5.8 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 4/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
Type II Cepheids (historically termed W Virginis stars) have extremely regular light pulsations and a luminosity relation much like the δ Cephei variables, so initially they were confused with the latter category. Type II Cepheids are uncommon stars that belong to the older Population II category, compared to the younger type I Cepheids. The Type II have somewhat lower metallicity, much lower mass of around 0.5–0.6 M☉, somewhat lower luminosity, and a slightly offset period versus luminosity relationship, so it is always important to know which type of star is being observed. They can be identified based on the shape of their light curve. Type II Cepheids are further sub-divided based on their pulsation periods as BL Her stars for periods of 1 to 4 days, W Vir stars for 4 to 20 days, and RV Tau stars for longer periods of up to 100 days. These three subtypes correspond to consecutive states of stellar evolution after the star has exhausted the helium at its core.
RV Tauri variables
These are yellow supergiant stars (actually low mass post-AGB stars at the most luminous stage of their lives) which have alternating deep and shallow minima. This double-peaked variation typically has periods of 30–150 days and amplitudes of up to 3 magnitudes. Superimposed on this variation, there may be long-term variations over periods of several years. Their spectra are of type F or G at maximum light and type K or M at minimum brightness. They lie near the instability strip, forming a higher luminosity extension of the type II Cepheids, while being cooler than type I Cepheids. Their pulsations are caused by the same basic mechanisms related to helium opacity, but they are at a very different stage of their lives.
RR Lyrae variables
These relatively common variable stars are somewhat similar to Cepheids, but are not as luminous and have shorter periods. They are older than type I Cepheids, belonging to Population II, but of lower mass than type II Cepheids. Due to their common occurrence in globular clusters, they are occasionally referred to as cluster-type Cepheids. They also have a well established period-luminosity relationship in the infrared K-band, and so are also useful as distance indicators. As standard candles, they can be detected out to 1 Mpc, which lies within the local group of galaxies. These are low mass giants having an A- or F-type spectrum, and are currently on the horizontal branch. They are radially pulsating and vary by about 0.2–2 in visual magnitude (20% to over 500% change in luminosity) over a period of several hours to a day or more. The category is divided into Bailey subtypes 'a', 'b', and 'c', depending on the shape of the light curve.
Delta Scuti variables
Delta Scuti (δ Sct) variables are similar to Cepheids but much fainter and with much shorter periods. They were once known as Dwarf Cepheids. Delta Scuti variables display both radial and non-radial pulsations modes. They often show many superimposed periods, which combine to form a complex light curve. Their spectral type is usually late A- and early F-type stars, and they lie on or near the main sequence on the H-R diagram. When metallicity is solar, they have masses ranging from about 1.6 times the Sun for slower periods up to 2.4 at higher pulsation rates. With rotation rates of 40 to 250 km/s, Delta Scuti stars show small amplitudes of 0.01–0.03 magnitude with multiple pulsation modes, including many non-radial. For slower rotation rates under 30 km/s, the amplitude is 0.20–0.30 magnitude or more, and they are often radial pulsators. Stars with Delta Scuti-like variations and an amplitude greater than 0.3 magnitude are known as AI Vel-type variables, after their prototype, AI Velorum.
SX Phoenicis variables
These stars are metal-poor, population II analogues of δ Scuti variables and are mainly found in globular clusters. They exhibit fluctuations in their brightness in the order of 0.7 magnitude (about 100% change in luminosity) or so with short periods of 1 to 3 hours. They have masses in the range of 1.0–1.3 solar. Within a cluster, they are referred to as pulsating blue stragglers, presumably being formed from the merger of two ordinary stars in a close binary system. SX Phe variables are slow rotators and most pulsation modes are radial.
Rapidly oscillating Ap variables
The roAp variables are rapidly rotating, strongly magnetic, chemically peculiar stars of spectral type A or occasionally F0, known as Ap stars. Their pulsatation behavior is much like those of Delta Scuti or Gamma Doradus variables found on the main sequence. They have extremely rapid variations with periods of a few minutes and amplitudes of a few thousandths of a magnitude. Unlike Delta Scuti stars, roAp stars pulsate with either a single high frequency or with multiple high frequencies that are closely spaced. However, the isolated high frequencies of roAp stars have also been observed in stars that are not chemically peculiar, and some Delta Scuti stars show pulsation in the roAp range. Thus the distinction is unclear.
==== Long period variables ====
The long period variables are cool evolved stars that pulsate with periods in the range of weeks to several years. All giant stars cooler than spectral type K5 are variable because of radial pulsations. Many variables of this class show longer period secondary variations that run for several hundred to several thousand days. This may change the brightness by up to several magnitudes although it is often much smaller, with the more rapid primary variations superimposed. The reasons for this type of secondary variation are not clearly understood, being variously ascribed to pulsations, binarity, and stellar rotation.
Mira variables