PULSATING VARIABLES
Pulsating variables are stars that show periodic expansion and contraction of their surface layers. Pulsations may be radial (where the star remains spherical in shape) or non-radial (where the star may become non-spherical).
Pulsating variables may be distinguished by their pulsation period, their mass, their age, and the nature of their pulsations.
Pulsating variable stars may be subdivided into 5 classes.
Cepheid Variables
- The cepheid variable stars are named after delta-Cephei, the first of their class to be discovered, by astronomer Goodricke in 1784.
- Cepheids are ‘yellow’ stars of type F and G.
- They have periods between 1 and 70 days.
- They have light variations between 0.1 and 2 magnitudes (Note: 5 magnitudes corresponds to a luminosity variation of 100 times).
- Cepheids are extremely important as “standard candles”.
Cepheids as Distance Markers
- During her investigations of Cepheids in the Magellanic Clouds, astronomer Leavitt found a relationship between the period and the absolute brightness of a Cepheid (early 20th century).
- The longer the period of a Cepheid, the greater was its absolute brightness.
- Thus by measuring the period of a Cepheid we immediately know its absolute brightness. We can also measure its apparent brightness and thus calculate how far it is away from us.
( B = Bo / d2 )
- Cepheids thus play a very important role as one of the ways to determine astronomical distance.
- Because of this, Cepheids are referred to as standard candles.
RR Lyrae Stars
- RR Lyrae stars are white giant stars (generally type A). They are older and less massive than Cepheids.
- They have periods from 0.05 to 1.2 days (1.2 to 29 hours).
- They show brightness changes from 0.3 to 2 magnitudes.
- They are of secondary importance to the Cepheids as distance markers (standard candles).
RV Tauri Stars
- RV Tauri type stars are yellow supergiant stars (spectral type G or K).
- They have periods from 30 to 150 days.
- They show brightness variations of up to 3 magnitudes. Their ‘light curve’ shows alternating deep and shallow minima.
- Sometimes they show superimposed very long period variations from 500 to 9000 days (1.3 to 25 years).
Long Period and Irregular Variables
ERUPTIVE (CATACLYSMIC) VARIABLES
Eruptive or cataclysmic variables may be subdivided into 7 groups:
Supernovae
- A supernova may occur near the end of the life of a massive star. The star implodes, and then explodes in a catastrophic release of energy that exceeds any other known explosion in the universe.
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- Supernovae are subdivided into type I and type II supernovae. Type I supernovae are used as standard candles in measuring the distance to distant galaxies.
- A supernova within a few hundred light years of the Earth may destroy most life on the planet.
- Supernovae are thought to occur about every 100 years per galaxy.
Novae
- The word nova means “new”. Novae are so named because their sudden increase in brightness makes it appear as if a new star has appeared in the sky.
- Novae are thought to be due to close binary stars (two stars orbiting around each other). The two stars are a Sun-like star and a white dwarf.
- Because of their proximity, mass is pulled or transferred from the Sun-like star onto the white dwarf. The impact of this mass on the dwarf causes its brightness to increase due to a runaway thermo-nuclear reaction on the surface.
- The brightness increases by 7 to 16 magnitudes in from one to several hundred days. It then slowly fades back to the original brightness in several years.
Other Nova Type Stars
- Recurrent Novae
- these stars are similar to novae, but they have two or more outbursts known to have occurred in their observed history. In all other respects they are identical to novae.
- Dwarf Novae
- these are close binary systems comprised of a Sun-like star, a white dwarf and an accretion disc surrounding the white dwarf. The brightness change is less than 6 magnitudes. They are divided into 3 subclasses (U Geminorum, Z Camelopardalis and SU Ursae Majoris).
- Symbiotic stars
- close binary systems of a red giant and a hot blue star (type O or B), both embedded in a gas cloud. Brightness changes up to 3 magnitudes.
Flare Stars
- Flare stars show violent outbursts over periods of just minutes and with brightness changes of from 1 to 6 magnitudes.
- They are instrinically faint, cool, red stars that are thought to undergo intense outbursts from localised areas on their surface.
- The brightness increase takes only a few seconds, and then returns back to normal in a few (10 to 20) minutes.
- Flare stars are also known as UV Ceti stars, which is the first star of the class found to exhibit this type of behaviour.
RR Coronae Borealis
- Corona Borealis means northern crown. This is the constellation in which the prototype of this type of variable star is found.
- RR Coronae Borealis variables are rare, bright, carbon rich and hydrogen poor stars (spectral types F,G,K,R).
- They spend most of their time at maximum brightness with an occasional fade by as much as 9 magnitudes.
- These fades are irregular, and the star then slowly recovers over a few months to a year.
- They are sometimes referred to as inverse novae.
- The brightness decrease is thought to be due to condensating carbon.
MEASURING VARIABLE STARS