Imagine listening to the cosmos itself pulsing with life. Scientists have discovered something extraordinary in the vast darkness of space: binary star systems that literally beat like hearts. These celestial objects, aptly named s, are rewriting our understanding of stellar behavior and the intricate dance between gravitational forces in the universe.
When plotted over time, the brightness variations of these remarkable systems create patterns identical to an electrocardiogram reading. This resemblance to a human heartbeat isn’t mere coincidence but reflects the fundamental physics at work when two stars orbit each other in highly elliptical paths. Recent discoveries continue to unveil new mysteries about these cosmic phenomena, with modern space telescopes detecting dozens of new heartbeat systems each year.
The Mechanics Behind Stellar Hearts
Heartbeat stars are binary star systems where each star travels in a highly elliptical orbit around the common mass center, and the distance between the two stars varies drastically as they orbit each other. During their closest approach, called periastron, the stars experience tremendous gravitational forces that literally reshape their bodies.
At their closest point in orbit, the tidal forces cause the shape of the heartbeat stars to fluctuate rapidly. When the stars reach the point of their closest encounter, the mutual gravitational pull between the two stars will cause them to become slightly ellipsoidal in shape, which is one of the reasons for their observed brightness being so variable. This deformation happens because the gravitational pull isn’t uniform across the star’s surface – the side facing its companion experiences stronger attraction than the far side.
In system, the distance between the two stars varies drastically as they orbit each other. Heartbeat stars can get as close as a few stellar radii to each other, and as far as 10 times that distance during the course of one orbit. This extreme variation in separation creates the dramatic brightness changes that produce the heartbeat signature. The stars spend most of their orbital period far apart, then swing dramatically close before flying back out to their distant positions.
TESS Telescope Revolutionizes Heartbeat Discovery
Thanks to the continued observations by the TESS survey telescope, in this work we have discovered 23 new HBSs based on TESS photometric data. NASA’s Transiting Exoplanet Survey Satellite has become instrumental in detecting these elusive systems, using its unprecedented precision to monitor stellar brightness variations.
Transiting Exoplanet Survey Satellite (TESS) is a space telescope for NASA’s Explorer program, designed to search for exoplanets using the transit method in an area 400 times larger than that covered by the Kepler mission. It was launched on 18 April 2018, atop a Falcon 9 launch vehicle and was placed into a highly elliptical 13.70-day orbit around the Earth.
The telescope’s wide field of view and continuous monitoring capabilities make it perfect for spotting heartbeat stars. We discover 23 new HBSs based on TESS photometric data. Heartbeat stars (HBSs) are ideal astrophysical laboratories to study the formation and evolution of binary stars in eccentric orbits and the internal structural changes of their components under strong tidal action. These recent discoveries bring the total number of known heartbeat systems to several hundred, each one providing unique insights into stellar physics.
The Kepler Legacy in Heartbeat Research
The Kepler Space Telescope with its long monitoring of the brightness of hundreds of thousands of stars enabled the discovery of many heartbeat stars. Before Kepler’s mission, heartbeat stars were virtually unknown to astronomers. The telescope’s four-year continuous observation of the same patch of sky provided the long-term data necessary to detect these subtle brightness variations.
One of the first binary systems discovered to show the elliptical orbits, KOI-54, has been shown to increase in brightness every 41.8 days. A subsequent study in 2012 characterized 17 additional objects from the Kepler data and united them as a class of binary stars. This groundbreaking work established heartbeat stars as a distinct category worthy of dedicated study.
The Kepler mission’s serendipitous discovery of heartbeat stars demonstrates how astronomical surveys designed for one purpose often uncover unexpected phenomena. This telescope was designed to search for exoplanets in a small patch of the sky, and it obtained four years worth of light curves. Since this data was over such a large and continuous period of time, several odd celestial objects were serendipitously found through Kepler, including heartbeat stars.
Extreme Tidal Forces Shape Stellar Bodies
The physics governing heartbeat stars involves some of the most extreme tidal forces observed in stellar systems. The individual stars in these systems are usually fairly large, and thus produce very strong tidal forces, which can shape the stars’ orbit. These tidal forces occur because the gravitational force exerted by one star on the other is not constant across it. The differential gravitational pull creates bulges and distortions that change as the stars orbit.
In heartbeat stars, these tides are so large that they can actually distort the shape of the star itself. As the two stars in the heartbeat system go through their point of closest passage, or periastron, their shape changes. The normally spherical stars become elongated, resembling footballs during their closest approach.
At the point of their closest encounter, the stars’ mutual gravitational pull causes them to become slightly ellipsoidal in shape, which is one of the reasons their light is so variable. This is the same type of “tidal force” that causes ocean tides on Earth, but magnified to stellar proportions. The deformation directly affects the amount of stellar surface area visible to Earth-based observers, causing the characteristic brightness variations.
Scientific Importance of Heartbeat Systems
Heartbeat stars (HBSs) are ideal astrophysical laboratories to study the formation and evolution of binary stars in eccentric orbits and the internal structural changes of their components under strong tidal action. These systems provide unique opportunities to test theories about stellar structure and binary evolution under extreme conditions.
Scientists are interested in them because they are binary systems in elongated elliptical orbits. This makes them natural laboratories for studying the gravitational effects of stars on each other. The extreme orbital configurations allow astronomers to observe tidal effects that would be impossible to study in more typical circular binary systems.
