H II regions pronounced 'H two' emerge when molecular cloud s collapse and form hot and young stars of spectral class O, B or A, the three hottest and brightest types of stars that exist. These newly born stars emit a lot of ultraviolet light that ionises the hydrogen atoms of the surrounding gas cloud, "kicking" or exciting electrons to a higher energy level.
When such an electron falls back to its original energy level it emits a photon with a specific wavelength of Since the predominant component of these star-forming regions is hydrogen, these regions glow in red.
An H I region is a region with neutral atomic hydrogen; H I regions do not emit light, hence are not emission nebulae. H II regions have typical temperatures of about K; far too hot to form new stars. New stars can only be born in the presence of a very cold molecular cloud. Therefore, within a timespan of a few million years, the gas of the H II region will be blown away by the radiation pressure of new-born stars that formed in cooler parts of the nebula This material is not lost and can be recycled in the far-distant future in other star-forming regions.
As the hydrogen recombines and returns to its neutral state, optical photons are emitted. Emission nebulae are clouds of ionised gas that, as the name suggests, emit their own light at optical wavelengths. Their mass generally ranges from to 10, solar masses and this material can be spread over a volume of less than light year to several hundred light years.
One of the most common types of emission nebula occurs when an interstellar gas cloud dominated by neutral hydrogen atoms is ionised by nearby O and B type stars. These extremely hot and luminous stars give off vast quantities of high-energy ultraviolet UV photons which break the neutral hydrogen atoms into hydrogen nuclei and electrons. It is illuminated by a pulsar which was created by the supernova. Nebulae are often the sites of star formation.
In fact, all stars, planets, and solar systems are formed from nebulae. A nebula may lie undisturbed for many millions or billions of years as it waits for just the right conditions. Eventually, the gravity from a passing star or the shock wave from a nearby supernova explosion may cause swirls and ripples within the cloud.
Matter begins to coalesce into clumps and grow in size. As these clumps get larger, their gravity increases. Gravity continues to pull in matter from the nebula until one or more of the clumps reach critical mass. The clumps are forming protostars. As gravity squeezes even tighter, the core temperature eventually reaches 18 million degrees. At this point, nuclear fusion begins and a star is born. The solar wind from the star will eventually blow away all of the excess dust and gas.
Sometimes other smaller clumps of matter around the star may form planets. This is the beginning of a new solar system. Several nebulae have been found to be stellar nurseries.
The Eagle Nebula, and the Orion Nebula are both sites of active star formation. There are a few nebulae that can be seen with the naked eye and many more that can be detected with a good pair of binoculars. A telescope is required to bring our fine details. Unfortunately, the human eye is not sensitive enough to bring out the rich colors of most nebulae. It is the photograph that does the most justice to these incredible objects. Until recently, time exposures on film were the best way to bring a nebula's true colors.
Today, digital photography has simplified the process. New tools like the Hubble space telescope are giving us views of nebulae that have never been seen before. Areas of active star formation have been identified in many galaxies that were once thought to be inert. Turbulence deep within these clouds gives rise to knots with sufficient mass that the gas and dust can begin to collapse under its own gravitational attraction.
Stars with higher mass have shorter lifespans. When the sun becomes a red giant, its atmosphere will engulf the Earth. During the red giant phase, a main sequence star's core collapses and burns helium into carbon.
After about million years, the helium runs out, and the star turns into a red supergiant. Nebula Formation: In essence, a nebula is formed when portions of the interstellar medium undergo gravitational collapse. Mutual gravitational attraction causes matter to clump together, forming regions of greater and greater density. If a red giant has insufficient mass to generate the core temperatures required to fuse carbon around 1 billion K , an inert mass of carbon and oxygen will build up at its center.
After such a star sheds its outer layers and forms a planetary nebula, it will leave behind a core, which is the remnant white dwarf. When this happens , the dust and gas condense into giant clouds like the Orion Nebula seen in the pictures above. As they start to condense further, in some spots in the nebula , stars begin to form. Most Diffuse nebula contain mostly hydrogen with smaller amounts of helium, oxygen, sulfur, and other heavier elements. White dwarf stars are the corpses of stars; what happens once they've used up all their fuel and lack the temperature and pressure to continue fusion in their core.
Eventually a star runs out of hydrogen fuel in its core and its fusion stops. Nebula is a cloud of interstellar dust, while galaxy is a huge collection of stars. Size of a galaxy is much larger than size of a nebula. Life of several stars is connected to life of a galaxy while life of only one star is associated with a nebula. Galaxies can be found in clusters in space. Life Cycles of Stars. A star's life cycle is determined by its mass.
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