Across the vast tapestry of the night sky, stars appear as fixed, unwavering points of light. However, as astronomers have long known, not all stars maintain a consistent brightness. Some, aptly named “variable stars,” undergo periodic or sometimes even random fluctuations in luminosity. These pulsating orbs play a key role in the cosmic narrative, providing scientists with invaluable data about the life cycles of stars, the distances between galaxies, and the very expansion of the universe itself. In this article, we embark on a journey to understand the diverse family of variable stars, the mechanisms driving their luminous dance, and the different types of variable stars.
Table of Contents
There are essentially two broad categories of variable stars: intrinsically variable and extrinsically variable stars. Intrinsically variable stars change in brightness due to internal processes. Dominating this category are pulsating variables—giant stars that rhythmically expand and contract. These encompass Cepheid variables, RR Lyrae variables, Delta Scuti variables, long-period or Mira-type variables, and semi-regular variables.
Another subset is the class known as eruptive variables. These stars display irregular brightness increases stemming from tumultuous internal processes, with flare stars, T Tauri stars, Wolf-Rayet stars, luminous blue variables (LBV), and novae being primary examples.
Extrinsically variable stars change in brightness due to external reasons. One of the most renowned subtypes of this class is that of eclipsing binaries. In these systems, two stars orbit one another and, from our perspective, periodically pass in front of each other, leading to brightness dips. Algol variables (Beta Persei type) and Beta Lyrae variables are prime examples of this behavior. There are also rotating variables—stars that exhibit brightness fluctuations as they spin, often due to star spots or atmospheric structures. BY Draconis variables and FK Comae Berenices variables fall into this category.
In this article, we’ll be going into detail about investigating all of the various types of variable stars as well as providing some examples of each that can be seen with telescopes, binoculars, or even the naked eye.
Type 1: Intrinsically Variable Stars
Pulsating Variable Stars
Pulsating variable stars are typically characterized by their internal structures and the specific regions where these pulsations originate. The mechanism behind the pulsation often lies in the delicate balance between the gravitational forces pulling the star’s material inward and the radiation pressure pushing it outward. When these forces are periodically out of sync, it results in the observed pulsations.
Cepheid Variables
Named after Delta Cephei, the first of its kind to be recognized, these stars have played a pivotal role in our understanding of the cosmos.
Cepheids are yellow to yellow-white supergiant stars that pulse in a very regular fashion, expanding and contracting over a period of days to weeks. The uniqueness of Cepheids lies in the direct relationship between their luminosity and their pulsation period: brighter Cepheids have longer periods, and dimmer ones have shorter periods. This relationship, known as the Period-Luminosity relationship, is crucial to their role as cosmic yardsticks. Cepheids are a fundamental rung on the “cosmic distance ladder.” Because of the established Period-Luminosity relationship, once the period of a Cepheid is determined, its absolute luminosity can be inferred. By comparing this absolute luminosity to the apparent brightness observed from Earth, astronomers can calculate the distance to the star and thus to any galaxy or cluster in which it resides. This method has proven invaluable in measuring the distance to nearby galaxies.
The significance of Cepheid variables was underscored by the work of the legendary astronomer Edwin Hubble, for whom the eponymous space telescope was named. In the early 20th century, the nature of “spiral nebulae” like the Andromeda “nebula” (M31) was a topic of intense debate. Some believed they were part of our own Milky Way galaxy, while others posited they might be separate galaxies altogether.
In 1923, Hubble identified several Cepheid variable stars in the Andromeda “nebula” using the Hooker telescope at Mount Wilson Observatory. Applying the Period-Luminosity relationship, he determined the distance to these stars, and, by extension, to Andromeda itself. His calculations placed Andromeda far beyond the borders of the Milky Way, confirming that it was indeed a separate galaxy. This revelation expanded our understanding of the universe exponentially, indicating that our Milky Way was just one of countless galaxies in a vastly larger cosmic arena.
The most well-known Cepheid is Polaris, the North Star, which exhibits a period of approximately 3.97 days and has an apparent magnitude that varies subtly around 2.0, but you won’t notice its tiny changes in brightness with the naked eye or a telescope. A better example is, of course, Delta Cephei, the prototype of this class, which has a period of about 5.37 days while its magnitude oscillates between 3.5 and 4.4, an obvious change to the unaided eye. Similarly, Eta Aquilae has a period of roughly 7.18 days, with its magnitude swinging between 3.5 and 4.4.
