Planetary Nebula Characteristics Physical Characteristics

A typical planetary nebula is roughly one light year across, and consists of extremely rarefied gas, with a density generally around 1000 particles per cm³ - 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 10,000 K.

1.2 Numbers and distribution

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.

1.3 Morphology

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.

2 Origins

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 supernovaRemnant of Kepler's Supernova, SN 1604. A supernova is a type of stellar explosion which appears to result in the creation of a new star upon the celestial sphere. Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also in 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 fusionIn physics, nuclear fusion (a thermonuclear reaction is a process in which two nuclei join, forming a larger nucleus and releasing energy. Nuclear fusion is the energy source which causes stars to shine, and hydrogen bombs to explode. It takes considerabl reactions converting hydrogenhydrogen helium H Li Full table General Name, Symbol, NumberHydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1 (IA), 1 , s Density, Hardness 0. 0899 kg/m3, NA Appearance colorless Atomic properties Atomic weight 1. 00794 amu Atomic radius (ca to heliumHelium is a colorless, odorless, tasteless chemical element, one of the noble gases of the periodic table of elements. Its boiling and melting points are the lowest among the elements; except in extreme conditions, it exists only as a gas. The second most 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 giantAccording to the Hertzsprung-Russell diagram, a red giant is a large non- main sequence star of stellar classification K or M; so-named because of the reddish appearance of the cooler "giants". Examples include Aldebaran and Arcturus. They are believed to. 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 ultravioletNote: Ultraviolet is also the name of a 1998 UK television miniseries about vampires. Ultraviolet UV radiation is electromagnetic radiation of a wavelength shorter than that of visible light, but longer than that of soft X-rays. It can be subdivided into light, which ionisesIonisation can be: the process of creating an ion, see ionization a piece of music by Edgar Varese; see Ionisation (Varese). the cloud, making it glow. The cloud has then become a planetary nebula.

3 Lifetime

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.

4 Galactic recyclers

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.

5 Observations

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.

6 Some famous planetary nebulae

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.

7 Related topics

List of planetary nebulae

8 References and external links:

Press release on Cat's Eye Nebula, including above image: http://chandra.harvard.edu/photo/cycle1/1220/index.html
Planetary Nebulae, SEDS Messier Pages

Astronomy Nebulae

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Home > Planetary nebula

1 Characteristics

1.1 Physical characteristics