which is the smallest star ever observed ?

Depends what you call a star. Neutron stars, white dwarfs and large brown dwarfs can all get labelled stars despite none of them being engaged in fusion. White dwarf stars would be considered a star, but white dwarfs eventually become black dwarfs and they would not be considered a star (no black dwarfs yet exist as the universe isn't old enough to have formed them).

Black Holes are essentially neutron stars with are so massive that they collapse over the tipping point and light is no longer able to escape. Black holes do not typically get labelled stars though. Anti black propaganda? /jk

It isn't just down to mass. Black holes would be classified as stars if that were the case. It's not down to circumference size, Neutron stars are smaller than planets.

Perhaps there's a case for luminosity even though some low end red dwarfs barely shine anything other than a dark dull red.

What you have here is the strike of what is known as the "tyranny of the discontinuous mind", and it affects some areas of the dogmatic scientific community to a degree it really shouldn't.

Basically, all that separates a Jupiter object from a brown dwarf is a bit more mass. All that separates a brown dwarf from a red dwarf is a bit more mass. All that separates the stars that become white dwarfs from the ones fated to become neutron stars is a bit more mass. All that stands between a neutron star and a black hole is once again, just a bit more mass.

It's all just one long continuum of classification, and it's very hard to break up into boxes of "stars" and "not stars" when you get right down to it.

Hackett out.

By size: Neutron star (if you count radio-telescopes as "observed")

only 11 km across

By mass: it would have to be a brown dwarf, between 15 and 80 times the mass of Jupiter.

I do not know which one is the specific "smallest".

Shirley Temple

I want to know that how small is the observed star in main sequence, excluding specific cosmic objects aout of the main sequence.

Do hydrogen bombs count? :) They produce a ball of ultracompressed hydrogen which releases energy through fusion reactions. The only difference is the pressure comes from an implosion instead of from gravity...

Perhaps the pulsar at the center of the crab nebula?

I'm surprised that you asked this question.

read this article. it explains everything.