6.4 Pulsating variables and cataclysmic variables
3 min read•july 25, 2024
Stars can pulsate, changing their brightness over time. These pulsations are driven by internal processes like the κ-mechanism and convection. Different types of pulsating stars exist, from Cepheids to white dwarfs, each with unique characteristics.
Cataclysmic variables are binary star systems where a pulls material from its companion. This can lead to dramatic events like novae and . The interplay between mass transfer and accretion processes creates a variety of fascinating phenomena in these systems.
Pulsating Variables
Mechanisms of stellar pulsations
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- κ-mechanism drives opacity-driven pulsations in ionization zones of stellar interiors blocks and releases radiation
- Convective driving transfers energy through convective motions interacts with pulsations
- Stellar structure supports radial and non-radial pulsations (spherically symmetric and asymmetric oscillations)
- Pressure modes (p-modes) and gravity modes (g-modes) propagate through different regions of the star
- Pulsation period-density relation connects a star's pulsation period to its mean density ()
Classification of pulsating variables
- exhibit used as standard candles for distance measurements (Classical and Type II Cepheids)
- are short-period pulsators found on the horizontal branch help map Galactic structure
- undergo large-amplitude pulsations on the asymptotic giant branch with periods of months to years
- are main sequence or slightly evolved A-F type stars with short periods (Altair)
- are hot, massive B-type stars with short-period pulsations (Beta Canis Majoris)
- exhibit long-period g-mode oscillations
- show alternating deep and shallow minima in their light curves (R Scuti)
- include DAV, DBV, and DOV types based on their spectral characteristics
Cataclysmic Variables
Characteristics of cataclysmic variables
- Binary star systems consist of a white dwarf primary and a main sequence or evolved secondary
- Mass transfer occurs through forms an around the white dwarf
- Novae result from thermonuclear runaway on the white dwarf surface can recur (RS Ophiuchi)
- Dwarf novae undergo outburst cycles driven by disk instability mechanism (SS Cygni)
- Magnetic cataclysmic variables include polars with strong magnetic fields and intermediate polars with moderate fields
- Evolutionary stages progress from longer to shorter orbital periods then potentially back to longer periods
- Orbital period distribution shows a gap between 2-3 hours called the ""
Outbursts in cataclysmic variables
- Accretion processes involve disk accretion in non-magnetic systems and stream accretion in magnetic systems
- Critical mass accumulation on the white dwarf surface triggers thermonuclear runaway
- Hydrogen fusion ignition causes rapid temperature increase leading to eruption
- eject accreted material produce characteristic light curves with rapid rise and slow decline
- Recurrence time scales vary from years to millennia depending on and white dwarf mass
- Dwarf nova outbursts cycle through quiescence and outburst states due to disk instability and mass transfer variations
- Accretion energy release converts gravitational potential energy to radiation ()
- X-ray emission in magnetic systems results from accretion column shock heating
Key Terms to Review (22)
Accretion disk: An accretion disk is a structure formed by diffused material in orbital motion around a central body, often a star or black hole. This disk is created when gas, dust, or other matter falls towards the central object due to gravitational attraction and gathers into a flat, rotating disk as it spirals inward, generating significant heat and energy during the process.
Beta cephei stars: Beta Cephei stars are a class of pulsating variable stars that exhibit periodic changes in brightness due to pulsations in their outer layers. These stars are typically found in the spectral types B and A, and they are primarily massive, young stars on the main sequence that undergo non-radial pulsations, causing them to vary in brightness over a time scale of hours to days.
Cataclysmic Variable Star: A cataclysmic variable star is a binary star system that undergoes dramatic changes in brightness due to explosive events, such as novae or dwarf novae. These stars are typically composed of a white dwarf and a companion star, where the white dwarf accretes material from its companion, leading to periodic outbursts. The behavior of these stars connects to broader themes in stellar evolution and the dynamics of binary systems.
Cepheid Variables: Cepheid variables are a type of pulsating star that changes brightness in a regular cycle due to periodic expansion and contraction. These stars are important for measuring cosmic distances because their pulsation period is directly related to their intrinsic brightness, allowing astronomers to determine how far away they are located in the universe.
Delta Scuti Stars: Delta Scuti stars are a class of pulsating variable stars that exhibit changes in brightness due to pulsations in their outer layers. These stars are typically located on the main sequence and are usually of spectral types A or F, with masses between 1.5 and 3 times that of the Sun. The pulsations are driven by both radial and non-radial modes, making them interesting objects for studying stellar structure and evolution.
Dwarf novae: Dwarf novae are a type of cataclysmic variable star system characterized by periodic outbursts caused by the accretion of material from a companion star onto a white dwarf. These outbursts are typically less energetic than those of classical novae and occur on a regular timescale, ranging from days to months. Understanding dwarf novae provides insights into the dynamics of accretion processes and the life cycles of binary star systems.
Light curve analysis: Light curve analysis is the study of the brightness of celestial objects over time, allowing astronomers to gather valuable information about their physical properties and behaviors. This method is crucial for understanding various types of stars, particularly pulsating variables and cataclysmic variables, as the variations in brightness can indicate changes in size, temperature, and composition, as well as providing insights into their evolutionary stages.
