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 56: a distant old star that was faint for a year

about 20%

Why this estimate. The dimming is real: ZTF recorded it in three filters and ASAS-SN, a separate network of small telescopes, saw the star faintest in December 2018 too. But the infrared measurement made during the dimming leans towards starspots, a machine classification calls the star a spotted binary, and only one ZTF field covers it.

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. Down means fainter. The note under the graphs sums up the measurements.

What changed. The star is a subgiant poor in heavy elements, 2,200 to 2,900 parsecs away in Pegasus, in the Milky Way’s thick disk or halo and near the direction of the MBM 53–55 dust clouds. ZTF found it already faint when it began observing in May 2018. It was faintest from November 2018 to January 2019, 0.19 magnitudes fainter in green light on 17 December 2018, about 16 per cent less light, faint again in June 2019, and steady from August 2019 to 2024. ASAS-SN saw it 0.14 magnitudes faint in December 2018.

Why it might matter. Blue light dimmed about 1.4 to 1.7 times as much as red light, which fits dust and is more than spots on a star this cool usually give. But WISE observed in November 2018 and saw the star 0.024 magnitudes faint at 3.4 microns, about what starspots would give and more than dust would (0.004), so the infrared leans towards spots. In May 2025 alerts showed the star faint again, but no other survey confirmed it.

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. One of them calls it a spotted binary of the RS Canum Venaticorum kind, with a probability of 0.90.

Star
Gaia DR3 2814851407508236160, a distant, metal-poor subgiant
Location
RA 345.48049°, Dec +12.76838° (J2000), in Pegasus
Distance
2,200 to 2,900 parsecs (7,200 to 9,500 light-years)
Brightness
Gaia G = 15.3, 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 infrared leans towards starspots. Only one ZTF field covers the star, its beginning was missed, and there is no TESS or ultraviolet test. Candidate 30, about 29 degrees away, was also deepest in December 2018. 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; the fainter star 12 arcseconds to its west-north-west is a neighbour (G = 18.7). 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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