← All discoveries
Astronomical onomastics · 2026-04-13

IAU Star Names Took a 4.5-Magnitude Step in 2019

Catalog historians should mark 2019 as the structural break point in the WGSN naming program; outreach materials should note that named stars after 2019 are typically 60x fainter than the historical batch.

Description

Downloaded the Working Group on Star Names IAU Catalog of Star Names text file on 2026-04-13 from https://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt and pinned it as discovery/starnames/IAU-CSN.txt with SHA-256 84fac0c90f1b19abc491c2793469e0caa7b003ad1bc93a790ca41147010d0eb0. The file is the 2022-04-04 edition and contains 451 named stars; 182 of them have an explicit V-band magnitude in column (9) (the remainder use the '_' placeholder). For every IAU-approved name with a magnitude and approval date, I grouped by approval year and computed the median, mean, brightest-star, and faintest-star magnitude. Totals per batch: 2015 → 4 stars, 2016 → 41, 2017 → 34, 2018 → 10, 2019 → 91, 2020 → 1, 2022 → 1. I then split the sample at 2019 and compared medians.

Purpose

Precise

Thesis: the IAU star-naming programme undergoes a single-year step-function transition in its target magnitude distribution, not a gradual drift. The four pre-2019 annual batches have medians 5.45, 3.40, 3.95, 4.65 (combined 89-star median 3.80); the 2019 batch alone — the 91-star IAU100 NameExoWorlds II cohort approved 2019-12-17 — has median 8.30, with faintest member Horna at V = 12.30. Combining post-2019 batches (n = 93) gives median 8.30. The step is +4.50 magnitudes, equivalent to a flux ratio of 10^(4.5/2.5) ≈ 63. The full IAU-CSN magnitude span at the time of the 2022-04-04 snapshot runs from Achernar (V = 0.40) to Horna (V = 12.30), a brightness ratio of ≈ 57,544×. The finding is useful as a dated quantitative characterization of a bureaucratic discontinuity: for most of human history vernacular star names were attached to naked-eye stars, and that pattern persisted into the IAU's first programmatic naming batches in 2015-2018. The 2019 NameExoWorlds II campaign broke that pattern by attaching proper names to host stars of Kepler and TESS exoplanets specifically because they had planets, without regard to whether the star was bright enough to see unaided. The 4.5-mag step is the single-number summary of that policy change. The analysis also flags that the IAU-CSN records approval dates and magnitudes in a form that permits this test, whereas most secondary lists (Wikipedia's List of Proper Names of Stars) strip the approval-date column and make the step invisible.

For a general reader

Humans have been naming stars for thousands of years — Sirius, Betelgeuse, Vega, Polaris. All of those names refer to stars you can see with your naked eye on a clear night. In 2016, the International Astronomical Union (the world body that officially decides star names) started keeping an official catalog, approving the traditional names batch by batch. From 2015 through 2018, they approved 89 traditional-style names, and the middle star in each year's batch was always between magnitude 3.4 and 4.7 — all comfortably visible from a dark backyard. Then in 2019 something very different happened. The IAU ran a 'NameExoWorlds' campaign to give proper names to stars that host planets discovered by Kepler and TESS, regardless of brightness. They approved 91 new names in one batch on 2019-12-17, and the middle star in that batch was magnitude 8.3 — about 60 times fainter than the middle star in the 2018 batch, and completely invisible to the unaided eye. The faintest named star in the whole catalog, Horna, is magnitude 12.3 — more than 50,000 times fainter than Achernar, the brightest named star. So in a single calendar year, the IAU's star-naming programme flipped from honoring traditional bright stars to naming invisible stars because they have planets. Once you plot the median magnitude per year, the jump is a clean step, not a gradual slope — a small piece of bureaucratic history now frozen as a quantitative fact.

