Location of HST image in a larger ground-based view
FAST FACTS
DISTANCE |
Closest planetary nebula to Earth at 450 light years |
|---|---|
AGE |
10,000 years since last ejection of gases |
LOCATION |
Constellation Aquarius |
SIZE |
13 Arcminutes ~ 1.7 ly |
Planetary nebulae offer the astronomer the opportunity to view into the workings of stellar evolution once the veil of gases have been lifted by the very process of their formation.
Planetary nebulae result when shells of gases are expelled by a red giant star as it nears the end of its life cycle. First discovered in the 19th century when telescopes were still small, the nebulae appeared as tiny bluish-green disks resembling Uranus and Neptune, and thus the reason for the name given them by William Herschel,discoverer of the planet Uranus in 1781. Planetary nebulae have nothing to do with planets; which may form in the earlier stages of a stars existence but not near their ending.
One of the most viewed nebulae in the northern night sky during the summer is the Ring Nebula located in the constellation Lyra. The Ring Nebula illustrates what was once considered the "typical" shape,morphology,for planetary nebulae. Improved telescopes have shown that the "ring" shaped nebulae constitute some twenty percent of the total nebulae observed to date.
Nebula formation results when a sun-like star in the latter stages of the red giant phase of existence blows off an outer shell of mass, often in bursts resulting from instabilities within the red giant. The uneven distribution of the outwardly moving gas and dust is indication of these instabilities. The initially expelled shell of gases,termed the strong wind,have a higher density and slower speed ,about 20 km/sec, than gases expelled at a later stage,termed the fast wind in the process of nebula formation. These fast wind speeds can be 100 km/sec or more.
Possible cause for the instabilities within the red giant star is that helium fusion is sensitive to changes in temperature and pressure. An increase in helium fusion pushes the hydrogen shell outward, resulting in a reduced pressure and temperature. This results in a reduction in helium fusion due to the drop in pressure. The outer hydrogen shell contracts, increasing the pressure and temperature, and the fusion rate of helium. Once again the cycle of disturbance is repeated, resulting in another pulse of outward moving material and a loss of mass.
In time the helium core has been converted to a carbon core through the fusion process. The red giant star now has an internal structure similar to that of an onion;i.e.; an inner core of carbon surrounded by a shell of helium which in turn is surrounded by a shell of hydrogen. The helium-carbon interface continues to process helium into carbon and oxygen at temperatures nearing 100 million Kelvin. Instabilities at the helium-carbon interface result in helium flashes, and an outward expansion of the outer hydrogen shell. The outer shell of hydrogen has an interface with the helium inner shell, the site of continued hydrogen fusion into helium. The star has expanded well beyond its original main sequence diameter, thus the name red giant; which may grow into a radial size equal to the Earth's orbital distance from the sun.
In time the red giant has evolved into a star that has thrown off sufficient matter to obtain a more stable state. The resulting core, much reduced in size,is a very hot star, about the size of the Earth. Emission of large amounts of ultraviolet radiation encounters the outwardly expanding shells of gases, causing an ionization effect and illumination of the ring(s);and thus the planetary nebula is born. The loss of mass returns the remaining stellar core, now known as a white dwarf, to a stable condition with a surface temperature of 25,000 to 200,000 Kelvin. The resulting white dwarf star retains some 30 to 60 percent of the original red giants mass, experiencing an extreme level of density - about a million grams per cubic centimeter.
Over 1000 nebulae have been cataloged and it is possible that there may exist some 10,000 to 50,000 such stellar objects in our Milky Way Galaxy. It is believed that most stars ranging between one to five solar masses will produce planetary nebulae as a normal process in the evolution of the star; however; this phase in the life of a star is short lived, lasting only some 10,000 to 30,000 years. It is this rapid develop that allows astronomers to look into the workings of a star.
Should a white dwarf have a mass greater than 1.4 solar masses its gravitational force would be too great for the star to exist as a white dwarf. This is known as the Chandrasekhar limit;which was calculated by Dr.Chandrasekhar in 1930's. He determined that stars with masses up to three or four times that of the Sun end their existence as white dwarf stars because they lose mass as planetary nebulae; which signals an end to the red giant phase of a stars existence.
The "bubbles" of gases thrown off by the dying star returns matter to the interstellar medium, thus enriching the space between stars with more complex chemical compounds other than hydrogen and helium. This enrichment will become the starting material for a new generation of stars having a more complex chemical history than earlier generations of stars.
The animation below will illustrate the formation process of a symmetrical ring type planetary nebulae;however;only some 20% of the nebulae observed to date are of this type. Most nebulae are either elliptical, butterfly shaped or bipolar in symmetry.
Animation showing formation of the nebula (1.2MB MPEG)