RR Lyrae Variables
While Cepheid variables often steal the limelight in the realm of variable stars, there exists another group of stellar luminaries that have been pivotal in our understanding of the universe: the RR Lyrae variables. These stars, named after the prototype RR Lyrae in the constellation Lyrae, are a subclass of pulsating horizontal branch stars, and they have proven invaluable in illuminating the structure and scale of our galaxy and beyond.
RR Lyrae stars are typically older, lack trace elements heavier than hydrogen and helium, and are less luminous than Cepheid variables. Their pulsation periods are much shorter, usually ranging between 0.2 and 1.2 days. All RR Lyrae stars, irrespective of their precise period or metallicity, have roughly the same average luminosity, making them standard candles for distance estimation.
There are multiple subtypes within RR Lyrae stars based on their light curves:
- RRab: Stars that exhibit an asymmetric light curve, with a steep rise in brightness followed by a gradual decline. These are the most common subtypes.
- RRc: Stars with a more sinusoidal light curve and shorter periods.
- RRd: Double-mode pulsators that exhibit both the above-mentioned light curve types simultaneously.
Given their age, RR Lyrae stars are considered “stellar fossils,” relics from the early universe. By studying them, astronomers can trace the formation and evolution of the Milky Way and other nearby galaxies. RR Lyrae stars are commonly found in globular clusters, ancient and densely packed groups of stars orbiting the Milky Way’s center. By studying the RR Lyrae populations within these clusters, researchers can glean insights into the clusters’ ages, distances, and origins.
While Cepheids have been crucial for determining distances to farther galaxies, RR Lyrae stars shine when it comes to understanding the structure of our own galaxy. Their consistent luminosity makes them excellent standard candles for mapping the Milky Way, especially its halo and the central bulge.
The best example of these stars is RR Lyrae itself, from which the class gets its name. This star demonstrates a period close to 0.577 days, and its magnitude varies from about 7.1 to 8.1. Another star in this category is X Arietis, with a period of around 0.651 days and an apparent magnitude ranging between 6.2 and 7.4.
RZ Cephei has an extremely short period of just 0.309 days and sees its magnitude dance between approximately 7.8 and 8.9 – a ~250% change – over this roughly 8-hour time span. RV Ursae Majoris (RV UMa) is slightly longer at 0.468 days, with its brightness oscillating between magnitude 7.7 to 8.9. Lastly, there’s TW Herculis (TW Her), possessing a period of 0.4 days and a magnitude range of 8.0 to 9.1. All of the aforementioned RR Lyrae variables can be seen with binoculars or a small telescope.
Delta Scuti Variables
Dancing to a rhythm much faster than many of their variable star counterparts, Delta Scuti variables offer a captivating glimpse into the world of intermediate-mass stars undergoing rapid pulsations. The hallmark of a Delta Scuti variable is its short period of pulsation, often lasting just a few hours. These rapid pulsations can be due to both radial and non-radial oscillations, leading to patterns in their light curves that can be intricate to decipher. Such stars are generally younger and hotter than the Sun, and their internal structures give rise to these quick-paced oscillations. Studying them not only provides insights into the pulsation mechanics but also offers clues about the evolutionary paths of similar intermediate-mass stars.
The most well-known Delta Scuti variable is Delta Scuti itself, the namesake of this class. Its period is rather swift, clocking in at just about 4.6 hours, and during this time, its magnitude subtly fluctuates around 4.7, though it’s not obvious to a visual observer. Another representative of this class is Caph (Beta Cassiopeiae), with its period of roughly 2.5 hours. During its cycle, observers can note a variance in magnitude from about 2.25 to 2.31, a subtle change but just about visible to the trained eye. Theta2 Tauri has a slightly more obvious brightness fluctuation, ranging from magnitude 3.4 to 3.5 (about 25%) over a 1.8-hour period.
Pulsating Red Giants (Mira-type, Semi-Regular Variables)
Pulsating red giants hold a special place in the cosmos. These mature stars, having exhausted the primary hydrogen fuel in their cores, have expanded and cooled, adopting a reddish hue and a rhythm of pulsation that spans longer periods than many other variable stars. Central to this group are the Mira-type long period variables and the semi-regular variables. Both provide a window into the later stages of stellar evolution and the mysteries of the universe.
Mira-type variable stars are characterized by their significant pulsation periods, which can range from around 100 to several hundred days. Central to this class is the star Mira (Omicron Ceti), whose periodicity stretches to an impressive 332 days. Throughout its cycle, Mira’s brightness can span a vast range from approximately magnitude 2.0 to a faint 10.1.
