If Two Stars Collide

The morning edition of my favourite daily ‘The Times of Space’ had the following headlines today:

  • CERN Scientists were successful in colliding proton with proton at a record 14,000 GeV in Large Hadron Collider.
  • Two cars made head-on collision in EM Bye pass last evening – one killed
  • Passenger train rammed into goods train near Warangal. More than 60 feared killed.
  • Mid-air collision killed all the passengers on board near Santa Barbara.

My Gosh! Is there no news other than collisions? Oh! Yes, and finally in a coloured box –

Hubble Space Telescope catches evidence of collision between two stars in the globular cluster NGC 6397. (No casualties!).

Well ! We Sky Lovers always think big.

What will happen if two Stars collide?

Can stars collide?  British astrophysicist James Jeans suggested from simple calculation in the early twentieth century that not a single one of the hundred billion stars in our Milky Way galaxy has ever hit another. But the conventional notion that the distances between stars in our galaxy are just to vast for stars to collide, is wrong. Stellar collision may be an unheard event in our environs of the Sun but not an uncommon sight in especially globular clusters where there is very high star traffic.

All the members of such a cluster were supposed to be born roughly at the same time. They are somewhat older stars whose temperatures and brightness evolve in lockstep. Allan Sandage of the Carnegie Institution found in early 1950s in globular clusters, anomalously hot and bright stars called “Blue Stragglers”, which after discarding a dozen theories are now best explained as the outcome of star collisions supported with strong evidence provided by Hubble Space Telescope.

F Paresce, G Meylan and M Shara of Space Telescope Science Institute, found in 1991 that the very centre of the globular cluster 47 Tucanae is jammed with blue stragglers exactly where they should exist in greatest number as predicted by collision theory. D Zurek, Rex Saffer and M Shara during 1997 found the mass of a blue straggler as twice the mass of the most massive ordinary stars in the same cluster by direct measurement, which was thought to be an outcome of stellar coalescence. S G Djorgovsky of California Institute of Technology and his team noted lack of Red Giants near the core of globular clusters. Star collision was held responsible for stripping away their outer layers and transforming them into a different breed.

The Uhuru satellite launched in 1970 to survey the sky for x-ray emitting objects found an anomalously high number of x-ray sources in globular clusters which again are thought to be an outcome of stellar collision by astrophysicists.

Basic Processes that Make Stellar Collisions more Probable :

Evaporation :

Globular clusters contain millions of stars packed in a sphere of typically 60 – 150 light-years in diameter : an equivalent volume near the sun would contain only a hundred stars. These stars zip around on ever changing orbits like bees in a swarm. The close encounter of two or more massive stars redistributes the energy of the system and lower mass stars picking up more energy eject out of the cluster like a molecule from the surface of liquid. The remaining stars loosing energy huddle at the core. Eventually the tightly packed stars began to collide – a process typically occurs over billions of years.

Gravitational Focusing :

In cosmic scale, each star sweeps a very narrow region of space – the average distance between stars is much larger than the stars themselves. But the probability of impact is not a simple matter of a star’s physical cross-section as shown by Jack Hills and Carol Day of University of Michigan during 1975. Stars in globular cluster move at 10 to 20 km per second and gravity gets enough time to act during close encounters of two stars. Without gravity the stars may make head-on collision only but as a result of gravity stars pull each other deflecting their path increasing thereby the chance of collision up to 10,000 times. Over the past 13 billion years half the stars in the core of some of such clusters may have undergone one or more collisions.

Tidal Capture :

Two isolated stars may pair up as a result of very-near-miss encounter suggested A Fabian, J E Pringle and M J Rees of University of Cambridge. Normally in a close encounter two stars approach, gather speed, swing past each other and fly apart unless make contact. But if one of them be a neutron star or a black hole, its enormous gravity can bend the approaching one out of shape resulting in dissipation of energy and thereby preventing its escape – a process called Tidal Capture. The neutron star or black hole in successive passages devours its prey spewing x-rays.

Impact Scenario :

The outcome of any two-body collision depends on the momentum of colliding objects and the collision conditions like whether it is head-on or a side kiss. The astronomers get no chance to video capture a stellar collision. They developed 28 different impact scenarios out of inter-star collisions of seven different basic types of stars viz. black hole, neutron star, white dwarf, brown dwarf, main sequence, red giants and super giants with the help of super computers and a few simple principles that govern the overall effect. Stellar density has the most important role to play in the crashing outcome – the much higher density star will barely suffer any damage like a bullet blowing a watermelon to shreds.

M Shara and his colleagues studied a head-on collision between a sun like star and a white dwarf during the seventies decade. When a sun like star is hit by a white dwarf – about 10 million times denser and traveling at a whopping speed of 600 km a second, a massive shock wave would be created and that would compress and heat the entire star above thermonuclear ignition temperatures. The white dwarf would take only an hour to smash through the star creating irreversible damage. As a result the superheated star would release as much fusion energy in that hour as it normally would in 100 million years. The pressure built up due to compression would force gas outwards at a speed much greater than escape velocity resulting in blowing up of star itself apart in a matter of a few hours. The white dwarf would escape like a highway drunk truck driver least bothering about any fatal damage it created.

The frames of events will look different for impact between two main sequence stars as studied by A Cameron of Yeshiva University and F Seidl of NASA in 1970. The stars would compress and distort each other into half-moon shapes with rise in temperature and density not enough to ignite thermonuclear burning and finally fuse themselves into one within a few hours with a few percent loss of mass. Computer created movies on star collisions are available at http://www.ukaff.ac.uk/movies/collision.mov and http://www.sciam.com.

Crash Crops :

Super giant Red giant Main Sequence  Brown Dwarf White Dwarf Neutron Star Black Hole
Black Hole black hole+ disk + white dwarf black hole+ disk +white dwarf black hole + disk black hole + disk black hole + disk black hole + disk black hole
Neutron Star neutron star or black hole + disk + white dwarf neutron star or black hole + disk + white dwarf neutron star or black hole + disk neutron star or black hole + disk neutron star or black hole + disk neutron star or black hole + disk
White Dwarf white dwarf + white dwarf white dwarf + white dwarf  white dwarf white dwarf or neutron star neutron star or white dwarf    
Brown Dwarf brown dwarf + white dwarf brown dwarf + white dwarf  main sequence main sequence or brown dwarf      
Main Sequence main sequence + white dwarf main sequence + white dwarf main sequence        
Red giant white dwarf + white dwarf white dwarf + white dwarf           
Super giant white dwarf + white dwarf            

Well ! We Sky Lovers should always think bigger.

What will happen if two Galaxies collide?

Reference : 

  1. Scientific American, vol. 287, No. 5, November 2002
  2. Websites
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