Tag: Earendel

Visionaries Old And New

On By rolcottIn Astronomy, UncategorizedLeave a comment

Cathleen’s back at the mic. “Let’s have a round of applause for Maria, Jeremy, Madison and C‑J. Thank you all. We have a few minutes left for questions… Paul, you’re first.”

“Thanks, Cathleen. A comment, not a question. As you know, archeoastronomy is my specialty so I applaud Jeremy’s advocacy for the field. I agree with his notion that the Colorado Plateau’s dry, thin air generally lets us see more stars than sea‑level Greeks do. When I go to a good dark sky site, it can be difficult to see the main stars that define a constellation because of all the background dimmer stars. However, I don’t think that additional stars would change the pictures we project into the sky. Most constellations are outlined from only the brightest stars up there. Dimmer stars may confuse the issue, but I very much doubt they would have altered the makeup of the constellations a culture defines. Each culture uses their own myths and history when finding figures among the stars.”

“Thanks for the confirmation from personal experience, Paul. Yes, Sy?”

“Another comment not a question. I’m struck by how Maria’s Doppler technique and Jeremy’s Astrometry complement each other Think of a distant stellar system like a spinning plate balanced on a stick. Doppler can tell you how long the stick is. Astrometry can tell you how wide the plate is. Both can tell you how fast it’s spinning. The strongest Doppler signal comes from systems that are edge‑on to us. The strongest Astrometry signal comes from systems we see face‑on. Those are the extreme cases, of course. Most systems are be at some in‑between angle and give us intermediate signals.”

“That’s a useful classification, Sy. Madison’s and C‑J’s transit technique also fits the edge‑on category. Jim, I can see you’re about to bust. What do you have to tell us about?”

“How about a technique that lets you characterize exoplanets inside a galaxy we see as only a blurry blob? This paper I just read blew me away.”

“Go ahead, you have the floor.”

“Great. Does everyone know about Earendel?” <blank looks from half the audience, mutters about ‘Lord Of The Rings?’ from several> “OK, quick refresher. Earendel is the name astronomers gave to the farthest individual star we’ve ever discovered. It’s either 13 or 28 billion lightyears away, depending on how you define distance. We only spotted it because of an incredible coincidence — the star happens to be passing through an extremely small region of space where light in our general direction is concentrated thousands‑fold into a beam towards us. Earendel may be embedded in a galaxy, but the amplification region is so narrow we can’t see stars that might be right next to it.”

<Feder’s voice> “Ya gonna tell us what makes the region?”

“Only very generally, because it’s complicated. You know what a magnifying lens does in sunlight.”

“Sure. I’ve burnt ants that way.”

“… Right. So what you did was take all the light energy hitting the entire surface of your lens and concentrate it on a miniscule spot. The concentration factor was controlled by the Sun‑to‑lens‑to‑spot distances and the surface area of the lens. Now bring that picture up to cosmological distances. The lens is the combined gravitational field of an entire galaxy cluster, billions of lightyears away from us, focusing light from Earendel’s galaxy billions of lightyears farther away. Really small spots at both ends of the light path and that’s what isolated that star.”

“That’s what got you excited?”

“That’s the start of it. This new paper goes in the other direction. The scientists used brilliant X‑ray light from an extremely distant quasar to probe for exoplanets inside a galaxy’s gravitational lens. Like one of your ants analyzing sunlight’s glare to assess dust flecks on your lens. Or at least their averaged properties. A lens integrates all the light hitting it so your ant can’t see individual grains. What it can do, though, is estimate numbers and size ranges. This paper suggests the lensing galaxy is cluttered with 2000 free‑floating planets per main‑sequence star — stars too far for us to see.”

~~ Rich Olcott

  • Thanks to Dave Martinez and Dr Ka Chun Yu for their informative comments.

A Thumbtack in A Needlestack

On By rolcottIn Astronomy: Techniques, James Webb Space Telescope, Physics: Light and Relativity, UncategorizedLeave a comment

“What’re the odds?”

