Nereide

This composite image from New Horizons shows Jupiter and its moon Io during the 2007 flyby. Jupiter, in false-color infrared, displays swirling cloud bands with the Great Red Spot as a prominent bluish-white oval storm. Warmer lower clouds appear reddish, higher hazes blue, highlighting atmospheric dynamics. Io, in near-true color, is superimposed. Its night side reveals a bright red glowing lava eruption at Tvashtar volcano, with a 330-km-high blue-tinged plume rising above, lit by scattered sunlight. The montage contrasts Jupiter’s vast atmosphere with Io’s intense volcanic activity during the gravity-assist maneuver.

In Feb 2007, New Horizons passed Jupiter & its active moon Io. ⚛️

In this nice montage, Jupiter was captured in 3 bands of infrared light making the Great Red Spot look white.🧪

Io is digitally superposed in natural color, showing an ongoing eruption.🔭

➡️ apod.nasa.gov/apod/ap08010...

#science 1/3

This 2007 flyby served as a gravitational assist to accelerate the spacecraft toward Pluto, while at the same time enabling detailed observations of Jupiter and its moons.

Image Credit: NASA, Johns Hopkins U. APL, SWRI

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Let's remember New Horizons spacecraft arrived at Pluto in 2015, and encountered Kuiper Belt object Arrokoth in 2019, the farthest flyby in history!

This composite image from New Horizons shows Jupiter and its moon Io during the 2007 flyby. Jupiter, in false-color infrared, displays swirling cloud bands with the Great Red Spot as a prominent bluish-white oval storm. Warmer lower clouds appear reddish, higher hazes blue, highlighting atmospheric dynamics. Io, in near-true color, is superimposed. Its night side reveals a bright red glowing lava eruption at Tvashtar volcano, with a 330-km-high blue-tinged plume rising above, lit by scattered sunlight. The montage contrasts Jupiter’s vast atmosphere with Io’s intense volcanic activity during the gravity-assist maneuver.

In Feb 2007, New Horizons passed Jupiter & its active moon Io. ⚛️

In this nice montage, Jupiter was captured in 3 bands of infrared light making the Great Red Spot look white.🧪

Io is digitally superposed in natural color, showing an ongoing eruption.🔭

➡️ apod.nasa.gov/apod/ap08010...

#science 1/3

"[Image description: At left, a field of space with a dozen white foreground stars and a number of small, yellow background galaxies. An unremarkable area at centre is outlined with a dashed circle surrounded by a white box. Lines extend from the box to a pullout at right containing faint, grainy white light surrounded by a circle labeled 'Candidate dark galaxy – diffuse emission.' Four white dots are circled in blue and labeled globular clusters.]" Source: the ESA press release linked in the thread.

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Just read about CDG-2, a candidate "almost dark" galaxy in the Perseus cluster, identified using #Hubble, #Euclid, and #Subaru. 🔭🧪⚛️

➡️ www.esa.int/Science_Expl...

It's really intriguing because it seems almost entirely dominated by #darkmatter (DM), with very few scattered stars.

#astronomy

Haha, "Gravitational Harassment" is such a perfect way to put it! It's one of those astronomy terms that sounds almost like it's from a courtroom drama, but it perfectly captures what happens in dense clusters.

...and whether it's lost material.
Fingers crossed for JWST or a big ground-based spectrograph to get those spectra soon – it would answer a lot of these open questions.
5/5

whittled down.
That's exactly why spectroscopy on the 4 GCs (and ideally on any faint diffuse component) would be huge – radial velocities would give us line-of-sight kinematics, let us estimate the enclosed mass, check if the GCs are truly bound, and start constraining the halo shape...
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though it's harder to strip DM than gas because it's collisionless and more centrally concentrated in many models.Right now there's no kinematic data to measure the current DM distribution or mass-loss history, so we can't tell if this is a "pristine" ultra-low-baryon system or one that's been
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The authors note that much of the normal (baryonic) gas was likely removed by gravitational harassment from other galaxies in the cluster, which prevented a lot of star formation outside the GCs. But for the DM itself, stripping is possible in principle (especially the outer parts of the halo),
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Great question.
The short answer from the paper and press releases: we don't have direct evidence yet to say whether CDG-2's DM halo was ever much more massive and then got partially stripped, or if it's always been this extreme.
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The authors already use "candidate" and "appears to be" for this very reason: it's an exciting claim, but preliminary. Spectroscopic follow-up (JWST or large ground telescopes) will be key to determining whether it's truly this extreme or if there's a more mundane explanation.

To confirm they are GCs, spectroscopy (redshift and velocity measurements) is needed to show they're at the same distance and gravitationally linked.

The diffuse light is good evidence against pure chance alignment, but it doesn't fully settle the issue.⤵️

Hubble spotted a ‘dark galaxy’ that’s at least 99.9% dark matter Search through space telescope’s archival images reveals unusually dim galaxy

BONUS
Astronomer Reynier Peletier from University of Groningen, quoted in a Science article (www.science.org/content/arti...), suggests the four "globular clusters" might actually be dwarf galaxies in the foreground or background that just align along our line of sight, not a truly bound group.⤵️

NOTE: About the total luminosity, the paper gives ~6 million with ±3 uncertainty, but ESA press release rounds to 1 million maybe to emphasize the extreme faintness.

Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster, Li, Dayi (David), Liu, Qing, Eadie, Gwendolyn M., Abraham, Roberto G., Marleau, Francine R., Harris, William E., van Dokkum, Pieter, Romanowsky, Aaron J., Danieli, Shany, Brown, Patrick E., Stringer, Alex

...really shone in picking up the faint diffuse light.

For more:

Full paper: iopscience.iop.org/article/10.3...

Image credits: NASA, ESA, Dayi Li (University of Toronto); processing Joseph DePasquale (STScI)

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...(stars mostly born in GCs) & test DM models.
Clear limits: high uncertainties (only 4 GCs, assumed distance 300 Mly, empirical scaling relations), needs spectroscopy to confirm redshift, true mass, and whether there are unseen GCs.
Not "definitive," but a solid step forward – and Euclid...

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...with the GCs contributing 16%.
The mass seems to be ~ 99% DM (the rest is stars + gas likely stripped away by cluster interactions). It's one of the most dark-matter-dominated candidates ever, but this is preliminary analysis.
It matters bc it could help understand extreme star formation...
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...telescopes: strong evidence there's a real galaxy there. It's the 1st one found solely through its globular cluster population, as lead author David Li says.
Under conservative assumptions (those 4 GCs are the full visible population), the total luminosity is ~1 million times that of the Sun,
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...Hubble images – these clusters often hint at a hidden galaxy.
They identified four GCs really close together, super unlikely to be random (1 in 67,000).
Then, stacking Hubble data & checking Euclid + Subaru, a very faint diffuse glow appeared around them, with the same shape across all...

3/7

It appears to contain just 4 globular clusters (GCs)— compared to the Milky Way's 150+ —, and it shines faintly with the light of roughly 1 million Suns.

They didn't find it by looking for normal starlight (way too faint), but with a statistical method that hunts for tight groups of GCs in...

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"[Image description: At left, a field of space with a dozen white foreground stars and a number of small, yellow background galaxies. An unremarkable area at centre is outlined with a dashed circle surrounded by a white box. Lines extend from the box to a pullout at right containing faint, grainy white light surrounded by a circle labeled 'Candidate dark galaxy – diffuse emission.' Four white dots are circled in blue and labeled globular clusters.]" Source: the ESA press release linked in the thread.

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Just read about CDG-2, a candidate "almost dark" galaxy in the Perseus cluster, identified using #Hubble, #Euclid, and #Subaru. 🔭🧪⚛️

➡️ www.esa.int/Science_Expl...

It's really intriguing because it seems almost entirely dominated by #darkmatter (DM), with very few scattered stars.

#astronomy

🧵1/8
In this viral clip from Cosmos Ep.1, Carl Sagan explains how Eratosthenes (276–194 BC) calculated Earth's circumference. 🔭 🧪 ⚛️

His estimate: 250,000 stadia ≈ 25,000 miles (~40,000 km).

Crazy accurate for 2200+ years ago!

#histsci #science

BONUS
Since Eratosthenes calculated along a meridian (north-south arc), the most direct modern comparison is the polar/meridional circumference: ~40,008 km per WGS84 data.

Equatorial is longer at ~40,075 km due to Earth's slight bulge.

Either way, his result was stunningly close – legend!

エラトステネス - Wikipedia

For deepening go to:

➡️ www.khanacademy.org/science/shs-...

➡️ en.wikipedia.org/wiki/Stadion...

➡️ ja.wikipedia.org/wiki/%E3%82%... (section on Earth's size).

A longer clip from Cosmos Ep.1 (1980, Eratosthenes segment) with subtitles ➡️ youtu.be/ZDC3PQNOiW0

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We're not 100% sure which stadion he used – ancient units shifted by region/use, and sources (Cleomedes, Strabo, Pliny) aren't crystal clear.

Eratosthenes' method was geometrically spot-on assuming spherical Earth and parallel sun rays.

Sagan loved it for showing ancient smarts.

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Note: some ancient sources report 252,000 stadia (instead of 250,000) as his final/revised figure – possibly a small adjustment for convenience or minor corrections.

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...some argue for the shorter 157.5 m Egyptian "Itinerary" one.
This makes Eratosthenes' result almost spot-on accurate.

Especially since he worked in Alexandria and the 5,000-stadia distance came from local bematists (ancient professional pacers / distance walkers) & annual Nile surveys.
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Japanese Wikipedia (& many historians) favors ~185 m per D. Rawlins & others, citing the common Roman/Italian stade (stadion) & sources like Pliny the Elder.
Citing Pliny the Elder (in Natural History, where he says a schoenus=40 stadia=5 Roman miles according to Eratosthenes' calculation),...
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The big uncertainty? "Stadion" length varied wildly in antiquity.
- Ptolemaic/Attic ~185 m (common in later sources).
- Egyptian "Itinerary" version often ~157.5 m.

With ~185 m: 250,000 stadia ~46,250 km (15-17% too high vs modern ~40,075 km equatorial).
With ~157.5 m: ~39,375 km (error <2%).
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