Planetary Nebula Alan Iannaccone

What is a Planetary Nebula? A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. Planetary Nebulas received their name due to the fact that their apparent size is so small they look like planets through small telescopes. Emission Nebula: Clouds of ionized gas that emit their own light at optical wavelengths. Their mass generally ranges from 100 to 10,000 solar masses and this material can be spread over a volume of less than a light year to several hundred light years. Red Giant Star: A dying star in the last stages of stellar evolution.

Discovery The Dumbbell Nebula, M27, was the first planetary nebula discovered by Charles Messier in 1764. William Herschel discovered NGC 1514 in 1790. Herschel identified and placed into his catalog 79 objects as planetary nebulae however only 20 of them were actually planetary nebula after further inspection. The first planetary nebula to be discovered was the Dumbbell Nebula, M27, by Charles Messier in 1764. He eventually added four to his catalog of astronomical objects. In 1790, Herschel found NGC 1514, a planetary nebula with a bright central star. He realized that these new objects were made up of gas or dust, rather than being clusters as thought at the time. Herschel identified 79 objects as planetary nebulae, but only 20 of them truly were, while 13 others that he had classified as other objects turned out to be these gaseous shells.

Planetary nebula evolution The ejection of the nebula begins when a red giant begins to lose mass at a rate up to .01 Earth mass per day in the form of a stellar wind. Star becomes hotter because hot core is exposed by loss of overlying atmosphere. Inner gas is ionized by radiation from hot star. Gas expands at about 30km/s Gas becomes diluted with expansion causing nebula to be surround by neutral hydrogen. Entire mass of gas is ionized and ultraviolet radiation escapes into space. Expansion of nebula is due to the motion of gas. A description of the evolution of a planetary nebula begins before the ejection of the nebula itself. As will be discussed below, the central staris a red giant before the ejection. In such a phase it experiences a rapid loss of mass, up to 0.01 Earth mass per day, in the form of a comparatively slowly expanding stellar wind. At this stage the red giant might be heavily obscured by dust that forms from the heavy elements in the wind. Eventually the nature of both the star and its wind changes. The star becomes hotter because its hot core is exposed by the loss of the overlying atmosphere. The inner gas is ionized by radiation from the hot star. The ionization zone moves steadily outward through the slowly moving material of what was formerly the stellar wind. The expansion speed of the gas is typically 30 km (19 miles) per second. Nebulae in this stage are bright but have starlike images as seen from Earth, because they are too small to show a disk. The gas is at a relatively high density—about one million atoms per cubic centimetre—but becomes more dilute as the gas expands. During this stage the nebula is surrounded by neutral hydrogen. It appears to expand faster than the individual atoms of gas in it are moving; the ionized shell is “eating into” the neutral material as the density falls. The middle stage of evolution occurs when the density has dropped to the point at which the entire mass of gas is ionized. After this stage is reached, some of the ultraviolet radiation escapes into space, and the expansion of the nebula is caused entirely by the motion of the gas. Most planetaries are now in this middle stage. Finally the central star becomes less luminous and can no longer provide enough ultraviolet radiation to keep even the dilute nebula ionized. Once again the outer regions of the nebula become neutral and therefore invisible. Eventually the gas is mingled with the general interstellar gas. A curious feature of several planetaries is that faint rings surrounding the bright inner nebula can be observed; they are the remnants of a previous shell ejected earlier by the star.

In the constellation Lyra Apparent Magnitude of 8.8 The Ring Nebula M57 2,000 light-years away In the constellation Lyra Apparent Magnitude of 8.8 Discovered by Antoine Darquier de Pellepoix in 1779. Made up of helium, hydrogen, oxygen, nitrogen, and sulfur. The tiny white dot in the center is the star’s hot core, commonly referred to as a white dwarf. The image of M57 in the video is colorized to show the chemical composition. The deep blue color in the center represents helium, the light blue color of the inner ring is the glow of hydrogen and oxygen, and the reddish color of the outer ring is from nitrogen and sulfur. M57 can be best spotted using a moderately sized telescope during the month of August. M57 is tilted towards earth which allows viewers to see the ring face on.

Sketches of M57

The Dumbbell Nebula M27 1,200 light years away. In the constellation Vulpeca near Cygnus the Swan. Apparent Magnitude of 7.5 Discovered by Charles Messier in 1764 and was the first planetary nebula ever discovered. Made up of hydrogen, oxygen, nitrogen, and sulfur. The term planetary nebula is kind of a misnomer due to the nebula’s round, planet-like appearance when viewed through smaller telescopes. The nebula can be spotted with a small telescope most easily in the month of September. The knots of gas displayed by M27 is considered to be part of the nebula’s evolution. The reason why the knots form is due to the fact that the stellar winds are able to blow away smaller particles but not large clumps. The shapes of the knots expand as the nebula evolves. Blue represents oxygen, green represents hydrogen, and red indicates sulfur and nitrogen.

References: http://astronomy. swin. edu