A variable star is a type of star that shines with varied brightness within a certain period. Some can be consistent and periodically change brightness, while others seem random. Variable stars are not rare. American Association of Variable Star Observers (AAVSO) reported that there are more than 1 million variable stars have been identified.

Do you know that every star in our sky is not constant in its brightness? For example, our Sun varies its brightness by about 1% every 11 years. However, our Sun is not considered a variable star because the fluctuation is too small.

For a star to be considered a variable star, its brightness fluctuation must be big enough to be seen from Earth. These changes in brightness can occur for various reasons. It’s a tell-tale sign of the intrinsic nature of the stars or the external factors influence. Therefore it gives us unique insights into the universe.

In this article, we’re going to give you an overview of every type of variable star thoroughly, so we can appreciate the night sky more when we gaze upon it.

Related articles: How Star Brightness is Measured, What is Stellar Magnitude, Why It’s Important

Variable Star Types Overview

Generally, there are 4 major categories of variable stars (there are more subcategories and we won’t go down that deep in this post). You can overview their characteristics and the notable stars (the prototypes) within each group in the table below.

Type of Variable Star	Characteristics	Examples / Prototypes
Pulsating Variables	Stars that rhythmically expand and contract, causing changes in brightness. They exhibit a regular period of variability, aiding in distance measurements.	Cepheid variables, RR Lyrae stars
Eclipsing Binaries	Binary star systems where one star periodically eclipses the other as they orbit, leading to observed variations in overall brightness.	Algol, Beta Lyrae
Cataclysmic Variables	Stars exhibit erratic changes in brightness without a discernible pattern. Their unpredictable nature challenges understanding of underlying mechanisms.	Novae, Dwarf Novae
Irregular Variables	Stars exhibit erratic changes in brightness without a discernible pattern. Their unpredictable nature challenges the understanding of underlying mechanisms.	Mira variables, RV Tauri stars

You can estimate the distribution of different types of variable stars in our night sky through the percentages presented in the bar chart below. These estimates are derived from various sources of astronomical data, including observational studies, sky surveys, stellar population analyses, and classifications of variable stars.

Pulsating Variables

Let’s start with Cepheid and RR Lyrae stars. Both are categorized as Pulsating Variables – the bright and dimming of the stars happened periodically. Here’s a comparison table of Cepheid variables and RR Lyrae stars based on their characteristics, significance, and examples.

Characteristics	Cepheid Variables	RR Lyrae Stars
Period of Pulsation	Longer periods (typically from days to weeks)	Shorter periods (ranging from a few hours to one day)
Luminosity	High intrinsic brightness	Lower luminosity compared to Cepheids
Period-Luminosity Relationship	Strong correlation between period and luminosity	Period-luminosity relationship used for distance measurements
Discoveries	Revolutionized cosmic distance measurements	Used to determine distances to globular clusters
Abundance	Less common compared to RR Lyrae stars	Commonly found in globular clusters
Examples	Delta Cephei, RS Puppis	RR Lyrae (the namesake star), stars in M3 and M15 clusters

What Are Cepheid Variable Stars

Cepheids are typically massive, young, and luminous stars, making them visible over far distances in the universe. They pulsate predictably and regularly, with periods ranging from a few days to several weeks. It was noticed that the longer the period of pulsation, the more luminous the star. This behavior of the stars helps astronomers to determine their distances accurately.

Due to their predictable brightness variations, Cepheid variables serve as essential standard candles for measuring cosmic distances. Edwin Hubble used Cepheid variables in the Andromeda Galaxy to determine it was not a part of the Milky Way.

There are 4 types of Cepheid Variable stars – Classical Cepheids, Type II Cepheids, Anomalous Cepheids (ACEPs), and Double-mode Cepheids. The famous Polaris (north star) is one of Classical Cepheid. ACEPs are short-period pulsating variables – within 0.4 – 2.5 days. They are mostly discovered in dwarf galaxies of the Local Group. However, they have been found in the Milky Way recently via Gaia (Ripepi et al., 2023).

Key Characteristics of Different Types of Cepheids

Here’s a comparative chart highlighting the key characteristics of different types of Cepheids.

Cepheid Type	Classical Cepheids	Type II Cepheids (W Virginis stars)	Anomalous Cepheids (ACEPs)	Double-mode Cepheids
Pulsation Period	Longer periods, typically 1 to 100 days	Shorter periods compared to Classical Cepheids	Varied periods, can be shorter or longer	Dual pulsation modes, simultaneous shorter and longer periods
Luminosity	High luminosity stars, often several thousand times more luminous than the Sun	Lower luminosity compared to Classical Cepheids	Varied luminosities	Moderate to high luminosity compared to other Cepheid types
Mass	Generally higher masses, around 4 to 20 times the mass of the Sun	Lower masses compared to Classical Cepheids	Varied masses	Variable masses, depending on individual stars
Examples	Delta Cephei, Eta Aquilae, Polaris	W Virginis, RZ Sagittarii, RT Aurigae	V19, V473 Lyr	SZ Tau, RV Tauri, DY Ori

RR Lyrae Stars

RR Lyrae stars are typically smaller and fainter than Cepheids but share a similar pulsating behavior. These stars have shorter periods of pulsation compared to Cepheids, typically ranging from a few hours to one day.

