Some stars vary in brightness. The cause can be within the star itself, (intrinsic). Otherwise the cause is external to the star (extrinsic). Lambda in Taurus is a binary system, one star blue, the other white. They are separated by only one tenth of the distance from the sun to the earth, so close that they are distorted to oval shapes. They appear as one star to us. As they revolve around each other, they appear to eclipse each other from our perspective. The eclipse lasts just less than 4 days. The magnitude drops from 3.3 to 4.2 and then returns to 3.3. This variable star is an eclipsing binary, the common extrinsic type.
There are several varieties of intrinsic variable stars. One type is the shell star. They are young, blue, and rotating so rapidly that they throw off shells of gas, causing variations in their starlight. Pleione in the star cluster Pleiades, or Seven Sisters, is one such star.
Another type is the cepheid variable. These are medium-aged stars that are slightly unstable. As they pulsate, they change color, brightness, temperature, and size. Mebsuta in Gemini is an example. It varies from 3.7 to 4.2 in a 10+ day period. Cepheids are an important yardstick in measuring distances to star clusters and even other galaxies. This is because the period of pulsation is mathematically related to its absolute magnitude.
Other Pulsating Variables
Cepheids are an example of pulsating variables. Other pulsating variables do not follow precise relationships of brightness and period and have other designations. For example, the red giant Mira in Cetus varies from 3rd the 10th magnitude over 331 days. In other words, it disappears from view, because 3rd magnitude is reasonably visible, but 10th magnitude stars are utterly invisible. It is visible for only a few weeks each year. Its disappearance and reappearance earned it the title “the wonderful”. It is a long-period variable. Another variable is the supergiant Antares in the constellation Scorpius. It varies from magnitude 0.9 to 1.8 in about 1733 days and is a semi-regular variable.