M27 (NGC 6853) The Dumbbell Nebula in Vulpecula M 27 is a planetary nebula (PN) in the Vulpecula constellation, at a distance of about 1360 light years. This object was the first planetary nebula to be discovered; by Charles Messier in 1764. A planetary nebula is an astronomical object consisting of a glowing shell of gas and plasma formed by certain types of stars at the end of their lives. The name originates from a similarity in appearance to giant planets when viewed through a small optical telescope, and is unrelated to planets of the solar system. They are a relatively short-lived phenomenon, lasting a few tens of thousands of years, compared to a typical stellar lifetime of several billion years. Planetary nebulae are the end stage of stellar evolution for most stars. A typical star weighing less than about twice the mass of the Sun spends most of its lifetime shining as a result of nuclear fusion reactions converting hydrogen to helium in its core. The energy released in the fusion reactions prevents the star from collapsing under its own gravity, and the star is stable After several billion years, the star runs out of hydrogen, and there is no longer enough energy flowing out from the core to support the outer layers of the star. The core thus contracts and heats up. Currently the sun's core has a temperature of approximately 15 million K, but when it runs out of hydrogen, the contraction of the core will cause the temperature to rise to about 100 million K. The outer layers of the star expand enormously because of the very high temperature of the core, and become much cooler. The star becomes a red giant. The core continues to contract and heat up, and when its temperature reaches 100 million K, helium nuclei begin to fuse into carbon and oxygen. The resumption of fusion reactions stops the core's contraction. Helium burning soon forms an inert core of carbon and oxygen, with a helium-burning shell surrounding it. Helium fusion reactions are extremely temperature sensitive, with reaction rates being proportional to T^40. This means that just a 2% rise in temperature more than doubles the reaction rate. This makes the star very unstable - a small rise in temperature leads to a rapid rise in reaction rates, which releases a lot of energy, increasing the temperature further. The helium-burning layer rapidly expands and therefore cools, which reduces the reaction rate again. Huge pulsations build up, which eventually become large enough to throw off the whole stellar atmosphere into space. The ejected gases form a cloud of material around the now-exposed core of the star. As more and more of the atmosphere moves away from the star, deeper and deeper layers at higher and higher temperatures are exposed. When the exposed surface reaches a temperature of about 30 000K, there are enough ultraviolet photons being emitted to ionise the ejected atmosphere, making it glow. The cloud has then become a planetary nebula