Vega, a Closer Look: Smooth Disc, No Planets, Starspots

A little over a week ago, scientist published a detailed analysis of Vega’s surprisingly planet-free debris disc.

Vega, one of the brightest stars in Earth’s sky, may have planets: but the October 31 paper rules out any Saturn-size or larger worlds in wide orbits. That reminded me of a Sherlock Holmes quote:

“‘Is there any point to which you would wish to draw my attention?’
‘To the curious incident of the dog in the night-time.’
‘The dog did nothing in the night-time.’
‘That was the curious incident,’ remarked Sherlock Holmes.”
(“The Memoirs of Sherlock Holmes”, “Silver Blaze” , Arthur Conan Doyle (1894) Via Gutenberg.org)

More to the point, not finding planets in Vega’s debris disc should help scientists learn more about how stars and planets form. And gives me something to write about.

• Vega Debris Disc: “Smooth, Ridiculously Smooth”
  • Dust, a Gap, and — the “Poynting-Robertson Effect”?
  • A Closer Look at Vega’s Dusty Disc
  • Hubble’s Vega Image
• How We Know What We Know About Vega, and a Little Lore
  • Finding Vega, and a little Skywatching Lore
• Spiky Stars and Fomalhaut’s Planet That Isn’t There (Probably)
  • Diffraction Spikes, a Debris Disc Distraction, Fomalhaut’s Far-Flung System
• 46 Centuries of Thinking About This Universe, Briefly

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Vega Debris Disc: “Smooth, Ridiculously Smooth”

“NASA’s Hubble, Webb Probe Surprisingly Smooth Disk Around Vega”
NASA Hubble Mission Team, NASA (November 1, 2024)

“…A team of astronomers at the University of Arizona, Tucson used NASA’s Hubble and James Webb space telescopes for an unprecedented in-depth look at the nearly 100-billion-mile-diameter debris disk encircling Vega. ‘Between the Hubble and Webb telescopes, you get this very clear view of Vega. It’s a mysterious system because it’s unlike other circumstellar disks we’ve looked at,’ said Andras Gáspár of the University of Arizona, a member of the research team. ‘The Vega disk is smooth, ridiculously smooth.’

“The big surprise to the research team is that there is no obvious evidence for one or more large planets plowing through the face-on disk like snow tractors. ‘It’s making us rethink the range and variety among exoplanet systems,’ said Kate Su of the University of Arizona, lead author of the paper presenting the Webb findings….”

It wasn’t all that along ago when detecting planets around another star might have been surprising: and would certainly have been exciting.

Now, with 5,780 confirmed planets in 4,314 planetary systems other than ours, not finding planets is a big deal.

Not finding large planets, that is: Saturn-mass worlds or bigger.

IRAS — an infrared space telescope; I talked about it, briefly, last week — showed scientists that something around Vega was shining in infrared ‘light’.

That was back in the 1980s.

Fast-forward to 2005. The Spitzer Space Telescope had given scientists (fairly) high-resolution infrared images of Vega’s dust cloud. Or, more likely, Vega’s debris disc.

Whatever it is, radio telescopes also showed that there’s something around Vega: something that’s quite smooth. On the other hand, other observations showed that what’s around Vega is clumpy. Or asymmetrical, at any rate. Probably.

But none of those were as high-resolution as this Webb Telescope image.¹

Dust, a Gap, and — the “Poynting-Robertson Effect”?

“Imaging of the Vega Debris System using JWST/MIRI”
Kate Y. L. Su, Andras Gaspar, George H. Rieke, Renu Malhotra, Luca Matra, Schuyler Grace Wolff, Jarron M. Leisenring, Charles Beichman, Marie Ygouf (Submitted October 31, 2024) Via arXiv, accepted for publication in The Astrophysical Journal

Abstract

“We present images of the Vega planetary debris disk obtained at 15.5, 23, and 25.5 microns with the Mid-Infrared Instrument (MIRI) on JWST. The debris system is remarkably symmetric and smooth, and centered accurately on the star. There is a broad Kuiper-belt-analog ring at 80 to 170 au that coincides with the planetesimal belt detected with ALMA at 1.34 mm. The interior of the broad belt is filled with warm debris that shines most efficiently at mid-infrared along with a shallow flux dip/gap at 60 au from the star. These qualitative characteristics argue against any Saturn-mass planets orbiting the star outside of about 10 au assuming the unseen planet would be embedded in the very broad planetesimal disk from a few to hundred au. We find that the distribution of dust detected interior to the broad outer belt is consistent with grains being dragged inward by the Poynting-Robertson effect. Tighter constraints can be derived for planets in specific locations, for example any planet shepherding the inner edge of the outer belt is likely to be less than 6 Earth masses. The disk surface brightness profile along with the available infrared photometry suggest a disk inner edge near 3-5 au, disconnected from the sub-au region that gives rise to the hot near-infrared excess. The gap between the hot, sub-au zone and the inner edge of the warm debris might be shepherded by a modest mass, Neptune-size planet.”
[emphasis mine]

Since I’ve got nerdy interests, I’d heard about things like the Yarkovsky effect, which describes how electromagnetic radiation, like light, affects smallish rotating things like asteroids. But the Poynting-Roberston effect was new to me.

