Most of what we photograph in the night sky is effectively frozen. Galaxies and nebulae look tonight the way they looked last year. Variable stars are the exception. Algol fades by more than a full magnitude in a few hours, Delta Cephei pulses with clockwork regularity, and Mira swells from invisible to one of the brightest stars in its corner of the sky over months. Measuring those changes used to belong to professional observatories. With the photometry tools built into Siril, a normal backyard rig can produce a genuine light curve, and in this tutorial we will walk the full path from calibrated frames to a plotted curve.
Why Variable Stars
Variable stars change brightness for fascinating reasons. In an eclipsing binary like Algol, the Demon Star of Perseus, a dimmer companion passes in front of the primary every 2.87 days and the combined light drops from magnitude 2.1 to 3.4. A pulsating star like Delta Cephei breathes, swelling and shrinking over a 5.37 day cycle that carries it between magnitudes 3.5 and 4.4. Mira in Cetus, the prototype of the long period variables, swings between roughly magnitude 3 and magnitude 9 over about eleven months, by turns a naked eye star and a telescope object.
These rhythms are real science. Cepheid periods revealed the true scale of the universe a century ago, and eclipsing binaries still yield stellar masses and radii today. The American Association of Variable Star Observers, the AAVSO, has coordinated amateur measurements since 1911, and backyard photometry still feeds professional research.
What You Need
The requirements are modest. You need a run of light frames on the same star field, taken with identical exposure and gain, ideally spanning a few hours so a fade or a dip has time to show itself. Calibrate the frames first with darks and flats; our guide to calibration frames covers the details, and photometry on raw frames will bake sensor artifacts straight into your measurements. You also want a current version of Siril, 1.2 or newer, which introduced the dedicated photometry mode and light curve plotting. No special camera, no special filter, just consistent data.
Step 1: Plate Solve and Identify the Star
Before measuring anything you need to know exactly which star is the variable, and in an unfamiliar star field that is harder than it sounds. Siril’s Image Plate Solver settles the question. Open the first frame of your sequence, launch the plate solver, and enter your best estimates for focal length and pixel size. If you are unsure of the numbers, iterate: hold the pixel size fixed and nudge the focal length until the solve locks and the console reports your image scale and field center.
With the field solved, right click the image and choose Search Object, type the variable’s name, Algol for example, and Siril annotates it on the frame. No more squinting at a chart wondering which point of light is the one that changes.
Step 2: Align the Sequence
Load the full light sequence and register it with global star alignment, the deep sky algorithm that records the alignment solution in the sequence file itself. For photometry this is a real advantage: no resampled copies are written, so the pixel values you measure are the pixels your camera recorded, and small tracking drifts between frames disappear while the star you selected stays put across the whole run.
Step 3: Cull the Bad Frames
Open the Graphics tab and plot the weighted FWHM of each image against star roundness. Passing clouds, wind gusts, and focus drift betray themselves as a cloud of points far from the healthy cluster. Click a suspicious point and Siril jumps to that frame so you can deselect it. Discarding a handful of frames costs you nothing, while letting them through corrupts the very measurement you set out to make.
Step 4: Measure the Variable
Toggle the photometry mode button, right click the variable star, and Siril draws the measurement apertures and fits a PSF to that star in every image of the sequence. The Plot tab then offers curves for FWHM, roundness, background level, and magnitude. The background curve alone is worth a look, because it tells the story of your session: moonrise, thin haze, or a neighbor’s porch light creeping into the sky.
The magnitude at this stage is relative. Only the variations mean anything until reference stars put them on a scale.
Step 5: Add Reference Stars
Right click three or four comparison stars in the same field and run the sequence photometry on each one. Choose stars of similar brightness to the variable that sit nearby in the frame and well away from the edges, and prefer stars known to be steady; charts from the AAVSO mark comparison stars with their magnitudes, which takes the guesswork out. References are the anchor: when sky transparency drifts during the night, they drift along with it, and the difference between variable and references cancels the noise.
Step 6: Plot the Light Curve
With references in place the Light Curve button activates. Switch the horizontal axis to Julian date, the standard timekeeping of variable star work, and click. Siril computes the calibrated magnitudes and saves a data file, and if gnuplot is installed it renders the plot on the spot. A clean Algol run looks like a flat line with a bowl carved into it: a drop of a bit over a magnitude as the eclipse begins, a flat bottom, then recovery, the whole event wrapped up in about ten hours.
Reading Your Curve
The shape is the science. An eclipsing binary gives sharp, flat bottomed dips on a strict schedule. A Cepheid rises quickly and fades slowly, a sawtooth rather than a bowl. Mira traces a slow swell and collapse measured in months. Read the period from the spacing of the dips, the depth from the vertical drop, and the character from the transitions, and you are doing the same analysis that taught astronomy the distance to the galaxies.
From Here
The identical workflow measures exoplanet transits and asteroid occultations, and the official Siril photometry tutorial walks the process on a hot Jupiter transit if you want a second opinion. If your curiosity wants consequences, the AAVSO welcomes observations from amateur equipment and their comparison charts slot straight into step five. And if you enjoy pulling science from your own frames, our earlier piece on Hertzsprung Russell diagrams is the natural companion to this one.