Another gem in the Mira-type collection is Chi Cygni, which sees its brightness change over a period of close to 408 days. During this time, its magnitude can dive from a readily naked-eye visible 3.3 to a dim 14.2, barely visible in an 8” telescope.
Mira-type variable stars’ brightness can also vary dramatically over these cycles, sometimes by more than a factor of ten. Mira-type red giant stars are cool, luminous, and have extended atmospheres. Their pulsations are driven by a mechanism involving helium shell burning and the opacity of their outer layers.
Other pulsating red giants have some regularity in their pulsations but are not as strictly periodic as Mira-type variables. They are thus termed semi-regular variables. Their periods range widely, from roughly 30 days to more than 1,000 days, and their brightness variations are typically less pronounced than Mira variables. Most semi-regular variables are also red giants or supergiants, lying on the asymptotic giant branch of the Hertzsprung-Russell diagram. They often exhibit multiple periods of variability, which suggests that different pulsation modes are at play simultaneously. Betelgeuse in the constellation Orion is perhaps the most famous of the semi-regular variables. Betelgeuse has recently undergone dramatic dimming and brightness increases that have never been seen in recorded history – as of 2023, the star now shines at its brightest ever seen at magnitude 0, making it among the brightest stars in the night sky.
Mu Cephei, also poetically named the “Garnet Star” for its deep red color as seen through a telescope, is another prominent semi-regular variable with a primary period of approximately 850 days. Its magnitude can shift from around 3.4 to 5.1.
Both Mira-type and semi-regular variables offer a unique insight into the later stages of stellar life. As these stars pulse, they shed material into space, contributing to the cosmic recycling of matter and potentially seeding future generations of stars and planets. Moreover, their variability offers researchers a tool to probe their internal structures and understand the intricacies of stellar evolution.
Eruptive Variables
Flare Stars
Flare stars, or UV Ceti-type variables, are a type of variable star that can display sudden, drastic increases in brightness for a few minutes to a few hours. This phenomenon is due to magnetic activity near the star’s surface, which releases a vast amount of energy in the form of a flare, similar to solar flares seen on our own Sun, but much more intense. Flare stars are often red dwarfs, although they can be other types as well. The increase in luminosity during a flare can range from a small fraction of the star’s normal brightness to several magnitudes, making it many times brighter than usual. The frequency of the flares varies from star to star. Some flare stars might exhibit multiple flares within a single day, while others might flare only once in several days or weeks.
During a particularly violent flare, the spectrum of a flare star often shows strong emission lines, especially in the ultraviolet range, which is a signature of the high-energy processes occurring on the star’s surface. Some extremely energetic flare stars unleash large quantities of gamma rays and X-rays as well.
UV Ceti (Gliese 65) is perhaps the archetype of flare stars and is often used as a reference point for describing the activity of other flare stars. UV Ceti is a binary system of two red dwarfs – one of the nearest star systems to us at just 8.7 light-years away. The pair normally shines at magnitude 12.8, but both stars are capable of producing extremely violent flares – increasing the brightness up to magnitude 6 in just a few minutes, or a brightness increase of over 600 times. One eruption in 1952 increased UV Ceti’s brightness by 4.5 magnitudes – or 75 times – in just 20 seconds.
Another red dwarf star, AD Leonis, is one of the most active flare stars known and is just 16.2 light-years away from us. It displays frequent and strong flaring activity, though most of its luminosity increase is not in the visible spectrum, and thus it usually shines around magnitude 9.3. Proxima Centauri, the nearest star to the Sun and normally magnitude 11, is also a flare star, which could pose problems for life on its known rocky planet, Proxima B. For instance, in 2016, Proxima Centauri erupted at magnitude 6.8, a brightness increase of nearly 50x.
Understanding flare stars is not just a passing observational interest. Given the increasing search for exoplanets around red dwarf stars, knowing about flaring behavior is crucial. Intense flares can have implications for the habitability of planets orbiting these stars, as the high-energy radiation from flares could strip away planetary atmospheres or impact potential life.
T Tauri Stars
Among the youngest and most tempestuous members of the cosmic family, T Tauri stars stand out as an integral phase in the early lives of low and intermediate-mass stars. They serve as pivotal subjects of study, revealing secrets about the nascent stages of star formation and offering a window into the birth of planetary systems.
