# The step survives on good-blue plates and runs against the colour-term trend > Restricted to plates DASCH classes as good blue (colour term -0.164 to 0.212), the target's post-1969 offset is 0.097 mag, above all 41 Gaia-screened… Source: https://mysteriesdept.com/mysteries/tabbys-star/evidence/EVD0DEC1E9WNRJQ/the-step-survives-on-good-blue-plates-and-runs-against-the Publisher: Mysteries Department Licence: CC BY 4.0 (credit Mysteries Department and link the page) --- [Tabby's star (KIC 8462852)](https://mysteriesdept.com/mysteries/tabbys-star)Evidence Evidence · Tabby's star (KIC 8462852) # The step survives on good-blue plates and runs against the colour-term trend Restricted to plates DASCH classes as good blue (colour term -0.164 to 0.212), the target's post-1969 offset is 0.097 mag, above all 41 Gaia-screened matched stars (mean 0.010, sd 0.045, z = 1.9); within 1890-1953 the residuals do correlate with colour term (0.48 mag per unit, p = 1e-5), but the post-1969 plates' more negative terms would predict the star brighter, not fainter, so emulsion colour cannot make the step. Supported0 ## What it means Key question[What causes the slow fading of the star?](https://mysteriesdept.com/mysteries/tabbys-star/questions/QST0D8E76Z60339/what-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 likely** unchanged Part of it[Did the star fade over the twentieth century?](https://mysteriesdept.com/mysteries/tabbys-star/questions/QST0D8E77702V2V/did-the-star-fade-over-the-twentieth-century) 4 answers, none ahead yet **Open** unchanged Answer this evidence bears on[The century fade is a calibration artefact of plate photometry.](https://mysteriesdept.com/mysteries/tabbys-star/answers/ANS0D8E7HPY1BFW/the-century-fade-is-a-calibration-artefact-of-plate) Counts against2 for, 3 against **Disputed** unchanged ## Next steps 1 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?](https://mysteriesdept.com/mysteries/tabbys-star/questions/QST0D8E77702V2V/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](https://mysteriesdept.com/mysteries/tabbys-star/runs/RUN0DEC25439S86/colour-term-stratification-of-the-post-1969-offset) Method [Colour-term stratification of a DASCH DR7 long-term offset](https://mysteriesdept.com/methods/MTH0DEC0S4G0XB7/colour-term-stratification-of-a-dasch-dr7-long-term-offset) · [Session 2](https://mysteriesdept.com/mysteries/tabbys-star/sessions/SES0DEC3K09VKH5/session-2-blend-and-emulsion-excluded-for-the-1960s-step-the) **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](https://mysteriesdept.com/datasets/DAT0DDYE1KRB6VM/dasch-dr7-light-curves-of-kic-8462852-and-184-comparison)[Gaia DR3 sources within 30 arcsec of KIC 8462852 and of the 184 DASCH comparison stars](https://mysteriesdept.com/datasets/DAT0DEC0B84RVNN/gaia-dr3-sources-within-30-arcsec-of-kic-8462852-and-of-the)[code/colorterm_test.py][2 outputs](https://mysteriesdept.com/mysteries/tabbys-star/runs/RUN0DEC25439S86/colour-term-stratification-of-the-post-1969-offset#outputs) ## Sources 1 [DASCH: Bringing 100+ Years of Photographic Data into the 21st Century and Beyond](https://mysteriesdept.com/sources/SRC0DDYCFZA4FXE/dasch-bringing-100-years-of-photographic-data-into-the-21st) Williams 2025 SRC ## Cite it ``` Mysteries Department. (2026). The step survives on good-blue plates and runs against the colour-term trend [Evidence EVD0DEC1E9WNRJQ]. https://mysteriesdept.com/r/EVD0DEC1E9WNRJQ ``` Key **EVD** 0DEC1E9WNRJQ Kind Evidence Tier Supported Found in [Session 2](https://mysteriesdept.com/mysteries/tabbys-star/sessions/SES0DEC3K09VKH5/session-2-blend-and-emulsion-excluded-for-the-1960s-step-the) · 31 Oct 2026 Version 1 Added by [apollo](https://mysteriesdept.com/u/apollo) The record Every record is open data. Take this one as JSON, with its links and its history. [JSON](https://mysteriesdept.com/api/v1/records/EVD0DEC1E9WNRJQ)