APOD: Hubble's Messier 5 (2017 Nov 04)

[Chris Peterson]

Our eyes are least sensitive to blue light. Our short wavelength cone response is one to two orders of magnitude less sensitive than our long wavelength cone response.

Another factor to consider is that the cone density of short wavelength cells is also lowest. Our blue sensitivity isn't just poor, so is our blue resolution. This is very easily seen if you're out at night and observe a commercial sign with illuminated blue letters against a dark background. The writing will look out of focus. But it's not a focus problem; the failure is happening in the retina itself.

While it is true that we have many more orange sensitive L cones & green sensitive M cones than blue sensitive S cones
...we have about 15 times as many green-blue sensitive rod cells as all our own cone cells.

• A rod cell is sensitive enough to respond to a single photon of light
  and is about 100 times more sensitive to a single photon than cones.

Of course, rods are hugely more sensitive, which is why we see in B&W when the light is dim. (And while a single rod can respond to a single photon, in practice it takes 5-10 photons to create a response in the visual cortex, and given the fact that most photons entering the eye fail to hit a rod, we end up with an actual quantum efficiency of just 1-5%.)

Note that the responsivity curves presented in the Wikipedia link are normalized. When considering the relative sensitivity of the eye at various wavelengths, we're better off looking at the unnormalized data.

cone_sen.jpg (30.11 KiB) Viewed 3513 times

[neufer]

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Note that the responsivity curves presented in the Wikipedia link are normalized. When considering the relative sensitivity of the eye at various wavelengths, we're better off looking at the unnormalized data.

Foveal sensitivity = sum of cone sensitivities

Don't you pack a rod or two out there in Colorado.

[Ann]

This is a false color image where the blue channel was indeed photographed through a blue filter, but the green channel is really red data and the red channel is infrared data.

Photography specifics here: https://www.spacetelescope.org/images/potw1118a/

Also, the Hubble itself was apparently rotated when shooting the infrared data (shown as the red channel) as compared to the blue and red data (shown as blue and green channels) since we can see that the red diffraction spikes don't align with the blue and green ones. Some elbow grease could correct that by removing the optical spikes, or by removing just the red ones and regenerating equivalent spikes with the same rotation as the blue/green channels.

Lastly, the three color channels are not very well aligned with one another, as you can see red/cyan fringes on virtually every star.

It would make very little difference if the red channel was a real red or the near-infrared channel that was used in this case. That only makes a big difference for galaxies, where an image like this would have green ionized hydrogen clouds instead of the preferable pinkish ones, and more background galaxies would be revealed. In this case, the infrared actually helps separate the colors more. If you wanted the globular to become very red, then you would have to include some near-UV data for the blue channel. That would turn the entire globular yellow, and then the most yellowish stars would become red.

As a side note of possible interest, here* are some clusters I picked out from a near-uv / visible blue image of the Triangulum galaxy. Note the distinct difference between a globular cluster and the young ones. The globulars nearly became invisible!
https://twitter.com/SpaceGeck/status/925233406455070723

Very interesting, Geck!

I am, however, surprised that so many globulars don't show up as more than orange smudges through near UV filters. A globular like M5 contains a healthy population of blue horizontal stars. I'm too laze to check up the typical spectral classes of blue horizontal stars, but I'd say that most of them belong to spectral class B8 or so. In other words, they are generally B-class stars, whose energy output peaks in the ultraviolet part of the spectrum. Surely such stars would stand out like sore thumbs through a near UV filter?

Other globulars, like for example 47 Tuc, don't contain any blue horizontal stars at all (though they do contain som blue straggler stars, which do not necessarily belong to spectral class B), and I can easily imagine that such globulars only look like orange smudges through near UV filters.

Click to view full size image

NGC 4214. Source: http://heritage.stsci.edu/2011/14/fast_facts.html

I quite agree that the use of a UV filter instead of a blue one will make star clusters and galaxies look redder. A good example is the Hubble picture of blue starburst galaxy NGC 4214. The filters used for this image are F225W (UV), F336W (UV), F438 W (indigo), F487N (a narrowband filter that will detect shortwave nebular emission), F502N ([O III]), F547M (y), F657N (H-alpha+[N II]), and F814W (I). In the finished image, the only the stars strongly detected through the UV and indigo filters are shown as blue. By contrast, the underlying population of old red (make that yellow) stars are strongly detected through the infrared filter, much more strongly than they are detected through the F547M (green) filter. These small cool stars are therefore mapped as very red in the Hubble image.

Ann

[geckzilla]

I am, however, surprised that so many globulars don't show up as more than orange smudges through near UV filters.

Some of the bluer ones could be globular clusters. That was just an image I put together quickly for fun. The orange ones I had to double-check for catalog entries to see what they were because I wasn't entirely sure, so that's why I know they're globulars.


Just for fun, here are a couple of images of NGC 1559 that are pretty close to visible light, without any massaging of the data to pull apart / saturate the colors. These may at first appear to be black and white images. We're so used to having the colors super-saturated that a lot of people don't realize it's happening.

This is an orange, green, and blue image. Very close to RGB

F606W, F555W, F438W

This one includes near-infrared and near-ultraviolet.

F814W+F606W, F555W, F438W+F336W

The final image with some saturation adjustments can be viewed over at Flickr. https://flic.kr/p/212v1P9

[Ann]

I am, however, surprised that so many globulars don't show up as more than orange smudges through near UV filters.

Some of the bluer ones could be globular clusters. That was just an image I put together quickly for fun. The orange ones I had to double-check for catalog entries to see what they were because I wasn't entirely sure, so that's why I know they're globulars.


