The notch is made of many voices
One edge of a rotating star approaches us while the other recedes. Each small patch contributes an absorption notch at a slightly different wavelength. A distant telescope usually receives all those contributions together. Those contributions merge into one broadened notch.
A companion pulls the whole star around their shared center of mass. Its motion shifts every local contribution together. That is the useful Doppler signal. But a dark region on the receding side removes some redshifted light. The combined notch moves toward the blue without the star’s center of mass making that motion.
Open the Lab and choose “A spot without a planet.” There is no companion in this preset. Watch the region cross the visible disk: the flux falls, the line becomes slightly uneven and its centroid changes. Drag the region toward the other side. The sign changes because a different part of the rotational velocity field loses light.
Reading: Dumusque, Boisse & Santos — SOAP 2.0, arXiv:1409.3594v1 · Haywood et al. — CoRoT-7, arXiv:1407.1044v1
Two clues do not always tell the same story
A cool spot changes brightness. A magnetically active bright region can instead suppress convective blueshift: rising bright gas normally contributes a net blueshift, and magnetic activity reduces it. That makes a local contribution redder. A real active region can do both. Brightness alone therefore cannot predict every activity velocity.
The Lab keeps these mechanisms separate. Its dark-region contrast changes the light weights; its advanced convective-shift control changes local line centers. “Bright magnetic region” shows why a small or reversed brightness variation need not mean a small velocity variation. The active-area indicator below the scene is a synthetic area proxy. It is not a measured calcium index or a thermometer.
The photocenter offers another clue. Remove light from the right side and the brightness-weighted apparent position moves left. The Lab computes that displacement and the spectrum from the same surface. A companion also moves the star in the sky, but the animation magnifies that tiny orbit. The plotted angular positions keep their actual model units.
Reading: Dumusque, Boisse & Santos — SOAP 2.0, arXiv:1409.3594v1 · Rajpaul et al. — joint activity processes, arXiv:1506.07304v1
Repetition starts an investigation
A rotating region can repeat. Its shape and the observing schedule can also place power at a harmonic or an alias. A neat sinusoid usefully describes a pattern. Independent observations must then test its orbital interpretation. Real observations help by combining velocities with activity indicators, photometry and, when available, astrometry. Those measurements share a star; they do not necessarily respond to it in the same way.
Our real example contains 71 published HARPS measurements of CoRoT-7. The campaign lasts about 25 days, roughly one stellar rotation. Later chapters compare three declared statistical models on exactly those rows. They do not turn this short interval into an independent planet discovery. The known short orbital period is supplied as external information, and the stellar-activity model remains an approximation.
If you want the original double-lined binary mass experiment, choose “Binary orbit.” That mode keeps the original paired velocities and resolved geometry. It answers a different question: how orbital velocities and the inclination connect to two masses.
Reading: Haywood et al. — CoRoT-7, arXiv:1407.1044v1 · Rajpaul, Aigrain & Roberts — Ghost in the time series, arXiv:1510.05598v1