Two galaxies in Sculptor constellation


By Tom Burns - Stargazing



The constellation Sculptor is among the most insignificant of the naked-eye star groupings. From our central Ohio latitude, it barely scrapes above the horizon in late summer and early autumn. Even then, its six stars are barely visible to the unaided eye.

Most stargazers in northern climes tend to forget about Sculptor. Its faint stars are relatively unimportant from a scientific perspective.

These days, the constellation is famous, albeit briefly, because it contains an object imaged by the James Webb Space Telescope (JWST). Full-color images of the Cartwheel Galaxy graced the front pages of many newspapers a few weeks back.

The Cartwheel is nearly invisible to the human eye, even in large telescopes, so only the most dedicated amateur telescopists have attempted to view it.

However, Sculptor contains one object, the Sculptor Galaxy, which is worth the effort to observe with your own two eyes, if for no other reason than bragging rights.

Decades back, I vowed to see Sculptor and its one bright galaxy for the first and, as it turned out, only time.

I remember vividly the first time I saw them. I was at the dark-sky observing site of the Richland Astronomical Society near Mansfield, Ohio, on a crystal-clear night in September.

Only one other telescope besides mine graced the hilltop. In those pre-climate-change days, the breeze was cool enough at 3 a.m. to send shivers up my spine.

I could not help but suppress a laugh when I observed it. Constellation designations have a reputation for not representing the star patterns they contain. However, Sculptor extends that trend to a ridiculous level.

In 1751, French astronomer Nicolas-Louis de Lacaille made an extended visit to the Cape of Good Hope, the southernmost locale in Africa.

Lacaille observed and plotted over 10,000 stars using an observatory he built there. The result was the first detailed map of the southern sky.

In the process, he named 14 new constellations, all of which the International Astronomical Union still officially recognizes. Many of those constellations are visible only south of the Equator.

A few are visible in the north but are composed of stars so faint that previous stellar cartographers hadn’t bothered to name them.

One of the latter constellations was l’Atelier du Sculpteur, the Sculptor’s Studio.

In 1756, Lacaille published a planisphere that included a drawing of the constellation complete with a tripod table, a hammer and chisel, and even a beautiful marble head sculpted in the Roman style.

In 1844, the English astronomer John Herschel wisely proposed shortening the name to Sculptor, and the name stuck.

The constellation stands out because it contains a galaxy of interest to amateur telescopists and professional astronomers alike.

In 1783, Caroline Herschel, comet hunter extraordinaire, was sweeping the stars of Sculptor looking for, you guessed it, comets. She spied a longish, narrow streak of light that modern astronomers nicknamed the Sculptor Galaxy and designated NGC 253.

Amateur telescopists consider the Sculptor Galaxy an easy spiral galaxy to see because of its relative brightness. At around magnitude 7, the galaxy is just below naked-eye visibility, but it jumps right out at you in even miniature binoculars.

Its brightness is tied partly to its proximity at only 11.4 million light-years away. Sadly, Sculptor’s southerly sky location makes it a better target if you observe from Florida rather than Ohio.

Still, it is a sight to behold in astronomical binoculars, in which it will look like a streak of light. In a large amateur telescope, the galaxy’s long, oval shape is interrupted by distinct dark mottling and a long, dark lane. The galaxy is replete with large quantities of dust and gas that have not turned into stars.

The Sculptor Galaxy is going through an intense period of star formation. In it, astronomers have identified enormous star clusters less than six million years old.

At some time in the recent astronomical past, a smaller, nearby galaxy passed through NGC 253. Given the enormous distances between stars in a galaxy, such galactic interactions do not result in stars colliding with stars.

However, galaxies contain much larger structures. Clouds of hydrogen gas many light-years wide are responsible for the birth of stars.

As galaxies collide, their gas clouds run into each other. As the gas clouds interact, they compress. The result is the birth of new stars.

The smaller galaxy survived the collision. However, the intense gravitational interaction between the two galaxies will cause them to merge a second time, producing another round of star formation.

