Stars can vary in shape, NZ research shows

"Twinkle, twinkle little star, far away and shaped like a cigar" may digress a little from the children's rhyme, but a Massey University astronomer has shown it can be true.

Dr Ian Bond said astrophysicists had suspected for some time that stars come in various shapes and sizes, but there were no observations to confirm their suspicions as most stars were too far away.

But researchers have used a technique known as "gravitational micro-lensing" to show that stars are not all spherical: they can also be oval or cigar shaped.

Gravitational micro-lensing occurs when a massive object in space, like a star or even a black hole, crosses in front of a star shining in the background.

The object's strong gravitational pull bends the light rays from the more distant star and magnifies them like a lens. On earth, observers see the star get brighter as the lens crosses in front of it, and then fade as the lens gets farther away. The presence of a planet orbiting the nearby star will modify this otherwise smooth process in a predictable way.

New Zealand researchers have previously used gravitational micro-lensing -- first described by Einstein in 1936 -- to detect two planets orbiting distant stars.

But measurement of the magnifying properties of each gravitational lens also allows estimates of the mass of the lensing star and properties of the more distant "source" star, including its size and shape.

Earth's sun proved to be slightly more cigar shaped than expected, Dr Bond told TVNZ.

"To just compare the distances, our sun is eight light minutes away and this star that we observed was 15,000 light years away," Dr Bond said.

He is part of the 10 year-old Japan-New Zealand astronomy collaboration, Microlensing Observations in Astrophysis (MOA), which uses the Mt John observatory at Tekapo and a very large digital camera.

The project includes four NZ universities and a similar number of universities and observatories in Japan.

- NZPA