-years across and 4,100 light-years from us. Despite that great distance it’s easily visible in binoculars (and just barely kinda sorta visible to the naked eye from a very dark site) because it’s luminous. Over 3,000 stars have been born in it, and more than 450 of them are massive and hot. At the top of that scale, the beefiest stars emit a lot of ultraviolet light that energizes the hydrogen in the cloud, causing it to glow. We call a gas cloud like that an emission nebula. The bluer material around it is gas and dust that reflect the light from the nearby stars; again, because the blue ones are the most luminous, that’s the color we predominantly see in that reflection nebula. 

It’s bright enough that the astronomer Charles Messier included it as the 20^th object in his catalog of things in the sky he didn’t want to confuse with comets, and so it’s also commonly called M20. The Trifid moniker is due to the dark dust lanes that trifurcate it, and not because it’s the origin of flesh-eating mobile plants that can be killed with seawater (spoiler).

The Rubin image is interesting in that it’s almost too bright; the colors and intensity are a little saturated so it can be difficult to see some of the details. Astrophotographer Derek Demeter has a lovely shot of it online you can peruse, and compare to the Rubin one, especially the wide-field image that includes the Lagoon Nebula. Back on The Old Blog™ I also wrote about what the Trifid looks like in infrared light, which may surprise you.

Image processor Roberto Colombari also sent me a version of it he made from combining Rubin data with observations from the Japanese Hyper Suprime Camera on the Subaru Telescope:

Wow! There’s a much larger version, too, where you can see fantastic details in the nebula.

I’ve observed the Trifid many times myself with binoculars from dark sites, and with my own telescope as well. That whole area of the sky is magnificent, since we’re looking toward the center of the Milky Way in Sagittarius. You can just scan around and see one delight after another. It reminds me of fair summer nights at the eyepiece, enjoying the treasures the sky brings to us, and that, I think, makes it a fitting subject for, appropriately enough, the last three-digit numbered series of issues’ banner.

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