Which is the fastest star?
First of all I need to explain what I mean about a “fast star”. For example, we feel the Earth is pretty much still, yet we know it is not. It is rotating quite fast, and turning around the Sun even faster. But the Sun itself travels at a tremendous pace around the Galaxy, which in turn is falling in the Cosmos towards our neighbour Andromeda.
That is not what I mean when I talk about a “fast star”, because all of those are the “natural” movements and the same speeds one would find for every regular planet and star located where the Earth and Sun are.
We are interested in the local speed of a star relative to its neighbourhood, relative to what it should be supposed to do as a “well-behaved” star.
And the winner is PSR B2224+65. The name of this star sounds rather unimpressive, but do not be fooled. It rotates 15 times every second (not very fast for a neutron star), and it moves through its neighbourhood of dust and gas at an amazing 5.8·106 km/h, that is, almost 6 million kilometers an hour (3.6 million miles per hour). Not bad.
But how could this ever happen? Take into account that a neutron star is the remaining core of the dramatic explosion of a giant star.
Before we go any further, consider a party balloon. It does not move around, does it? However, if you open its knot and let air out, it will fly off your hands making a funny trumpeting noise. This is called jet propulsion and is how we got to the Moon.
When an old star explodes something similar happens: all of the gas in the outer layers of the star is expelled away at tremendous speeds. Because this is happening equally in all directions, the core inside remains more or less stationary.
However, if the explosion is not perfectly symmetrical, more gas will go in one particular direction. And the star core, just like our balloon above, will fly in the opposite direction to compensate. Because we are talking stellar masses here, “a bit this way”, “a bit this other” can result in the stupendous speed of old PSR B2224+65.