ROYAL ASTRONOMICAL SOCIETY

31st March 1998

MILKY WAY GALAXY MAY BE SMALLER THAN WE THOUGHT

In a new study, astronomers Dr Michael Merrifield and Dr Robert Olling of the University of Southampton have come up with revised estimates for the size of our galaxy, the Milky Way, and for the rate at which it is turning. They have found that the Milky Way is significantly smaller, and is spinning more slowly, than has previously been assumed. These results are being presented by Dr Merrifield on Thursday 2nd April at the UK National Astronomy Meeting at the University of St Andrews.

Astronomers have known for almost a century that the solar system is located toward the outskirts the Milky Way, and that it follows a roughly circular path around the galactic centre. However, the quantitative details of this picture - how far we are from the centre of the Galaxy and how fast we are travelling on our orbit - have proved hard to pin down. For example, recent estimates of the distance to the galactic centre have ranged from 21,000 light years up to 30,000 light years, with a "best" estimate of around 28,000 light years.

These uncertainties are particularly troubling to astronomers trying to understand the motions of stars in the Milky Way. The way stars move through space is largely controlled by the gravitational pull of our Galaxy as a whole, and the strength of that pull can only be estimated when astronomers know the size of our galaxy, and how rapidly the Sun is orbiting around its centre.

Michael Merrifield and Robert Olling have shown how this problem can be turned on its head. They have looked at several studies of the actual observed motions of stars in the Milky Way. They say that the motion of the stars can only be understood if the Sun is located some 23,000 light years from the centre of the Milky Way, travelling at approximately 185 kilometres per second. Although within the range of existing estimates, these new results suggest that the values usually quoted for our distance from the galactic centre (28,000 light years) and the Sun's speed of rotation in its galactic orbit (220 kilometres per second) are significantly too high.

A lower value for the size of our galaxy has repercussions on a much larger scale: the size of other objects in the Universe are often measured relative to the size of the Milky Way. Thus, if astronomers have previously overestimated the size of our galaxy, they will also have overestimated the sizes of other objects, and, indeed, of the Universe as a whole. It is therefore quite possible that the Universe is some 15% smaller than has been previously believed.

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