Gorgeous Gas

New observations of space clouds reveal stellar histories

Fierce winds of particles and radiation from massive stars can sculpt the universe. What would otherwise be dim regions of amorphous gas become transformed into luminous works of art. Resembling bright bubbles, these diaphanous expanses of gas and dust belong to the category of astronomical phenomena known as nebulas. Unlike most of the nebulas that populate the universe, these clouds are limned by arcs or rings and bathed in the blue light emitted by helium ions. That makes these cosmic beauties really hot stuff.

NEBULA IN HEAT. An image of an HeII nebula, a cloud of gas and dust set aglow by helium ions near extraordinarily hot stars. Located in the Small Magellanic Cloud galaxy, this nebula surrounds the hot double-star star AB7. The image was taken with one of the four 8.2-meter telescopes that make up the Very Large Telescope in Paranal, Chile. Blue-purple represents the light from singly ionized helium, known as HeII; green-yellow represents the light from doubly ionized oxygen; and red, from hydrogen. The blue-purple emissions are the hottest. European Southern Observatory
COMELY CLOUDS. In the Large Magellanic Cloud galaxy, an HeII nebula dubbed N44C (above) is associated with two massive stars. This nebula poses a puzzle because neither of the two stars it enshrouds appears energetic enough to have created N44C. Astronomers suspect that that a third, unseen, very dense star may also be present. Another HeII nebula (below) in the same galaxy surrounds the binary star BAT99-49. Two gas arcs appear above and below the star. European Southern Observatory

European Southern Observatory

Last year, astronomers observed four of the nearest such nebulas with one of the quartet of 8-meter telescopes that make up the Very Large Telescope (VLT) in Paranal, Chile. Images of the nebulas, located just a few hundred thousand light-years away, provide the first detailed look at these comely clouds and the hot, massive stars that power them.

Beyond their undeniable beauty, the images may reveal properties of the very first stars to light up the cosmos. Models of stellar formation suggest that pioneer stars, which lived and died nearly 14 billion years ago, were unusually hot and massive compared with typical stars today (SN: 6/8/02, p. 362: Cosmic Dawn). The new observations, which home in on nebulas from two satellite galaxies of the Milky Way, may also provide insight into the strong visible-light emissions that have been observed coming from some distant galaxies. Astronomers suspect that the distant emissions may emanate from the same type of nebulas.

Ya�l Naz� of the Institute of Astrophysics and Geophysics in Li�ge, Belgium, and her colleagues, including You-Hua Chu of the University of Illinois at Urbana-Champaign, describe the findings in the April II issue of Astronomy and Astrophysics.

Windy stars

Any hot, young star can set a surrounding nebula aglow by stripping hydrogen atoms of their single electron. The light emitted by the ionized hydrogen atoms gives these regions a ruddy hue. One well-known example is the Orion nebula. But it takes an even hotter, more massive star, with a surface temperature greater than 75,000 kelvins and a mass heavier than 20 times that of the sun, to strip helium atoms of their two electrons. These completely ionized helium atoms then grab a single electron, becoming singly ionized helium, which astronomers refer to as HeII. These ions emit a particular wavelength of blue light, 488.6 nanometers.

It’s these special nebulas, known as HeII nebulas, that astronomers observed last year with the VLT.

To fully ionize helium, a massive star must be extremely hot. That requirement disqualifies most massive stars, which typically have temperatures lower than 50,000 kelvins and can’t radiate enough energy to strip helium atoms of both of their electrons, notes Donald R. Garnett of the University of Arizona in Tucson. Hotter stars “are not predicted by normal stellar evolution, so the presence of the HeII nebulas is a bit of a mystery,” comments Garnett, who has observed such nebulas with the Hubble Space Telescope.

The ultraviolet light radiated by massive stars can’t be seen by ground-based telescopes. But if a high density of hydrogen and helium atoms surrounds the star, much of the ultraviolet radiation is reemitted as visible light, which reveals these gases in the form of glowing nebulas. Because so much of the ultraviolet light is absorbed, studying the nebulas is the only way to learn about the ultraviolet emissions and general nature of these unusually powerful stars, says Garnett.

