1. Introduction

Variable stars are stars whose brightness as seen from Earth fluctuates over time. These variations can be periodic, semi-periodic, or irregular, and are caused by intrinsic or extrinsic factors.

2. Types of Variable Stars

2.1 Intrinsic Variables

  • Pulsating Variables
    Stars that expand and contract, changing their luminosity.

    • Cepheid Variables: Used as standard candles for measuring cosmic distances.
    • RR Lyrae: Found in globular clusters, useful for mapping galactic structure.
    • Mira Variables: Red giants with long pulsation periods.

  • Eruptive Variables
    Stars that experience sudden outbursts due to processes like flares or mass ejection.

    • Novae: Sudden increase in brightness due to thermonuclear explosions on white dwarfs.
    • Supernovae: Catastrophic explosions marking the death of massive stars.

2.2 Extrinsic Variables

  • Eclipsing Binaries
    Two stars orbiting each other; brightness dips when one passes in front of the other.

  • Rotating Variables
    Brightness changes due to star spots or irregular shapes as the star rotates.

3. Light Curves

A light curve is a graph of brightness versus time. It is the primary tool for studying variable stars.

Light Curve Diagram

4. Causes of Variability

  • Intrinsic Causes: Changes in the star’s structure, temperature, or nuclear processes.
  • Extrinsic Causes: Obscuration by companion stars, planetary transits, or interstellar material.

5. Classification Schemes

  • General Catalogue of Variable Stars (GCVS)
  • American Association of Variable Star Observers (AAVSO)

Each catalog uses unique codes to classify variables based on their light curve and physical properties.

6. Practical Applications

6.1 Distance Measurement

  • Cepheid Variables
    Their period-luminosity relationship enables accurate measurement of distances to galaxies.

6.2 Stellar Evolution

  • Variable stars provide insight into late-stage stellar evolution, mass loss, and supernova mechanisms.

6.3 Exoplanet Detection

  • Eclipsing binaries and transit events help identify exoplanets and study their atmospheres.

6.4 Cosmology

  • Supernovae (especially Type Ia) are used to measure the expansion rate of the universe.

7. Practical Experiment: Observing a Variable Star

Objective:
Monitor the brightness of a known variable star (e.g., Algol) over several nights.

Materials:

  • Small telescope or binoculars
  • Digital camera or photometer
  • Star charts
  • Spreadsheet software

Procedure:

  1. Identify Algol in the night sky using star charts.
  2. Record its brightness at regular intervals each night.
  3. Compare Algol’s brightness to nearby non-variable stars.
  4. Plot the brightness data to produce a light curve.
  5. Analyze the period and amplitude of variability.

Expected Outcome:
A periodic dip in brightness corresponding to Algol’s eclipsing binary nature.

8. Surprising Facts

  1. Cepheid Variables enabled the discovery of the universe’s expansion.
    Edwin Hubble used Cepheids in the Andromeda Galaxy to prove it was outside the Milky Way.

  2. Some variable stars change brightness by a factor of 10,000 or more.
    Supernovae can outshine entire galaxies for brief periods.

  3. Variable stars can impact Earth’s climate.
    The Sun is a variable star; small changes in its output can influence climate over long timescales.

9. Impact on Daily Life

  • Timekeeping:
    Atomic clocks are calibrated using pulsating variables like white dwarf stars.
  • Navigation:
    Variable stars help refine the cosmic distance ladder, improving GPS and satellite accuracy.
  • Climate Studies:
    Solar variability is a component in climate modeling and forecasting.

10. Recent Research

A 2022 study published in Nature Astronomy (“A new class of variable stars discovered in the Gaia data”, Jayasinghe et al., 2022) identified thousands of previously unknown variable stars using data from the Gaia space observatory. This discovery challenges existing classification schemes and suggests that stellar variability is more complex than previously thought.

11. Diagram: Eclipsing Binary System

Eclipsing Binary Diagram

12. Summary Table

Type Cause Example Application
Pulsating Intrinsic Cepheid Distance measurement
Eruptive Intrinsic Supernova Cosmology
Eclipsing Binary Extrinsic Algol Exoplanet detection
Rotating Extrinsic BY Draconis Stellar evolution studies

13. References