The Main Sequence
When the stars are in their life's intermediate phases (90%), they occupy a position in the so-called main sequence of stars, which is represented by a strip extending from the upper left to the lower right of the Hertzsprung-Russell diagram. This diagram relates the brightness with the surface colour of the stars.
Ants and Elephants
In the main sequence, the brightest and biggest stars tend to display a blue blaze, emitted by stars with temperatures up to 50 000 ºK and masses up to 100 times the mass of the Sun. On the other hand, the smaller stars tend to display a red blaze, emitted by stars with temperatures close to 3000 ºK and masses that can be as low as 0,08 times the mass of the Sun.
In cooler and smaller stars, the low temperatures often allow the existence of molecules whose formation would be impossible if the temperatures were higher. Among those molecules are the titanium oxide (TiO) and the molecular hydrogen (H2).
The Sun, with a surface temperature of about 6000 ºK, occupies an intermediate position in the sequence.
Hertzsprung-Russell diagram, which relates the luminosity with the temperature and colour of the stars: in black - Population I stars, in red - Population II stars (TRW Inc.)
Evolution of the Sun represented in the Hertzsprung-Russell diagram (Andy Dornan)
In net figures, 4 hydrogen nuclei are necessary to produce 1 helium nucleus. For that reason, as hydrogen is gradually converted into helium, it falls down the available quantity of atomic nuclei needed to hold the internal pressure. So, in order to counterbalance the gravity, the star is forced to increase its activity and to get hotter. This way, as the star becomes old, it moves away from the main sequence, becoming brighter.
The Beginning of the End
On the other hand, the hydrogen available inside its core becomes rarer and, therefore, the star reaches an epoch when the stellar activity is slowed down. For that reason, the star's core is forced to contract in order for the temperature to get higher again, so that the balance can be replaced. This contraction releases energy, which provokes the expansion of the exterior layers of the star. The surface temperature diminishes but the surface size grows. This increase prevails over the temperature decrease and determines the growth of the star's brightness. The star starts to get closer to the red giants field.
The smaller stars often reveal themselves as flare stars, which display sudden brightness increases (in seconds) and return back to normality in some minutes. It's not known for sure what is the cause of the flares, but they may be related to the strong magnetic activity.
The Variability of the Big and Average Sized Stars
Blue giant stars also display some periods of slight variability, which result from violent phenomena of matter loss, caused by the intense activity of these stars.
There are other blue giant stars where the variability results from expansion and contraction cycles. Those are called beta Canis Majoris variables. On the other hand, the previous ones are called Be variables.
Less bright, lower temperature stars may also display some variability, namely those whose magnetic fields are particularly intense.
Cepheids and their Cousins
Finally, it's still worth to mention the cepheid variables, which are stars at the phase of the final exhaustion of the central hydrogen and that become unstable during their path to the red giant phase.
There are several kinds of cepheids. Among them we can emphasize the classical cepheids - stars of the population I, with high metallicity and periods higher than one day; the W Virginis – stars of the population II, with low metallicity and periods also higher than one day but dimmer than the classical cepheids; the RR Lyræ – with low brightness and periods lower than one day.
The pulsing of these stars results from the fact that their exterior layers, by turns, partially block or freely allow the transit of the energetic flow proceeding from the body's interior. It's this mechanism that provokes the variations of the stellar diameter and, therefore, it is responsible for the variations in the temperature (which is smaller when the diameter is bigger). When the surface temperature decreases, the luminosity also decreases. When the first one increases, the latter also increases.
Since there is an exact relation between the absolute average brightness and the periodicity of the cycles (this one is longer when the brightness is higher), these stars are great indicators of the distance of their host galaxies. This way, if a star has a determined pulsing period, its absolute brightness shall be known. Comparing the apparent brightness (seen from the Earth) with the absolute brightness, it's possible for us to calculate its distance.
A natural extension of the cepheids are the RV Tauri variables, with light curves and brightness-periodicity relations similar to the W Virginis variables', but displaying more irregular variability patterns and periods higher than 20 days.
Relation between the absolute brightness and the variability periods of the cepheids, w virginis and rr lyræ (Ediciones Orbis - Astronomia)