With the Yarkovsky effect, light warms the ‘day’ side of a rotating object. Then, as the object rotates, the energy’s re-radiated in another direction. The point is that the Yarkovsky effect describes how light affects the orbits of fairly small rotating objects.

The Poynting-Robertson effect describes how light affects any small object, like dust grains, whether it’s rotating or not.

There are at least two ways of looking at the Poynting-Robertson effect, depending on which frame of reference you pick.

Taking a dust grain’s viewpoint, that’s (a) in Michael Schmid’s drawing (right) a star’s radiation (S) is coming at the grain at an angle. That’s due to the astronomical sense of the word “aberration”: objects looking like they’re a bit ‘ahead’ of their actual position, due to the observer’s motion.

Since there’s a trifle more light hitting the forward-facing part of the dust grain in (a), re-radiated photons will slow the grain down. Not much, but it’s a non-zero amount.

The same situation, from the star’s viewpoint, (b) in the drawing, light from the star is coming straight ‘down’ on the dust grain.

Re-radiated photons? That’s anisotropic emission: geek-speak for the way photons leave the dust grain just slightly leaning toward the grain’s direction of motion. That will slow the grain down: again, not much, but by a non-zero amount.

Anisotropy — is a rabbit hole I’ll skip today. I put links in the footnotes.²

A Closer Look at Vega’s Dusty Disc

Backing up a bit, IRAS showed that there’s a lot of dust around Vega; but infrared telescopes couldn’t give scientists particularly high-resolution pictures of the Vega system.

Not until the James Webb Space telescope got to work in July of 2022. By then, we’d learned that we’re looking at Vega — and its dust disc — from the ‘top’, with one of the star’s poles pointed almost directly at us.

Astronomers measured Vega’s diameter, using an astronomical interferometer. That was about two decades back. Astronomical interferometry is using two or more telescopes and a whole lot of math to get high-resolution images.

Their results said that Vega was 2.73 times as wide as our Sun. Give or take 0.01. That was winder than they’d expected.

Then science happened, and now we figure that Vega looks as bright as it does, and as wide as it is, because it’s spinning really fast: once every 16.3 hours. Our sun goes around once every 25 days at the equator, 34 and a half days near the poles.

Spinning that fast, Vega bulges at the equator. I’ll get back to that.

Now, about these images.

“Imaging of the Vega Debris System using JWST/MIRI” discusses images taken with three of Webb’s filters:

• F2550W
  Broadband imaging, peak sensitivity at 25.5 micrometers
• F1550C
  Coronagraph imaging, peak sensitivity at 15.5 micrometers
• F2300C
  Coronagraph imaging, peak sensitivity at 22.75 micrometers

A coronagraph is a telescope gadget that blocks light from something bright, like a star, letting us see stuff near the star. That accounts for black bits at the centers of the images.

Except for one. Kate Y. L. Su et al. say that “the 25.5 μm [micrometer] image is missing the core because of saturation”.³

Hubble’s Vega Image

This Hubble image of Vega’s dust disc was taken with the 50CORON filter on Hubble’s STIS: Space Telescope Imaging Spectrograph.

The 50CORON filter lets Hubble see light from about 2000 to 10,300 angstroms, or 0.2 to 1.03 micrometers: which is probably why it’s called a “clear” filter.

A HubbleSite press released explained that both the Kate Y. L. Su et al. paper and Hubble images were basically grayscale. The Hubble image is blue and the Webb image is orange, because folks assigned those colors to each.⁴

Something I haven’t learned is why that Hubble image of Vega’s dust disc looks like a starburst, with all those spikes radiating from the center. I’ve seen the same sort of pattern elsewhere, and I’ll get back to that.

I’m up to two “back to thats” now, so I’d better start tying up loose ends.

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How We Know What We Know About Vega, and a Little Lore

The Sun’s sunspots are bright, but they’re darker than the rest of the visible surface.

We haven’t actually seen starspots, tiny dark patches on other stars, but we’ve detected larger dark patches on some stars.

