A Tale of Two Nebulae
New Gemini Images Contrast the Late Evolution of Two Very Different Stars
4 June 2006
Two new images from Gemini Observatory released today at the American Astronomical Society meeting in Calgary, Canada, show a pair of beautiful nebulae that were created by two very different types of stars at what may be similar points in their evolutionary timelines. One is a rare type of very massive spectral-type "O" star surrounded by material it ejected in an explosive event earlier in its life. It continues to lose mass in a steady "stellar wind." The other is a star originally more similar to our Sun that has lost its outer envelope following a "red giant" phase. It continues to lose mass via a stellar wind as it dies, forming a planetary nebula. The images were made using the Gemini Multi-Object Spectrograph (GMOS) on Gemini South as part of the Gemini Legacy Imaging program.
An Amateur's View
The objects NGC 6164-5 and NGC 5189 are accessible to southern-hemisphere amateur observers using good backyard-type telescopes.
NGC 6164-5 is a small (6’ x 3’) inverted S-shaped nebula in Norma whose appearance in a telescope is adversely affected by the brightness of its central star, 6.8-magnitude HD 148937. (Its dual NGC entry refers to the nebula’s two brightest crescents, discovered by Sir John Herschel in 1834 while observing from the Cape of Good Hope in Africa.) NGC 6164 lies immediately northwest of the central star, while NGC 6165 lies immediately southeast of it. To find the nebulae, simply look for HD 148937 just 1 1/4˚ east-southeast of 4.5-magnitude Epsilon Normae, near the Ara border. Through a 4-inch refractor, the twin nebulae appear as a general haze around the central star (like breath on a mirror). Compare what you see with nearby stars of similar brightness to see the difference. Gerd Bahr-Vollrath (of Noosa Heads, Queensland, Australia) gives a most fitting description of the faint nebula as he saw it through an 8-inch f/12 Schmidt-Cassegrain telescope. While the nebula was difficult to spot, Bahr -Vollrath described the two crescents as “apparently disconnected bars of nebulosity. After careful study, faint, indistinct nebulosity immediately surrounding the central star became visible, connecting the two brighter bars. A challenging object!” He points out that clean optics and an extremely transparent night were necessary to see anything at all.
NGC 5189 is an intriguing telescopic sight. It lies 1 3/4˚ southeast of 4.5-magnitude m Centauri, which is about 6 1/4˚ east-southeast of 1st-magnitude Alpha Crucis (Acrux), the brightest and southernmost star in the Southern Cross. Although NGC 5189 shines at magnitude 9.9, its light stretches across 2.5’ of sky, so it appears dimmer than expected as seen through a small telescope. James Dunlop discovered NGC 5189 on July 1, 1826, with a 9-inch reflector at Paramatta, New South Wales. But it was John Herschel who first noticed the nebula’s curious figure. After observing it through his 18-inch speculum-mirror reflector, Herschel wrote that the object’s bright, central axis appeared “somewhat curved,” and it terminated “in two masses brighter than the rest.” Today, under a dark sky, the nebula’s irregular structure can be detected in telescopes as small as 3 inches. Through a 4-inch refractor, NGC 5189 looks like a 1.5’-long bow tie of light about 5’ northwest of a 7.5-magnitude star. With magnification and concentration, the bow tie appears irregularly bright, beaded, and blotchy. Through a 12-inch reflector, NGC 5189 is a stunning sight. Its long axis appears riddled with irregularities, while its squashed, inverted S-shaped extensions emerge from the central bar like butterfly wings open to the wind.
A Rare and Massive Star
The first image shows the emission nebula NGC 6164-5, a rectangular, bipolar cloud with rounded corners and a diagonal bar producing an inverted S-shaped appearance. It lies about 1,300 parsecs (4,200 light-years) away in the constellation Norma. The nebula measures about 1.3 parsecs (4.2 light-years) across, and contains gases ejected by the star HD 148937 at its heart. This star is 40 times more massive than the Sun, and at about three to four million years of age, is past the middle of its life span. Stars this massive usually live to be only about six million years old, so HD 148397 is aging fast. It will likely end its life in a violent supernova explosion.
