Rethinking an Astronomical Icon

The Eagle's EGGs: Not so fertile

In 1995, less than 2 years after astronauts installed a device to correct the Hubble Space Telescope’s blurry vision, NASA released a picture that captivated millions. In it, eerie blue-green pillars of gas and dust rise up like stalagmites in a cave. In a glimpse of the universe that’s both science and art, a team of Arizona astronomers captured what they thought were nests brimming with stellar embryos. “I distinctly recall the first time I saw the image, in my office on a quiet holiday, a few days before the press release came out,” says astronomer Mark J. McCaughrean of the Astrophysical Institute in Potsdam, Germany. “I was blown away . . . and immediately went in search of someone to show it to.”

IN A NEW LIGHT. The Hubble Space Telescope’s 1995 image of star-forming pillars in the Milky Way’s Eagle nebula, taken with a visible-light camera, is overlaid with more recent images taken by Hubble’s near-infrared camera. Although the earlier image suggested that the pillars contain much star-forming material?dense gas and dust?the infrared images show only isolated clumps of it. Visible-light image credit: Hester et al./NASA; infrared image credit: Thompson, Smith, Hester/NASA; overlay: Z. Levay/STScI
TWO VIEWS. Comparison of the Hubble Space Telescope’s visible-light image of the Eagle’s pillars (above) with an infrared image taken by the Very Large Telescope (below). Because the groundbased instrument has a wider field of view than the Hubble’s infrared camera does, it shows more of the pillars and star-forming regions within them. Hester et al./NASA

McCaughrean and Andersen/ESO

The picture landed on the covers of magazines around the world (SN: 11/4/95, p. 294). Jay Leno displayed it on the Tonight Show. The heavenly portrait not only restored Hubble’s tarnished reputation but became an astronomical icon.

Now, images taken with infrared detectors show that icon in a new light.

Composed of cold hydrogen gas and dust, the pillars reside in the Milky Way’s Eagle nebula, a star-forming region 7,000 light-years from Earth. The columns are the dense remnants of what had been a giant gas cloud. Harsh ultraviolet light and fierce winds from a nearby cluster of hot, massive stars blasted away most of that cloud, leaving the pillars behind. As the ultraviolet light continues to bombard the pillars, it both hinders and fosters star birth. It hinders it by continuing to erode material and fosters it by compressing gas, hastening its collapse into stars.

When their original visible-light image was released, Hubble astronomers Paul Scowen and J. Jeff Hester of Arizona State University in Tempe drew special attention to a group of dense, comet-shaped protrusions at or near the tips of the pillars. They suggested that the protrusions, which they called “evaporating gas globules,” or EGGs, are havens where stars are forming.

Taking the findings one step further, Hester and his colleagues proposed that the EGGs represent a novel aspect of star formation. The unrelenting ultraviolet radiation from the neighborhood bullies wasn’t only laying bare the EGGs, it was stripping material from these would-be stellar nurseries, stopping newborn stars from packing on additional girth (SN: 11/30/96, p. 350). According to Hester’s team, the erosion of material by bombardment of ultraviolet radiation, a process dubbed photoevaporation, may be the primary factor in limiting the heft of newborn stars across the galaxy.

Hester had estimated that hundreds to thousands of stars are currently forming in the 73 EGGs recorded by the Hubble camera. That interpretation prompted NASA to issue a press release that called the dusty columns “pillars of creation.”

But there remained one problem with this interpretation, notes McCaughrean. The images were taken in visible light, and visible light doesn’t penetrate dust. More recently, several researchers, including Hester and McCaughrean, have independently examined the EGGs with infrared detectors, which can probe the interior of the dusty columns.

The resulting images paint a different portrait of the pillars of creation. Only 15 percent of the EGGs contain newborn stars; the rest are sterile, too sparse in dust and gas to support star formation. Nonetheless, researchers disagree about whether ultraviolet light limits the growth of these infants.

Eyes of the beholder

“The [visible-light] image was and is one of the most stunning things to come out of the Hubble Space Telescope,” says McCaughrean. “On the other hand, much of the EGG-star formation theory was rather speculative, and after it got sound-bitten a few times, the public ended up thinking this revolutionized everything we ever knew about how stars are made.”

To examine the pillars, McCaughrean and Morten Andersen, a colleague at Potsdam, recently used one of the quartet of 8-meter telescopes collectively known as the Very Large Telescope (VLT) in Paranal, Chile. Their findings will be published in an upcoming Astronomy and Astrophysics. A group of Japanese researchers also studied the pillars in the infrared, using the University of Hawaii’s 2.2-m telescope. They report their findings in the Jan. 20 Astrophysical Journal Letters.

Hester maintains, however, that the new findings only confirm his original view, that ultraviolet light has been limiting the final mass of stars forming in the Eagle nebula’s EGGs. He and his collaborators had studied the pillars with Hubble’s near-infrared camera and multi-object spectrometer (NICMOS) in 1998, about a year before the device ran out of its solid nitrogen coolant and stopped operating. They found that few of the EGGS contain stars.

