MysteriesM of Planetary Nebulae Spider Webs in Space Red Rectangle Nebula NASA / ESA / HANS VAN WINCKEL (CATHOLIC UNIVERSITY OF LEUVEN) / MARTIN COHEN (UNIVERSITY OF CALIFORNIA, BERKELEY) Astronomers are still puzzled by how stars create the bizarre variety of planetary nebulae in our galaxy. Robert Zimmerman on with this system?” said Gull. To make sure the shape wasn’t an artifact he took another image of a nearby star. In October 1973, the 4-meter Mayall Telescope at Kitt It was a sharp point. HD 44179, however, remained “a Peak in Arizona was just starting operations. Ted Gull crisp, rectangular object.” (now at NASA/Goddard Space Flight Center) was using And thus the Red Rectangle was found. the telescope to produce some spectacular test images. Observations in the ensuing decades have not only pro- Seeing the images, Martin Cohen (University of duced even more amazing images of this strange angu- California, Berkeley) was impressed and asked Gull to use lar object, they have added further mysteries. In 2003 a the telescope to photograph a particular star Cohen had Hubble Space Telescope image revealed some inexplicable spotted earlier that year using a U.S. Air Force sounding rocket. This star, HD 44179, was intriguing because of its STELLAR WEB The nebula surrounding HD 44719, known as the unusual brightness in the infrared. Red Rectangle, has baffl ed astronomers since its discovery in the Gull’s fi rst images baffl ed him. In red wavelengths the 1970s. Explanations range from a bipolar fl ow seen at an odd angle star looked like a rectangle. “What in the world is going to light leaking out through gaps in a dusty doughnut of material. 20 November 2014 sky & telescope features, including four spokes on which were hung a Stellar Evolution series of “ladder rungs,” or nested rectangles of increas- Planetary nebulae form during a very short phase late in ing size. The structure almost looks as if a giant spider the life of Sun-like, low-mass stars. During this stage the was weaving a web in space, stringing its thread from one star is known as an asymptotic giant branch (AGB) star, beam of light to another. the name referring to the location of these stars on the What is really most astonishing about the Red Rect- Hertzsprung-Russell Diagram (S&T: June 2014, p. 32). angle is that it is actually typical for a planetary nebula. AGB stars are somewhat unstable. In this stage the In the past 20 years Hubble has given us our fi rst high- star’s core is mostly oxygen and carbon, surrounded by an resolution look at a large number of these weird and inner shell of helium with an outer shell of hydrogen. Most enigmatic objects and found that, more often than not, of the fusion goes on in the outer hydrogen shell, which they are as complex, as astonishing, and as baffl ing as the produces helium that rains down onto the helium shell Red Rectangle. below, increasing its density. Eventually this increased From this new data astronomers are beginning to put density ignites the helium in an explosive pulse called a together a coherent outline of the origins and shapes of helium fl ash, pushing against the hydrogen shell so that planetary nebulae. That picture is also giving astrono- it expands, becomes less dense, and stops burning. mers a deeper understanding of the evolution of Sun-like The helium shell burns for a while, then shuts down. stars. “By fi guring out how these shapes transform we The hydrogen shell then settles, becomes dense enough can learn to understand how stars really die,” explains to again ignite, and kicks off the whole cycle again. Raghvendra Sahai (NASA/JPL). This helium-fl ash process repeats many times, and Even more exciting, scientists are beginning to suspect during this phase the star begins to send out a wind of that the presence of exoplanets might explain the strange high-density dust made of carbon and other heavy ele- shapes of some planetary nebulae. “I like to say that we ments dredged up from the dying star’s core. This dust are putting the ‘planet’ back into ‘planetary nebulae,’” drags gas from the star’s fl uff y outer envelope with it. The adds Sahai. winds spew out anywhere from half to 90% of the star’s mass and form a dense envelope surrounding the star. The First Discoveries As these low-mass stars burn off the last bits of their Although the fi rst planetary nebula, the Dumbbell Neb- hydrogen and helium fuel while losing mass, their cores ula (M27), was discovered by Charles Messier in 1764, it begin to evolve into a white dwarf, made up of the carbon was William Herschel who gave these objects their name and oxygen