Luminosity Classes And Variable Stars Luminosity Classes The

Luminosity Classes And Variable Stars

Luminosity Classes The width of the absorption lines in a star’s spectrum indicates its density. The thinner the line the less the density. Supergiants & Giants are the least dense. In general the less dense a star is the more luminous it will be (because it has more surface area). Luminosity and the thickness of the absorption lines are combined to group stars into Lumniosity Classes. Luminosity Classes are combined with spectral class to describe Stars. The Sun is Class V so … The Sun is a “G 2 V” star.

Luminosity Class Chart Ia Brighter Supergiants Rigel, Betelgeuse Ib Dimmer Supergiants Polaris II Bright Giants Mintaka III Ordinary Giants Arcturus IV Subgiants Achenar For classes I – IV luminosity and luminosity class (density) are inversely related (I’s are brightest) V Main Sequence Stars Sirius, the Sun Main-sequence stars vary in luminosity but their density is about the same. For main sequence stars: Mass determines luminosity

Variable Stars The Luminosity of stars change over time Some Stars change luminosity significantly and cyclically. They get noticeably dimmer, then brighter, then dimmer again. These are called Variable Stars. The change in luminosity is due to a change in size. (Though temperature changes too. )

Types of Variables The period of the change determine the type: Long Period Variables have a period of 100 s of days. They are usually Giant stars. Cepheid Variables have a period of 1 to 100 days. Their period is related to their luminosity so they make good “Standard Candles” and are used to find stellar distances… Polaris is one!

RR Lyrae Variables have periods of a day or less. They are smaller and dimmer than Cepheids and their luminosities are all nearly the same making them good “standard candles” for finding stellar distances Irregular Variables have inconsistent periods and are usually very young and very old stars.