Where are HII regions found?
spiral galaxies
H II regions are found only in spiral galaxies like the Milky Way and irregular galaxies. They are not seen in elliptical galaxies. In irregular galaxies, they may be dispersed throughout the galaxy, but in spirals they are most abundant within the spiral arms.
How are HII regions observed?
HII regions are also observed at radio wavelengths, emitting radio free-free emission from thermalized electrons and radio recombination lines from highly excited states of H, He, and some metals (e.g., H109α and C lines).
What causes an HII region?
As O and early B-type stars form and turn on within molecular clouds, their UV radiation begins to ionize the surrounding, mostly neutral hydrogen (HI) medium. The ionizing radiation from these stars or clusters of stars provide the ionizing source that powers the HII region throughout their stellar lifetimes.
What is the difference between HI and HII regions?
Such regions of ionized hydrogen are called HII regions, while cold un-ionized hydrogen clouds are termed HI regions (with the I and II referring to the ionization state of the hydrogen).
What are H regions?
H II region, also called diffuse nebula or emission nebula, interstellar matter consisting of ionized hydrogen atoms. The energy that is responsible for ionizing and heating the hydrogen in an emission nebula comes from a central star that has a surface temperature in excess of 20,000 K.
Which is the same as an H II region?
H II region, also called diffuse nebula or emission nebula, interstellar matter consisting of ionized hydrogen atoms.
How does the use of HII regions to find a galaxy’s distance?
H II regions are clouds of gas filled with ionized hydrogen. H II regions are luminous because a lot of star formation takes place. Cepheid variables are stars that have periodic brightness phases, and their periods are related to their luminosity, which can be used to find distance.
Which of the following statements best explains why HII regions appear red?
Why do HII regions appear red? HII regions are composed of completely ionized hydrogen. When the hydrogen captures an electron, light is emitted. The red Balmer hydrogen line is the strongest wavelength emitted.