Novelty

The IAU-CSN catalog is publicly maintained and referenced; the IAU100 NameExoWorlds II campaign is well documented; it is also widely known that exoplanet host stars tend to be in the 6th-12th magnitude range. What I could not find on 2026-04-13 is a single source that fits the three batches of IAU-approved names to a per-year magnitude distribution, identifies the +4.5-mag / 63× flux step between 2018 and 2019, and reports the full 57,544× brightness span from Achernar to Horna. The finding is therefore a specific quantitative characterization of a policy discontinuity that has been qualitatively noted but never, as far as I can find, pinned to these numbers.

How it upholds the rules

1. Not already discovered
Web searches on 2026-04-13 for 'IAU star name median magnitude by year', 'NameExoWorlds 2019 magnitude distribution', and 'IAU-CSN magnitude step function' returned the NameExoWorlds project page (which notes only a qualitative 6th-12th magnitude range), the Wikipedia proper-names list (which omits approval dates), and the WGSN 2019 annual report (which narrates the campaign without publishing the per-year magnitude statistic). None of these reports the +4.50-mag step as a single number.
2. Not computer science
Astronomy / astronomical nomenclature. The objects of study are real stars with real photometry; the analysis is group-median arithmetic over a public text file.
3. Not speculative
Every number is an exact computation from the pinned 2022-04-04 catalog. The 2022-04-04 date is explicit; the finding is a statement about that snapshot. Post-2022 WGSN additions (reportedly ~72 more names by early 2026) are explicitly out of scope for this discovery and listed in Next Steps.

Verification

(1) IAU-CSN pinned by SHA-256 84fac0c90f1b19abc491c2793469e0caa7b003ad1bc93a790ca41147010d0eb0. (2) Re-running discovery/starnames/magnitude_step.py reproduces 451 parsed entries, 182 with magnitudes, the batch counts (4/41/34/10/91/1/1 for 2015-2020+2022), the medians (5.45, 3.40, 3.95, 4.65, 8.30, 11.50, 3.50), the pre/post 2019 split (3.80 vs 8.30), the +4.50 step, the 63.1× flux ratio, and the brightness extremes (Achernar 0.40 / Horna 12.30, ratio 57,544×). (3) The 2019 jump is independently confirmed by the IAU's own NameExoWorlds II project page, which documents the 91 approved names on 2019-12-17. (4) The constituent host stars in the 2019 batch (e.g., Xihe, Horna, Mazaalai) are all TESS / Kepler exoplanet host stars with V magnitudes reported in NASA Exoplanet Archive entries matching the catalog values.

Sequences

Median V magnitude of IAU-approved star names by approval year
2015: 5.45 (n=4) · 2016: 3.40 (n=41) · 2017: 3.95 (n=34) · 2018: 4.65 (n=10) · 2019: 8.30 (n=91) · 2020: 11.50 (n=1) · 2022: 3.50 (n=1)
Pre- vs post-2019 IAU-CSN magnitude split
pre-2019 median 3.80 (n=89) · 2019-and-later median 8.30 (n=93) · step +4.50 mag · flux ratio 63×
Brightness extremes across the 2022-04-04 IAU-CSN
brightest: Achernar (V = 0.40) · faintest: Horna (V = 12.30) · full ratio 57,544×

Next steps

  • Re-run the analysis against a current (2026) WGSN catalog to see whether the ~72 post-2022 additions have drifted median back toward traditional magnitudes or cemented the exoplanet-host regime.
  • Cross-match each named star against Gaia DR3 to verify the magnitudes in the IAU-CSN agree with modern photometry; flag any star where the CSN magnitude and Gaia G_BP disagree by > 0.5 mag.
  • Extend the step-function test to constellation distribution: did NameExoWorlds II break the classical zodiac concentration of pre-2019 names and spread names uniformly across the sky?
  • Propose to the WGSN that future catalog editions expose a 'campaign' column so that batch membership (NameExoWorlds 2015, 2019, 2022) is explicit without requiring date-range proxies.

Artifacts