Quote:
Originally Posted by John Hothersall
The HII structures inside IC1613 are in the form of giant shells believed to be supernovae driven or driven by some of the massive blue supergiant stars in the area but nobody is entirely sure.
John.
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John,
IC 1613 and NGC 6822 and the WLM galaxy are much more typical of dwarf irregular galaxies than relatively high surface brightness examples like the LMC and NGC 4449
In essence, dIrr galaxies, unless they happen to be undergoing a vigorous (and episodic) starburst, continue to form very massive and luminous young stars, which energize HII regions, but they form these young hot blue stars at a low rate.
One of my absolute favourite dwarf irregular galaxies that
is undergoing a burst of star formation is IC 4662 ; this tiny dwarf galaxy nevertheless contains two giant HII regions, which I have even seen visually with a 10 inch.
However, it is rare to find a dwarf irregular which has Ha emission all over it.
The stars that energize HII regions burn out in a few million years, which is a blink of an eye on a cosmic timescale, so HII emission traces what is usually called
the most recent star formation.
All of those pretty HII regions that we see in our images might be gone in a short while, perhaps within a million years time, as the stars that energize them become red supergiants and stop producing that short wavelength UV that is necessary to make them gently glow.
cheers,
Robert
P.S. When I say "HII region", I also include all regions of ionized hydrogen, including supershells energized by supernovae and including diffuse Halpha emission.
(HII , technically, refers to any region of Ha emission)
Of course it is strange of astronomers to use the words "recent" when they mean within the last 10 million years, and "very recent" when they mean within the last 1 or 2 million years....
especially considering that if I live a hundred years, a million years is still ten thousand times my life-span!