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 49: a very old star that dimmed for most of 2019

about 30%

Why this estimate. The dimming is real: ASAS-SN, a separate network of small telescopes, saw the same dip in all five of its cameras while nearby stars stayed steady, and ZTF flagged it six times. Green light dimmed more than red, as behind dust. But the dip is shallow, only one ZTF field covers the star, and the infrared cannot decide between dust and spots.

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 of about 5,500 K with only about a twelfth of the Sun’s share of heavy elements, about 13 billion years old, some 1,500 to 1,900 parsecs away in Aquila. ZTF saw it steady in 2018. It faded from April 2019 and from June to November was about 0.06 magnitudes fainter in green light and 0.035 to 0.04 in red, about 5 per cent less light. It recovered between December 2019 and March 2020 and was steady from April 2020 to 2024. ASAS-SN saw the dip in green light, 0.07 to 0.10 magnitudes, from April to November 2019, and its older record shows the star steady from 2014 to 2018.

Why it might matter. A star this old should be quiet, and ZTF found no rotation period in any season. Blue light dimmed about 1.6 times as much as red, more than ordinary interstellar dust (1.4) or starspots (1.1 to 1.3) usually give, which points to dust passing in front. WISE observed the star in April and October 2019 and saw no clear change, but that does not decide the colour in the infrared.

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 4235833950054628096, a very old, metal-poor subgiant
Location
RA 301.77934°, Dec −1.21691° (J2000), in Aquila
Distance
1,500 to 1,900 parsecs (4,900 to 6,200 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
The dip is shallow. Only one ZTF field covers the star, and only ZTF measured the colour. The infrared cannot decide between dust and spots, and there is no ultraviolet measurement. 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 fainter star 15 arcseconds south of it is a neighbour (G = 17.8). 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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