Posts Tagged ‘galaxy’

by Morgan McFall-Johnsen

The NASA scientist Jessie Christiansen made a video that traces our solar system’s movement through the Milky Way as dinosaurs emerged, went extinct, and were replaced by mammals on Earth.

Our sun orbits the galaxy’s center, so many dinosaurs roamed the Earth while the planet was on the other side of the Milky Way.

Our solar system’s orbit keeps us just the right distance from the galaxy’s chaotic center for life to exist.

When dinosaurs ruled the Earth, the planet was on a completely different side of the galaxy.

A new animation by the NASA scientist Jessie Christiansen shows just how long the dinosaurs’ reign lasted — and how short the era of humans has been in comparison — by tracing our solar system’s movement through the Milky Way.

Our sun orbits the galaxy’s center, completing its rotation every 250 million years or so. So Christiansen’s animation shows that the last time our solar system was at its current point in the galaxy, the Triassic period was in full swing and dinosaurs were just emerging. Many of the most iconic dinosaurs roamed the Earth when the planet was in a very different part of the Milky Way.

Christiansen got the idea to illustrate this history when she was leading a stargazing party at the California Institute of Technology in Pasadena. Attendees were astonished when she mentioned that our solar system was across the galaxy when dinosaurs roamed.

“That was the first time I realized that those time scales — archaeological, fossil-record time scales and astronomical time scales — actually kind of match along together,” Christiansen told Business Insider. “Then I had this idea that I could map out dinosaur evolution through the galaxy’s rotation.”

Christiansen said it took her about four hours to make the film using timed animations in PowerPoint. She also noted a couple of minor corrections to the text in her video: Plesiosaurs are not dinosaurs, and we complete a galactic orbit every 250 million years, not 200 million years.

‘A spiral through space’

But galactic movement is more complicated than the video shows. The other stars and planetary systems in the galaxy are also moving, at different speeds and in different orbits. The inner portions spin faster than the outer regions.

What’s more, the galaxy itself is moving through space, slowly approaching the nearby Andromeda galaxy.

“The animation kind of makes it seem like we’ve come back to the same spot, but in reality the whole galaxy has moved a very long way,” Christiansen said. “It’s more like we’re doing a spiral through space. As the whole galaxy’s moving and we’re rotating around the center, it kind of creates this spiral.”

So in the solar system’s rotation around the galactic center, we’re not returning to a fixed point. The neighborhood is different from the last time we were here.

Earth, however, is not drastically different; it still supports complex life. That’s partially thanks to the path of our sun’s galactic orbit.

“Our solar system doesn’t travel to the center of the galaxy and then back again,” Christiansen said. “We always stay about this distance away.”

In other words, even as our solar system travels through the Milky Way, it doesn’t approach the inhospitable center, where life probably wouldn’t survive.

“There’s a lot of stars, it’s dynamically unstable, there’s a lot of radiation,” Christiansen said. “Our solar system certainly doesn’t pass through that.”

That’s a huge part of why dinosaurs, mammals, or any other form of life can exist on Earth.

https://www.businessinsider.com/video-nasa-scientist-dinosaurs-milky-way-2019-10

milky-way-died-once-reborn-noguchi_1024

The Milky Way is a zombie. No, not really, it doesn’t go around eating other galaxies’ brains. But it did “die” once, before flaring back to life. That’s what a Japanese scientist has ascertained after peering into the chemical compositions of our galaxy’s stars.

In a large section of the Milky Way, the stars can be divided into two distinct populations based on their chemical compositions. The first group is more abundant in what is known as α elements – oxygen, magnesium, silicon, sulphur, calcium and titanium. The second is less abundant in α elements, and markedly more abundant in iron.

The existence of these two distinct populations implies that something different is happening during the formation stages. But the precise mechanism behind it was unclear.

Astronomer Masafumi Noguchi of Tohoku University believes his modelling shows the answer. The two different populations represent two different periods of star formation, with a quiescent, or “dormant” period in between, with no star formation.

Based on the theory of cold flow galactic accretion proposed back in 2006, Noguchi has modelled the evolution of the Milky Way over a 10 billion-year period.

Originally, the cold flow model was suggested for much larger galaxies, proposing that massive galaxies form stars in two stages. Because of the chemical composition dichotomy of its stars, Noguchi believes this also applies to the Milky Way.

That’s because the chemical composition of stars is dependent on the gases from which they are formed. And, in the early Universe, certain elements – such as the heavier metals – hadn’t yet arrived on the scene, since they were created in stars, and only propagated once those stars had gone supernova.

In the first stage, according to Noguchi’s model, the galaxy is accreting cold gas from outside. This gas coalesces to form the first generation of stars.

After about 10 million years, which is a relatively short timescale in cosmic terms, some of these stars died in Type II supernovae. This propagated the α elements throughout the galaxy, which were incorporated into new stars.

But, according to the model, it all went a bit belly-up after about 3 billion years.

“When shock waves appeared and heated the gas to high temperatures 7 billion years ago, the gas stopped flowing into the galaxy and stars ceased to form,” a release from Tohoku University says.

During a hiatus of about 2 billion years, a second round of supernovae took place – the much longer scale Type Ia supernova, which typically occur after a stellar lifespan of about 1 billion years.

It’s in these supernovae that iron is forged, and spewed out into the interstellar medium. When the gas cooled enough to start forming stars again – about 5 billion years ago – those stars had a much higher percentage of iron than the earlier generation. That second generation includes our Sun, which is about 4.6 billion years old.

Noguchi’s model is consistent with recent research on our closest galactic neighbour, Andromeda, which is thought to be in the same size class as the Milky Way. In 2017, a team of researchers published a paper that found Andromeda’s star formation also occurred in two stages, with a relatively quiescent period in between.

If the model holds up, it may mean that the evolution models of galaxies need to be revised – that, while smaller dwarf galaxies experience continuous star formation, perhaps a “dead” period is the norm for massive ones.

If future observations confirm, who’s up for renaming our galaxy Frankenstein?

Noguchi’s paper has been published in the journal Nature.

https://www.sciencealert.com/milky-way-star-formation-two-generations-cold-flow-accretion-model-noguchi