Professor of Mathematics

Paata Ivanisvili

pivanisv@uci.edu · x.com/PI010101

Department of Mathematics, University of California, Irvine

Paata Ivanisvili

← Galaxy hunt

Candidate 38: an old Sun-like star that was slightly faint for two years

about 35%

Why this estimate. The dimming is real: ZTF recorded it on two of its detectors and ASAS-SN, a separate network of small telescopes, saw it in all five of its cameras that watched the star. Green light dimmed more than red, and the infrared did not change, which rules out a solid body. But the dip is shallow, and spots or another slow cause are harder to exclude for a change this small.

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. Bottom right: the star and its comparison stars on two separate ZTF fields, recorded by different detectors. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is a Sun-like star, about 5,700 K and some 10 billion years old, that is beginning to leave the main sequence, about 920 parsecs away in Cepheus. ZTF saw it steady from 2018 to late 2021. From early 2022 it was about 0.04 magnitudes fainter in green light, and in July to September 2023 0.07 in green and 0.03 in red, about 7 per cent less light. It was close to normal again during 2024. ZTF’s alert system flagged it 17 times, and ASAS-SN saw the same episode.

Why it might matter. Blue light dimmed about 1.7 times as much as red light, more than starspots usually give (1.1 to 1.3) and at least as much as ordinary interstellar dust. WISE, observing every six months, saw no clear change at 3.4 microns, so whatever dimmed the star blocked visible light more than infrared light, as dust does. TESS found no rotation in any of seven sectors, so the star does not look spotted. A thin dust cloud drifting slowly across an old star would fit.

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 555998192372279808, an old Sun-like star beginning to evolve off the main sequence
Location
RA 53.27765°, Dec +79.77970° (J2000), in Cepheus
Distance
about 920 parsecs (3,000 light-years)
Brightness
Gaia G = 13.9, 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
The dimming is shallow, only a few per cent for most of its length. The colour ratio differs between the two ZTF fields (1.5 and 2.0), and GALEX gives only a mild hint of ultraviolet light. 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 between the yellow ticks; the faint dots around it are 7 or more magnitudes fainter. 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 seven sectors, in 2020, 2022, 2023 and 2024. 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.

· All candidates ·