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When Cosmic Giants Meet Galactic Dwarfs

By David Reneke

What happens when two different-sized galaxies collide?

When two different-sized galaxies smash together, the larger galaxy stops the smaller one from making new stars, according to a study of more than 20,000 merging galaxies. The research also found that when two galaxies of the same size collide, both galaxies produce stars at a much faster rate.

Dr Luke Davies of The University of Western Australia node of the International Centre for Radio Astronomy Research says our nearest major galactic neighbour, Andromeda, is hurtling on a collision course with the Milky Way at about 400,000 km/h but “the two won’t smash into each other for another four billion years or so”.

Previously, astronomers thought that when two galaxies smash into each other their gas clouds are churned up and seed the birth of new stars much faster than if they remained separate. However, Davies says that whether a galaxy forms stars more rapidly in a collision, or forms any new stars at all, depends on if it is the big guy or the little guy in this galactic car crash: “When two galaxies of similar mass collide, they both increase their stellar birth rate”.

However, when one galaxy significantly outweighs the other the researchers found that star formation rates are affected for both, but in different ways. The more massive galaxy begins rapidly forming new stars, whereas the smaller galaxy suddenly struggles to make any at all. This might be because the bigger galaxy strips away its smaller companion’s gas, leaving it without star-forming fuel, or because it stops the smaller galaxy from obtaining the new gas required to form more stars.

So what will happen in four billion years to the Milky Way and Andromeda? Davies says the pair are both relatively large and with similar mass. “As they get closer together they will begin to affect each other’s star formation, and will continue to do so until they eventually merge to become a new galaxy, which some call ‘Milkdromeda’,” he said.

Revealing Mercury’s Ancient Magnetic Field

New data from MESSENGER, the spacecraft that orbited Mercury for 4 years before crashing into the planet recently, reveal that Mercury’s magnetic field is almost four billion years old. The discovery helps scientists piece together the history of Mercury, the closest planet to the Sun about which we knew very little before the mission.

NASA’s MESSENGER probe left Earth in 2004, flew by Mercury for the first time in 2008 and had been orbiting the planet since 2011, sending valuable data back to scientists. A study detailing the planet’s ancient magnetic field was compiled using data obtained by MESSENGER in early 2015 when the probe flew incredibly close to the planet’s surface at altitudes as low as 15 km.

“The mission was originally planned to last one year; no one expected it to go for four,” said Catherine Johnson, a University of British Columbia planetary scientist and lead author of the study. “The science from these recent observations is really interesting, and what we’ve learned about the magnetic field is just the first part of it.”

Scientists have known for some time that Mercury has a magnetic field similar to Earth’s, but much weaker. The motion of liquid iron deep inside the planet’s core generates the field. Mercury is the only other planet besides Earth in the inner solar system with such a magnetic field. There is evidence that Mars once had a magnetic field but it disappeared at some point more than three billion years ago.

When MESSENGER dipped low over the planet, its magnetometer collected data on the magnetism of rocks in Mercury’s surface, revealing the field is between 3.7 and 3.9 billion years old. The planet itself formed around the same time as Earth, just over 4.5 billion years ago.

One of the biggest challenges of the MESSENGER mission was getting the spacecraft into orbit around Mercury. Because the planet is so close to the Sun, there was a risk that the spacecraft would remain in orbit about the Sun rather than be drawn into orbit around Mercury. Engineers also had to deal with the issue of high temperatures.

MESSENGER was designed with a protective sunshield to keep the side of the spacecraft facing the Sun cool. Between 2011 and 2015, MESSENGER completed more than 4000 orbits of the planet.

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at www.davidreneke.com