Julian Date: 2459026.16
2019-2020: CLXIX
THE DAILY ASTRONOMER
Thursday, July 2, 2020
Remote Planetarium 57: Intrinsic Variable Stars I - Cepheids
Yesterday we discussed the two main types of extrinsic variable stars: eclipsing binaries and rotating stars. Today, we begin our discussion about intrinsic variable stars, those stars whose variability results from processes occurring within the star, itself. As we shall see, the different types of intrinsic variable far outnumber the extrinsic, hence the necessity of dividing the intrinsic variable lesson into more than one part.
The chart below shows a variable star "tuning fork" diagram. While the extrinsic variables subdivide only into two categories, the intrinsic variables subdivide into two categories that each contain multiple subdivisions. Today's RP focuses on Cepheids, one of astronomy's most important types of variable stars. Astronomers were able to use these stars to determine the distances of extremely remote objects, such as globular clusters and galaxies.
We begin with a quick review about stellar luminosity, defined as a star's energy output over the period of one second. A star's luminosity depends on its size and effective temperature. Recall the following equation introduced in RP # 37:
The luminosity increases with the square of the radius and the fourth power of the temperature. If a star expands, its size increases, but its effective temperature will also increase. As its name implies, "pulsating variables" expand and contract. Consequently, a pulsating star's size and temperature -and therefore, luminosity- vary throughout each pulsation cycle. Cepheids, one of the best known types of pulsating variables, operate on this same principle. The mechanism responsible for Cepheid pulsation involves ionized and doubly ionized helium. Helium is ionized when it loses one electron and when it loses both electrons is doubly ionized.
As the helium layer contracts due to the Cepheid's gravitational attraction, it absorbs heat and in the process becomes more ionized. Doubly ionized helium is more opaque than singly ionized helium or neutral helium. As it is more opaque, it collects more heat. (As an analogy, think of how darker colors absorb more heat than lighter ones.) As the doubly ionized helium layer heats up, it begins to expand. The expansion causes the layer to cool. The cooled helium then regains an electron to become singly ionized. The singly ionized helium, being less opaque, allows more heat to escape. The expansion ceases and the gravitational contraction resumes. The compressed layer then heats, the helium becomes doubly ionized and the layer becomes more opaque. The heating induces the next expansion and the cycle continues again.
Cepheid variables have proven to be immensely useful for distance determination purposes. A cepheid's variability period is directly related to its luminosity, hence the term period-luminosity relation. The more luminous the cepheid, the longer the period. Astronomers recognize two types of Cepheids, I and II.
- Type I Cepheids: a.k.a. "Classical Cepheids." High metallicity, Population I stars that are between 4- 20 more massive than the Sun. They can also be as much as 100,000 times more luminous.
- Type II Cepheids: Low metallicity, Population II stars of low mass (less than the Sun.)
Each type has its own period luminosity relation curve, as seen below.