Back in 2015 some scientists detected smallish bright spots on Vega. That’s why Matúš Motlo put little bright starspots on his illustration, comparing Vega and Sol/the Sun.

With a lower-case “s”, a sol is a day on Mars, and I’m wandering off-topic.

Both Vega and the Sun are about halfway through their time on the main sequence: a period when a star is ‘burning’ hydrogen and hasn’t started running out of fuel.

But where the Sun is around 4,600,000,000 years old, Vega is 700,000,000: or, rather, between 850,000,000 and 625,000,000.

Vega is something like two and an eighth times as massive as the Sun, so it’s burning through its hydrogen much faster.

Vega’s width and brightness strongly suggested that it’s rotating so fast that it’s shaped sort of like a loaf of pumpernickel. The way Vega’s bright starspots act backs up that idea. That’s why astronomers are pretty sure about the star’s 16.3 hour rotation period.

Another point about Vega: it’s not nearly as ‘metal’ as the Sun. For astronomers, metallicity is now much of a star is made of elements heavier than hydrogen and helium. Vega has only about 32% of the Sun’s share of elements heavier than helium.

Since it’s spinning so fast, Vega is thicker across the equator than it is pole-to-pole. That also makes it hotter at the poles: or cooler at the equator, take your pick.⁵

Let’s see. What else? Vega is by far the brightest star in the constellation Lyra, and part of the Summer Triangle.

Finding Vega, and a little Skywatching Lore

I queued up Sky & Telescope’s Sky Tour Podcast (September 2024) to start at 6:40. That’s where the narrator starts taking about Arcturus, and eventually gets to Vega and the Summer Triangle.

Vega is the constellation Lyra’s brightest star, and is part of the Summer Triangle; along with Altair and Deneb.

The constellation Lyra goes back at least to Ptolemy’s Almagest, and was one of 88 official constellations defined and published by the IAU in 1930.

In Greek mythology, Lyra is the magic lyre of Orpheus.

As a feature in Earth’s sky, Lyra isn’t real: and neither is the Summer Triangle.

Constellations, as astronomers use the term, are regions on the celestial sphere: an abstract/imaginary sphere that Earth’s sky is projected on.

Asterisms are patterns on the celestial sphere; like the Little Dipper, Summer Triangle, and Winter Circle.

Asterisms, constellations, and the celestial sphere aren’t “real” in the sense of being a physical part of this universe. They’re convenient abstractions we use when we’re talking about the real wonders in our sky.⁶

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Spiky Stars and Fomalhaut’s Planet That Isn’t There (Probably)

Now, about that spiky Hubble image of Vega’s dust disk.

The picture’s caption on HubbleSite describes the disc as “…very smooth, with no evidence of embedded large planets….”

With a starburst of dark spikes, it didn’t look particularly smooth to me.

So I figured that the spikes came from Hubble’s optics, or maybe image processing, not Vega’s disc: and that whoever wrote the caption knew this.

I spent more time than I might have, looking for an explanation for those spikes. Unsuccessfully.

Diffraction Spikes, a Debris Disc Distraction, Fomalhaut’s Far-Flung System

I’m pretty sure that I’m not looking at diffraction spikes: not the sort of four- and six-pointed-star effects you’ll see in some astronomy photos, at any rate.

Another thing: I’ve been calling Vega’s dusty halo a debris disc, not a protoplanetary disc, because resources I’d been reading used that term.

A little checking verified that debris disc, circumstellar disc, and protoplanetary disc have overlapping definitions.

Circumstellar disc looks like the most generic term, with debris disc running a close second.

Protoplanetary discs are what we’ve been finding around young stars: like Vega, but they have protoplanets in them.

Maybe Vega’s debris disc is called that because scientists haven’t found protoplanets in it yet.

Where was I?

• Hubble’s spiky image of Vega’s debris disc
• Diffraction spikes
• Circumstellar discs
• Vega’s debris disc
  • Wondering why it’s not called a protoplanetary disc

Right.

I still don’t have an explanation for Hubble’s spiky image of Vega’s debris disc.

But I did find another spiky Hubble image of a star’s debris disc: Fomalhaut and a debatable exoplanet named Dagon.

Dagon, Fomalhaut b, was discovered in 2005, (tentatively) confirmed in 2012, and more positively confirmed as the dust cloud from a whacking great collision in 2020.

The Fomalhaut system has two more stars besides Fomalhaut.

Fomalhaut B, or TW Piscis Austrini, is a BY Draconis variable: and I am not going to dive down that rabbit hole. Fomalhaut C, LP 876-10, is even smaller and dimmer than TW Piscis Austrini.

