Anevaporating gas globule (EGG) is a region of hydrogen gas in outer space approximately 100astronomical units in size, such that gases shaded by it are shielded from ionizingUV rays.[2] Dense areas of gas shielded by an evaporating gas globule can be conducive to the birth of stars.[2] Evaporating gas globules were first conclusively identified via photographs of thePillars of Creation in theEagle Nebula taken by theHubble Space Telescope in 1995.[2][3]
EGG's are the likely predecessors of newprotostars. Inside an EGG the gas and dust are denser than in the surrounding dust cloud. Gravity pulls the cloud even more tightly together as the EGG continues to draw in material from its surroundings. As the cloud density builds up the globule becomes hotter under the weight of the outer layers, a protostar is formed inside the EGG.
A protostar may have too little mass to become a star. If so it becomes abrown dwarf. If the protostar has sufficient mass, the density reaches a critical level where the temperature exceeds 10 million kelvin at its center. At this point, a nuclear reaction starts converting hydrogen to helium and releasing large amounts of energy. The protostar then becomes a star and joins the main sequence on theHR diagram.[4]
A study of 73 EGGs in the Pillars of Creation (Eagle Nebula) with theVery Large Telescope showed that only 15% of the EGGs show signs of star-formation. The star-formation is not everywhere the same: The largest pillar has a small cluster of these sources at the head of the pillar.[5]
| Visible nebula | |
|---|---|
| Pre-stellar nebulae | |
| Stellar nebula | |
| Post-stellar nebulae | |
| Clouds | |
| Morphology | |
| Intergalactic blobs | |
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| Related | |