T Tauri stars are pre-main-sequence stars, implying they have not yet begun the process of stable hydrogen fusion at their cores. These stars are still contracting and are fueled by gravitational energy. Typically less than a few million years old, they exhibit strong chromospheric lines and powerful stellar winds and are often associated with circumstellar disks. This latter trait is especially significant, as these disks can be the sites where planets form.
The variability of T Tauri stars is a consequence of their turbulent interiors, magnetic activity, and often the presence of the aforementioned disks, which can cause irregular changes in brightness as material is accreted onto the star.
T Tauri stars are celestial laboratories, providing researchers with invaluable insights into the processes of early stellar evolution and planetary system formation. Their inherent variability and dynamic environments make them particularly intriguing subjects, offering both challenges and rewards for astronomers dedicated to deciphering the mysteries of stellar birth.
T Tauri itself is the prototype of the class. Located in the Taurus-Auriga Star Forming Region, T Tauri’s brightness can vary widely, fluctuating between magnitudes of 9.3 and 14. This star offers astronomers a direct view into the early stages of stellar evolution and the mechanisms that drive young stars’ variabilities. T Tauri is also the source of Hind’s Variable Nebula (NGC 1555), a bizarre comma-shaped cloud of material that brightens and darkens as dust passes between the nebula and the star.
Wolf-Rayet Stars
Wolf-Rayet (often abbreviated as WR) stars are characterized by their extremely hot surfaces, with temperatures often exceeding 30,000 K. What makes them truly stand out, however, are their strong, broad emission lines, particularly of helium, nitrogen, carbon, and oxygen. These lines hint at the violent winds that are a signature of WR stars, carrying away a significant portion of the star’s mass at speeds that can exceed 2000 km/s.
The fierce winds lead to substantial mass loss, with some WR stars losing the equivalent of the Sun’s mass in just 100,000 years. As a result, while these stars begin their lives as massive entities, they shed a substantial part of their initial mass by the time they reach the WR phase. Wolf-Rayet stars are categorized mainly into three classes based on their spectral lines: WN (dominated by nitrogen lines), WC (dominated by carbon lines), and WO (dominated by oxygen lines). The life cycle of a Wolf-Rayet star is short, at least in astronomical terms. These stars represent one of the final stages for massive stars before they meet their cataclysmic end, either as supernovae or, potentially, gamma-ray bursts. Their sheer power, exhibited through violent winds and dramatic mass loss, serves as a reminder of the dynamic and ever-evolving nature of our universe.
One of the brightest and most noteworthy Wolf-Rayet stars visible from the Northern Hemisphere is WR 136, which is the source of the well-known Crescent Nebula (NGC 6888) in Cygnus. This nebula is the result of the fast stellar winds from WR 136 colliding with the slower moving particles that were ejected during an earlier phase of its life. The star itself shines with an apparent magnitude of around 7.5, easily spotted with binoculars or a small telescope.
Luminous Blue Variables
Luminous blue variables, or LBVs for short, are massive stars, often more than 20 times the mass of the Sun, and are extraordinarily luminous, shining up to a million times brighter than our Sun. They are recognized for their episodic and unpredictable variations in brightness and are known to shed vast amounts of material during their outbursts. These phases of eruptions can last for years to decades, during which their temperature can change significantly. The physical mechanisms driving the intense eruptions and variability in LBVs are not entirely understood, making them subjects of ongoing research. It’s believed that these stars are in a transitional phase and are on their way to becoming Wolf-Rayet stars or are potential supernova candidates.
P Cygni is one of the earlier-recognized members of the LBV class. Found in the constellation Cygnus, this star has been the subject of astronomical curiosity since the 17th century, when its sudden brightening was first observed. Presently, its brightness hovers around magnitude 4.8. P Cygni is known for its prominent spectral lines, which bear evidence of powerful stellar winds and the outflow of materials.
Eta Carinae is arguably the most famous of the LBVs, particularly due to its spectacular eruptions in the 19th century. Nestled within the constellation Carina, this star is accompanied by the Homunculus Nebula, a structure formed from an enormous eruption in the 1840s. Currently, Eta Carinae shines with a magnitude varying between 6.2 and 7.0. For those in the Northern Hemisphere, this gem remains elusive, as it’s exclusively visible from the Southern Hemisphere.
S Doradus is the prototype of the LBV class, lending its name to the S Doradus-type outbursts typical of these stars. Located within the Large Magellanic Cloud, it’s another spectacle reserved for Southern Hemisphere observers. Its variability is pronounced, with magnitudes ranging between 8.6 and 11.5 over a period that spans several months to years.
