by alter-ego » Sat Oct 25, 2014 5:08 am
Chris Peterson wrote:
...
Sunspots are blazingly bright. If the entire surface of the Sun were the temperature (and brightness) of a sunspot, you'd still go blind almost as fast staring at it.
That's so true, although one might be tempted to think otherwise.
I plotted the blackbody visibility (adjusting for photopic vision sensitivity) for three temperatures: 5800°K (Sun), 3800°K and ~3000°K (latter two for cool sunspots).
The visible energy at 3000°K is ≈1.8% of the visible energy at 5800°K. The APOD shows the central sunspot region also to be ~1.8% of the bright region surrounding the spot. Since the Sun is a good blackbody, it seems reasonable to conclude the coolest sunspot region ~3000°K. This temperature is within the
sunspot temperature range. Put in terms of magnitudes, the coolest sunspots appear about 4½ magnitudes fainter, so at 3000°K, the whole sun would be about -22.3 mag.
[quote="Chris Peterson"]
...
Sunspots are blazingly bright. If the entire surface of the Sun were the temperature (and brightness) of a sunspot, you'd still go blind almost as fast staring at it.[/quote]
That's so true, although one might be tempted to think otherwise.
I plotted the blackbody visibility (adjusting for photopic vision sensitivity) for three temperatures: 5800°K (Sun), 3800°K and ~3000°K (latter two for cool sunspots).
[attachment=0]Blackbody Visiblity_Sun and Sunspots.JPG[/attachment]
The visible energy at 3000°K is ≈1.8% of the visible energy at 5800°K. The APOD shows the central sunspot region also to be ~1.8% of the bright region surrounding the spot. Since the Sun is a good blackbody, it seems reasonable to conclude the coolest sunspot region ~3000°K. This temperature is within the [url=http://en.wikipedia.org/wiki/Sunspot]sunspot temperature[/url] range. Put in terms of magnitudes, the coolest sunspots appear about 4½ magnitudes fainter, so at 3000°K, the whole sun would be about -22.3 mag.