Mass transfer rate: Mass transfer rate refers to the speed at which mass moves from one component to another in a system, particularly in the context of astrophysical processes. This rate is crucial for understanding phenomena such as accretion onto compact objects and interactions in binary systems, where one star may draw material from its companion. It plays a key role in determining the evolution of stars and the dynamics of stellar interactions, especially in pulsating and cataclysmic variables.
Mira Variables: Mira variables are a type of pulsating red giant stars that exhibit periodic changes in brightness due to thermal pulsations within their outer layers. These stars are known for their long periods of variability, typically ranging from 80 to 1,000 days, and they can increase in luminosity by up to a factor of 1,000. The changes in brightness are linked to the star's complex internal structure and energy processes.
Nova: A nova is a cataclysmic explosion on the surface of a white dwarf star in a binary system, resulting from the accumulation of hydrogen from its companion star. This explosive event leads to a sudden increase in brightness, often making the star appear significantly brighter for a short period, before it eventually fades back to its original luminosity. Novae are a critical part of understanding stellar evolution and mass transfer processes in binary systems.
Nova eruptions: Nova eruptions are explosive events that occur on the surface of a white dwarf star, resulting from the accumulation and subsequent fusion of hydrogen on its surface. These events can significantly increase the brightness of the star for a short period, often outshining entire galaxies before returning to their normal state. The process of nova eruptions is closely related to cataclysmic variables, where a binary system involves a white dwarf drawing material from a companion star.
Period Gap: The period gap refers to a noticeable absence of pulsating variable stars with periods between 0.1 and 1 day. This phenomenon occurs in the context of different types of pulsating stars, where certain ranges of periods are underrepresented due to the evolutionary stages and physical processes involved in stellar evolution. It highlights important differences in stellar structure and pulsation mechanisms, especially when comparing various types of pulsating variables and cataclysmic variables.
Period-luminosity relationship: The period-luminosity relationship is an important astronomical concept that describes how the intrinsic brightness (luminosity) of certain types of variable stars, especially Cepheid and RR Lyrae variables, correlates with their pulsation periods. This relationship allows astronomers to determine distances to these stars based on their observed brightness and periodic behavior, connecting the behavior of these pulsating stars to broader cosmic measurements.
Pulsating white dwarfs: Pulsating white dwarfs are a type of variable star that undergoes periodic changes in brightness due to pulsations in their outer layers. These stars are remnants of low- to medium-mass stars that have exhausted their nuclear fuel and shed their outer layers, leaving behind a dense core that continues to cool and contract, resulting in these pulsations. The study of pulsating white dwarfs provides important insights into stellar evolution and the internal structure of these compact objects.
Pulsation mode: Pulsation mode refers to the specific patterns of expansion and contraction exhibited by certain types of variable stars, which causes them to vary in brightness over time. These modes are critical in understanding the mechanisms behind pulsating variables, including their stability and the physics governing their behavior, as well as how they relate to cataclysmic events like nova eruptions.
Roche lobe overflow: Roche lobe overflow occurs when a star in a binary system expands and fills its Roche lobe, causing mass transfer from one star to another due to gravitational interaction. This process can lead to significant changes in the structure and evolution of both stars involved, impacting their lifecycles and leading to phenomena like nova explosions or the formation of accretion disks.
RR Lyrae Stars: RR Lyrae stars are a type of pulsating variable star that exhibit regular changes in brightness due to pulsations in their outer layers. These stars are primarily found in globular clusters and are considered important standard candles for measuring astronomical distances because of their predictable luminosity variations.
RV Tauri Variables: RV Tauri variables are a specific type of pulsating variable stars that exhibit regular brightness fluctuations, typically caused by changes in their surface temperature and radius. These stars are characterized by their unique light curves, which often display alternating periods of increasing and decreasing brightness. Their behavior connects them to other pulsating variables and highlights their role in understanding stellar evolution and the late stages of stellar life cycles.
Slowly Pulsating B Stars: Slowly pulsating B stars are a type of pulsating variable star characterized by their low-amplitude brightness variations, typically occurring over a period of several hours to days. These stars are B-type main-sequence stars that exhibit non-radial pulsations, allowing them to change in brightness due to changes in their outer layers. Their unique behavior makes them an important subject in the study of stellar evolution and the physical processes within stars.
Spectroscopy: Spectroscopy is the study of the interaction between electromagnetic radiation and matter, specifically how light is absorbed, emitted, or scattered by substances. This technique allows scientists to analyze the composition, temperature, density, and motion of celestial objects by examining their spectra, connecting it deeply to understanding astronomical phenomena.
Thermal instability: Thermal instability refers to a condition where a system becomes unstable due to variations in temperature, leading to rapid changes in pressure and density. In astrophysics, this phenomenon often plays a crucial role in the heating and cooling processes within interstellar matter, as well as influencing the behavior of pulsating and cataclysmic variable stars. It is fundamentally linked to how energy is distributed and released in these systems, affecting their evolution and dynamics.
White dwarf: A white dwarf is a stellar remnant that forms when a medium-sized star exhausts its nuclear fuel and sheds its outer layers, leaving behind a hot, dense core composed primarily of carbon and oxygen. These remnants represent the final stage of evolution for stars that were not massive enough to become neutron stars or black holes, often leading to important insights about stellar death and evolution.