| | Nebula
|
Nebulas
A nebula is an interstellar cloud of dust, hydrogen gas and plasma. Originally nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were discovered by Edwin Hubble). Nebulae often form star-forming regions, such as in the Eagle Nebula(now considered a galaxy). This nebula is depicted in one of NASA's most famous images, of the "Pillars of Creation". In these regions the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to form stars. The remaining materials are then believed to form planets, and other solar system objects.
|
Formation
Many nebulae form from the gravitational collapse of diffuse gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the centre and illuminate the surrounding gas allowing it to be observed. An example of this type of nebula is the Rosette Nebula or the Pelican Nebula. Some nebulae are formed as the result of supernova explosions. One of the best examples of this is the Crab Nebula, in Taurus. It is the result of a recorded supernova in 1054 AD. At the center of the nebula is a neutron star, created during the explosion. Other nebulae may form as planetary nebulae. Again these are created near the end of a star's life; when a star with a mass of under 1.4 solar masses becomes a red giant. An outer layer of light Hydrogen gas is ejected from the star as the fusion process slows, and the star becomes unable to sustain its entire mass. A nebula is made of luminescent gases, rock and ash. They are not part of our solar system.
|
Planetary Nebula
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. They are in fact unrelated to planets; the name originates from a supposed similarity in appearance to giant planets. 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. About 1,500 are known to exist in the Milky Way Galaxy.
Planetary nebulae are important objects in astronomy because they play a crucial role in the chemical evolution of the galaxy, returning material to the interstellar medium which has been enriched in heavy elements and other products of nucleosynthesis (such as carbon, nitrogen, oxygen and calcium). In other galaxies, planetary nebulae may be the only objects observable enough to yield useful information about chemical abundances.
In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies. About a fifth are roughly spherical, but the majority are not spherically symmetric. The mechanisms which produce such a wide variety of shapes and features are not yet well understood, but binary central stars, stellar winds and magnetic fields may all play a role.
|
Planetary Nebula - 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 - fusion reactions have ceased, as the star is not heavy enough to generate the core temperatures required for carbon and oxygen to fuse. Eventually it will cool down so much that it doesn't give off enough ultraviolet radiation to ionise the increasingly distant gas cloud. The star becomes a white dwarf, and the gas cloud recombines, becoming invisible. For a typical planetary nebula, about 10,000 years will pass between its formation and recombination of the star.
|
Galactic Recycling
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 thus contain a large proportion of elements such as carbon, nitrogen and oxygen, and as they expand and merge into the interstellar medium, they enrich it with these heavy elements, collectively known as metals by astronomers.
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 (see stellar population).
|
|
|
Planetary Nebula Origions
Planetary nebulae are the end stage of stellar evolution for most stars. Stars weighing more than a few solar masses will end their lives in a dramatic supernova explosion, but for the medium and low mass stars, such as our Sun, 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 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 T40. 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.
|
|
|
