De58te wrote: ↑Sun Mar 24, 2019 9:30 am
Amazing that it was even discovered because you can't even see it in the beginning where the Milky Way looks like you normally see it in the sky. It doesn't even appear as a blue star until you zoom in halfway there (whatever the power of the zoom is). And even stranger that when it gets to the Hubble image that there are only a handful of blue stars and at least 99% of the stars are white or yellow. I wonder how many other distant faint blue stars are really white globular clusters?
According to Sky Catalogue 2000.0, volume 2, Terzan 5 is a highly reddened cluster whose (reddened) B-V index is +2.77. That makes the reddened color of Terzan 5 considerably redder than the star Betelgeuse.
Why does the cluster look blue (at a certain distance from us) in the video? You will have to ask the people responsible for the picture and the video.
Information on this page
tells us that the bluest filter used for the picture was centered on 606 nm, which corresponds to an orange color (although it was likely a wideband filter). There were also three infrared filters used for the image.
What this means is that anything that looks blue in the picture might in reality be orange. On the other hand, it might also be truly blue but possibly reddened to an orange color.
What Terzan 5's intrinsic integrated color is depends on how metal-poor its stars are. All globular clusters are metal-poor, but the more metal-poor they are, the bluer they typically are. That is because the really metal-poor globulars contain (many) blue horizontal branch stars, but the less metal-poor ones contain few or no such stars.
Take a look at the color-magnitude diagram of globular cluster M13 at left. M13 is a "blue" cluster, which contains fairly large numbers of blue horizontal stars. You can see the horizontal branch of M13 at around magnitude 15 in the diagram at left, and you can see that the B-V color of the stars here is around 0.0. That is the same color as Vega or Sirius, and it is fairly blue.
But now look at the color-magnitude diagram of a less metal-poor cluster, 47 Tuc. 47 Tuc sports a tiny little horizontal branch at about magnitude 14 in the diagram at right. You can see that the B-V color of the stars here is around 0.6-0.8. That is not at all blue.
According to Sky Catalogue 2000.0, Volume 2, the integrated B-V index of "blue" globular cluster M13 is 0.69, which is comparable to the color of the Sun. The integrated B-V index of 47 Tuc, by contrast, is 0.89, which is clearly yellower than the Sun and somewhat comparable to the star Pollux.
"Non-blue" globular 47 Tuc. Note the lack of blue horizontal stars.
The pictures of M13 and 47 Tuc are not quite comparable, because the filters for the picture of M13 were 435 nm (blue), 625 nm (orange-red) and 814 nm (near infrared), whereas the filters for the picture of 47 Tuc were 336 nm (ultraviolet), 555 (green) and 814 nm (near infrared). That means that it was easier for bluish stars to show up through the filters used for the M13 picture than through the filters used for the 47 Tuc one. Even so, there can be no doubt that M13 is a decidedly bluer cluster than 47 Tuc.
But there is another lesson here. We can't expect any
(old) globular clusters to be truly "blue". At their bluest, they will be a little bluer than the Sun, but not by much. Maybe possibly maybe they might be as blue as Procyon
, Alpha Perseus. At their (unreddened) reddest, they will be about as yellow as the star Pollux
So what color is Terzan 5 really? If it wasn't so reddened by dust?
My guess is that it is a little redder than M13 but a little bluer than 47 Tuc. I base that on the (large) picture of Terzan 5 published at the ESO page (see address below the topmost picture). In the large picture, you can spot a few (but not very many) blue horizontal stars in this globular.
By the way, the blue horizontal stars of Terzan 5 likely belong to the globular's oldest, most metal-poor stars. It seems less likely that the much younger stars created in the second starburst would be metal-poor enough to give rise to such stars.