From Ethan Siegel: “Hubble Catches New Stars, Individually, Forming In Galaxies Beyond The Milky Way” 

Ethan Siegel
May 28, 2018

These six images represent the variety of star-forming regions in nearby galaxies. The galaxies are part of the Hubble Space Telescope’s Legacy ExtraGalactic UV Survey (LEGUS), the sharpest, most comprehensive ultraviolet-light survey of star-forming galaxies in the nearby universe. (NASA, ESA, and the LEGUS team)

LEGUS Hubble Space Telescope’s Legacy ExtraGalactic UV Survey

NASA/ESA Hubble Telescope

The legacy of what humanity’s greatest telescope has to teach us about the Universe continues to grow.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. Although this is from our own galaxy, Hubble can now take us to the stars newly forming in galaxies up to nearly 50 million light years away. (ESA / Hubble & NASA)

Pillars of Creation. in the Eagle Nebula, NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Eagle Nebula NASA/ESA Hubble Public Domain

When large-enough clouds of gas or dust collapse, star formation is inevitable.

An enormous star-forming region in the dwarf galaxy UGCA 281, as imaged by Hubble in the visible and the ultraviolet, as part of the LEGUS survey. The blue light is starlight from hot, young stars reflected off of the background, neutral gas. (NASA, ESA and the LEGUS team)

They don’t simply form in isolation, but in large clusters, usually containing thousands of new stars.

The Whirlpool Galaxy (Messier 51) 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. The LEGUS data has thoroughly confirmed this. ((NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI / AURA))

From its perch above Earth, the Hubble Space Telescope can view these newborn, individual stars in galaxies beyond the Milky Way.

The spiral galaxy NGC 6744, part of the LEGUS survey, showcases new star formation along the spiral arms, where gas and dust are plentiful, but none in the galactic center, which is overwhelmed with stars and contains little gas. (NASA, ESA, and the LEGUS team)

There are a massive variety of star-forming regions nearby, and Hubble’s new Legacy ExtraGalactic UV Survey (LEGUS) is now the sharpest, most comprehensive one ever.

In some galaxies, stars form merely on the outskirts exclusively, leaving the interior regions with only the older stars that pre-existed.(NASA, ESA, and the LEGUS team)

By imaging 50 nearby, star-forming spiral and dwarf galaxies, astronomers can see how the galactic environment affects star-formation.

The dwarf galaxy UGC 5340 is forming stars irregularly, likely due to a gravitational interaction with a companion galaxy that is not pictured here. Gravitational interactions often trigger new star formation, leading to the collapse of interior gas clouds. (NASA, ESA, and the LEGUS team)

In dwarf galaxies, the most active star-formation occurs away from each galaxy’s center.

The dwarf galaxy UGCA 281, shown here as imaged by Hubble in the visible and ultraviolet, is rapidly forming new stars. An older, background population of redder stars is what these newer, bluer stars are superimposed on top of. (NASA, ESA, and the LEGUS team)

Instead, star-formation is concentrated in clusters where the neutral gas is densest, theoretically triggered by gravitational interactions.

The dark swaths that permeate spiral galaxies are neutral clouds of gas and dust, and block visible and ultraviolet light. However, when gravitational collapse triggers the formation of new stars, these regions will light up in pinks and blues as they either ionize or reflect starlight, respectively. (NASA, ESA, and the LEGUS team)

In the spirals, by contrast, waves of star-formation occur along dark, dusty features tracing the spiral arms.

The outskirts of a spiral galaxy, like NGC 3627 shown here, are most often where the newest, youngest, bluest stars can be found in great abundance. (NASA, ESA, and the LEGUS team)

As the newborn stars heat the nearby gas, it ionizes, leading to a pink color as the electrons recombine with hydrogen ions.

Here in spiral galaxy NGC 3368, the newest stars appear to be forming only in the outermost regions of this spiral galaxy, all along what appears to be a single arm. As we learn more about the link between gas, stars, and environment, mysteries like this galaxy will begin to make sense. (NASA, ESA, and the LEGUS team)

Understanding how star-formation is connected to its environment represents the final link between the initial gas and the end population of stars.

By understanding when-and-how stars form under differing environmental conditions, scientists can reconstruct the link between star formation and the galaxies that will give rise to them. Here, the interior, central region of a galaxy doesn’t form new stars, while the outer, dust-rich regions do. (NASA, ESA, and the LEGUS team)

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan