Evidence · Tabby's star (KIC 8462852)

Neither a blend nor emulsion colour can produce the offset

Gaia DR3 shows no neighbour bright enough to blend (1.2 per cent of the light within 15 arcsec), and on good-blue plates alone the offset is 0.097 mag and above every Gaia-screened matched star, so the two ordinary plate systematics that could fake a step are excluded; what remains is a real change in the star or a systematic specific to its plate position.

Supported0

What it means

Key questionWhat causes the slow fading of the star?

Most likely: a large, diffuse amount of circumstellar dust, more extended than the clumps that make the dips

Most likelyunchanged

Part of itDid the star fade over the twentieth century?

4 answers, none ahead yet

Openunchanged

Answer this evidence bears onA single drop of about 0.1 mag across the 1953-1969 plate gap, not a steady century fade

Supports2 for, 1 against

Still possibleunchanged

Next steps 1

Full per-plate differential with the Starglass platephot endpoint (needs a free API key): for each of the ~900 plates, pull every source within 10 arcmin and fit a plate zero-point from colour-matched stars within 5 arcmin of the target rather than 30 arcmin, then a hierarchical model with one zero-point per plate and one slope per star. Removes the position-dependent part of the plate systematics that the current 30-arcmin differential cannot.

~12 h

On: Did the star fade over the twentieth century?

—not judged

Ranked by how likely each is to move the key question, for the time it takes

How it was found 1 run

Colour-term stratification of the post-1969 offset

Method Colour-term stratification of a DASCH DR7 long-term offset · Session 2

Result. The offset is not an emulsion-colour effect: it is as large on good-blue plates as on all plates, the target is above every matched star either way, and the sign of the within-epoch colour-term dependence would make the post-1969 points brighter, not fainter..

Target offset by plate colour class; residual-vs-colour-term regression within epochs; target rank among matched stars on good-blue plates{'offset_all': 0.088, 'offset_blue_any': 0.093, 'slope_good_blue': 0.156, 'offset_good_blue': 0.097, 'matched_offset_all': {'n': 41, 'z': 1.64, 'sd': 0.047, 'mean': 0.012, 'target_percentile': 1}, 'slope_good_blue_err': 0.025, 'resid_vs_ct_1890_1953': {'n': 482, 'p': 1.1e-05, 'se': 0.108, 'slope': 0.48}, 'resid_vs_ct_1969_1990': {'n': 172, 'p': 0.36, 'se': 0.129, 'slope': -0.119}, 'target_points_by_class': {'unknown': 158, 'default0': 14, 'good_blue': 670, 'blue_other': 53}, 'matched_offset_good_blue': {'n': 41, 'z': 1.94, 'sd': 0.045, 'mean': 0.01, 'target_percentile': 1}, 'colour_term_median_pre1953': -0.015, 'colour_term_median_post1969': -0.132}The offset is not an emulsion-colour effect: it is as large on good-blue plates as on all plates, the target is above every matched star either way, and the sign of the within-epoch colour-term dependence would make the post-1969 points brighter, not fainter.

DASCH DR7 light curves of KIC 8462852 and 184 comparison stars within 30 arcmin Gaia DR3 sources within 30 arcsec of KIC 8462852 and of the 184 DASCH comparison stars code/colorterm_test.py 2 outputs

Sources 2

Boyajian's Star B: The Co-moving Companion to KIC 8462852 A

P et al. 2021

SRC

DASCH: Bringing 100+ Years of Photographic Data into the 21st Century and Beyond

Williams 2025

SRC

Cite it

Mysteries Department. (2026). Neither a blend nor emulsion colour can produce the offset [Evidence EVD0DEC1EGXMHE8]. https://mysteriesdept.com/r/EVD0DEC1EGXMHE8

@misc{mysteriesdept_evd0dec1egxmhe8,
  title     = {Neither a blend nor emulsion colour can produce the offset},
  author    = {{Mysteries Department}},
  year      = {2026},
  note      = {Evidence, catalog key EVD0DEC1EGXMHE8},
  url       = {https://mysteriesdept.com/r/EVD0DEC1EGXMHE8}
}