Journey To The Center Of The Milky Way

Can Stars Reside Outside a Galaxy ?

We all know that stars originate and die in Galaxies. The Galactic motion plays a vital role in star formation. But can stars reside outside a Galaxy? This was our question. The fateful answer is YES. There are many stars which reside in the space between the galaxies. Though there number is small but still they exist. We can compare galaxies with parents and stars with the sons or daughters of the respective parents. Thus we can say that the stars which are found outside the galaxies are lost from there parent galaxies. Or they are orphan stars. But how did these stars become Orphan? 

Now there are two possibilities:

1. There might have been collision between galaxies which left the stars alone.

2. The Stars might have come in contact with Black Holes which threw them out   of there parent galaxies.

Now both the possibilities stated above have respective conditions. If we find stars lying alone in the space very far away from a galaxy then this might be due to galactic collisions. Again if we find few stars outside a nearby galaxy then this might be due to Black Hole interaction.

We all basically know what can happen during Galactic Collision. This can be understood if one has basic idea of Physics Of collisions. Simple mechanics. But how can a Black Hole send a star out of the galaxy. This point is puzzling. 

We all know about Binary stars. For nerds its just a system of two stars which are Gravitationally held by there mutual gravitational attraction. When a Binary system come closer to a black hole (Very fast spinning) one of the star of the binary system comes in contact of the accretion disk of the Black Hole and suddenly experiences very high spinning motion and this in turn exerts a very high  force on the other star which is gravitationally held. The Spinning action or force is very very high compared to the mutual gravitational pull of the Binary and as the spinning motion is outward the other star attains a very high momentum and starts to move very fast and hence sometimes there high speed motion takes them out of the galaxy. 

For the first time in the year 1997 Astronomers found intergalactic stars in the Virgo cluster which is thought to be due to galactic collision. 

Again astronomers in the year 2005 found a star HE 0437-5439 which is massive, unbound and moving with a very high velocity (hypervelocity) about 723 km/s. This is thought to be due to the interaction of a binary system or a Triple star system with the huge Black Hole situated in the center of our Milky Way Galaxy. Like this there are many stars whose fate is decided by some accident. 

Every night we look into the sky full of twinkling Stars, dazzling our mind. The wonderful sparkling night sky is due to the formation of discrete set of stars from the intersteller molecular clouds or dusts.The fight between the inward gravitational force and the outward pressure force decides the fate of a star. Gravity brings the particles in the molecular clouds closer while the pressure waves radiated from the random agregation of the particles, tend to oppose the gravity. The balance between the two forces creates a protostar which is said to be in Hydrostatic Equilibrium.

Protostar is fully opaque and hence trasfer of energy via radiation is impossible. Thus energy is trasfered outward via convection and the protostar is said to be in convective equilibrium.

The convective equilibrium time period is fully dependent on the mass of the protostellar object. More the mass, less will be the time spent in convective equilibrium. Gravitaion still continue to contract the protostar. During this contraction a radiative core starts to develop. And as the radiative core grows enough to push the convective layer outward or throw it out of the protostar the protostar is said complete its pre- main sequence evolution. After this stage the star still continue to contract and increase its internal temperature. When the internal temperature becomes high enough to ignite the Hydrogen a thermo nuclear reaction or burning of hydrogen begins. At this stage radiation carries the energy outward and thus balancing the inward pull of the gravity. For the first time the star is said to attain an equilibrium between the two forces. The protostar settles in the Main sequence of the HR diagram and becomes a STAR. This way Stars are formed.