Extrinsic Variables

Sometimes, a star whose luminosity is steady appears to vary because of variable external factors that affect how much light reaches us from the star. Extrinsic variability is really only apparent variability, meaning that the object itself is not getting brighter or fainter overall, but its brightness appears to be changing because the object's orientation to us is changing, or some other object is intervening along our line of sight to the object.

• Rotating

An astronomical object whose brightness changes as it rotates. A star that has large spots on it, analogous to sunspots, may have a steady luminosity but appear to change because the star is rotating. As the cooler, darker spots rotate towards and away from Earth, the brightness that we measure from the star changes. Among main sequence stars, examples include the BY Dra type variables. The FK Comae Berenices stars are extremely rapidly rotating giant stars with giant starspots.

• Eclipsing

In a binary star system, two stars orbit each other under their mutual gravitational pull. If the light from one star is fully or partially blocked because the other star moves in front of it, that is an eclipsing binary star system, which will show periodic variability that repeats on the timescale of the orbit. Another example is a planet moving in front of a distant star, which is called a planetary transit.

• Microlensing

Microlensing is a phenomenon that occurs due to the gravitational lens effect. An astronomical object appears to brighten when its light is gravitationally lensed by a massive compact foreground object and focused towards Earth. When a distant star or quasar (the source) becomes aligned with an intervening object (the lens), the bending of its light results in multiple distorted, unresolved images which leads to an observable magnification of the background source. Thus, with microlensing we are able to study objects that are far away and emit very little light.