Astronomers doubt that the smaller galaxy will survive the event. The Sculptor Galaxy will probably absorb its smaller companion in a galactic merger relatively common in the universe.

JWST’s image of the Cartwheel Galaxy is in another category of visual observing entirely. At magnitude 15.2, the Cartwheel is 1,738 times fainter than the Sculptor Galaxy. At 500 million miles away, it is almost 44 times more distant than the Sculptor Galaxy.

Only the most diehard of stargazers with giant amateur telescopes and the darkest of rural skies have even attempted it.

The Cartwheel is important to astronomers because it was a spiral galaxy similar in size and shape to our Milky Way.

It has one significant similarity to the Sculptor Galaxy. A cataclysmic interaction with a smaller galaxy 700-800 million years ago produced its current disrupted state.

When Swiss astronomer Fritz Zwicky discovered it in 1941, he realized its significance to the study of galactic structure. He wrote that the Cartwheel was “one of the most complicated structures awaiting its explanation on the basis of stellar dynamics.”

The Cartwheel gets its nickname because of its wheel-like appearance. It has a bright central hub, which makes it similar to most lens-shaped spiral galaxies.

There the similarity ends. The galaxy has an outer ring that resembles a bicycle tire. Its spiral arms have shrunk to wavy, ephemeral spokes.

Astronomers classify the Cartwheel as a ring galaxy, one of the rarest types of galactic structure.

How rare are they? In 2017, amateur astronomers in the Galaxy Zoo 2 project looked at all the galaxies in the Sloan Digital Sky Survey and came up with 3,962.

That sounds like a lot. But given the trillions of galaxies in the universe and the 4.4 million galaxies mapped so far, astronomers estimate that a typical spiral galaxy has less than a one in 10,000 chance of transforming into a ring.

The cause of the ringed structure was a mystery for decades, but thanks to images from the Hubble Space Telescope and JWST, astronomers have finally unraveled what happened.

Near the Cartwheel is a small galaxy not pictured in the JWST image. Astronomers believe that the smaller galaxy shot through the central hub of the Cartwheel like a bullet passing through a bullseye.

The central nature of the collision sent more or less symmetrical shock waves through the Cartwheel, forcing many of its stars into the external ring. As in the Sculptor Galaxy, those waves of force also triggered a wave of star formation, represented in the image by the blue-colored stars.

The original impact probably wiped out the spiral structure of its curved arms. However, the galaxy continued to rotate.

After seven or eight million years of rotating, spiral arms are beginning to form again.

The JWST image of the Cartwheel is significantly better than the previous image taken by the Hubble Space Telescope because the Hubble imaged the galaxy in visible light. The dusty regions of star formation, in large part, absorb visible light. The reforming spiral arms are barely visible to the Hubble.

JWST imaged the object in the infrared portion of the electromagnetic spectrum.

Light in the infrared is far less prone to be absorbed by dust but is invisible to the human eye. NASA translates the data into light that your eyes can see into a “false color” image.

Consequently, the image isn’t what the galaxy “really looks like,” as many JWST naysayers have pointed out. What the galaxy “really looks like” to the limited range of human vision is practically nothing.

I got curious about the telescopic visibility of the Cartwheel in visible light, so I checked on Cloudy Nights, a website devoted to diehard amateurs and their giant telescopes. I found a remarkable sketch by a Namibian amateur using a telescope with a 24-inch, light-gathering mirror, a whale of a telescope by amateur standards.

You can see it at https://www.cloudynights.com/topic/807252-observing-the-cartwheel-galaxy/.

Remarkably, it shows some of the detail in JWST’s image, notably the galaxy’s central hub surrounded by the Cartwheel’s ring-shaped halo. The disrupted spiral arms appeared as a faint, undifferentiated glow in the Namibian drawing. They do not appear in any other sketch I could find.

I am deeply impressed by JWST’s ability to ferret information from the Cartwheel galaxy’s faint glow.

Even so, I was just as impressed — and deeply moved — by the miraculous power of a dedicated human eye to see even a hint of the glory of a galaxy so faint and so far away.

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.