“Odds on what, Vinnie?”

“A gazillion galaxies out there, only 41 lensing galaxy clusters, but one of them shows us a singleton star. I mean, what’s special about that star? What are the odds?”

I can’t help it. “Astronomical, Vinnie.”

Cathleen punches my shoulder, hard. “Sy Moire, you be ashamed of yourself. That pun was ancient a century ago. Vinnie, the odds are better than they seem. We didn’t just stumble on Earendel and the Sunrise Arc, we found them in a highly targeted Big Data search for things just like that — objects whose light was extremely stretched and also gravitationally bent in our direction. The Arc’s lensing galaxy cluster has a spherical effect, more or less, so it also acts on light from other far-away objects and sends it in other directions. It even bends an image of our Milky Way towards Earendel’s galaxy.”

“I call weaseling — you used ‘more or less‘.”

“Guilty as charged, Vinnie. A nice, spherical black hole is the simplest case of gravitational lensing — just one mass at the center of its simple light‑bending gravity field. Same thing for a single star like our Sun. Clusters are messy. Tens or hundreds of billion‑star galaxies, scattered at random angles and random positions about their common center of mass. The combined gravity field is lumpy, to say the least. Half of that research paper is devoted to techniques for estimating the field and its effects on light in the region around the Arc.”

“I guess they had to get 3D positions for all the galaxies in the cluster. That’d be a lot of work.”

“It would, Al, but that’s beyond what current technology can do. Instead, they used computer models to do — get this, Sy — curve fitting.”

<chuckle> “Good one, Cathleen.”

“What’s so funny?”

“There’s a well-established scientific technique called ‘curve fitting.’ You graph some data and try to find an equation that does a respectable job of running through or at least near your data points. Newton started it, of course. Putting it in modern terms, he’d plot out some artillery data and say, ‘Hmm, that looks like a parabola H=h+v·t+a·t2. I wonder what values of h, v and a make the H-t curve fit those measurements. Hey, a is always 32 feet per second per second. Cool.’ Or something like that. Anyhow, Cathleen’s joke was that the researchers used curve fitting to fit the Sunrise Arc’s curve, right?”

“They did that, Sy. The underlying physical model, something called ‘caustic optics,’ says that—”

“Caustic like caustic soda? I got burnt by that stuff once.”

Image by Heiner Otterstedt,
under the Creative Commons Attribution-Share Alike 3.0 Unported license

“That’s the old name for sodium hydroxide, Vinnie. It’s a powerful chemical and yeah, it can give you trouble if you’re not careful. That name and caustic optics both come from the Greek word for burning. The optics term goes back to using a lens as a burning glass. See those focused patterns of light next to your water glass? Each pattern is a caustic. The Arc’s lensing cluster’s like any light‑bender, it’s enclosed in a caustic perimeter. Light passing near the perimeter gets split, the two parts going to either side of the perimeter. The Earendel team’s curve‑fitting project asked, ‘Where must the caustic perimeter be to produce these duplicate galaxy images neighboring the Arc?‘ The model even has that bulge from the gravity of a nearby foreground galaxy.”

“And the star?”

“Earendel seems to be smack on top of the perimeter. Any image touching that special line is intensified way beyond what it ought to be given the source’s distance from us. It’s a pretty bright star to begin with, though. Or maybe two stars.”

“Wait, you don’t know?”

“Not yet. This study pushed the boundaries of what Hubble can do for us. We’re going to need JWST‘s infrared instruments to nail things down.”

Al’s in awe. “Wow — that caustic’s sharp enough to pick one star out of a galaxy.”

“Beat the astronomical odds, huh?”

