Nebula

This Serpent’s Tail is Made of Starry Nebulae

The ancients didn’t have the scientific understanding of nature that we have now. All they could do was look up at the night sky and wonder, which isn’t a bad way to spend time. Part of understanding something is naming it, and when the ancients looked up at the patterns in the stars, they gave them simple names based entirely on their appearances. That’s likely how the Greeks named the constellation Serpens: it looks like a snake, so they called it that.

The Greeks lacked astronomical telescopes, so they never saw any of the rich detail in Serpens that a new image from the European Southern Observatory reveals.

The ancient Greeks didn’t know about the electromagnetic spectrum and infrared light, and they couldn’t have imagined enormously powerful mountain-top observatories like the one at Paranal. Paranal is in Chile, on a continent they didn’t even know existed.

But the name they gave to the squiggle of stars in the northern sky has endured, and modern infrared astronomy has revealed astronomical detail the ancient Greeks could only have imagined.

The new image shows Sh2-54, a nebula named after American astronomer Stewart Sharpless who catalogued over 300 nebulae in the 1950s. It’s an infrared image, and it shows the light from the stars behind all that gas and dust. Sh2-54 is at the tail end of Serpens, along with two other nebulae: the Eagle Nebula and the Omega Nebula. These nebulae are also stellar nurseries, where the gas and dust are contracting into dense cores that become protostars and, eventually, full-blown stars undergoing fusion.

The visible light image shows Sh2-54’s other side. In visible light, the structure of the nebula is clearer, but the light from the stars inside and behind the nebula is blocked. This is what makes infrared astronomy so important: it allows us to see things obscured by gas and dust that block optical light.

A visible-light image of the Sh2-54 nebula, captured by the VLT Survey Telescope at ESO’s Paranal Observatory in Chile. At these wavelengths, the structure of the nebula is clear, and the clouds of dust and gas block the light of stars within and behind it. Image Credit: ESO

The Serpens constellation is actually split into two non-contiguous parts, separated by the Ophiuchus constellation, the ‘Serpent Bearer.” Serpens Caput, meaning serpent head, is to the west and Serpens Cauda, the serpent tail, is to the east.

The Eagle Nebula (M16) is near Serpens Cauda, the serpent’s tail. It’s one of the most famous nebulae and hosts the well-known and often-imaged ‘Pillars of Creation.’ The Pillars of Creation were made famous by the Hubble Space Telescope, and the James Webb Space Telescope has also captured an illustrious image of the remarkable structure.

The Pillars of Creation are set off in a kaleidoscope of colour in the NASA/ESA/CSA James Webb Space Telescope’s near-infrared-light view. The pillars look like arches and spires rising out of a desert landscape but are filled with semi-transparent gas and dust and are ever-changing. This is a region where young stars are forming. Image Credit: NASA/ESA/CSA

The stunning Omega Nebula (M17) is also in the region. It’s ‘below’ M16 at the end of Serpens Cauda. It’s often considered part of the Sagittarius Constellation. Omega is considered one of the brightest and most active star-forming regions in the Milky Way.

This image of the rose-coloured star-forming region Messier 17 was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It is one of the sharpest images showing the entire nebula, and not only reveals its full size but also retains fine detail throughout the cosmic landscape of gas clouds, dust and newborn stars. Image Credit: By ESO – http://www.eso.org/public/images/eso1537a/, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=43587748

An open cluster of stars called NGC 6618 is embedded in the Omega Nebula. Light from hot young stars makes the gases in the nebula shine. It’s one of the youngest clusters known and holds about 800 stars. Another 1,000 stars, maybe more, are still forming in the cluster’s outer regions.

M17, the Omega Nebula, is just below M16, the Eagle Nebula, near the end of the Serpens Cauda, the serpent’s tail. Image Credit: freestarcharts.com

Because it’s so close and is such an active star-forming region, the Omega nebula is a well-studied structure. Researchers have used the Spitzer Infrared Telescope to probe the nebula in detail. An image from 2010 showed that the nebula has two distinct regions: an older Post-Star-Forming Bubble and a younger Dark Star-Forming Cloud called M17 SWex.

M17 SWex hasn’t spawned any of the most massive types of stars yet, known as O-type stars. But these hot behemoths have formed in the center nebula and are lighting it up ferociously. They’ve also blown a hole, or “Bubble,” in the gas called M17 EB. Eventually, o-type stars will form in the Dark Cloud and will light it up like the nebula in the center.

This is a three-colour composite image of M17 (Omega Nebula) from the Spitzer Telescope that shows infrared observations from two Spitzer instruments. Blue represents 3.6-micron light, and green shows light of 8 microns, both captured by Spitzer’s infrared array camera. Red is 24-micron light detected by Spitzer’s multiband imaging photometer. Image Credit: NASA/JPL-Caltech/M. Povich (Penn State Univ.)

These images show the power that infrared astronomy has to reveal nature’s structure. The ESO created a draggable, two-part image that exemplifies what infrared astronomy can do.

Click on the image to visit the ESO page with the draggable image. Image Credit: ESO/VVVX

The work of Hellenistic astronomers, that is, astronomers inside and outside of Greece who spoke Greek, is a major phase in the history of astronomy. Most of the names of stars and constellations in the northern hemisphere stem from Hellenistic astronomers, though their work also rests on previous work by Babylonian, Egyptian, and other astronomers.

Many Greek astronomical writings were lost. The Great Library of Alexandria held many of them, but that knowledge was lost as the Library and the society that held it declined over time. Eventually, the Library was destroyed sometime around AD 270. We’ll never know for sure what was lost.

But the names live on, a good reminder of what can be gained and what can be lost and how humans have worked so hard for so long to understand nature.

Two of the sky’s more famous residents share the stage with a lesser-known neighbour in this enormous three gigapixel image from ESO’s VLT Survey Telescope (VST). On the right lies the faint, glowing cloud of gas called Sharpless 2-54, the iconic Eagle Nebula (Messier 16) is in the centre, and the Omega Nebula (Messier 17) to the left. This cosmic trio makes up just a portion of a vast complex of gas and dust within which new stars are springing to life and illuminating their surroundings. Image Credit: ESO

Celestial objects are named much differently now. When something new is discovered, it’s often given an unwieldy, technical name based on how it was found. For example, there’s a quasar named SDSSp J025518.58+004847.6, a name full of technical information but difficult to relate to and not likely to inspire the imagination. Other objects are named after the discoverer, like the nebula in the leading image. In other cases, things are named after celebrities, like the asteroid Freddie Mercury. There are so many asteroids that we can afford fanciful names.

It’s hard to imagine civilization convulsing as it did in ancient times. But who knows what the future holds? The Great Library of Alexandria wasn’t destroyed all at once; it suffered centuries of decline, helped in its demise by anti-intellectual sentiment and changing religious circumstances, until it was finally destroyed for good. Could something similar happen in the future?

If it does, then Sh2-54 and the other nebulae near the constellation Serpens will just be fragments of a body of knowledge that may atrophy over time. Future people, maybe in the grip of resurgent superstitions, may wonder what the point of all these mountain-top observatories is, why we launched telescopes into space that might insult the creator, and who Freddie Mercury was.

John Flamsteed’s Serpens from Atlas Coelestis (1753).

But for now, we get to enjoy the unbridled age of information we live in. We can sit in the comfort of our homes and gaze through the eyes of our distant space telescopes as they probe nebulae and other wondrous objects. Then we can click a link and wonder at the ancients who gazed at the same sky and saw a much simpler version of nature.

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Evan Gough

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