Some peculiar stars

We shall now examine some peculiar stars. These stars are, because of their configuration, or their spectral type, outside the usual classification :

Multiple stars

Sometimes, when you look at a star, you may notice that, in fact, there are two or more. It can be a simple optical illusion : two very remote stars, but brought into alignment with the Earth.

But these stars can actually be very close together : this is called a binary system. In such a system, the two stars are orbiting around their common center of mass .

 These stars are actually twins sisters, born together inside the same molecular cloud. They can evolve together or not, depending upon their initial masses.
If one of them becomes a red giant, it can overflow its Roche's lobe. The matter captured by its companion will create an accretion disk and accelerate its evolution.

Triple, and even quadruple, systems can exist, but the more stars there are inside the system, the most unusual it is.

More than half of the stars that you can see in the sky are binary systems.

Variable stars

Some stars can exhibit a variable luminosity. This variation can be quite regular with a period between one day and one year.

 Among these stars, Cepheïds are very important for astronomers, because their pulsating period is directly linked to their magnitude . So, such a star can be used as a cosmic milestone, in order to measure the distance of the galaxy which harbours it.

•  Cepheïds are stars which have finished their main sequence lives, and are pulsating from a deep instability. Their magnitudes not only change by one unity with time, but their surface temperatures vary by 1500 K, and their sizes between 10 and 30 %.

  Their pulsating period takes between 3 and 50 days, and the period of the pulsation pattern is directly related to the star's intrinsic brightness.

  

  These stars are very important in cosmology, they are used as milestones in the universe to measure the distances of nearby galaxies.

• W Virginis stars are like Cepheids, but they shine less.
   
• Mira Ceti stars are red giants, whose variations are localized in the peripheral shell. Their magnitudes can vary between 3 and 9.
  Their pulsation periods take between several months and one year.
   
• RRLyrae stars are red giants stars, of spectral type F, with typical periods of some hours - never more than 24 hours.

Cold giant stars

Some giant stars are very cool : their surface temperatures are less than 2500 K, sometimes less than 2000 K. These stars are red giants whose carbon core is exposed.
Their outer shell is torn by powerful stellar winds, so they show a strong gas corona, and a heavy mass loss.
They are called carbon stars, because one can find a lot of carbon in their corona.

Formerly, these stars were listed in the spectral classes R (2500 K) and N (2000 K). Nowadays, they are called type C, when a lot of carbon lines are present in their spectrum, or type S, if zirconium oxyde is more important.

Brown dwarfs

The current theory about stellar formation says that a star can start fusion reactions only if its mass is greater than 0.084 solar masses.

A star whose mass is about 1/10th the mass of the Sun will be able to start these reactions, but will stop in tens of millions of years.
After this stage, these aborted stars will go on radiating, because of a slow gravitational contraction. The main radiation will be in the infra-red part of the spectrum, the surface temperature of the star being more or less about 1000 K.
Their luminosity, of course, is very faint : about 1/100.000th times the luminosity of the Sun, and this is an absolute magnitude of 17.

These stars are called brown dwarfs. They are very difficult to detect, and, until recently, we thought that they were uncommon.

Recent observations have in fact shown that they are relatively numerous, at least inside molecular clouds, where an infra-red telescope is able to detect them.