Novae
Novae are fascinating astronomical events that result from explosive interactions in close binary star systems, typically comprising a white dwarf and a main-sequence companion. The white dwarf, due to its strong gravitational pull, accretes material from its companion. As this material, primarily hydrogen, accumulates on the white dwarf’s surface, it can ignite in a thermonuclear explosion, leading to a sudden and dramatic increase in luminosity.
This outburst, which can last from days to several weeks, causes the star to brighten considerably, sometimes by up to 100,000 times its usual brightness. Following the peak, the nova gradually returns to its original state, and the accretion process can commence anew, leading to potential future eruptions. While novae and supernovae may seem similar due to their explosive natures, they are distinctly different phenomena with varied causes, energies, and outcomes. For a more in-depth exploration of novae, as well as a comparison with supernovae, readers are encouraged to refer to our separate article dedicated to these rare celestial events.
Type 2: Extrinsically Variable Stars
Eclipsing Binary Stars
Eclipsing binaries stand out not due to intrinsic changes in the stars themselves but because of the mutual dance they perform. These systems consist of two stars orbiting around a common center of mass, aligned in such a way that, from our viewpoint on Earth, one star periodically passes in front of the other. This leads to a temporary reduction in the combined light of the system, causing the “eclipsing” effect. Eclipsing binary stars have been fundamental in astrophysics, offering methods to determine stellar masses, sizes, and sometimes even the shapes of stars. Their predictable patterns allow for accurate modeling and, consequently, a deeper understanding of stellar interactions and life cycles.
Algol-type eclipsing binaries, named after their prototype Algol (Beta Persei), are characterized by a well-defined, regular period during which the brightness dips sharply and then returns to normal. The drop in light is usually brief compared to the overall cycle. The light curve of an Algol-type system typically shows a flat brightness level with two sharp dips. Algol shines at magnitude 2.1 normally, but drops to magnitude 3.4 during an eclipse, which occurs every 3 days for a period of 10 hours. Another example is Delta Librae, which sees a drop in its brightness from magnitude 4.9 to 5.9 roughly every 2.3 days.
Beta Lyrae type binaries have continuously varying brightness levels, without a flat section in their light curves. This is because the stars are often so close that they are almost touching, leading to tidal distortions and continuous mutual eclipses. As such, the light curve of these bizarre binary systems showcases a more gradual, continuous change in brightness without the sharp drops seen in Algol types. Beta Lyrae type variables are named after the double star Sheliak (Beta Lyrae), which varies between magnitude 3.25 and 4.36 over a 13-day period.
V356 Sagittarii showcases a smooth and continuous variability in brightness, typical of Beta Lyrae-type binaries. Its apparent magnitude fluctuates between 6.8 and 7.7 over a period of about 20.4 hours. V933 Scorpii in neighboring Scorpius fluctuates in brightness from magnitude 6.7 to 7.5 over an approximately 1.3-day period.
Rotating Variable Stars
Rotating variables are active stars whose brightness varies as they rotate, and different active regions (like star spots) come into and out of view. Two notable subclasses of rotating variables are the BY Draconis and FK Comae Berenices types, each with distinct characteristics.
BY Draconis variable stars are typically small, Sun-like and lower mass stars exhibiting small amplitude light variations due to the presence of starspots (which, in the case of an obvious variable, are unusually large ones) combined with the star’s rotation. We see brightness changes as the spots appear and rotate out of view, as well as flare activity originating from the spots. BY Draconis itself is a triple star system, but its magnitude barely changes to the eye. The same is true of most other BY Draconis variables; at best, they typically fluctuate by less than a twentieth of a magnitude (12.5%).
FK Comae Berenices variables are typically orange and yellow giant stars. Their distinction lies in their unusually fast rotation for giant stars, possibly a result of the merger of a close binary system. They exhibit strong flare activity, too. The brightness changes in FK Comae Berenices variables are due to both star spots and expansive, cool clouds in their chromospheres. Unlike the BY Draconis type, the light variations in these stars are accompanied by strong emission features in their spectra. The prototype of this class, FK Comae Berenices, displays brightness changes between magnitudes 8.14 and 8.33. This fast-rotating giant completes one rotation in a short period of about 2.4 days. UZ Librae fluctuates between magnitudes 8.0 and 8.5, with a longer 4.8-day rotation period.