TW Biscis Austrini is less than a light-year from Fomalhaut. LP 876-10 is 2.5 light-years from Fomalhaut, 3.2 light-years from TW Biscis Austrini. All three are part of a very spread-out trinary star system.⁷

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46 Centuries of Thinking About This Universe, Briefly

Folks had been looking at rivers, mountains, and the stars; and thinking about how all this began, for uncounted ages, when someone committed what we call the Kesh temple hymn to writing.

About that: I assume humanity didn’t pop into existence around 2600 B.C. — and that the wealth of literature which appeared as soon as folks started writing came from oral traditions stretching back to our beginnings.

Fast-forwarding over the Babylonian Map of the World, Parmenides, Anaximander, Aristotle, and all that — natural philosophers started calling themselves scientists in 1834. They eventually accepted that Aristotle wasn’t right about everything.

About a century back now, we started learning how the Sun and other stars produce energy.

That, and — during the last few decades — getting increasingly detailed images of growing stars and planetary systems, is helping us learn how Earth, Jupiter, AEgir, Janssen, and worlds we haven’t found yet, evolve.⁸

I’m pretty sure that finding nascent planetary systems without large planets in distant orbits will help scientists explain the wild variety of planetary systems we have found.

I’m also pretty sure that we’ll eventually find a world where life that’s not like Earth’s flourishes: or that we won’t.

I’ve talked about that, and somewhat-related topics, before:

• “Squishy Stars, Science, and Sirach”
  (August 24, 2024)
• “Colliding Planets Near ASASSN-21qj: Maybe”
  (January 6, 2024)
• “Super-Duper Super Earths and the Search for Life”
  (May 27, 2023)
• “A Star by Any Other Name, and a Galilean Interlude”
  (November 13, 2021)
• “Secondary Causes: Both/And, not Either/Or”
  (August 21, 2021)

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¹ Searching for new worlds:

• Wikipedia
  • Debris disk
  • Exoplanet
  • James Webb Space Telescope
  • IRAS
  • Lists of planets (Exoplanet count was from October 17, 2024: there’s data about thousands of ‘possibles’ that hasn’t been analyzed yet, so the odds are that we’ll soon know of a great many more.)
  • Spitzer Space Telescope (AKA SIRTF, Space Infrared Telescope Facility)
  • Vega
• “Imaging of the Vega Debris System using JWST/MIRI“
  Kate Y. L. Su, Andras Gaspar, George H. Rieke, Renu Malhotra, Luca Matra, Schuyler Grace Wolff, Jarron M. Leisenring, Charles Beichman, Marie Ygouf (Submitted October 31, 2024) Via arXiv, accepted for publication in The Astrophysical Journal

² Light, physics, and nerdy vocabulary:

• Wikipedia
  • Aberration (astronomy)
  • Frame of reference
  • Photon (A particle of light, sort of.)
  • Poynting-Robertson effect (AKA Poynting-Robertson drag.)
  • Radiation pressure
  • Solar sail (AKA lightsail, light sail, and photon sail.)
  • Vega
  • Yarkovsky effect
• Definition of anisotropy, g [dimensionless]
  ECE532 Biomedical Optics; © 1998 Steven L. Jacques, Scott A. Prahl; Oregon Graduate Institute

³ Getting a closer look at Vega:

• Wikipedia
  • Astronomical interferometer
  • Coronagraph
  • James Webb Space Telescope
  • Launch and commissioning of the James Webb Space Telescope
  • Mid-Infrared Instrument (James Webb Space Telescope’s MIRI.)
  • Sun
  • Vega
• “Imaging of the Vega Debris System using JWST/MIRI”
  Kate Y. L. Su, Andras Gaspar, George H. Rieke, Renu Malhotra, Luca Matra, Schuyler Grace Wolff, Jarron M. Leisenring, Charles Beichman, Marie Ygouf (Submitted October 31, 2024) Via arXiv, accepted for publication in The Astrophysical Journal
• “Rapidly Spinning Star Vega has Cool Dark Equator”
  noao0603 — Science Release, NOIRLab, NSF (National Science Foundation) (January 10, 2006)
• HubbleSite
  • “Hubble and Webb Observations of Vega Circumstellar Disk” (November 1, 2024)
  • “NASA’s Hubble, Webb Probe Surprisingly Smooth Disk Around Vega” (November 1, 2024)
• Webb Space Telescope
  • How Are Webb’s Full-Color Images Made?
  • JWST Mid Infrared Instrument
  • MIRI Filters and Dispersers

⁴ Astronomy images and imaging:

