Interactive observing experiment · November 2026

Can stars change brightness?

A night with GP Andromedae

Follow a real Seestar light curve, try to find the star’s pulsation period, then analyse your own observations.

Star field with GP Andromedae circled in yellow
GP Andromedae in my Seestar S50 image. The yellow circle marks the star I measured.

Many stars shine at nearly the same brightness on human timescales, from one night to many years. The Sun is a good example. Its total energy output varies by only about 0.1% over its roughly 11-year activity cycle. Over billions of years, a star’s luminosity can change substantially as it evolves. NASA’s measurements of solar brightness.

Variable stars can change enough for us to measure over minutes, nights or years. Sometimes the star or system emits more or less light. Sometimes the amount of light that reaches us changes because of how we see it.

  • Pulsating stars expand and contract, changing their temperature and luminosity. Examples include Delta Scuti stars, Cepheids, RR Lyrae stars and Mira variables.
  • Eclipsing binaries such as Algol dim when one star passes in front of the other.
  • Rotating variables change brightness as dark starspots rotate into view, or as a distorted star presents a different shape to us.
  • Eruptive variables can brighten suddenly in a flare or dim when ejected material forms dust that blocks their light.
  • Cataclysmic variables often involve a white dwarf drawing gas from a companion. The incoming gas can produce dramatic outbursts, including novae.
  • X-ray binaries vary as gas falls towards a neutron star or black hole. Their changes are often most prominent in X-rays.

A star can also appear temporarily brighter during gravitational microlensing, when a foreground object bends and magnifies its light. AAVSO guide to variable-star classification.

Some variable stars help astronomers measure distances. Classical Cepheids are particularly useful for measuring distances to nearby galaxies. ESO’s guide to the cosmic distance ladder.

On 13 November 2026, I pointed my Seestar S50 at GP Andromedae, a high-amplitude Delta Scuti variable about 1,780 light-years away in the constellation Andromeda. I recorded 722 exposures of about 10 seconds each over 2 hours and 17 minutes. SIMBAD entry with Gaia parallax.

GP Andromedae was near its faintest about 50 minutes into the session and near its brightest about 85 minutes in. In the stacked image below it looks like an ordinary point of light. The light curve shows the roughly 35-minute rise.

What makes GP Andromedae pulsate?

In Delta Scuti stars such as GP Andromedae, a layer of partly ionised helium helps keep the pulsation going. When compressed, the layer becomes more opaque, so some of the energy flowing out from the interior is temporarily held back. The pressure builds and pushes the gas outward. As the layer expands, it becomes more transparent and energy escapes more easily. Gravity pulls the expanded gas back inward, and the cycle continues. Astronomers call this the opacity mechanism, or kappa mechanism. Study of Delta Scuti pulsations.

From exposures to a light curve

Photometry is the measurement of brightness. In a telescope image, software adds up the light recorded around a star and subtracts the glow of the surrounding sky. Comparing that measurement with reference stars in the same image helps account for changes in observing conditions. Repeat this across many exposures and you can plot a light curve showing how the star’s brightness changes over time.

I imported the Seestar’s per-exposure measurements into my photometry app and matched GP Andromedae across the frames to track its brightness over time. On the magnitude scale, smaller numbers mean brighter stars, so the highest part of the curve is maximum light. You can do the same with your own Seestar CSV files in the light-curve analyser below.

GP Andromedae varies in blue while the orange check star stays nearly steady on the same magnitude and time axes
● GP Andromedae   ● Check star
Both stars are shown on the same time and magnitude axes. Vertical bars show the estimated measurement uncertainty. The orange series was aligned from the original check-star plot.

The check star was measured in the same exposures. Its brightness stays nearly steady while GP Andromedae changes. That supports the conclusion that the large swing comes from GP Andromedae itself.

The Seestar measurements ranged from about 10.57 near minimum to 10.10 near maximum. That difference of 0.47 magnitude corresponds to about 1.5 times as much measured light at maximum. Published study of GP Andromedae.

My Seestar measurements show a smaller swing than the catalogued range of 10.43–11.03 in standard Johnson V. The Seestar measurements were not calibrated to Johnson V, and light from a nearby star may have reduced the apparent swing. Catalogue comparison · Study of the nearby star

Find the pulsation period

Move the slider to test a pulsation period. The blue dots stay at the elapsed times of the observations. Look for the period where the purple model follows the observations most closely.

Typical fit error mag (root mean square). Smaller values mean the model follows the points more closely.

● Seestar observations   ━ Fitted model

The model combines the trial frequency and its first two harmonics to reproduce the fast rise and slower fade. The built-in example uses an approximate, binned trace digitised from the Seestar plot above.

Analyse your own light curve

Using a Seestar? Select the Magnitude_*.csv files from your observing run, one per exposure. Use Shift-click to select a range.

After loading: The plots appear below. With Seestar files, use the dropdowns to choose a variable star and, if available, a reference star. The light curve shows how their brightness changes over time, with the variable star in blue and the reference star in orange. If there are enough measurements, the periodogram shows which trial periods best fit the variable star’s data and marks the strongest candidate when one is found.

Using a light-curve CSV from another app?
  • Time column: elapsed_minutes, elapsed_seconds, JD or timestamp_utc.
  • Brightness column: magnitude or differential_mag.
  • Optional error bars: mag_err or magnitude_error.

Load files for the same star on the same magnitude scale. For separate nights, use timestamps or Julian dates to preserve the gaps. Elapsed times must share a starting point. Timestamps without a time zone are treated as UTC. Files are processed in your browser.

Try it with your own telescope

Altitude is measured above the horizon. 0° is the horizon and 90° is directly overhead. The dropdown shows altitude at your selected time.

Coordinates (J2000)
Published period
Published magnitude range

Best time on the selected night (local clock)

Suggested recording plan

Best months for evening observing

What this variable type means

View source

Use the coordinates to frame your target and save the individual exposures for photometry. To catch an eclipse, check a prediction for the binary you’ve selected.

How the planner estimates visibility

The evening guide samples the middle of each month around 9 p.m. local solar time. The selected-night guide checks the night around your chosen time, from noon to noon. Both require the Sun to be at least 12° below the horizon. The selected-night guide also requires the star to be at least 20° above the horizon. Check moonlight and local obstructions before observing.

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