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Matthew
2005-Jan-12, 07:39 AM
What does the H in HI, HII, HIII mean? I take in the context of astronomical images and in the Virtual Star Party thread (http://www.universetoday.com/forum/index.php?showtopic=5914&st=45) was asked by Duaney to find a HII of the Cat's Eye Nebulae.

astromark
2005-Jan-12, 08:52 AM
Hubble... No this is wrong. I thought you were asking re; images of stellar nursaries, and thought the hubble library was the answer to your question. Oops.

Ola D.
2005-Jan-12, 09:52 AM
Originally posted by matthew@Jan 12 2005, 07:39 AM
What does the H in HI, HII, HIII mean?
H is for hydrogen. HII region is a region of ionised hydrogen (HII) surrounded by cooler, neutral hydrogen (HI). The largest example of an HII region in the sky is the Orion Nebula.

And there's no HIII.

Duane
2005-Jan-12, 05:45 PM
Ola is correct. HII regions are usually ionized by hot stars emitting strong UV radiation, such as is found in young stellar nursuries.

Dave Mitsky
2005-Jan-17, 06:55 AM
The largest HII region that I'm aware of is the Tarantula Nebula (NGC 2070), which is over 30 times larger than the Orion Nebula (M42). It is the only extragalactic, non-galactic DSO that is visible to the unaided eye.

http://antwrp.gsfc.nasa.gov/apod/ap991026.html

http://www.eso.org/outreach/press-rel/pr-2...phot-14-02.html (http://www.eso.org/outreach/press-rel/pr-2002/phot-14-02.html)

Dave Mitsky