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Universe
Special Issues
Variable Stars in the 21st Century: From Microvariability to Megavariability
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Variable Stars in the 21st Century: From Microvariability to Megavariability

A special issue of Universe (ISSN 2218-1997). This special issue belongs to the section "Solar and Stellar Physics".

Deadline for manuscript submissions: 31 December 2025 | Viewed by 3349

Special Issue Editors

Prof. Dr. Laszlo Szabados

E-Mail Website
Guest Editor
HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Observatory, H-1121 Budapest XII, Konkoly Thege Miklós út 15-17, Budapest, Hungary
Interests: observational astrophysics; general astronomy; space astronomy; variable stars
Special Issues, Collections and Topics in MDPI journals
Dr. Tamas Szalai

E-Mail Website
Guest Editor
Department of Experimental Physics, Institute of Physics, University of Szeged, Dóm tér 9, H-6720 Szeged, Hungary
Interests: astrophysics of supernovae; interactions and dust formation in the environments of SN explosions; eclipsing binary stars; space astronomy

Special Issue Information

Dear Colleagues,

Variable stars are the most important objects of astrophysics because their observable behavior provides valuable information on their evolutionary state and internal characteristics. In addition, they are not rare, instead, rather ubiquitous. Nowadays, more than 10 million variable stars are known—a fact expressed as megavariability in the title of this Special Issue. Moreover, we can detect variability pertaining to amplitudes as tiny as several millionth of a magnitude, which is considered as microvariability.

It is not an exaggeration to state that each star is a variable star, though both the time scale and the range of variability span very wide intervals.

With this Special Issue, the Editors' goal is to demonstrate how diversified and picturesque are the variable star studies—from investigations of a single variable object to general results obtained for a large group of a similar type of variable stars. Binarity is also a main driver of stellar variability phenomena.

The research papers to be submitted for this Special Issue can be based on observations carried out in any region of the electromagnetic spectrum as far as variability is found in the studied object and the data are analyzed properly and reliably.

To attract the attention of the astronomer community, several invited reviews will also be published that will summarize the variability-related results of recent major sky surveys.

Prof. Dr. Laszlo Szabados
Dr. Tamas Szalai
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Universe is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • stellar variability
  • sky surveys
  • time-domain astrophysics
  • astrophysical transients
  • individual variable stars

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Published Papers (5 papers)

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Research

Jump to: Review

75 pages, 17108 KB  
Article
A Catalog of 73 B-Type Stars and Their Brightness Variation from K2 Campaign 13–18
by Bergerson V. H. V. da Silva, Jéssica M. Eidam, Alan W. Pereira, M. Cristina Rabello-Soares, Eduardo Janot-Pacheco, Laerte Andrade and Marcelo Emilio
Universe 2025, 11(9), 301; https://doi.org/10.3390/universe11090301 - 3 Sep 2025
Viewed by 310
Abstract
The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 [...] Read more.
The variability of B-type stars offers valuable insights into the interiors of stars and the processes that drive pulsation and rotation in massive stars. In this study, we present the classification of the variability of 197 B-type stars observed in various Kepler/K2 campaigns, including 73 newly classified stars from Campaigns 13–18. For these stars, we derived atmospheric and evolutionary parameters using space-based photometry and ground-based spectroscopy. We obtained spectroscopic data for 34 targets with high-resolution instruments at OPD/LNA, which were supplemented by archival LAMOST spectra. After correcting for instrumental systematics, we analyzed the light curves using Fourier transforms and wavelet decomposition to identify both periodic and stochastic signals. The identified variability types included SPB stars, β Cephei/SPB hybrids, fast-rotating pulsators, stochastic low-frequency variables, eclipsing binaries, and rotational variables. We also revised classifications of misidentified stars using Gaia astrometry, confirming the main-sequence nature of objects once considered subdwarfs. Our results indicate that hot-star variability exists along a continuum shaped by mass, rotation, and internal mixing rather than distinct instability domains. This study enhances our understanding of B-type star variability and supports future asteroseismic modeling with missions like PLATO. Full article
(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)
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11 pages, 771 KB  
Article
Period Variation Rates of Four Radial Single-Mode High-Amplitude Delta Scuti Stars
by Tian-Fang Ma, Jia-Shu Niu and Hui-Fang Xue
Universe 2025, 11(3), 86; https://doi.org/10.3390/universe11030086 - 6 Mar 2025
Viewed by 757
Abstract
In this work, we present a study on the long time-scale period variations of four single-mode high-amplitude delta Scuti stars (HADS) via the classical O−C analysis. The target HADS are (i) XX Cygni, (ii) YZ Bootis, (iii) GP Andromedae, and (iv) [...] Read more.
In this work, we present a study on the long time-scale period variations of four single-mode high-amplitude delta Scuti stars (HADS) via the classical O−C analysis. The target HADS are (i) XX Cygni, (ii) YZ Bootis, (iii) GP Andromedae, and (iv) ZZ Microscopii. The newly determined times of maximum light came from the Transiting Exoplanet Survey Satellite (TESS), American Association of Variable Star Observers (AAVSO), and Bundesdeutsche Arbeitsgemeinschaft für Veränderliche Sterne (BAV) projects. Together with the times of maximum light obtained in the historical literature, the O−C analysis was performed on these HADS, in which we obtained the linear period variation rates P˙/P as (9.2±0.2)×10−9yr−1, (3.2±0.2)×10−9yr−1, (4.22±0.03)×10−8yr−1, and (−2.06±0.02)×10−8yr−1, respectively. Based on these results and some earlier research, we also discuss the evolutionary stages and the mechanisms of the period variation of these four HADS. Full article
(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)
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17 pages, 5124 KB  
Article
Pulsation in Hot Main-Sequence Stars: Comparison of Observations with Models
by Luis A. Balona
Universe 2024, 10(12), 437; https://doi.org/10.3390/universe10120437 - 25 Nov 2024
Cited by 1 | Viewed by 1125
Abstract
The locations of hot pulsating variables in the H–R diagram are found using the effective temperatures derived from spectroscopic analysis and luminosities from Gaia parallaxes. Frequency peaks extracted from TESS photometry were used to compare with model predictions. A large number of stars [...] Read more.
The locations of hot pulsating variables in the H–R diagram are found using the effective temperatures derived from spectroscopic analysis and luminosities from Gaia parallaxes. Frequency peaks extracted from TESS photometry were used to compare with model predictions. A large number of stars with pulsation frequencies similar to δ Scuti variables were found between the predicted δ Scuti and β Cephei instability regions, contrary to the models. These Maia variables cannot be explained by rapid rotation. There is a serious mismatch between the observed and predicted frequencies for stars within the known δ Scuti instability strip. In δ Scuti and Maia stars, the frequency at the maximum amplitude as a function of the effective temperature was found to have a surprisingly well-defined upper envelope. The majority of γ Doradus stars were found within the δ Scuti instability strip. This is difficult to understand unless pulsational driving is non-linear. Non-linearity may also explain the huge variety in frequency patterns and the presence of low frequencies in hot δ Scuti stars. γ Doradus stars were found all along the main sequence and into the B-star region, where they merged with SPB variables. There seemed to be no distinct instability regions in the H–R diagram. It was concluded that current models do not offer a satisfactory description of observations. Full article
(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)
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Review