Adapted from a public-domain image.
Credit: Science: NASA / ESA / Brian Welch (JHU) / Dan Coe (STScI); Image processing: NASA / ESA / Alyssa Pagan (STScI)

~~ Rich Olcott

A Needle in A Needlestack

On By rolcottIn Astronomy: Techniques, James Webb Space Telescope, UncategorizedLeave a comment

“How’d they find that far-away star, Cathleen? Seems like you’d have to know just where to point your telescope.”

“It’s worse than that, Al, first you’ve got to find that telescope, or more precisely, its lens. We can’t simply swing a black hole or galaxy cluster into position for a good look at something interesting. No, we have to discover lensing objects that magnify good stuff beyond them. The good news is that some of those are out there, but the bad news is that the sky is cluttered with far more objects that don’t play the game we want. This research team appears to have hit paydirt but they did it with humungous power shovels and heavy‑duty panning techniques.”

“Impressive metaphor, Cathleen. Could you un‑metaphor it for us?”

“Sure, Sy. The power shovels are Hubble and Spitzer, both of which piled up beaucoodles of data from decades of infrared observing time.”

“I thought Hubble was designed for visible and UV surveillance.”

“It is, mostly, but since 2009 its instrument suite included WFC3, a camera that’s sensitive out to 1700 nanometers and covers a square 2 arcminutes on a side. That’s a lot, by big‑telescope astronomy standards.”

“Wait, arcminutes?”

“That’s right, Mr Feder. We astronomers have trouble with distances but we’re good at measuring angles. The Moon’s about a degree across. One degree is sixty arcminutes, next step down is sixty arcseconds per arcminute. After that we go semi‑metric, milliarcseconds and so forth. One WFC3 pixel records a patch of sky 130 milliarcseconds across. JWST‘s NIRCam instrument has a resolution twice as sharp. Anyway, Hubble‘s 1700‑nanometer limit is plenty good enough to pick up 120‑nanometer hydrogen light that’s been stretched out by a factor of z=2.8. Distance and stretch correlate; the lens that highlighted Earendel and its Sunrise Arc for NASA and Vinnie is that far away.”

“How far away?”

“It’s tricky to answer that. The spectra we see let us measure an object’s z‑factor, which by way of the Doppler effect tells us how fast the object is moving away. Hubble’s constant ties that to distance, sort of. My convenient rule of thumb is that an object whose z is near 2 is running away at 80% of lightspeed and on the average is about 55 trillion lightyears from us but don’t quote me because relativity complicates matters. Using the same dicey calculation I estimated the lens and Earendel velocities at 87% and 96% of lightspeed, which would put their ‘proper distances‘ around 60 and 66 trillion lightyears away. And no, I’m not going to go into ‘proper distance‘ versus ‘comoving distance‘.”

“Let’s get back to your metaphor, Cathleen. I get that Hubble and Spitzer and such generated a ton of data. What’s the panning part about?”

“Well, in the old days it would have been hired hands and graduate students spending years peering at dots on photographic plates. These days it’s computers, thank Heaven. The research team used a series of programs to filter their digital data. The software had to decide which dots are stars or noise specks and which are galaxies or arcs. Then it picked out the reddest red galaxy images, then clusters of galaxy images at the same redness level that are near each other in space, then clusters with arcs around them. I said that WFC3 covers a square 2 arcminutes on a side, remember? The sky, both hemispheres, contains almost 2½ million squares like that, although the surveys didn’t get all of them. Anyhow, after burning through cubic acres of computer time the team found 41 deep red lensing clusters.”

“Only 41.”

“Yup.”

We ponder that for a minute, then Vinnie pipes up. “Wait, the dots are in color?”

“No, but these images are generally taken through a filter that transmits only a known narrow wavelength range, infrared or whatever. Using relative dot intensity at several different wavelengths you can create ‘false color‘ images. When you find something, you know where to point spectroscopic tools to be sure you’ve found the good stuff.”

“Like a star shining less than a billion years after the Big Bang.”

“Paydirt.”

Image adapted from NASA and STScI

~~ Rich Olcott