Like other O-type stars, HD148937 is heating up its surrounding clouds of gas with ultraviolet radiation. This causes them to glow in visible light, illuminating swirls and caverns in the cloud that have been sculpted by winds from the star. Some astronomers suggest that the cloud of material has been ejected from the star as it spins on its axis, in much the same way a rotating lawn sprinkler shoots out water as it spins. It's also possible that magnetic fields surrounding the star may play a role in creating the complex shapes clearly seen in the new Gemini image.
Astronomers are also studying several "cometary knots" out on the boundaries of the cloud that are similar to those seen in planetary nebulae such as the Eskimo Nebula (NGC 2392) and the Helix Nebula (NGC 7293). These cometary knots (so called because they seem to resemble comets with their tails pointing away from the star) are inside what appears to be a low-density cavity in the cloud. The knots may be a result of the denser, slower shells being impacted by the faster stellar wind, as observed in planetary nebulae (formed during the deaths of much less massive stars like the Sun).
Massive stars like HD 148937 burn hydrogen to helium in a process called the CNO cycle. As a byproduct, carbon and oxygen are converted into nitrogen, so the appearance of enhanced nitrogen at the surface of the star or in the material it also blows off indicates an evolved star. According to astronomer Nolan Walborn of the Space Telescope Science Institute, who has been studying this star from the ground for several years now, it is a member of a very small class of O stars with certain peculiar spectral characteristics. "The ejected, nitrogen-rich nebulosities of HD 148937 suggest an evolved star, and a possible relationship to a class of star known as luminous blue variables," he said.
Luminous blue variables are very massive, unstable stars advanced in their evolution. Many have nitrogen-rich nebulae that are arrayed symmetrically around the stars, similar to what we see in NGC 6164-5. One of the best-known examples is the star Eta Carinae, which ejected a nebula during an outburst in the 1840s.
The Death of a Sunlike Star–With a Twist
Just as astronomers are still seeking to understand the process of mass loss from a star like HD 158937, they are also searching out the exact mechanisms at play when a star like the Sun begins to age and die. NGC 5189, a chaotic-looking planetary nebula that lies about 550 parsecs (1,800 light-years) away in the southern hemisphere constellation Musca, is a parallelogram-shaped cloud of glowing gas. The GMOS image of this nebula shows long streamers of gas, glowing dust clouds, and cometary knots pointing away from the central star. Its unruly appearance suggests some extraordinary action at the heart of this planetary nebula.
At the core of NGC 5189 is the hot, hydrogen-deficient star HD 117622. It appears to be blowing off its thin remnant atmosphere into interstellar space at a speed of about 2,700 kilometers (about 1,700 miles) per second. As the material leaves the star, it immediately begins to collide with previously ejected clouds of gas and dust surrounding the star. This collision of the fast-moving material with slower motion gas shapes the clouds, which are illuminated by the star. These so-called "low ionization structures" (or LIS) show up as the knots, tails, streamers, and jet-like structures we see in the Gemini image. The structures are small and not terribly bright, lending planetary nebulae their often-ghostly appearance.
"The likely mechanism for the formation of this planetary nebula is the existence of a binary companion to the dying star," said Gemini scientist Kevin Volk. "Over time the orbits drift due to precession and this could result in the complex curves on the opposite sides of the star visible in this image."
NGC 5189 was discovered by Scottish observer James Dunlop in 1826. when Sir John Herschel observed it in 1835 he described it as a "strange" object. It was not immediately identified as a planetary nebula, but its peculiar spectra, shows emission lines of ionized helium, hydrogen, sulfur and oxygen. These all indicate elements being burned inside the star as it ages and dies. As the material is blown out to space, it forms concentric shells of various gases from elements that were created in the star's nuclear furnace.
The Gemini data used to produce these images is being released to the astronomical community for further research and follow-up analysis. Note to astronomers: Data can be found at the Gemini Science Archive by querying "NGC 6164" and "NGC 5189."
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