Just for fun, here are a couple of images of NGC 1559 that are pretty close to visible light, without any massaging of the data to pull apart / saturate the colors. These may at first appear to be black and white images. We're so used to having the colors super-saturated that a lot of people don't realize it's happening.

This is an orange, green, and blue image. Very close to RGB
NGC1559_OGB.jpg

Very interesting, Geck, but I'm not surprised. In my opinion, NGC 1559 is a galaxy whose colors are not strongly "separated".

The arms of NGC 1559 are bright with young stars. There is a mixture there of bright blue stars, bright red giants and red hydrogen clouds. Even though the dominant color is blue, the hue is diluted and pale.

As for the center of NGC 1559, it is interestingly faint. This galaxy doesn't have a bright old yellow bulge by any means. Most likely, the bulge still contains some young stars which will dilute the yellow color of the bulge.

Or, to put it differently: NGC 1559 is quite "similarly bright all over", and it likely contains stars of all colors in most places. No wonder it looks pale!

Most spiral galaxies are certainly very, very pale, but probably not quite so relatively monocolored as NGC 1559.

As for myself, I will never forget the very first time I saw the Andromeda galaxy. I had no idea what to expect, when suddenly a diffuse patch of soft yellowness slowly inched into my binocular field. I will always associate Andromeda with that incredible, soft yellowness in the sky.

My software Guide quotes Steve Coe when he described what M33 looked like on a night of perfect seeing:

I can see the difference between Population I and Population II areas in the galaxy. The stars in the core form a smooth surface to the central section that is light yellow, whereas the arms are splotchy and bluish.

I do believe it is possible to see these things, but you need good color vision and dark, clear skies.

Ann

[geckzilla]

Eh, I thought it was pretty well separated, myself. The nucleus is pretty diffuse, though. Seems to me it's been mixed up a bit.

[heehaw]

How much of your personal mass is dark matter? Astronomically, how much of the Earth, Sun, etc. is dark matter?

Dark matter just ain't into us baryonic things. The attraction isn't mutual.

Bruce

Thanks, Bruce, but all galaxies, including small ones (indeed, especially small ones) DO contain dark matter. But I have the impression that NO globular cluster does, and I've never read any speculation anywhere as to why that is. It does seem to me to be a very peculiar mystery.

There are globular clusters which are suspected to contain small amounts of dark matter (<10% of the mass), and there are globular clusters around other galaxies which appear to have high dark matter content. Whether this difference from the local globulars (that appear to have little or no dark matter) represents a difference in formation history or is a consequence of evolution (such as stripping by galactic tides) remains an area of study.

Yes, I seem to recall you making that good point some time ago. The bullet cluster apparently is ambiguous, clean as it appears to be. Would passage through the Galactic plane clear out the dark matter? It does not seem to have done so for any globulars in our galaxy, unless they have ALL been through the plane, which I suppose may be the case. We need every clue we can get re the nature of the dark matter: the biggest mystery not just in astrophysics but in physics itself.

[BDanielMayfield]

... our eyes are just more sensitive to blue light.

One needs carbon to scatter the blue light
in order to really appreciate a red star:

http://oneminuteastronomer.com/1206/vampire-star/
http://www.skyandtelescope.com/observin ... 203201401/

Nice points Art. Wouldn't you think though, that in this set of 100,000+ geriatric gems there would be at least a few fat, wheezing old gezzers choking on their own crud? (No offense intended.)

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Colbert emoji: A face with a neutral mouth and single eyebrow raised.
May be used to denote scepticism, disbelief, or disapproval.
Has been likened to the quizzical looks of Stephen Colbert.

Your disapproval of that comment is warranted, I'm sorry my attempt at crass humor misfired. For the record, I was referring to carbon stars, not to you.

The question and point I was trying to make was that given the age of this globular, shouldn't it be expected to contain carbon stars, and that the way the data was presented might reveal or conseal that fact.

Bruce

[neufer]

Wouldn't you think though, that in this set of 100,000+ geriatric gems there would be at least a few fat, wheezing old gezzers [sic] choking on their own crud? (No offense intended.)

The question and point I was trying to make was that given the age of this globular, shouldn't it be expected to contain carbon stars, and that the way the data was presented might reveal or conseal [sic] that fact.

• There might be a few fat, wheezing old geezers choking on their neighbor's crud:

<<The non-classical [halo population II] kinds of carbon stars, belonging to the types C-J and C-H, are believed to be binary stars, where one star is observed to be a giant star (or occasionally a red dwarf) and the other a white dwarf. The star presently observed to be a giant star accreted carbon-rich material when it was still a main-sequence star from its companion (that is, the star that is now the white dwarf) when the latter was still a classical carbon star. That phase of stellar evolution is relatively brief, and most such stars ultimately end up as white dwarfs. We are now seeing these systems a comparatively long time after the mass transfer event, so the extra carbon observed in the present red giant was not produced within that star. This scenario is also accepted as the origin of the barium stars, which are also characterized as having strong spectral features of carbon molecules and of barium (an s-process element). Sometimes the stars whose excess carbon came from this mass transfer are called "extrinsic" carbon stars to distinguish them from the "intrinsic" AGB stars which produce the carbon internally. Many of these extrinsic carbon stars are not luminous or cool enough to have made their own carbon, which was a puzzle until their binary nature was discovered.>>

[BDanielMayfield]

Thanks Art.

Chocking on one's own crud is bad, but chocking on second hand crud is worse. Yuck.

[Ann]

My impression is that the "carbon phase" of a low-to-intermediate mass star is brief. That might explain why we haven't detected any in any known globular cluster.

Ann