NASA unveiled this infrared image at a press briefing last month, where officials described plans for an ambitious mission to tune up Hubble. In that mission, astronauts attached a refrigerator to NICMOS to revive it (see “Telescope Tuned Up,” in this week’s issue: Telescope Tuned Up: Back to work for orbiting observatory). A report by Hester and his colleagues Rodger I. Thompson of the University of Arizona in Tucson and Bradford A. Smith of the University of Hawaii in Honolulu is scheduled for publication in the Astrophysical Journal.

“I’ll admit that my original guess was that a larger fraction of the EGGs would contain young stellar objects,” Hester says. The infrared images taken by the three teams imply that within the pillars an amount of gas equivalent to one-thousandth the sun’s mass is converted into stars each year. Although that rate is one-tenth the amount he had calculated in 1995, Hester says it’s “still a remarkably high star-formation rate.”

The pillars, estimated to contain only a few hundred solar masses of material, could only sustain that rate of star formation for 100,000 years, he notes.

“We are seeing a brief, fairly intense period of star formation, with an ongoing competition between material being tied up in stars and material being dispersed by photoevaporation,” says Hester. “The bottom line is that the [new] star-formation rate for the columns is both significant and the right magnitude to support the idea that photoevaporation plays a key role in shaping what is going on.”

Same images, new ideas

McCaughrean says the new infrared images, which show that only about 11 EGGs contain stars, make it even more of a stretch to suggest that nearby massive stars and the ultraviolet radiation they emit limit the maximum mass of stars in the Eagle’s pillars, let alone the rest of the Milky Way galaxy.

Moreover, he notes, many models of star formation indicate that massive stars take longer to assemble than do lower-mass stars, such as the typical stars within the EGGs. If that’s the case, then EGG stars would reach maturity before neighboring stars could have grown massive enough to emit the disruptive ultraviolet radiation. The ultraviolet light would therefore have little affect on the overall mass of the newborn, he contends.

In this scenario, the radiation might still disperse the disks of gas and dust that surround newborn stars, interfering with their ability to form planets from this material, McCaughrean notes.

Hester interprets the same data differently. “The process that is revealed by the infrared and other observations is frankly remarkably close to the process that we originally suggested,” he says.

Mario Livio of the Space Telescope Science Institute in Baltimore agrees that the early interpretations of the image were plausible. “I don’t think the [original] image was hyped too much,” he says. And although he notes that the picture’s influence “was always because of its breathtaking beauty, not necessarily because of its scientific impact,” the image did inspire astronomers to search for similar structures in other star-forming regions of the Milky Way.

“We now know that in many star-forming regions, such as 30 Doradus, you find such pillars,” Livio adds.

The best observations to date indicate that there’s little if any variation in the typical mass of newborn stars, whether they form in low-density regions of the Milky Way, which contain no massive stars, or regions bombarded by ultraviolet radiation from heavyweights, notes Stephen E. Strom of the National Optical Astronomy Observatory in Tucson. “The issue of triggering star formation, in which ultraviolet radiation could play a role, should be isolated from the ability of the radiation to determine the mass of stars–there’s no evidence of that,” he says.

“This whole theory of truncated star formation was always just hype,” says astronomer Lynne Hillenbrand of the California Institute of Technology in Pasadena. Ultraviolet radiation from massive stars “surely is impacting the pillars, but once a star has started to form, I doubt the radiation dominates over gravity anywhere but in the outermost [layers] of the star-forming core,” she notes.

When the original pillar images came out in 1995, Hubble’s flawed optics had only recently been corrected. To both the scientific community and the public, the orbiting telescope was no longer a symbol of high-tech failure, says Hester. He adds, “The image became attached to the larger story–the recovery of Hubble and the dreams that had originally accompanied its launch.”

Scientists don’t find it surprising that interpretations of an astronomical image–even an icon like the pillars–may need revising, says McCaughrean. Hester “was doing science when he made a hypothesis based on his observations, and we’re also doing science in checking his hypothesis and finding it to be less substantial than he hoped,” McCaughrean notes. “The problem with press releases is that they tend to obscure this to-and-fro process, making science sound like it’s all unimpeachable Eureka moments.”

The new findings also highlight how much astronomers still have to learn about star formation.

The arguments may not be settled until researchers can take ultrasharp, longer-wavelength images of the region. Infrared images and spectra obtained at these longer wavelengths can penetrate more of the dust in the pillars and provide a better estimate of the number and masses of stars that reside there, says McCaughrean.

He emphasizes that Hubble has an unavoidable case of tunnel vision–a small field of view. Just around the corner from the pillars, a much larger drama is unfolding. “There’s a huge cluster of young stars, several thousand of them, which are home to the [handful of] massive stars whose ultraviolet light is destroying the pillars,” McCaughrean notes. There’s no clear evidence that the ultraviolet radiation from these massive stars determine the size of the thousands of low-mass stars in the cluster, he asserts.

In their Astronomy and Astrophysics report, McCaughrean and his colleagues summarize the findings this way: “The ongoing destruction of the columns and the relatively limited star formation taking place within them may ultimately prove to be a sideshow in the grander scheme of things–albeit a beautiful one.”

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