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Home SCIENCE What was it like when the Milky Way grew up? | by Ethan Siegel | Starts With A Bang! | Feb, 2024

What was it like when the Milky Way grew up? | by Ethan Siegel | Starts With A Bang! | Feb, 2024

by California Digital News

The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including a multi-armed spiral structure and a central bar, as well as spurs and minor arms, plus a central bulge of stars. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light: produced by new stars. The Southern Pinwheel galaxy is one of the closest and brightest barred spiral galaxies at a distance of just 15 million light-years, and has a similar diameter (118,000 light-years) to our own Milky Way. (Credit: CTIO/NOIRLab/DOE/NSF/AURA; Acknowledgment: M. Soraisam (University of Illinois); Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin)

Our own galaxy, the Milky Way, is both completely normal and absolutely remarkable in a number of ways. Here’s the story of our cosmic home.

The Milky Way galaxy may be just one among trillions present within the observable Universe, but it’s uniquely special for personal reasons to us: it’s our cosmic home. It’s the fertile soil from which our Sun and Solar System, including the bodies that would eventually become planet Earth, sprung some 4.6 billion years ago. All told, it’s composed of a few hundred billion stars, about a trillion solar masses worth of dark matter, a supermassive central black hole of about 4 million solar masses, and a plethora of gas and dust. And that’s no outlier; we’re actually somewhat typical of modern galaxies, with perhaps a hundred billion others similar to our own. We’re neither among the biggest nor the smallest of galaxies, nor are we in an ultra-massive cluster or found in isolation, but rather a modest galaxy group, where we’re the second-largest member.

What does make us special, though, is how evolved our galactic home has become. Some galaxies grow up quickly, exhausting their gaseous fuel and becoming “red and dead” when they lose the ability to form new stars. Some galaxies undergo major mergers, often transforming from gas-rich spirals into gas-free ellipticals in the aftermath of those collisions. Still others experience enormous tidal disruptions, leading to sweeping, distended spiral arms. Not the Milky Way, though. We grew up in exactly a typical fashion. Here’s how we got here.

The Whirlpool Galaxy (M51) appears pink along its spiral arms due to a large amount of star formation that’s occurring. In this particular case, a nearby galaxy gravitationally interacting with the Whirlpool galaxy is triggering this star formation, but all spirals rich in gas exhibit some level of new star birth. (Credits: X-ray: NASA/CXC/SAO/R. DiStefano, et al.; Optical: NASA/ESA/STScI/Grendler)

At the present time, galaxies like the Milky Way are incredibly common. Here are some properties that Milky Way-like galaxies typically display:

  • they contain hundreds of billions of stars,
  • concentrated into a disk-like, or pancake-like shape,
  • surrounded by globular clusters in a halo-like distribution,
  • containing spiral arms that extend radially outward for tens of thousands of light-years in either a flocculent or…

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