• Wikipedia
  • Angstrom
  • Electromagnetic spectrum
  • Hubble Space Telescope
  • Space Telescope Imaging Spectrograph
• HubbleSite
  • “Vega Hubble Compass Image” (November 1, 2024)
  • “Hubble and Webb Observations of Vega Circumstellar Disk” (November 1, 2024)
  • Astronauts Smith and Lee Install STIS During Hubble Space Telescope Servicing Mission 2 (February 28, 2017) (1997)
  • STIS in Cleanroom (February 28, 2017)
• Hubble Space Telescope User Documentation
  • 12.11 Coronagraphic Imaging – 50CORON
  • 5.2 Optical CCD Imaging

⁵ Stars and skywatching:

• Wikipedia
  • Lyra
  • Main sequence
  • Mars sol
  • Metallicity
  • Sol (Roman mythology)
  • Starspot
  • Stellar evolution
    • Main sequence
  • Summer Triangle
  • Sun
  • Sunspot
  • Summer Triangle
  • Vega
• “Interferometric Imaging of λ Andromedae: Evidence of Starspots and Rotation”
  J. R. Parks, R. J. White, F. Baron, J. D. Monnier, B. Kloppenborg, G. W. Henry, G. Schaefer, X. Che, E. Pedretti, N. Thureau, M. Zhao, T. ten Brummelaar, H. McAlister, S. T. Ridgway, N. Turner, J. Sturmann, L. Sturmann; The Astrophysical Journal (May 2021) Abstract via NASA/ADS, Harvard.
• “Discovery of starspots on Vega — First spectroscopic detection of surface structures on a normal A-type star”
  T. Böhm, M. Holschneider, F. Lignières, P. Petit, M. Rainer, F. Paletou, G. Wade, E. Alecian, H. Carfantan, A. Blazère, G.M. Mirouh (submitted November 28, 2014 (v1), last revised Mar 23, 2015) Via arXiv

⁶ More stars and skywatching:

• Wikipedia
  • Almagest
  • Altair
  • Arcturus
  • Celestial sphere
  • Constellation
  • Deneb
  • IAU designated constellations
  • International Astronomical Union (IAU)
  • Lyra
  • Orpheus
  • Ptolemy
  • Summer Triangle
  • Ursa Minor (Includes the Little Dipper asterism.)
  • Winter Hexagon (AKA Winter Circle.)
• “What Are Asterisms?”
  Preston Dyches, Public engagement specialist, NASA (July 28, 2021)

⁷ Astronomical miscellanea:

• Wikipedia
  • BY Draconis variable
  • Circumstellar disc
  • Debris disk
  • Diffraction spike (AKA starburst effect, sunstars.)
  • Fomalhaut
  • Fomalhaut b (A possible exoplanet whose existences is debatable and debated.) (“Parts of this article (those related to the difference between the planetary and cloud debris models, and why most recent analysis does not place this object in a bound orbit around Fomalhaut) need to be updated….”)
  • Fomalhaut C (AKA LP 876-10)
  • List of proper names of exoplanets
  • Protoplanet
  • Protoplanetary disk
  • Starburst (symbol)
  • TW Piscis Austrini (AKA Fomalhaut B)
  • Vega
• “Fomalhaut’s Planet Has Gone Missing, But it Might Have Been Something Even More Interesting”
  Matt Williams, Universe Today (April 28, 2020)
• “New HST data and modeling reveal a massive planetesimal collision around Fomalhaut”
  András Gáspár, George H. Rieke; PNAS (April 20, 2020)
• HubbleSite
  • “Vega Hubble Compass Image” (November 1, 2024)
  • “Hubble and Webb Observations of Vega Circumstellar Disk” (November 1, 2024)

⁸ Myth, cosmology, and philosophy; from the Kesh temple hymn to AEgir and Janssen:

• Wikipedia
  • 55 Cancri e (AKA Janssen.)
  • Anaximander
  • Aristotle
  • Astrophysics
  • Babylonian Map of the World
  • Cosmology
  • Democritus
  • Epsilon Eridani b (AKA AEgir.)
  • Formation and evolution of the Solar System
  • Kesh temple hymn
  • Kish tablet
  • List of languages by first written account
  • List of proper names of exoplanets
  • Nebular hypothesis
  • Parmenides
  • Stellar evolution
  • Stellar nucleosynthesis
  • Timeline of cosmological theories (Along with myth and philosophy.)
  • William Whewell
    • Whewell’s neologisms (Including “scientist”, published in his anonymous review of Mary Somerville’s “On the Connexion of the Physical Sciences”. (published 1834))
  • Writing