Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

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Candidate 29: an old Sun-like star that dimmed for a year

about 45%

Why this estimate. The dimming is certain: ZTF flagged it 53 times, and the eight faintest months that ASAS-SN, a separate network of small telescopes, has recorded since 2018 all fall inside it. Green light dimmed about twice as much as red, as behind fine dust; NEOWISE saw no change in the infrared, which rules out a solid body and argues against spots, and NASA’s TESS space telescope found no rotation during the dimming. There is no recent spectrum.

How to read the pictures. The finder chart shows a patch of sky two arcminutes across, about a fifteenth of the Moon’s width, with north up and east to the left. The star sits between the two yellow ticks, and the white bar is 30 arcseconds long. On a light curve the vertical axis is magnitude, a brightness scale on which larger numbers mean fainter stars, and it is drawn so that down means fainter. Purple shading marks the dimming. Each survey uses slightly different filters, so each is compared with its own normal level. The ZTF pictures show ZTF’s reference image of the star, an image taken during the dimming, and the difference between the two, in which only what changed remains: blue means fainter and red brighter. On the TESS light curves the star is black and the comparison stars are coloured.

Five graphs. In the upper four, of brightness against time, the points stay level apart from a dip, shaded purple. The fifth compares the change in green and red light.
Top three rows: brightness from 2010 to 2025 in green, red and infrared light, each against the star’s normal level, from Pan-STARRS1 (a survey telescope in Hawaii), ZTF, and WISE and NEOWISE (NASA’s infrared space telescope); gold diamonds and orange crosses are ASAS-SN, and a note marks the years of Gaia, the European Space Agency’s star-mapping satellite, which publishes only averages. Purple marks the dimming. Middle: a closer view, with the star against its comparison stars, ZTF’s alerts and ASAS-SN’s monthly medians. Bottom: the change in green against that in red, with lines for dimming equal in all colours, ordinary dust and fine dust; the solid line is this star’s. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is an old Sun-like star, about 5,700 K and some 10 billion years old, about 670 parsecs away in Gemini. ZTF saw it steady in 2018 and 2019. It began to fade slightly in 2020, dimmed further in the first half of 2021 and from July to December 2021 was 0.14 to 0.15 magnitudes fainter in green light and 0.07 to 0.08 in red light, about 15 per cent less light. It recovered during 2022 and was back to normal by July. ZTF’s alert system flagged it 53 times, every time fainter than the reference.

Why it might matter. Blue light dimmed about 1.8 to 2.0 times as much as red light, more than ordinary interstellar dust (1.4) or starspots (1.1 to 1.3) would cause and as very fine dust would. In late September 2021, near the deepest point, NEOWISE saw no change at 3.4 microns, which rules out a solid body passing in front (it would have dimmed the infrared as much) and argues against spots. TESS watched the star for two and a half months during the minimum and found no rotation. A cloud of fine dust passing in front of an old, quiet star is the likeliest explanation.

No record of the dimming could be found. The star is not in SIMBAD, VSX (the variable-star index), Gaia’s variable-star lists or the other catalogues consulted, nor among the dimming stars of Tzanidakis et al. (2025), and there is no Transient Name Server entry, Gaia alert or paper giving its Gaia number. ZTF’s alert system holds only machine-generated classifications for it.

Star
Gaia DR3 3433284916692771840, an old Sun-like star
Location
RA 93.59992°, Dec +27.76124° (J2000), in Gemini
Distance
about 670 parsecs (2,200 light-years)
Brightness
Gaia G = 14.1, far too faint for the eye
Probability
This candidate is unconfirmed. The percentage under the title is an estimate of the chance that it is a real, previously unrecorded case of the kind described. The estimate has not been checked by outside experts. See the introduction to all fifty-eight candidates.
Caveat
Only one ZTF field covers the star, and only ZTF measured the colour. A faint neighbour 1.85 arcseconds away is lost in its glare. The only spectrum, from the LAMOST survey, was taken in 2013, long before the dimming.
A patch of sky with a star at the centre between two yellow ticks.
Pan-STARRS1 colour image in the g, r and i filters, from 2010 to 2014, two arcminutes by two. The star is the bright one between the yellow ticks; a faint neighbour (G = 18.3) 1.85 arcseconds away is lost in its glare, and fainter ones 7 to 8 arcseconds to the east sit beside the left-hand tick. North is up, east is left, and the white bar is 30 arcseconds.
Three square images. The star, between orange ticks, looks much the same in the left and middle ones and leaves a dark blue spot in the right.
Cutouts from one ZTF alert, one arcminute across; orange ticks mark the star. Left: the reference image, made in 2018 before the dimming. Middle: an image taken during the dimming. Right: the difference between them, in which blue means fainter; the star leaves a dark blue spot, while its neighbours subtract away.
Wide panels of dotted light curves, one per TESS sector. In each the star is the black row at the bottom, below the coloured rows of comparison stars.
TESS light curves from five sectors: three from 16 September to 2 December 2021, in the deepest stretch of the dimming, and two from October to December 2023. No panel shows the steady wave of a spotted star. In each panel the star is in black at the bottom, below comparison stars on the same images, each divided by the others, averaged over two hours and offset for clarity.

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