MARTIN GEORGE Space Keeping an eye on our stellar visitors

IWAS fascinated to read recently of a study that has identified a number of stars that are likely to be “visiting’’ us from another galaxy.

The measurement that gives them away is their great speed, indicating that they will probably escape our galaxy’s gravitational pull.

The study, performed by researchers at Leiden in the Netherlands, has made use of detailed information on the positions and velocities of stars as measured by Gaia, a spacecraft that has provided data on more than a billion stars and the threedimensional velocities through space of more than seven million of them.

As a result of this, the astronomers have identified 13 stars that may be passing through our galaxy and eventually leave, maybe having originated in a different galaxy to our own.

It is the velocity (the combination of both speed and direction) of a star through space that gives astronomers the information they need in order to calculate where a star was in the past, and where it will be in the future.

On a smaller scale, the same kind of information is vitally important within our solar system (the sun’s “family’’ of objects) when a comet or asteroid is discovered.

Knowing its position and velocity at a particular time reveals its overall path through the solar system and, for example, how closely it will approach Earth.

However, there is a difference with objects so close by. Their motion, as seen from Earth, appears far more rapid, and several observations of an object’s position and velocity go together to produce more accurate knowledge of its past and future path.

The positions and velocities of the stars form a gold mine of information about their paths through our galaxy, and the important result of the measurements of these 13 stars has shown that they are travelling at greater than the escape velocity of the galaxy. Put simply, escape velocity is the velocity needed to leave the gravitational pull of an object forever.

Our sun is in orbit around the centre of our galaxy at a speed of about 225km per second. If, at its distance from the centre of the galaxy, the sun were instead travelling at about 530km per second or more, it would leave the galaxy permanently.

The escape velocity depends on an object’s distance from the centre, and varies between about 600km per second near the centre, to only about two-thirds that value in the outer reaches of the galaxy.

Some stars in the study have been found to be travelling at greater than the velocity required to escape, meaning that they are “unbound’’ to the galaxy. Two of the fastest ones are moving at about 700km per second.

While it is possible for a star that is a normal member of our galaxy to be ejected from it at great speed by physical processes, in the case of the 13 special stars their paths indicate that they are not only likely to leave, but that they may be visiting from another place.

One possibility is that they have come from a nearby galaxy called the Large Magellanic Cloud, one of two galaxies visible as fuzzy patches of light in our southern sky.

It is also possible that they did actually originate from the extreme outer regions of our Milky Way galaxy, but that some physical interactions have caused them to be flung around at high speed.

We don’t know for sure, but if indeed they have come from, for example, the Large Magellanic Cloud, this will give us important information on the physical processes going on in that galaxy.

We have no need to worry about these stars, even though they are travelling at such a high speed through our galaxy. They are nowhere near us.

In any case, even at that speed, it would take an object tens of millions of years to travel from one side of our galaxy to the other!