AFTER pondering the weighty question of the mass of the Milky Way galaxy, astronomers have come up with an answer: 42.

That is, our galaxy weighs three times 10 to the power of 42kg - a number written as 3 followed by 42 zeroes, which has echoes of author Douglas Adams's fictional answer to the question of life, the universe and everything in his series Hitchhiker's Guide to the Galaxy.



It seems esoteric but knowing the weight of the galaxy - the amount of matter it contains - is key to solving important astronomical problems.



Of particular interest to astrophysicist Ken Freeman is the nature of so-called dark matter.



Unlike the "ordinary matter" of stars and planets, scientists have only hunches about the nature of the invisible material that, along with "dark energy", they estimate makes up 96 per cent of the universe.



What is it? How is it distributed across the universe? Does it really even exist?



"That's worth knowing," said Professor Freeman, an astrophysicist with Mt Stromlo Observatory and the Australian National University's Research School of Astronomy and Astrophysics in Canberra.



So along with colleagues in Australia, Europe, the US and Britain, he decided to "weigh" a galaxy.



They presented their results this week at the Astronomical Society of Australia's annual meeting in Sydney.



While it's possible to estimate the mass of the entire universe, accurately measuring galaxies, particularly distant ones, is another matter.



The problem is there's no good way to quantify all the dark matter in such galaxies, thus making it difficult to total all the matter, dark and ordinary.



So Professor Freeman and his colleagues chose the Milky Way.



"Because we're inside our galaxy, we can get a more reliable measure of the dark matter content than we can for galaxies outside," he said.



To do so, the group first estimated the "escape velocity" of the galaxy - the speed stars passing near the sun needed to attain in order to escape its gravitational pull.



It did so using the line-of-sight, or radial, velocity of stars crossing the central rotating disc of the galaxy.



The data was collected by the 1.2m Schmidt Telescope of the Anglo-Australian Observatory at Siding Spring, NSW.



The escape velocity, calculated at between 544km/sec and 608km/sec, allowed the team to calculate the Milky Way's mass and weight, as well as the amount of dark matter: 94 per cent.





Originally published as The answer is 42