Color shmolor, who cares?
When discussing a Wolf-Rayet nebula, we may say that the nebula itself is all that interests us, and then we don't care about the ionizing central star. Or else we may say that the central star is indeed interesting, and it is interesting for two reasons: 1, it has indeed created the nebula, and 2, it is not done with its shenanigans, since it is going to go supernova in the moderately near future.
So let's say that we acknowledge that the Wolf-Rayet star itself, called EZ CMa
, is of some interest. We may decide that the only thing that interests us about it is its stellar wind
, whose strength and duration we may try to roughly judge by looking at the shape and size of the nebula.
Or else, we may say that we might just possibly gain a little bit of knowledge about the properties of EZ CMa by looking at its color. In doing so, it is very helpful to look at stars in the same field. The brightest star in today's APOD is orange, and indeed, this star is a cool K-type supergiant star, Omicron-1 CMa
, with a B-V index of +1.74. That color index is suitable for a somewhat red or reddened orange K-type star. Its orange color reflects the star's cool temperature, which is probably around 4,000 K.
As we can see if we look at the APOD, the EZ CMa is just as orange as its bright neighbor. Does that mean that the temperature of EZ CMA is similar to that of Omicron-1 CMa?
Jim Kaler wrote
about EZ CMa:
EZ CMa is classed as a mid-temperature (relative to the set) WN4 star of 89,000 Kelvin
with a radius of just 2.65 times that of the Sun, a luminosity of 400,000 Suns, and a current mass of 19 Suns.
89,000 Kelvin is about 20 times as hot
as a typical K-type star like Omicron-1 CMa. So why do the hues of Omicron-1 CMa and EZ CMa look much the same? Is it because all the stars in today's APOD look orange? No, they don't, as a quick check will reveal. Many stars in the field look blue.
Wolf-Rayet stars are typically dusty and reddened by the dust of their own making. Could EZ CMa be sufficiently dust-reddened to actually look like an orange K-type star, even though it is 20 times hotter?
Answer: No. The B-V index of EZ CMa is -0.05, which is bluer than Vega
I'd say that the reason why blisteringly hot and even-to-the-eye blue-white star EZ CMa is the same color as cool orange supergiant Omicron-1 CMa in the APOD is because those who produced the APOD didn't care about the color of the ionizing central star of the nebula. But it is certainly possible to bring out the color difference between a hot and a cool star even in narrowband photography, as the picture at right demonstrates.
A little more than a year ago, another picture of the Dolphin Nebula
was the APOD. But in that case, the photographer did a good job at bringing out a color difference between the hot central star and its cool supergiant neighbor.