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 28: a distant star that dimmed in two steps

about 45%

Why this estimate. The dimming is certain: ZTF recorded it on two of its detectors, and ASAS-SN, a separate network of small telescopes, saw both steps and the recovery while nearby stars stayed steady. Green light dimmed more than twice as much as red, the steepest ratio among many such dimmings checked, which fits very fine dust. The estimate is held down by the lack of a spectrum and by two disagreeing distances.

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.

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 yellow star of about 5,900 K that has begun to swell, seen through more dust than maps give for its line of sight, in Taurus about 4 degrees from the Pleiades (which are much closer and unrelated). ZTF saw it steady until spring 2020. By July 2020 it was 0.04 magnitudes fainter in green light, and it stayed slightly faint until spring 2021. From August to November 2021 it was 0.21 to 0.29 magnitudes fainter in green light, losing about 24 per cent of it at the deepest, in November. In February and March 2022 it was back at the first step’s level, and from July 2022 it was normal. ZTF’s alert system flagged it 59 times, every time fainter than the reference. ASAS-SN saw the same two steps and recovery, and its older record shows the star steady from 2012 to 2018.

Why it might matter. Blue light dimmed about 2.3 times as much as red light, and near-infrared light least. A solid body passing in front would dim all colours equally, ordinary interstellar dust gives 1.4 and starspots about 1.1 to 1.3, so a steeper ratio points to a cloud of very fine dust crossing the line of sight. A hot companion star that faded could also take away more blue light, but it would have to be hotter than about 11,000 K, and GALEX saw too little ultraviolet light for that. Dust clouds in front of ordinary, mature stars are rare, which is why such cases are worth following up.

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 69548890757086720, a yellow star beginning to swell into a subgiant
Location
RA 52.53906°, Dec +25.66977° (J2000), in Taurus, 25 degrees below the Milky Way’s mid-plane
Distance
uncertain: 1,800 parsecs from Gaia’s model of its spectrum, about 4,000 from its parallax
Brightness
Gaia G = 15.5, 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 ZTF measured the colour, the infrared measurements are too noisy to help, and TESS cannot separate the star from a brighter neighbour 35 arcseconds away. Several stars in the same part of the sky were faint in late 2021, but ASAS-SN, which shares no equipment with ZTF, saw this dimming too. There is no spectrum, and the two distances disagree.
A patch of sky with a star at the centre between two yellow ticks.
Pan-STARRS1 colour image in the g, r and z filters, from 2010 to 2014, two arcminutes by two. The star is between the yellow ticks; the bright star 35 arcseconds to the east shares its TESS pixels. 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.

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