Planetary nebulae are emission nebulae that are formed when a star reaches its final stages and the outer layers of the star are ejected. The material forms an envelope surrounding the dying star and the material gets spread out due to strong stellar winds. Planetary nebulae have nothing to do with planet formation. However, they are important in the evolution of the galaxy as they return gas and dust along with other material back into space.
Some Amazing Facts
The Hourglass Nebula is a planetary nebula which is unique in many aspects. Let us take a look at some interesting facts about this nebula:
- The Hourglass Nebula is a young planetary nebula that is located in the constellation Musca.
- The nebula is located some 8000 light years from Earth.
- Another name for the nebula is MyCn 18.
- The unique shape of the hourglass of this nebula is due to the expansion of a fast stellar wind within a cloud that is slowly expanding.
- The expanding cloud is more dense near its equator than at the poles.
- The Wolf-Rayet (WR9) star has ejected its material and is responsible for illuminating the nebula.
- The star was thought to lie at the center of symmetry of the nebula but it is clearly off center.
- The star has a temperature of around 110,000° Kelvin.
- The expansion velocity of the nebula is 10 km/s.
- Around the rims of the nebula there appears to be a series of arc-like etchings. These etchings could be as a result of the remnants of discrete shells that were ejected during the younger stages of the star. Another possible explanation is that the etchings could be due to matter striking against the walls of the hourglass in a narrow beam.
- The central region of the star contains a smaller hourglass structure and two rings.
- Ring 1 is bright and elliptical whereas Ring 2 is smaller and is a higher-excitation ring.
- The bright elliptical ring resembles a potato shape and is symmetrically very different from the larger hourglass.
- The top or south-eastern part of the nebula appears to be tilted towards the Earth.
- The central star emits a high-energy solar wind and also contains a massive core composed of heavy elements. The core generates an extreme magnetic field. Particles within the nebula get trapped in the magnetic fields and emit light causing the rings to flatten out giving it the familiar hourglass shape.
- The dust grains within the nebula reach a temperature of 51,000° Kelvin at a distance of approximately 5 x 1017 centimeters from the central star.
- A star located 2.3 degrees north of the central star can be seen in the image of the nebula. However, the star may or may not be located in the nebula and is thought to have no affect on the evolution of the nebula.
- Compressed gas which forms a thin shell making up the walls of the larger hourglass structure may be subject to a number of instabilities including Kelvin-Helmholtz instabilities.
- The arc-like etchings and moltings that appear on the walls of the larger hourglass could be a result of such instabilities.
- The central star is offset from the center of the nebula and why this is remains a puzzle. One possible explanation is that the star has a companion and is part of a binary system.
- Another mystery surrounding the nebula is the formation of Ring 1. One theory states that if the central star visible in the images was part of a binary system and was close to its companion star, then mass transfer onto the surface of this star would have produced an explosive event which would result in an expanding shell of ejected matter and the formation of Ring 1.
Discovery of the Nebula and its Future
The Hourglass Nebula was discovered jointly by Annie Cannon and Margaret W. Mayall while working extensively on the Henry Draper Catalogue. The catalog was compiled between 1918 ans 1924. The actual hourglass shape of the nebula was discovered later on as a result of improved technology and observation techniques by Raghvendra Sahai and John Trauger of the Jet Propulsion Laboratory on Jan 18th, 1996. Certain aspects of the nebula still remain a mystery such as the formation of the inner rings and the offsetting of the central star from the center of the nebula. In due time, scientists tend to solve these mysteries so that we can get a better understanding of the nebula which can help us understand similar nebulae in the vast reaches of space.