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 40: an old Sun-like star that lost a fifth of its light

about 35%

Why this estimate. The dimming is certain: ZTF flagged it 26 times, and ASAS-SN, a separate network of small telescopes, saw five of its six faintest months since 2018 fall inside it. Green light dimmed more than red, as behind dust, and NEOWISE saw no change in the infrared, which argues against spots. Only one ZTF field covers the star, and there is no 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 5,500 K and some 11 billion years old, 980 to 1,250 parsecs away in Camelopardalis. ZTF saw it steady from 2018 to April 2021. It began to fade in mid-2021, and from mid-August to late September was about 0.10 magnitudes faint in green light, with a deeper dip for about a week at the end of September: on 2 October it was 0.23 magnitudes fainter in green light and 0.14 in red, a fifth of its green light gone. It then recovered slowly and was back to normal by April 2022.

Why it might matter. Blue light dimmed about 1.5 to 1.7 times as much as red light, as dust would make it. WISE observed the star on 10 September 2021, in the deepest stretch, and saw no change at 3.4 microns while red light was about 0.07 magnitudes faint; starspots would have dimmed the infrared by about 0.03 and a solid body by about 0.07, so both are disfavoured. A dust cloud 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 274442882597165440, an old Sun-like star
Location
RA 70.82205°, Dec +55.26868° (J2000), in Camelopardalis
Distance
980 to 1,250 parsecs (3,200 to 4,100 light-years)
Brightness
Gaia G = 15.6, 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. TESS did not observe during the dimming. There is no spectrum.
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; its nearest Gaia neighbour (G = 20.5), 15 arcseconds to the north-east, is the faint reddish dot above and to the left of the ticks. 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 three sectors, in November and December 2019, December 2023 to January 2024 and November and December 2024, all outside the dimming. 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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