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 39: a Sun-like star that dimmed for a year

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 each of its five cameras that observed the star. Green light dimmed about twice as much as red, as behind fine dust, and NASA’s TESS space telescope found no sign that the star is spotted. The dip is shallow, the infrared cannot decide between spots and dust, 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. 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 dwarf, about 5,800 K and 6.7 billion years old, some 810 parsecs away in Cepheus. ZTF saw it steady from 2018 to early 2023. From May 2023 it dimmed by 0.04 to 0.06 magnitudes in green light, reached 0.063 in green and 0.033 in red in February and March 2024, about 6 per cent less light, and was back to normal by June 2024. ZTF’s alert system flagged it 7 times, and ASAS-SN saw the star 0.03 to 0.07 magnitudes faint in green light in every month from June to December 2023.

Why it might matter. Blue light dimmed about twice as much as red light, more than ordinary interstellar dust (1.4) or starspots (1.1 to 1.3) would cause and as fine dust would. TESS watched the star in many sectors outside the dip and never saw the regular wave of a spotted star. WISE observed twice during the dip and saw the star at most a few thousandths of a magnitude fainter at 3.4 microns, too little to tell dust from spots at this depth.

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 2274904749169663744, a Sun-like dwarf
Location
RA 310.70327°, Dec +71.31979° (J2000), in Cepheus
Distance
about 810 parsecs (2,600 light-years)
Brightness
Gaia G = 14.8, 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 dip is shallow, the colour ratio depends on how it is measured (1.6 to 2.3 on the two ZTF fields), and the infrared cannot decide between dust and spots. 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; a neighbour (G = 15.9) lies 19 arcseconds away. 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 sectors observed from 2019 to 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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