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 white dwarf pulls material from its companion. This can lead to dramatic events like novae and dwarf novae. The interplay between mass transfer and accretion processes creates a variety of fascinating phenomena in these systems.

Pulsating Variables

Mechanisms of stellar pulsations

• 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 ($P \propto \rho^{-1/2}$)

Classification of pulsating variables

• Cepheid variables exhibit period-luminosity relationship used as standard candles for distance measurements (Classical and Type II Cepheids)
• RR Lyrae stars are short-period pulsators found on the horizontal branch help map Galactic structure
• Mira variables undergo large-amplitude pulsations on the asymptotic giant branch with periods of months to years
• Delta Scuti stars are main sequence or slightly evolved A-F type stars with short periods (Altair)
• Beta Cephei stars are hot, massive B-type stars with short-period pulsations (Beta Canis Majoris)
• Slowly pulsating B stars exhibit long-period g-mode oscillations
• RV Tauri variables show alternating deep and shallow minima in their light curves (R Scuti)
• Pulsating white dwarfs 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 Roche lobe overflow forms an accretion disk 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 "period gap"

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 nova eruption
• Nova eruptions eject accreted material produce characteristic light curves with rapid rise and slow decline
• Recurrence time scales vary from years to millennia depending on mass transfer rate 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 ($L_{acc} \approx GM\dot{M}/R$)
• X-ray emission in magnetic systems results from accretion column shock heating