Comparing the Sun to Sun-like stars

The importance of addressing facular and/or spot domination

¹ Institut für Planetenforschung (PF), Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
² Centre for Planetary Habitability (PHAB), Department of Geosciences, University of Oslo, Sem Sæland 2A, 0371 Oslo, Norway
³ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
⁴ Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, I. Metaxa & Vas. Pavlou St, 15236 Penteli, Greece

^⋆ Corresponding author; konstantin.herbst@geo.uio.no

Received: 14 November 2024
Accepted: 30 March 2025

Abstract

Context. Whether the Sun is an ordinary G-type star is still an open scientific question. Stellar surveys by Kepler and TESS, however, have revealed that Sun-like stars tend to show much stronger flare activity than the Sun.

Aims. This study aims to reassess observed flare and spot activity of Sun-like Kepler stars by fine-tuning the criteria for a more robust definition of Sun-like conditions and better comparability between the current Sun and Sun-like stars.

Methods. We updated one of the recent stellar Sun-like star samples by applying new empirical stellar relations between the starspot size and the effective stellar temperature to derive more reliable starspot group sizes. From the 265 solar-type stars, we found 48 stars supporting Kepler 30-minute-cadence light curves. We analyzed these 48 by implementing the gradient of the power spectra method to distinguish between spot- and faculae-dominated stars. We employed the α factor to quantify the area ratio of bright and dark features on the stellar surface.

Results. We were able to determine which of the 48 stars were spot- or faculae-dominated. Our results revealed a preferential distribution of Kepler Sun-like stars toward the spot-dominated (44 stars) and transitional (4 stars) regimes. As the current Sun is faculae-dominated, only the transitional stars were utilized for further evaluation. Additionally, accounting for comparability in stellar mass, radius, and rotation period, we show that only 1 of the 265 Sun-like stars in the Kepler sample (i.e., KIC 11599385) can be directly compared to the current Sun. We further show that the single flare observed on KIC 11599385 falls right within the flare energy range estimated for the AD774/775 event observed in the cosmogenic radionuclide archives of ¹⁰Be, ¹⁴C, and ³⁶Cl.