What Are Variable Stars?

Variable stars are stars whose brightness as seen from Earth changes over time. Unlike most stars that shine with a steady light, variable stars flicker, pulse, or dim in patterns that can be regular or unpredictable.

Analogy

Think of variable stars like streetlights on a timer or dimmer switch. Some streetlights brighten and dim in a predictable rhythm, while others flicker due to electrical issues. The reasons for these changes can be internal (the bulb itself) or external (something blocking the light).

Real-World Example

Imagine watching a lighthouse from the shore. Sometimes the beam is bright, sometimes dim, and sometimes blocked by passing ships. Variable stars behave similarly—sometimes their own processes change their light, sometimes something passes in front of them.

Types of Variable Stars

Intrinsic Variables

Stars that change brightness due to processes within themselves.

  • Pulsating Variables: Like a beating heart, these stars expand and contract, causing their brightness to rise and fall.
    Example: Cepheid variables, which pulse in a regular rhythm.
  • Eruptive Variables: Sudden outbursts or flares make these stars brighten dramatically.
    Example: Novae, where a star suddenly becomes much brighter.

Extrinsic Variables

Stars that change brightness because of external factors.

  • Eclipsing Binaries: Two stars orbit each other. When one passes in front of the other, the combined light dips.
    Example: Algol (the “Demon Star”), famous for its regular dimming.
  • Rotating Variables: Spots or uneven surfaces rotate into view, altering the star’s brightness.

How Are Variable Stars Studied?

  • Light Curves: Graphs showing brightness over time, like a heart monitor for stars.
  • Telescopes & Surveys: Instruments like the Vera C. Rubin Observatory scan the sky nightly, tracking thousands of variable stars.
  • Citizen Science: Projects like AAVSO invite the public to help monitor variable stars.

Common Misconceptions

  • Misconception 1: All stars shine steadily.
    Fact: Many stars are variable, and some change by large amounts.
  • Misconception 2: Only dying stars vary in brightness.
    Fact: Young, middle-aged, and old stars can all be variable.
  • Misconception 3: Variability means instability.
    Fact: Many variable stars are stable in their cycles and can be used as cosmic yardsticks.

Practical Applications

  • Measuring Cosmic Distances: Cepheid variables have a direct relationship between their brightness and pulsation period. This allows astronomers to measure distances to faraway galaxies.
  • Detecting Exoplanets: Eclipsing binaries and periodic dimming can signal planets passing in front of stars.
  • Understanding Stellar Evolution: Studying variable stars reveals how stars change over time, from birth to death.
  • Monitoring Space Weather: Eruptive variables can affect the environment around them, impacting nearby planets.

Memory Trick

“Varying Views”
Imagine a movie theater where the projector bulb flickers, the curtain sometimes covers the screen, and occasionally someone walks in front of the projector. Each scenario changes what you see—just like variable stars change what we see from Earth.

Most Surprising Aspect

Some variable stars, like RR Lyrae and Cepheids, are so reliable in their brightness cycles that they’re used as “standard candles” to measure the size of the universe. The fact that distant galaxies can be mapped using the heartbeat of a single star is both surprising and profound.

Recent Research

A 2021 study published in Nature Astronomy (“Asteroseismology of red giants from TESS: new insights into stellar evolution,” Hon et al., 2021) used data from NASA’s TESS spacecraft to analyze thousands of variable red giant stars. The study revealed unexpected differences in the internal structure of these stars, challenging previous models and improving our understanding of how stars age and die.

Citation: Hon, M., et al. (2021). “Asteroseismology of red giants from TESS: new insights into stellar evolution.” Nature Astronomy, 5, 1109–1117. https://doi.org/10.1038/s41550-021-01435-3

Unique Details

  • Variable stars can be found in every stage of stellar life, from newborn protostars to dying supernovae.
  • Some variable stars are so faint and distant that only space telescopes can detect their changes.
  • The study of variable stars led to the discovery that the universe is expanding.
  • Variable stars are used to calibrate the brightness of other astronomical objects, making them essential for all cosmic measurements.
  • The patterns of variability can be as short as minutes or as long as centuries.

Summary Table

Type Cause of Variability Example Application
Pulsating Variable Internal expansion/contraction Cepheid Measuring distances
Eruptive Variable Sudden outbursts Nova Studying stellar death
Eclipsing Binary One star blocks another Algol Detecting exoplanets
Rotating Variable Surface features rotate BY Draconis Understanding starspots

Conclusion

Variable stars are cosmic beacons whose changing light helps astronomers map the universe, study stellar life cycles, and discover new worlds. Their flickering is not just a curiosity—it’s a vital tool for science, and their reliability is one of the most surprising and useful aspects in astronomy.