by Ann » Thu Jun 13, 2019 7:02 pm
When I first saw today's APOD, I thought,
No way the stars in M13 can be that blue! Look at the color indexes of the stars, where the bluest ones are -1.5, that's ridiculous! NO stars can get that blue ever!!! And look at that main sequence, it is almost all bluish!!!
Then I realized that the color index used in today's APOD is not the customary
B - V, but instead
B - R.
The northern Milky Way and bluish Vega at 2 o'clock.
Photo: Alan Dyer.
When I looked at today's APOD and saw how "blue" the stars were, I realized how much the "normal" B-V index actually "reddens" stars, or at least reddens our perspective of them. Vega, for example, has a B-V index of 0.00, and it is used as a "zero point" in the description of star colors and as a perfect example of a purely "white" star.
Well, that's ridiculous! I have taken part in showing people Vega through a telescope and asked them what color they thought it was. All of them said that Vega is blue.
Vega's "whiteness" is a product of the workings of the B-V system, where Vega comes out neutral. (All right, it is also a product of our human night vision, because the blue-sensitive rods in our retinas are not good at responding to blue light when the blue light source is very faint and is seen against a very dark background.) Well, Vega's color wouldn't be neutral, but clearly blue, in a B-R color index system. In B-R, our own Sun would probably have a color index close to zero, which means it would be "white".
I keep insisting that our Sun really is white. Not because it possesses some magical quality of "whiteness", but because humanity has evolved under the light of the Sun, and it has been to our advantage to see the Sun as white. Oh, we don't see it as white, you say? No, but we see
daylight as white, or neutral, and daylight is sunlight, pure and simple.
Ann
When I first saw today's APOD, I thought, [i]No way the stars in M13 can be that blue! Look at the color indexes of the stars, where the bluest ones are [b]-1.5[/b], that's ridiculous! NO stars can get that blue [b]ever[/b]!!! And look at that main sequence, it is almost all bluish!!![/i]
Then I realized that the color index used in today's APOD is not the customary [b][size=150][color=#0046ff]B[/color][/size] - [size=150][color=#a6ff00]V[/color][/size][/b], but instead [b][size=150][color=#0046FF]B[/color] - [color=#FF0000]R[/color][/size][/b].
[float=left][img2]http://img.fruugo.com/product/3/77/32303773_max.jpg[/img2][c][size=85]The northern Milky Way and bluish Vega at 2 o'clock.
Photo: Alan Dyer.[/size][/c][/float]
When I looked at today's APOD and saw how "blue" the stars were, I realized how much the "normal" B-V index actually "reddens" stars, or at least reddens our perspective of them. Vega, for example, has a B-V index of 0.00, and it is used as a "zero point" in the description of star colors and as a perfect example of a purely "white" star.
Well, that's ridiculous! I have taken part in showing people Vega through a telescope and asked them what color they thought it was. All of them said that Vega is blue.
Vega's "whiteness" is a product of the workings of the B-V system, where Vega comes out neutral. (All right, it is also a product of our human night vision, because the blue-sensitive rods in our retinas are not good at responding to blue light when the blue light source is very faint and is seen against a very dark background.) Well, Vega's color wouldn't be neutral, but clearly blue, in a B-R color index system. In B-R, our own Sun would probably have a color index close to zero, which means it would be "white".
I keep insisting that our Sun really is white. Not because it possesses some magical quality of "whiteness", but because humanity has evolved under the light of the Sun, and it has been to our advantage to see the Sun as white. Oh, we don't see it as white, you say? No, but we see [i]daylight[/i] as white, or neutral, and daylight is sunlight, pure and simple.
Ann