Classical Cepheids, or Cepheid-type variable stars, are some of the most fascinating objects in the cosmos. These massive and bright stars are subject to fluctuations in brightness at regular intervals, which has proved crucial to some of the most groundbreaking discoveries in astrophysics.
The history of classical cepheids is intertwined with the work of Edwin Hubble – an American astronomer – whose research in the early 20th century revolutionized our understanding of the universe [1]. In 1924, Hubble exploited the period-brightness relationship of the cepheids [2]. This was a key discovery initially made by Henrietta Swan Leavitt in 1912. This relationship indicates that the longer the period over which the brightness of a cepheid star changes, the greater its brightness.
Using this relationship, Hubble was able to determine the distance to the “Andromeda Nebula” and several other spiral nebulae. His measurements proved conclusively that these nebulae were not part of our Milky Way, but were in fact entire galaxies of their own [2]. This discovery effectively doubled the size of the known universe and moved the scientific community away from a cosmos centered on the Milky Way.
A few years later, observation of Cepheids in neighboring galaxies led Hubble to another monumental discovery: the universe is expanding. This observation laid the foundation for the Big Bang theory, fundamentally changing the field of cosmology.
OGLE’s discovery of classical cepheids
The Optical Gravitational Lensing Experiment (OGLE) project stands out particularly for its contribution to the study of pulsating stars such as classical cepheids. Led by Professor Andrzej Udalski, the research team (I. Soszynski, D. M. Skowron, P. Pietrukowicz, M. Gromadzki, M. K. Szymanski, J. Skowron, P. Mroz, R. Poleski, S. Kozlowski, P. Iwanek, M. Wrona, K. Ulaczyk, K. Rybicki, M. Mroz) of OGLE has played a key role in expanding our knowledge of the Milky Way and its structure [1]. This project, carried out mainly with telescopes located in Chile, was a breakthrough in the discovery and study of variable stars. Approximately half of all known classical cepheids in our galaxy have been identified by astronomers from the Warsaw University Astronomical Observatory, making a significant contribution not only to quantitative but also qualitative aspects of astrophysics, providing fundamental data for studying various aspects of the galaxy.
The star is present in the ASAS-SN and Gaia DR3 catalogs of variable stars, but in these catalogs it is classified as a long-period variable. Based on more than 10 years of photometric monitoring of this star by the OGLE project in the I and V bands and the radial velocity curve from Gaia Focused Product Release, we conclusively demonstrate that this object is a classical fundamental-mode Cepheid.
– “Discovery of the Longest-period Classical Cepheid in the Milky Way,” by I. Soszynski , D. M. Skowron , A. Udalski , P. Pietrukowicz , M. Gromadzki, M. K. Szymanski, J. Skowron, P. Mroz, R. Poleski, S. Kozlowski, P. Iwanek, M. Wrona, K. Ulaczyk, K. Rybicki, and M. Mroz.
Although OGLE-GD-CEP-1884 was mentioned in earlier reviews such as ASAS-SN and Gaia DR3, it was initially misclassified as a different type of variable star due to its unique characteristics. Only a meticulous analysis of photometric data and radial velocity measurements over more than a decade allowed scientists to confirm its true nature as a classical cepheid.
Mapping the galaxy in three dimensions
One of the most notable achievements of the OGLE team was the work led by Professor Dorota Skowron. Several years ago, Professor Skowron’s team published a groundbreaking three-dimensional map of the Milky Way [1]. This map was developed based on the spatial distribution of thousands of cepheids discovered over the years by the OGLE project. The precision and depth of this map have provided unprecedented insight into the structure of our galaxy.
The most striking discovery of this 3D map was the confirmation that the Milky Way’s disk is not flat. Contrary to traditional views of a flat galactic plane, the map clearly showed that the disk bends and twists in an S-shape. This warping could be caused by torques exerted by the massive inner disk or by gravitational forces from nearby massive galaxies.
Figure 1. Map of the Milky Way
Astronomical Observatory of the University of Warsaw, Cepheid with a record pulsation period in the Milky Way
Significance of OGLE discoveries
The OGLE team’s discoveries have profound implications – understanding the shape and structure of our galaxy helps to study the formation and evolution of galaxies throughout the universe. It also has implications for calibrating the Milky Way model used to measure astronomical distances and navigate our position in space [1]. Moreover, the discovery of a large number of classical Cepheids by the OGLE team enhances our ability to more accurately measure distances within the Milky Way. These stars serve as essential calibrators for the cosmic distance ladder, directly affecting our understanding of the expansion rate of the Universe.
