Planetary nebula Guide, Meaning , Facts, Information and Description
A planetary nebula is an astronomical object consisting of a roughly spherical glowing shell of gas formed by certain types of stars at the end of their lives.
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2 Origins 3 Lifetime 4 Galactic recyclers 5 Observations 6 Some famous planetary nebulae 7 Related topics 8 References and external links: |
A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally around 1000 particles per cm² - about a million billion billion times less dense than the earth's atmosphere! At the centre of the nebula is a hot star, which heats the gases to temperatures of about 10,000K.
About a thousand planetary nebulae are known to exist in our galaxy. They are found mostly near the plane of the Milky Way, with the greatest concentration near the galactic centre. They are only very rarely seen in star clusters, with only one or two known cases.
Generally speaking planetary nebulae are symmetrical and approximately spherical, but a wide variety of shapes exist with some very complex forms seen. Approximately 10 per cent of planetary nebulae are strongly bipolar, and a small number are asymmetric. One is even rectangular. The reason for the huge variety of shapes is not fully understood, but may be caused by gravitational interactions with companion stars if the central stars are double stars. Another possibility is that planets disrupt the flow of material away from the star as the nebula forms.
Our sun is a very average star, and only a small number of stars weigh very much more than it. Stars weighing more than a few solar masses will end their lives in a supernova explosion, but for most stars the end involves the creation of a planetary nebula.
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 collapsing under its own gravity.
After several billion years, the star runs out of hydrogen, and the core can no longer support the outer layers of the star. The core thus contracts and heats up, eventually reaching temperatures at which helium fusion begins. The outer layers of the star expand enormously because of the very high temperature of the core, and the star becomes a red giant. Helium fusion is extremely temperature sensitive (reactions rates vary as T40!), which makes the star unstable. Huge pulsations eventually kick off the outer layers of the star.
The ejected gases form a cloud of material around the now-exposed core of the star. The central star is very hot, with a temperature as high as 200,000K. It gives off enormous amounts of ultraviolet light, which ionises the cloud, making it glow. The cloud has then become a planetary nebula.
The gases of the planetary nebula drift away from the central star at speeds of a few kilometres per second. At the same time as the gases are expanding, the central star is cooling as it radiates away its energy. Eventually it will cool down so much that it doesn't give off enough UV radiation to ionise the increasingly distant gas cloud.
Planetary nebulae play a very important role in galactic evolution. The early universe consisted almost entirely of hydrogen and helium, but stars create heavier elements via nuclear fusion. The gases of planetary nebulae contain lots of elements such as carbon, nitrogen and oxygen, and as they expand and merge into the interstellar medium, they enrich it in these heavy elements.
Subsequent generations of stars which form will then have a higher initial content of heavier elements. Even though the heavy elements will still be a very small component of the star, they have a marked effect on its evolution. Stars which formed very early in the universe and contain small quantities of heavy elements are known as Population II stars, while younger stars with higher heavy element content are known as Population I stars.
The first planetary nebula discovered was the Dumbbell Nebula in the constellation of Vulpecula, observed by Charles Messier in 1764 and listed as M27 in his Messier Catalogue. To early observers with low-resolution telescopes, M27 and subsqequently discovered planetary nebulae somewhat resembled the gas giants, and William Herschel, discoverer of Uranus, eventually coined the term 'planetary nebula' for them, although as we now know, they are very different objects to planets.
The nature of planetary nebulae was unknown until the first spectrocopic observations were made in the mid-19th century. These observations established that these objects consisted of extremely rarefied gas, but it was only in the 1950s that their place in stellar evolution was established.
Today, there are many controversial issues in the study of planetary nebulae. The process by which the shapes arise is one, and there are also serious uncertainties about how much carbon, nitrogen and oxygen they contain. Modern instruments such as the Hubble Space Telescope are commonly used to observe planetary nebulae, and deep spectroscopic observations from the ground and space are also crucial to understanding them.
Some of the more famous examples of this type of object are the Ring Nebula, the Helix nebula, the Eskimo Nebula and the Cat's Eye nebula.
This is an Article on Planetary nebula. Page Contains Information, Facts Details or Explanation Guide About Planetary nebula Characteristics
Physical characteristics
Numbers and distribution
Morphology
Origins
Lifetime
Galactic recyclers
Observations
Some famous planetary nebulae
Related topics
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