Although HII regions may produce thousands of stars in this manner, the star formation process itself is highly inefficient. stream

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a(H0,T) is the recombination rate coeffand n iand n eare the number densities of ions and electrons. The winds and UV radiation emitted by the massive stars not only irradiate the gas in the HII region, but also work to carve out a cavity in the surrounding molecular cloud. –most spirals lack H II regions in their nuclei –~10% of spirals show active star formation and large H II regions in their nuclei à H II or starburst galaxies •Irregulars: H II regions usually asymmetrically distributed –some show features resembling spiral arms or bars (LMC) •Ellipticals and SOs: almost never show H II regions.

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• To expand the HII region by dR, the star must ionize 4πR2n HdRatoms. h�b```f``rg`e``3c`@ �+G���b �s �,��1��B]�E|g��ʨ�ʞ� ����7C�Ip�@�@�� ��)|l�U���� L��|������b�݂sV�d�,ç��o;�!�{���m�t1����UݱT�EY���žTM��>WKw��\V��QV�T�s�N�W��U���CF\!n;?�4M�.uɞ�qB����X$���ӉJ�9[Ϸ�z�L��;mjO�Ȋ�⇒�Z�1?�����zcO*����v~�4ވShȼ%�����i� �$n�\��BXt/ >> Measuring Temperature: The mere occurrence of optical forbidden lines suggests values of order 104 K. (NB: E/eV = 12,400 (Ǻ/λ) and 1eV corresponds to 11,605 K) More precisely, observe levels of the same ion arising from different upper levels, e.g., ratio the … A very high temperature is required to ionize the gas and eventually form the HII region. 301 0 obj <>stream Centre for Astrophysics and Supercomputing, COSMOS - The SAO Encyclopedia of Astronomy, Study Astronomy Online at Swinburne University. %%EOF 0000003603 00000 n This process causes stars to be born. %PDF-1.3

HII regions developed from a giant molecular cloud. E�Q����2ֺ��طӨw���!��/�/�E7~8�? 0000002984 00000 n trailer 0000001301 00000 n 0000002907 00000 n 455 0 obj<>stream It is necessary to be confident that our traditional techniques for deriving chemical abundances in ionized nebulae (based on the analysis of intensity ratios of collissionally excited lines, CELs) provide the real values.

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Ionized Hydrogen (HII) Regions III-2 The integral is taken over all photons with h h 0, where h 0 is the Ionization Potential of H 0, 13.59eV for ionization out of the 1s ground state. •In the interior of the region, recombinationsoccurper unittime. All material is © Swinburne University of Technology except where indicated. 0000000609 00000 n 0000002624 00000 n H II regions are essential tools for the study of the chemical composition and chemical evolution of the Universe, specially in the extragalactic domain. They range in size from so-called ultra-compact (UCHII) regions perhaps only a light-year or less across, to giant H II regions several hundred light-years across.

Alternative Titles: HII region, diffuse nebula, ionized hydrogen cloud H II region , also called diffuse nebula or emission nebula , interstellar matter consisting of ionized hydrogen atoms .