The star, named OGLE-GD-CEP-1884, broke previous records with a pulsation period of 78.14 days, nearly 10 days longer than the previous record holder, S Vulpeculae.
The discovery of such an ultra-long-period Cepheid in our Galaxy suggests that there are probably more such rare stars hidden by interstellar extinction, which may be crucial to better understanding the structure of the Milky Way – “Discovery of the Longest-period Classical Cepheid in the Milky Way
I. Soszynski , D. M. Skowron , A. Udalski , P. Pietrukowicz , M. Gromadzki, M. K. Szymanski, J. Skowron, P. Mroz, R. Poleski, S. Kozlowski, P. Iwanek, M. Wrona, K. Ulaczyk, K. Rybicki, and M. Mroz
Classical Cepheids or Delta Cepheid-type variable stars are of crucial importance in the study of cosmic astronomical distance gauges. These bright stars pulsate in a predictable manner, allowing astronomers to determine their distances with great precision. The unusual pulsation period of OGLE-GD-CEP-1884 places it in the rare category of ultra-long pulsation period (ULP) cefeids, which play a key role in extending our reach to the far corners of the galaxy and beyond.
OGLE-GD-CEP-1884
Located at a distance of about 4.47 kpc (about 14,600 light-years) from Earth [1], the identification of this star was supported by using period-luminosity relations in the mid-infrared, which are less susceptible to the influence of interstellar dust that can obscure and mislead observations. The method not only confirmed the distance of the star, but also its location on the map of the Milky Way, providing valuable data on the structure of our galaxy.
The discovery of OGLE-GD-CEP-1884 opens up new possibilities for understanding the distribution of stars in the Milky Way [1]. Its presence suggests that there may be more ULP spheres in our galaxy, especially in regions heavily affected by interstellar extinction. These discoveries underscore the importance of improving methods for detecting and classifying variable stars, which remains a challenge due to the limitations of automated surveys in accurately categorizing such unique celestial phenomena.
Moreover, the discovery has profound implications for calibrating Leavitt’s law (the period-luminosity relationship), which is crucial for improving existing models of the Universe’s expansion rate. While astronomers continue to search the skies, the insights provided by OGLE-GD-CEP-1884 will undoubtedly improve our understanding of the cosmic distance ladder and contribute to more accurate cosmological measurements.
Current uses of classical cepheids
Today, classical cepheids continue to be at the forefront of astrophysical research. They are essential tools for measuring intergalactic distances – a process crucial to mapping the structure of the universe. Cepheids serve as ‘standard candles’ because of their predictable changes in brightness, which allow astronomers to accurately calculate distances.
The accuracy of these measurements is crucial for determining the Hubble constant, or the rate of expansion of the universe. Recent advances have led to the most precise measurements of this constant to date, mostly using observations of classical Cepheids. This work not only helps in understanding the rate of expansion, but also in studying the properties of dark energy, the mysterious force that accelerates the expansion of the universe.
Challenges and future prospects
Despite their invaluable contributions, the study of classical cepheids is not without its challenges. Calibration of the period-luminosity relationship, especially its dependence on the composition of stars and their positions in galaxies, requires continuous improvement [1]. Moreover, discrepancies in measurements of the Hubble constant compared to data from the cosmic microwave background suggest that there are still unknowns in our standard model of cosmology.
Future research will continue to exploit the potential of the Cepheids, and next-generation telescopes will enable more detailed observations. These advances promise to refine our understanding of the expansion of the universe and may hold the key to solving the dark energy puzzle.
Cover photography: Astronomical Observatory of the University of Warsaw
Bibliography:
[1] UW Astronomical Observatory, Cepheid with record pulsation period in the Milky Way, https://www.astrouw.edu.pl/cefeida-o-rekordowym-okresie-pulsacji-w-drodze-mlecznej/
[2] Polish Radio, Edwin Hubble. Spotted fleeing galaxies, https://www.polskieradio.pl/39/156/artykul/1324774,edwin-hubble-wypatrzyl-uciekajace-galaktyki
