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Universe
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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: 30 June 2025 | Viewed by 1654

Special Issue Editors

Prof. Dr. Laszlo Szabados

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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 (2 papers)

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Research

11 pages, 771 KiB  
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 428
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 KiB  
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
Viewed by 816
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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