Interestingly, they are commonly found in globular clusters, making them useful for studying these ancient stellar systems. Similar to Cepheid variables, RR Lyrae stars allow astronomers to calculate distances to globular clusters and galaxies containing these stars.

The Gaia mission surveyed a total of 140,784 RR Lyrae stars. Among these, 50,220 stars were newly identified as variable stars, marking their first discovery as such (Clementini et al., 2018).

There are mainly two subtypes of RR Lyrae stars based on their light curve shapes and period ratios: RRab and RRc. Both stars are fundamental to studies in astrophysics, especially in understanding stellar evolution, the ages of globular clusters, and the cosmic distance ladder due to their predictable and well-defined period-luminosity relationships.

Key Characteristics of Different Types of RR Lyrae

Here’s a comparative chart highlighting the key characteristics of different types of RR Lyrae.

Characteristics	RRab Stars	RRc Stars
Light Curve	Asymmetric curve with steep rise and slower decline	More symmetric curve with quicker rise and fall
Period	Longer periods (around 0.2 to 1 day)	Shorter periods (around 0.2 to 0.5 days)
Brightness	Higher amplitude in brightness variations	Lower amplitude in brightness variations
Appearance	Brightness changes are more pronounced	Brightness changes are less pronounced
Location	Found in old, metal-poor globular clusters	Present in globular clusters, similar environments
Usefulness	Crucial for distance measurements in astronomy	Also significant for understanding stellar systems

To understand more about the light curve of rising and dimming of brightness between both types of stars, you can take a look at the graph below. As you can see the curve is smoother in RRc which indicates the changing of the brightness is less abrupt than RRab star.

In essence, RRab stars are frequently employed as primary distance indicators due to their well-established period-luminosity relationship. Meanwhile, RRc stars play a complementary role in refining distance estimates and contribute to a more comprehensive understanding of stellar populations and galactic structures.

Related articles: How Star Brightness is Measured, What is Stellar Magnitude, Why It’s Important

Eclipsing Binaries

Next, we move on to the Eclipsing Binaries family. The famous members are Algol and Beta Lyrae. This type of variable stars provide astronomers with valuable insights into the dynamics of eclipsing binary systems. Hence, studying these systems help us to understand the stellar evolution and binary star dynamics in the universe.

Algol a.k.a Demon Star

Algol got its nickname the “Demon Star,” which stems from the Arabic phrase “Al-Ghoul,” meaning “the Demon.” This name was associated with its regular changes in brightness which is referred to as “winking” behavior.

This eclipsing binary three – star system is located in the constellation Perseus. The two primary stars – Algol A and Algol B are classified as a B star (3 times larger than the sun) and a G star (cooler surface temperature than the sun), respectively. From Earth’s vantage point, Algol’s brightness visibly dims every 2.87 days from a magnitude of 2.1 to 3.4. This happens as the dimmer companion passes in front of the brighter star, causing a partial eclipse.

The eclipse behaviour occurs on a precise and predictable schedule, thus permitting astronomers to anticipate observe study the principles of eclipsing binary systems.

Beta Lyrae

Beta Lyrae is another eclipsing binary system situated in the constellation Lyra. It consists of two stars closely orbiting each other, where a swollen and evolving star—classified as a subgiant or giant transfers mass onto its more massive and hotter companion.

Unlike Algol, Beta Lyrae exhibits irregular variations in brightness. Its irregular brightness changes highlight the complexities inherent in interacting binary systems. The mass transfer, along with the distorted shapes of the stars, results in intricate and variable patterns of brightness.

Here’s a comparison table highlighting the characteristics and significance of Algol and Beta Lyrae, two notable eclipsing binary star systems.

Common Name	Known as the “Demon Star”	–
Location	Constellation Perseus	Constellation Lyra
Star Components	Algol A (larger, brighter star) and Algol B (dimmer star)	Two closely orbiting stars
Eclipse Period	Displays regular eclipses every 2.87 days	Eclipses cause irregular brightness changes
Predictability	Eclipses occur predictably and regularly	Irregular variations in brightness due to complex dynamics
Nature of Eclipses	Regular and predictable eclipses	Irregular brightness changes due to mass transfer
Complexity	Known for its predictable eclipses	Shows complexities in interacting binary systems
Educational Value	Often used for educational purposes due to predictability	Offers insights into complexities of mass transfer

Cataclysmic Variable Stars

In the next group, Novae and Dwarf Novae are both types of cataclysmic variable stars. Each of them characterized by distinct outbursts caused by interactions within binary star systems involving a white dwarf.

Their distinct characteristics and behaviors provide astronomers with valuable opportunities to explore the intricacies of stellar interactions and transient astronomical phenomena in the universe.

