There’s a hungry galaxy which — in space terms — is not too far away and it’s hurtling towards our Milky Way.
- Scientists discover more detail about neighbouring galaxy Andromeda, particularly its eating habits
- The galaxy has been on feeding frenzies in the past and it is likely to collide with the Milky Way in the future
- Scientists are unsure which galaxy will come out on top
- The researchers have also hinted at the recent discovery’s potential to challenge our understanding of gravity
Its name is Andromeda.
The two are destined for a collision in about 4.5 billion years and astronomers are still unsure which system will cannibalise the other for ultimate supremacy.
What they now also know is that Andromeda has been on a feeding frenzy in the past.
The galaxy’s eating habits were the focus of an international study co-led by Dr Dougal Mackey from the Research School of Astronomy and Astrophysics at the Australian National University.
“What we know about Andromeda is that it’s roughly the same size as the Milky Way, both in terms of distances and terms of mass,” he said.
“The Milky Way is a spiral system; you may see pictures of that type of galaxy that’s flat like a dinner plate with spiral arms, and Andromeda is a very similar system in that sense.”
The study, published in the latest edition of the journal Nature, found evidence of a mass-eating event as long as 10 billion years ago, when Andromeda was still forming.
There was also a second feeding frenzy about 4 billion years ago, along with a spattering of smaller events.
That information provides astronomers with important information about how large galaxies form.
They made the discovery by studying Andromeda’s stellar halo, and realising two gravitationally-bound clusters of stars, known as globular clusters, were orbiting at right angles to each other.
Two gravitationally bound clusters of stars are orbiting Andromeda at right angles to each other.
(Supplied: The University of Sydney)
Feeding frenzies billions of years apart
Astronomers were trying to put together a picture of how Andromeda had grown over time and the orbital direction of the clusters surprised them.
Another of the study’s lead authors, Professor Geraint Lewis from the University of Sydney, said they were able to calculate that the clusters came together at different times — billions of years apart.
“We know that galaxies like Andromeda and the Milky Way were embedded in this thing called the cosmic web — it’s like this sponge-like structure of galaxies and material around us and objects fall into our galaxy from this web.
“What we’ve started concluding is that these episodes of feeding came from different parts of the cosmic web — one came from a completely different direction to another.
“There’s a big feed from one direction, then another feed from another direction.”
Astronomers are enthusiastic about this discovery because they know Andromeda is similar to the Milky Way, but it’s hard to get a good view of our galaxy.
When big galaxies fight, there’s rarely a winner
When the Milky Way consumes a smaller galaxy, it gets a little bigger. Given it and Andromeda are similar sizes, it looks like it will be a fair fight.
“In the end, they’re both going to lose because they’re both beautiful spiral galaxies,” Professor Lewis said.
“When they get close to each other, their mutual gravitational pulls are going to tear them apart. So, they will be a new single — much larger — featureless galaxy after the collision in around 4.5 to 5 billion years time.”
An illustration showing a stage in the predicted merger between our Milky Way and the neighbouring Andromeda galaxy. (Supplied: NASA)
Another, unlikely but remotely possible scenario is one where Earth is destroyed by the cannibalistic showdown, or the Sun is thrown off course.
“It’s not out of the question, but in general the stars in galaxies are spaced sufficiently sparsely that direct collisions between stars are rare,” Dr Mackey said.
“However, it’s possible that the Sun could be thrown out of the merged Andromeda and Milky Way system by the collision, into intergalactic space, and/or a nearby close passage with another star could perturb the Earth’s orbit such that the Earth can no longer support life.
“So it’s not factually incorrect to say there is a risk from the collision.”
Information like this helps scientists understand where galaxies came from because they can piece together a galaxy’s timeline.
“What processes brought the Milky Way into being?” Professor Lewis questioned.
“It’s [about] trying to pin together the actions of gravity over the history of the universe and look at the conditions the universe was in just after the Big Bang.”
Professor Lewis says the big question about our origin will help determine where we are going and what’s going to happen to the Milky Way over the next five, 10, and 15 billion years.
“Putting together the past also tells us a bit about the future,” he said.
Do we have gravity all wrong?
This finding may also eventually help to explain the distribution of dark matter.
Professor Lewis said he had colleagues who believed the presence of Andromeda’s two well-defined orbits did not fit the current understanding of gravity, and raised some questions.
“I’m still on the fence with this,” he said.
“They (some astronomers) are saying we’ve actually got gravity wrong, rather than this being an indication that we understand what’s going on with galaxy evolution.”
“It may tell us more about the distribution of dark matter, but the presence of ordered motion does sort of worry some people that we have gotten something wrong somewhere.”
Though, he quickly added: “Extraordinary claims require extraordinary evidence and if you’re going to say that we’ve got gravity wrong, then you’re going to require some pretty extraordinary evidence”.
There is a small but vocal group of astronomers that continue to say “yep, we need to go back to the drawing board when it comes to gravity”.
“We’re not quite there yet,” Professor Lewis said.