Several of the new observations provide evidence for one of the many models astronomers have formulated to account for the stars, says Chu. According to this model, a type of star called Wolf-Rayet, known for its strong winds and intense radiation, ionizes the helium. Wolf-Rayet stars, which last for only 100,000 years, appear to be the descendants of massive O stars, which also produce winds. Akin to the winds of ionized particles blowing out from the sun, these winds of O and Wolf-Rayet stars are much more powerful, with speeds of 2,000 to 4,000 kilometers per second.

Most Wolf-Rayet stars either aren’t hot enough or don’t have enough gas surrounding them to produce bright, easy to see HeII nebulas, notes Claus Leitherer of the Space Telescope Science Institute in Baltimore. “The only good candidates are outside the Milky Way, making it hard to obtain high-quality data,” he says. “The new VLT data finally allow us to derive precise temperatures and measure the radiation field” produced by these stars, he says.

One of the new images depicts the nebula surrounding the star BAT99-2, which is in the nearby Large Magellanic Cloud galaxy. The star is the hottest Wolf-Rayet star known, with a surface temperature of 120,000 kelvins, nearly 20 times that of the sun (see Correction, below). Before the star became hot enough to be classified as a Wolf-Rayet, it emitted a strong wind that swept up interstellar debris like a snowplow, suggest Naz�, Chu, and their colleagues.

This wind apparently created a bubble of hydrogen and helium gas, which can be seen as a large semicircle to the south of the star. After evolving into its Wolf-Rayet phase, the star blew an even stronger wind, some 20,000 to 40,000 km/s which slammed into gaseous material previously ejected by the star. This created a new bubble, visible as a small arc to the northwest of the star. “We are apparently witnessing an ongoing merger of these two bubbles,” Chu says.

Wolf-Rayet stars are the probable power sources for two other HeII nebulas examined in detail by Naz�’s group. But a fourth image poses a puzzle.

The nebula known as N44C contains two massive stars, but even the hottest, most massive one lacks the oomph to ionize helium. The astronomers propose that this nebula harbors a third, unseen star that’s extremely dense and circled by the nebula’s most massive visible star. When these two stars are closest together, the dense, unseen star siphons a large amount of material from its partner. As the material falls onto the dense star, it reaches searing temperatures of up to millions of kelvins, emitting both X rays and ultraviolet light�enough to fully ionize helium atoms. At other times, when the two stars are farther apart, the compact star can cannibalize very little mass and the energetic radiation dwindles on a time scale of decades to centuries.

So far, however, observations of N44C reveal little or no fading of the radiation.

Says Naz�, “We were able to understand three nebulas, but we must now look more closely at N44C.”

Nebulous riddles

There are other puzzles to solve, notes Garnett. Even for the HeII nebulas that are clearly powered by Wolf-Rayet stars, it’s uncertain how these stars arise. They may be the descendants of massive stars, or they might be produced through the interaction of binary stars. What’s more, astronomers don’t fully understand how hot, massive stars can produce so much high-energy ultraviolet radiation.

Part of the uncertainty stems from the difficulty of modeling the ultraviolet output of hot, massive stars. Not only is the light readily absorbed by atoms, preventing direct measurements, but modelers must also take into account the effects of the stellar wind and the shape of the nebulas. The new observations will help, Garnett says, because once astronomers calculate the ultraviolet output of the stars imaged , they can use that information to model other Wolf-Rayet stars.

On a grander scale, the new nebula observations could hold clues to star formation in distant galaxies, including so-called starburst galaxies, in which stars are forming like gangbusters. The energetic stars that power HeII nebulas “may be common in starburst galaxies, especially in those with low abundances of heavy elements,” notes Garnett.

It’s also possible that the first stars in the universe were at least as hot and massive as the stars that power HeII nebulas today, he adds.

Theory predicts that O stars were far more abundant in the early universe than there are today. “If there were many O stars, then there should also be many Wolf-Rayet stars as well,” notes Leitherer.

Astronomers propose that a group of ancient, massive stars ended the so-called cosmic Dark Ages by ionizing hydrogen and helium atoms, lighting up the universe for the first time since the Big Bang. But to say this for certain, says Garnett, “we need to understand the ultraviolet emissions from present-day massive stars” and the breathtaking nebulas that surround them.

Correction: This article states that star BAT99-2 “is the hottest Wolf-Rayet star known, with a surface temperature of 120,000 kelvins. . . .” BAT99-2’s temperature is actually less than 100,000 kelvins, and the star is the hottest Wolf-Rayet star known in the Large Magellanic Cloud galaxy .

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