The Complex Structure of a Planetary Nebula shown By Hubble
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The Complex Structure of a Planetary Nebula shown By Hubble

NGC 6891 is a brilliant and uneven planetary nebula in the constellation Delphinus, the Dolphin. This Hubble image shows an aspherical outer halo that is expanding faster than the interior nebula, as well as at least two differently oriented ellipsoidal shells.

As seen in this photograph, filaments and knots encircle the central white dwarf star in the nebula’s center. Based on their movements, astronomers believe that one of the shells is 4,800 years old and the outer halo is 28,000 years old, implying a series of eruptions from the dying star at various times.

NGC 6891 was studied by Hubble as part of his research into nebula distances and the formation and evolution of their structures. NGC 6891’s central white dwarf star has ionised the gas, removing electrons from the nebula’s hydrogen.

When charged electrons interact, they emit energy in the form of light. They can go from a higher form to a lower level by combining with existing hydrogen nuclei, allowing the nebula’s gas to glow.

Structure and forms

Planetary nebulae, as opposed to diffuse nebulae (see H II area), are small objects with a radius of about one light-year and a gas mass of about 0.3 solar mass. The Helix Nebula (NGC 7293) in Aquarius is one of the largest known planetary nebulae, with an angle of around 20 minutes of arc—roughly two-thirds the size of the Moon.

Planetary nebulae have a 1,000-fold brighter surface and are significantly denser than most H II zones, with dense sections containing 1,000–10,000 atoms per cubic cm.

Some are so far away that they appear brilliant when photographed directly, but the most notable examples have angular diameters of up to 20 minutes of arc across, with 10–30 minutes of arc through being the norm.

Bright disc areas have more regular shapes than chaotic H II areas, but there are frequent significant brightness changes throughout the disc. Planetaries have regular, sharp outside edges and a moderately regular inner boundary, giving them a ring-like appearance. Many have two light lobes that look like arcs of a circle joined by a bridge that looks like the letter Z.

Planetary nebulae’s distances

Because of the variety of ionized gas forms and masses, determining the distance to any specific planetary nebula is difficult. The amount of ionizing radiation that escapes from the central star, as well as the amount of hot low-density material that fills a portion of the volume but does not emit significant radiation, are both unknown. As a result, planetary nebulae do not form a homogeneous group of objects.

Distances are calculated by measuring approximately 40 items that have particularly advantageous features. Association with other objects whose distance can be measured independently, such as membership in a stellar cluster or association with a star with known properties, is one of the advantageous characteristics.  For all others, statistical methods calibrated by these objects offer approximate estimates (around 30% inaccuracies) of distances.

Planetary nebulae evolution

A description of the evolution of a planetary nebula begins before the nebula’s own ejection. As will be discussed later, the central star is a red giant prior to the ejection. During this time, it loses mass rapidly, up to 0.01 Earth mass per day, in the form of a relatively slow-growing stellar wind.

At this point, the red giant could be obscured by dust that has accumulated as a result of the wind’s heavy ingredients. The natures of the star and its wind eventually change. The star becomes hotter as its hot core is exposed due to the loss of its overlying atmosphere.

The blazing star’s radiation ionizes the inner gas. The ionisation zone expands gradually outward through what was previously the slowly moving material of the stellar wind. The gas expands at about 30 kilometers (19 miles) per second. Nebulae at this stage are brilliant but have starlike appearances as seen from Earth because they are too small to have a disc.

Its gas has a dense population of about one million atoms per cubic centimeter, but it becomes more dilute as it spreads. At this point, the nebula is surrounded by neutral hydrogen. It appears to grow faster than the individual gas atoms within it; as density decreases, the ionised shell “eats into” the neutral substance.

Image Credit: NASA, ESA, A. Hajian (University of Waterloo), H. Bond (Pennsylvania State University), and B. Balick (University of Washington);
Processing: Gladys Kober (NASA/Catholic University of America)

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