Binary stars are already extremely important for helping us learn about the properties of stars and Heartbeat stars can also help us understand how tides affect stars and their pulsations. These systems reveal how stellar interiors respond to external gravitational perturbations. The study of heartbeat stars contributes to our understanding of stellar physics, binary evolution, and the role of tides in shaping stellar systems throughout the galaxy.
Tidally Excited Oscillations
Beyond their characteristic heartbeat signature, some of these systems exhibit an even more fascinating phenomenon. The best part is that in some cases, the interaction of stars while they’re at their closest excites one or both stars, making them pulsate in time with the heartbeat. Over 170 heartbeat stars have been discovered and of these, only a few dozen show these stellar pulsations which are called tidally induced oscillations.
The first one concerns the tidally excited oscillations (TEOs) in eccentric binaries where TEOs are mostly due to resonances between dynamical tides and gravity modes of the star. TEOs appear as orbital-harmonic oscillations on top of the eccentric ellipsoidal light curve variations (the “heartbeat” feature). These oscillations provide additional information about the internal structure of the stars and the physics of tidal coupling.
The tidally excited oscillations represent one of the most direct ways astronomers can study the internal structure of distant stars. These pulsations act like seismic waves, carrying information from the stellar interior to the surface where we can observe them. These stars are windows into the world of extreme tidal interactions, and by continuing to study stars with these types of pulsations, we can learn more about the nature and effect of these forces. By learning more about the HB effects for ι Orionis, we can learn a great deal about massive star and heartbeat star evolution.
Orbital Characteristics and Evolution
The preliminary results show that these HBSs have orbital periods in the range from 2.7 to 20 d and eccentricities in the range from 0.08 to 0.70. The eccentricity-period relation of these objects shows a positive correlation between eccentricity and period and also shows the existence of orbital circularization. This relationship reveals how tidal forces gradually modify binary orbits over time.
The shorter-period orbits are likely to circularize more quickly since they are undergoing stronger tidal forces; the eccentricity is smaller for shorter-period systems. This demonstrates the ongoing evolutionary process in heartbeat star systems, where tidal friction slowly converts orbital energy into heat, causing the elliptical orbits to become more circular over millions of years.
These spectroscopic solutions confirm that the stars are members of eccentric binary systems with eccentricities e > 0.34 and periods P = 7–20 days, strengthening conclusions from prior works that utilized purely photometric methods. Heartbeat stars in this sample have A- or F-type primary components. The stellar types involved suggest that heartbeat stars typically involve relatively massive, hot stars that can maintain their elliptical orbits against tidal circularization forces.
The Puzzle of Orbital Stability
The mere existence of heartbeat stars is a bit of a puzzle. All the tidal stretching of these heartbeat stars should have quickly caused the system to evolve into a circular orbit. This creates a fundamental question about how these systems maintain their highly eccentric orbits over astronomical timescales.
A third star in the system is one way to create the highly stretched-out, elliptical orbits we observe. Researchers are currently pursuing follow-up studies to search for third-star components in heartbeat star systems. The presence of additional stellar companions could explain how heartbeat systems maintain their extreme orbital configurations by periodically pumping energy back into the eccentric orbit.
Study authors also postulate that some binary systems of heartbeat stars could have a third star in the system that has not yet been detected, or even a fourth star. This suggests that heartbeat stars might be more complex than initially thought, potentially representing the observable components of hierarchical multiple star systems. The gravitational influence of distant companions could prevent the tidal circularization that would otherwise eliminate the heartbeat signature.
Future Research Directions
Recent studies have identified numerous HBs using the TESS full-frame images by applying convolutional neural networks to TESS phase-folded light curves, followed by a visual inspection to confirm the candidates. Machine learning techniques are revolutionizing how astronomers identify and classify heartbeat stars in the massive datasets produced by space telescopes.
The continued operation of TESS and future space missions promise to dramatically expand our catalog of known heartbeat systems. The team plans to search for similar pulsations in other bright GRBs. Each detection will bring astronomers closer to understanding the life and death of compact stars, the role of magnetars in cosmic evolution, and the origins of extreme astrophysical phenomena.
Advanced observational techniques are enabling astronomers to study heartbeat stars with unprecedented detail. Specifically, they used an instrument on the W.M. Keck Observatory telescope in Hawaii called the High Resolution Echelle Spectrometer (HIRES), which measures the wavelengths of incoming light, which are stretched out when a star is moving away from us and shorter in motion toward us. This information allows astronomers to calculate the speed of the objects along the line of sight, and measure the shape of the orbit.
Conclusion
Heartbeat stars represent one of the most captivating discoveries in modern astronomy, revealing the dynamic and violent nature of stellar interactions. These systems challenge our understanding of binary evolution while providing unprecedented opportunities to study extreme tidal physics. HBSs are ideal astrophysical laboratories to study the formation and evolution of binary stars in eccentric orbits; it is also an important class of objects to study the properties of binaries with eccentric orbits, and to search for special celestial bodies, etc. Therefore, they are important and valuable in scientific research.
The ongoing discoveries by TESS and other advanced telescopes continue to expand our knowledge of these remarkable systems, each new detection adding pieces to the puzzle of stellar evolution and binary dynamics. As we listen to the cosmic heartbeats of distant stars, we gain deeper insights into the fundamental forces that shape our universe.
What do you think about these stellar heartbeats pulsing through space? Share your thoughts in the comments below.
