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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Variable star | 5/10 | https://en.wikipedia.org/wiki/Variable_star | reference | science, encyclopedia | 2026-05-05T13:33:52.261350+00:00 | kb-cron |
Mira variables are aging red giant stars nearing the end of their active life on asymptotic giant branch (AGB). They have radial pulsation periods that can range from under 100 to over 2,000 days, although most are in the 200 to 450 day range. They fade and brighten over a range of 8 magnitudes, a thousand fold change in luminosity. Mira itself, also known as Omicron Ceti (ο Cet), varies in brightness from almost 2nd magnitude to as faint as 10th magnitude with a period of roughly 332 days. The very large visual amplitudes are mainly due to the shifting of energy output between visual and infra-red as the temperature of the star changes. In a few cases, Mira variables show dramatic period changes over a period of decades, thought to be related to the thermal pulsing cycle of the most advanced AGB stars.
Semiregular variables
These are long-period variables with shorter periods and smaller amplitudes than Miras, and their light curves are less regular. Types SRa and SRb are red giants, with the latter type displaying a less regular periodicity. The visual amplitude is typically less than 2.5 magnitudes. They are believed to be precursors of Mira variables, but are longer lived and thus more common. The types SRc and SRd consist mostly of red supergiants and yellow supergiants, respectively. Semiregular variables may show a definite period on occasion, but more often show less well-defined variations that can sometimes be resolved into multiple periods. A well-known example of a semiregular variable is Betelgeuse, which varies in brightness by half a magnitude with overlapping periods of 1.10 and 5.75 years. At least some of the semi-regular variables are very closely related to Mira variables, possibly the only difference being pulsating in a different harmonic.
Slow irregular variables
These are red giants or supergiants with little or no detectable periodicity. Some are poorly studied semiregular variables, often with multiple periods, but others may simply be chaotic. These variables are classified as type Lb or Lc, depending on whether they are cool giants or cool supergiants, respectively. A prominent example of a slow irregular variable is Antares, which is classified as an Lc type with a brightness that ranges from 0.88 to 1.16 in visual magnitude.
==== Beta Cephei variables ====
Beta Cephei (β Cep) variables (sometimes called Beta Canis Majoris variables, especially in Europe) undergo short period pulsations in the order of 0.1–0.6 days with an amplitude of 0.01–0.3 magnitudes (1% to 30% change in luminosity). They are at their brightest during minimum contraction. Many stars of this kind exhibits multiple pulsation periods.
==== Slowly pulsating B-type stars ====
Slowly pulsating B (SPB) stars are hot main-sequence stars slightly less luminous than the Beta Cephei stars, with longer periods and larger amplitudes. They have masses in the range of 2.5–7 M☉, and non-radial pulsation periods from 0.5 to 3 days. Many are rapid rotators, which can cause them to appear cooler and, in some cases, lie outside instability strip.
==== Very rapidly pulsating hot (subdwarf B) stars ====
The prototype of this rare class is V361 Hydrae, a 15th magnitude subdwarf B star. They pulsate with periods of a few minutes and may simultaneous pulsate with multiple periods. They have amplitudes of a few hundredths of a magnitude and are given the GCVS acronym RPHS. They are p-mode pulsators.
==== PV Telescopii variables ====
Stars in this rare class are chemically peculiar type B (Bp) supergiants with a period of 0.1–1 day and an amplitude of 0.1 magnitude on average. Their spectra are peculiar by having weak hydrogen but extra strong carbon and helium lines, making this a type of extreme helium star. The prototype for this category of variable is PV Telescopii, which undergoes small but complex luminosity variations and radial velocity fluctuations.
==== Alpha Cygni variables ====
Alpha Cygni (α Cyg) variables are nonradially pulsating supergiants of spectral classes B to A. Their periods range from several days to several weeks, and their amplitudes of variation are typically of the order of 0.1 magnitudes. The light changes, which often seem irregular, may be caused by the superposition of many oscillations with close periods. The progenitors of these stars have at least 14 solar masses. At least for the brighter members, these variables appear to have returned to the blue supergiant region of the H–R diagram after losing considerable mass as red supergiants. Deneb, in the constellation of Cygnus is the prototype of this class.
==== Gamma Doradus variables ====
Gamma Doradus (γ Dor) variables are non-radially pulsating main-sequence stars of spectral classes F to late A, with luminosity classes of IV-V or V. Their periods are 0.3 to 3 days and their amplitudes typically of the order of 0.1 magnitudes or less. This variable type occupies a narrow range near the low-luminosity part of the instability strip, which partially overlaps the range of Delta Scuti variables. The physical properties of Gamma Doradus variables are similar to long-period Delta Scuti variables. Their slow period and low amplitudes makes Gamma Doradus variables difficult to discover from the ground; most have been spotted by space missions.
==== Solar-like oscillations ====
The Sun oscillates with very low amplitude in a large number of modes having periods around 5 minutes. The study of these oscillations is known as helioseismology. Oscillations in the Sun are driven stochastically by convection in its outer layers. The term solar-like oscillations is used to describe oscillations in other stars that are excited in the same way and the study of these oscillations is one of the main areas of active research in the field of asteroseismology. Stars with surface convection layers that can produce solar-like oscillations are generally cooler than the right edge of the instability strip, which includes the lower main sequence along with subgiants and red giants. However, solar-like oscillations can also be excited by stellar pulsations, such as by Cepheids.