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Intimations of a galactic disaster loom ahead

Ian Sample

Milky Way heading for a crash, but it may take 7 billion years

— PHOTO: AP

NEW PICTURE: An artist’s rendering provided by the Harvard-Smithsonian Centre for Astrophysics shows the latest view of the Milky Way’s structure.

London: Astronomers have announced that the Milky Way is set to crash into a nearby galaxy sooner than they had thought until now.

According to the most detailed measurements yet, scientists admitted to have grossly underestimated the mass of the Milky Way, and so the gravitational pull it exerts on our cosmic neighbours, including the giant Andromeda galaxy.

The oversight means that the two galaxies, which are on a cataclysmic collision course, will slam into one another earlier than scientists had previously predicted.

When the two galaxies meet, powerful shockwaves will compress interstellar gas clouds within them, triggering a dazzling flourish of newborn stars, in a last heavenly hurrah before the giant wreckage slowly dims and dies out.

Fortunately the galactic disaster still lies unfathomably far into the future.

Our solar system is around 28,000 light years from the centre of the Milky Way, itself one of more than 35 galaxies in our cosmic neighbourhood. The Andromeda galaxy, which is twice as wide, is around 2 million light years away.

Karl Menten, an astronomer at the Max Planck Institute for Radio Astronomy in Germany, said that while the galactic collision would happen sooner than expected, there was no cause for alarm. “We still expect it to happen billions of years in the future,” he said.

A team led by Mr. Menten and Mark Reid at the Harvard-Smithsonian Centre for Astrophysics in Massachusetts used a radio telescope called the Very Large Baseline Array (VLBA) to make precise measurements of the Milky Way as it moved through space. As the galaxy rotates, parts that emit radio waves move relative to Earth, allowing the researchers to work out how fast the galaxy is spinning.

The scientists recorded intense radio waves coming from the galaxy’s four spiral arms, where new stars are born. Heat from the stars warms up molecules of alcohol in interstellar gas clouds, which release the energy as radio waves.

The new measurements showed that our solar system is hurtling along at 960,000 kmph, 160,000 kmph faster than thought. “These measurements are revising our understanding of the structure and motions of our galaxy,” said Mr. Menten.

The speedier rotation of the galaxy means its mass must be similar to that of Andromeda, around 270 billion times the mass of the sun, or 33 per cent greater than earlier calculations have suggested. “No longer will we think of the Milky Way as the little sister of the Andromeda galaxy,” said Mr. Reid.

The research was presented at the annual meeting of the American Astronomical Society in Long Beach, California.

Astronomers believe the crunch to end all crunches could happen around the same time our sun is due to burn up the last of its nuclear fuel, within the next 7 billion years. It is highly unlikely that planets and stars will collide. Instead the two galaxies will merge to form a new, large galaxy.

“The galaxies will be dramatically stirred up, but they are very squidgy, so they will stick together and eventually all the stars will die out, and it will become one huge, dead galaxy,” said Gerry Gilmore at the Institute of Astronomy at Cambridge university, who was not involved in the study. “One thing we don’t know yet is whether Andromeda will hit us square on, or whether it will be a glancing blow.”

If the galaxy strikes the side of the Milky Way, it is expected to be pulled back again for further collisions. The whole collision could take many millions of years.

According to Mr. Gilmore, the research does more than bring forward the date of our galactic demise. The work also sheds fresh light on the nature of dark matter, the invisible substance believed to hold galaxies together. Mr. Gilmore said the findings point to more dark matter at the centre of the galaxy that may be colder and more compacted than astronomers thought.

Other astronomers at the meeting reported an updated map of the Milky Way’s spiral arms. It shows two prominent and symmetrical arms spiralling our of the galaxy’s core, which then branch into four separate arms. Earlier observations had confused astronomers by revealing different numbers of spiral arms reaching out from the galaxy’s centre. — © Guardian Newspapers Limited, 2009

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