Novae

Novae occur in binary star systems comprising a white dwarf and a companion star, often a red giant or main-sequence star. As the white dwarf’s strong gravity pulls in material from its companion star, this accreted material accumulates on the white dwarf’s surface.

When a critical amount of accreted material accumulates on the white dwarf’s surface, it undergoes a sudden and rapid nuclear fusion reaction, resulting in a thermonuclear explosion. The explosion causes a rapid and substantial increase in brightness, making the star appear much brighter than its usual luminosity for a limited period.

In addition, these explosions also enrich the surrounding space with heavier elements synthesized during the fusion process, contributing to the chemical evolution of galaxies.

Dwarf Novae

Dwarf Novae are binary star systems comprising a white dwarf and a normal star, typically a red dwarf or a main-sequence star. Unlike novae, Dwarf Novae exhibit recurrent outbursts caused by cyclic increases in the rate of accretion of material onto the white dwarf from its companion.

The increased accretion leads to the formation of an accretion disk around the white dwarf, and instabilities in this disk trigger the outbursts. Dwarf Novae show periodicity in their outbursts – with a characteristic pattern of increasing brightness followed by a gradual return to their quiescent state. Studying these systems provides insights into the physics of accretion disks and the mechanisms triggering periodic outbursts in cataclysmic variable stars.

Here’s a comparison table highlighting the characteristics and significance of Novae and Dwarf Novae, two types of cataclysmic variable stars.

Characteristics	Novae	Dwarf Novae
Binary System	White dwarf and companion star configuration	White dwarf and normal star (often a red dwarf) configuration
Accretion Process	Accretion of material onto the white dwarf from a companion	Recurrent accretion onto the white dwarf triggering outbursts
Thermonuclear Explosion	Sudden and intense brightness increase due to thermonuclear explosion	Regular outbursts caused by increased accretion
Brightness Change	Rapid increase in brightness lasting days to weeks	Periodic outbursts with cycles of brightness fluctuations
Transient Nature	Transient event, returns to original luminosity gradually	Recurrent outbursts with predictable cycles
Chemical Enrichment	Contribution to chemical enrichment of surrounding space	Insights into accretion processes and cataclysmic events

Irregular Variables

Irregular variables showcase erratic changes in brightness without a predictable pattern. Examples of this type of star are Mira Variables and RV Tauri Stars. Their variability can stem from various factors, such as irregular pulsations, changes in mass loss, or instabilities within the star’s atmosphere.

Mira Variables

Mira Variables are red giants in the late stages of their evolution. They have expanded and become pulsating stars. Mira stars undergo long-period pulsations, causing irregular changes in brightness over extended periods, ranging from several months to more than a year.

Their luminosity can vary by several magnitudes, fluctuating significantly as they expand and contract during their pulsation cycle. Researching Mira Variables is essential for comprehending the later phases of stellar evolution, especially the behaviors and processes taking place in red giants as they move towards becoming planetary nebulae and white dwarfs. Our Sun will also become a Mira Variable in approximately 5 billion years.

RV Tauri Stars

RV Tauri Stars are evolved stars that have transitioned from the asymptotic giant branch (AGB) phase – a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) late in their lives. It represents a transitional stage between AGB stars and the formation of planetary nebulae or white dwarfs.

These stars exhibit irregular cycles of brightness variations with alternating deep and shallow minima. The cycles can span several hundred days to a few years, characterized by irregular patterns of variability.

RV Tauri Stars’ variability is attributed to complex pulsations and instabilities in their atmospheres. Their variability can also contribute to the enrichment of surrounding space with elements synthesized in their stellar interiors.

Summary

The captivating brightness changes of Variable stars provide insight into the complexity and mysterious nature of the universe. From the rhythmic pulsations of Cepheids to the erratic ‘winks’ of Irregular Variables, each category offers unique insights into the cosmos. Astronomers continue to decode the nuances of these stellar performers through careful observations and thorough studies.

References

Ripepi, V., Catanzaro, G., Trentin, E., Straniero, O., Mucciarelli, A., Marconi, M., Bhardwaj, A., Fiorentino, G., Monelli, M., Storm, J., De Somma, G., Leccia, S., Molinaro, R., Musella, I., & Sicignano, T. (2023). First spectroscopic investigation of Anomalous Cepheid variables. ArXiv. /abs/2310.20503

Clementini, G., Ripepi, V., Molinaro, R., Garofalo, A., Muraveva, T., Rimoldini, L., Guy, L. P., De Fombelle, G. J., Nienartowicz, K., Marchal, O., Audard, M., Holl, B., Leccia, S., Marconi, M., Musella, I., Mowlavi, N., Eyer, L., De Ridder, J., Regibo, S., . . . Riello, M. (2018). Gaia Data Release 2: Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars. ArXiv. https://doi.org/10.1051/0004-6361/201833374

Disclaimer:

While we strive to provide accurate and reliable information, please be aware that the content of this blog post is subject to a margin of error. The probability of absolute accuracy is not guaranteed.

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