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29 pages, 6197 KB  
Review
Selected Results on Variable Stars Observed by TESS
by Zsófia Bognár and Ádám Sódor
Universe 2025, 11(9), 319; https://doi.org/10.3390/universe11090319 - 17 Sep 2025
Viewed by 151
Abstract
As we enter the final year of the second extended mission of the Transiting Exoplanet Survey Satellite (TESS), it is time to reflect on what the TESS mission has contributed to the advancement of astronomy. Thousands of papers based on TESS data have [...] Read more.
As we enter the final year of the second extended mission of the Transiting Exoplanet Survey Satellite (TESS), it is time to reflect on what the TESS mission has contributed to the advancement of astronomy. Thousands of papers based on TESS data have already been published, making it a challenge to select the ones we mention or summarise in this review. As the title suggests, this paper focuses on variable stars, that is, phenomena that causes a star’s brightness to change. We discuss all the major classes of extrinsic and intrinsic variables, from planetary transits to pulsating stars, excluding only the longest-period ones, which are not well suited for the typical time spans of TESS time-series observations. TESS has provided significant and interesting data and results for all these variable types. We hope that this selection successfully demonstrates the diverse applicability of TESS in variable star research. Full article
(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)
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24 pages, 10348 KB  
Review
The Variable Sky Through the OGLE Eye
by Patryk Iwanek
Universe 2025, 11(9), 304; https://doi.org/10.3390/universe11090304 - 8 Sep 2025
Viewed by 275
Abstract
The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering [...] Read more.
The Optical Gravitational Lensing Experiment (OGLE) is one of the most productive and influential photometric sky surveys in the history of observational astronomy. Originally designed to detect dark matter through gravitational microlensing events, OGLE has evolved into a cornerstone of time-domain astrophysics, delivering three decades of two-band, high-cadence observations of approximately two billion stars across the Galactic bulge, disk, and Magellanic System. This review summarizes OGLE’s key contributions to variable star research, including the discovery, classification and characterization of pulsating stars, eclipsing, ellipsoidal, and rotating variables, or irregular and eruptive stars. Particular emphasis is placed on the OGLE Collection of Variable Stars (OCVS), a publicly available and systematically expanded dataset that has become a fundamental resource for studies of stellar variability and evolution, Milky Way and other galaxies structure, microlensing, compact objects, exoplanets and more. The synergy between OGLE and other major sky surveys, including ASAS, ASAS-SN, ATLAS, Gaia, KMTNet, MACHO, MOA, TESS, PLATO, or ZTF further amplifies its scientific reach. Full article
(This article belongs to the Special Issue Variable Stars in the 21st Century: From Microvariability to Megavariability)
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