What Are Variable Stars?

Variable stars are stars whose brightness as seen from Earth changes over time. These changes can occur over hours, days, months, or even years. The variability can be due to intrinsic factors (physical changes in the star itself) or extrinsic factors (external influences such as eclipses by companion stars).

Types of Variable Stars

1. Intrinsic Variables

  • Pulsating Variables: Change brightness due to physical expansion and contraction (e.g., Cepheids, RR Lyrae, Mira variables).
  • Eruptive Variables: Experience sudden outbursts or flares (e.g., Novae, Supernovae, Flare stars).
  • Rotating Variables: Show variability due to spots or non-uniform surface features.

2. Extrinsic Variables

  • Eclipsing Binaries: Two stars orbit each other; brightness dips when one passes in front of the other.
  • Rotating Stars with Spots: Brightness varies as star rotates and spots come into view.

Diagram: Light Curve Example

Light Curve of a Variable Star

Caption: Typical light curve for a Cepheid variable star showing periodic brightness changes.

Why Study Variable Stars?

  • Distance Measurement: Cepheid variables are ā€œstandard candlesā€ for measuring cosmic distances.
  • Stellar Evolution: Observing variability reveals details about star lifecycles.
  • Exoplanet Detection: Eclipsing binaries and transit events help discover exoplanets.

Surprising Facts

  1. Variable Stars Led to the Discovery of the Universeā€™s Scale: Henrietta Swan Leavittā€™s study of Cepheids enabled astronomers to measure the distance to galaxies.
  2. Some Variables Change in Minutes: Flare stars like UV Ceti can brighten and dim in less than an hour.
  3. Variable Stars Can Host Exoplanets: The first confirmed exoplanet (PSR B1257+12 b) was found orbiting a variable starā€”a pulsarā€”in 1992.

Recent Breakthroughs

Gaia Mission & Variable Stars

The European Space Agencyā€™s Gaia mission has catalogued millions of variable stars with unprecedented accuracy. In 2022, Gaia Data Release 3 identified over 10 million variable stars, refining our understanding of stellar populations and galactic structure.

Reference:

  • Gaia Collaboration et al. (2022). ā€œGaia Data Release 3: Variable stars in the Milky Way.ā€ Astronomy & Astrophysics, 668, A1. Link

Machine Learning in Variable Star Classification

Recent research uses machine learning to classify variable stars based on their light curves. Algorithms can distinguish subtle differences in variability, leading to the discovery of new types.

Reference:

  • Jayasinghe et al. (2021). ā€œMachine Learning Classification of Variable Stars in the ZTF Survey.ā€ Monthly Notices of the Royal Astronomical Society, 504(4), 5617-5636.

Project Idea: Build a Variable Star Light Curve Analyzer

Objective:
Create a Python tool to analyze light curves from open-source databases (e.g., AAVSO, ZTF).
Steps:

  1. Download light curve data for selected variable stars.
  2. Plot brightness vs. time.
  3. Use Fourier analysis to determine periodicity.
  4. Classify the star type using machine learning algorithms.

How Variable Stars Connect to Technology

  • Data Science & AI: Processing vast amounts of photometric data requires advanced algorithms and machine learning.
  • Space Telescopes: Missions like Gaia and TESS rely on precise photometric sensors to detect variability.
  • Timekeeping: Pulsars (a type of variable star) serve as cosmic clocks, aiding navigation for spacecraft.
  • Exoplanet Detection: Transit photometry (used by Kepler and TESS) depends on identifying tiny dips in brightness caused by planets passing in front of variable stars.

Observational Techniques

  • Photometry: Measures brightness variations over time.
  • Spectroscopy: Analyzes changes in spectral lines during variability.
  • Astrometry: Tracks position changes due to binary motion.

Key Terms

  • Light Curve: Graph of brightness vs. time.
  • Period: Time taken for one complete cycle of variability.
  • Amplitude: Difference between maximum and minimum brightness.

Variable Stars and Exoplanets

The first exoplanet discovery (1992) was around a variable starā€”a pulsar. Variable stars often have unique environments that challenge planet formation theories. Studying these systems helps refine models of planetary system evolution.

Notable Variable Stars

  • Betelgeuse: Red supergiant, semi-regular variability, famous for its recent dimming event.
  • Algol (Beta Persei): Eclipsing binary, prototype for this class.
  • Delta Cephei: Prototype Cepheid variable, crucial for distance measurements.

Further Reading

Summary Table: Variable Star Types

Type Cause of Variability Example Period Range
Cepheid Pulsation Delta Cephei 1ā€“100 days
RR Lyrae Pulsation RR Lyrae 0.2ā€“1 day
Mira Pulsation Mira ~330 days
Eclipsing Binary Orbital eclipse Algol 0.1ā€“1000 days
Flare Star Magnetic flares UV Ceti Minutesā€“hours
Pulsar Rotational beam PSR B1257+12 Millisecondsā€“seconds

Conclusion

Variable stars are essential for understanding the universeā€™s structure, measuring cosmic distances, and discovering exoplanets. Advances in technology and data science continue to revolutionize their study, making them a dynamic field for both amateur and professional astronomers.

Variable Star Types

Caption: Overview of variable star types and their causes of variability.