RX J1856.5-3754 is a very interesting neutron star. (Well I'd say all neutron stars are interesting, they are composed of matter at incredibly high densities, the stars are about as dense as they can be and still remain matter, pile too much more on to them and their matter would collapse out of existence to form a black hole.)
It was discovered by the ROSAT X-ray satellite telescope, and the source of the x-rays were tied down by the Hubble Space Telescope to be a neutron star located at RA 18h 56m 35.3s | Dec -37º 56' 34.40". It created a lot of interest with those studying neutron stars, as it appeared to be alone in space and quiet... This type of neutron star provides the ideal material to test theories of what kind of 'life-cycle' neutron stars can be expected to have. The more common type of neutron star observed has it's 'native' behaviour obscured by the very energetic (and poorly understood) processes assiociated with the accretion of material onto the star from the remnants of its creating supernova explosion, or material from a companion star. It is also not of the 'pulsar' variety of neutron stars.
As it is quiet, it is reasonable to assume that is must be old and any energetic processes that could have taken place have taken place; there's nothing left to happen to the star. However observations showed the star to have a temperature of 700,000 degrees, which according to models is far too hot for an old, isolated neutron star.
Once source of energy is interstellar dust falling onto its surface; as the star can accelerate material up to about half of the speed of light, this is quite an energetic process.
Later observations by the Hubble telescope threw up a couple more interesting facts, firstly the star had moved since it was first observed! Secondly it was surrounded by a faint glow...
Given it's distance, about 200 light years (making it the closest observed neutron star) and the amount it moved it must have a velocity of about 100 kilometers per second! To give the surrounding glow, if you assume it's caused by infalling interstellar dust, then the dust would have to have a density about 100 times larger than average.(NB Subsequent observations have put the distance out to about 450 light years.)
On the strength of these results time was given on the VLT (very large I think..) instrument to take a very 'deep' image of the star. After about 5 hours of aquisistion the spectacular images were seen, showing RX J1856.5-3754 to be a the tip of a cone-shaped nebula! This is in fact a bowshock from the stars motion through the hydrogen dust filled interstellar medium, similar to a ships in water.
Recent observations made by the Chandra X-ray space telescope, coupled with the Hubble have shown the star to be very small; about 7 miles across, in fact too small (given its mass) to actually exist, objects of such density should have formed a black hole. There are two types of answer to this problem, firstly something is just wrong; either the observations or the underlying theory; or more interestingly this is the sign of something completely new. One new theory postulated is that the object may be a 'quark star
, comprised of free quarks.
All in all a very interesting star, that could be showing us the way to some new physics!