core that’s not dense enough to burn. The when he conducted his survey of the Northern sky in the wind changes from a slow high-density wind into a fast late 1700s. They reminded him of Uranus, the planet he had just discovered — even though he knew that what he saw was absolutely not a planet. All told, Herschel found NGC 5189 33 planetary nebulae, though of these he mislabeled 13 as something else. (He also dubbed another 59 objects as planetary nebulae that turned out not to be.) Since then, and especially since the repair of Hubble in 1993, scientists have found planetary nebulae in a bewildering variety of shapes. About a quarter or more have bipolar lobes extending out from their centers. About 20% have multiple lobes. Others appear oval, some look like barrels, and some resemble cylinders that have been squeezed in the center. A few even have spiral arms almost like galaxies. In a number of cases the central star is often off set from the center of the nebula, even when that nebula is highly symmetrical. And then, like the Red Rectangle, there are some whose complex patterns and shapes are too diffi cult to describe in a mere sentence. You have to look at a high- resolution image of each to understand it. Astronomers have so far identifi ed approximately 3,500 planetary nebulae in the Milky Way. Based on a count of the planetary nebulae within 2 kiloparsecs (6,500 light- years) of the Sun, they extrapolate that the Milky Way’s population includes anywhere from 11,000 to 28,000. NASA / ESA / HUBBLE HERITAGE TEAM (STSCI / AURA) SkyandTelescope.com November 2014 21 Mysteries of Planetary Nebulae low-density wind that collides with the older circum- shaping process turns the spherical AGB cloud into some- stellar cloud. At the same time the star’s surface tempera- thing much more complex. But astronomers lack a good ture increases to as high as 30,000 kelvin, and the intense theory to explain this change. Instead, they have many radiation ionizes the surrounding gas clouds, causing theories, none of which seems to fi t all the observations. them to glow. (See page 25 for three of the possibilities.) Thus begins the planetary nebula phase of the star’s For example, one theory posits that because the density life. The complex interaction between these two clouds, of the older outer envelope might be diff erent in diff er- plus ultraviolet radiation from the star’s hot surface, com- ent places, the later high-speed winds will tend to fl ow bine to shape the planetary nebula and make it visible. faster and farther into the more tenuous regions. In most The transition from an AGB star to a planetary nebula cases, this means the inner wind will fl ow out the poles, happens very quickly, no more than a few centuries producing a bipolar shape. In other cases, the inner wind and sometimes as short as a few decades. The planetary punches through at diff erent spots to produce a multi- nebula phase that follows will then last a few tens of polar planetary nebula. thousands of years. Lacking nuclear fuel, the star’s winds This theory is inadequate, however. For example, one eventually die off , the nebula steadily dissipates away, and feature seen in about half of all planetary nebulae is point we are left with nothing but a small and very compact symmetry, where each point on one side of the nebula white dwarf star that will slowly cool and fade away. matches a corresponding point on the other side. Point symmetry can produce some incredibly complex shapes, Shaping the Nebula such as the Spirograph Nebula and the Cat’s Eye Nebula. Very few planetary nebulae are round. Even those few that Both are point symmetric, but unlike bipolar objects, they appear round might only look that way because we happen look very diff erent from each other. to be viewing them at a particular angle. Somehow, the An inner fast wind impacting an outer older envelope Boomerang Nebula PGC 3074547 NASA / ESA / HUBBLE HERITAGE TEAM (STSCI / AURA) AND ESA / HUBBLE COLLABORATION FACING PAGE: HUBBLE SPACE TELESCOPE / LARGE BINOCULAR TELESCOPE / SUBARU TELESCOPE / ROBERT GENDLER 22 November 2014 sky & telescope Ring Nebula M57 SkyandTelescope.com November 2014 23 Mysteries of Planetary Nebulae Eskimo Nebula Frosty Leo Nebula NGC 2392 IRAS 09371+1212 ESA / HUBBLE / NASA can’t produce such complex point-symmetric shapes. If the AGB star was part of a binary system, however, the interaction between the two stars could do it. As the stars NASA / ESA / ANDREW FRUCHTER (STSCI) / ERO TEAM (STSCI / ST-ECF) orbit each other they can churn the winds in many diff er- ent ways to create point symmetry.