APOD: Hoags Object: A Strange Ring Galaxy (2013 Jul 28)

[FLPhotoCatcher]

This image of Hoag's Object is several years old - does anyone know if a higher resolution image of it exists?
What about a reprocessed image? I have seen several in the past that were of significantly higher quality than the originally released version.

[Ann]

The idiot astronomers have to grapple with questions of distance all the time and have come up with a few ways to make such determinations. However, any idiot also knows there aren't any single stars visible within the galaxy. It's too far away and the resolution is not fine enough.

I'm not so sure about that. Looking at the image, it appears that quite a few individual stars within the galaxy are resolved. Certainly, both Hubble and ground-based telescopes regularly resolve individual stars inside galaxies. This galaxy is only a little outside the range where Cepheid variables can be used to assess distance. In any case, resolution only determines the distance stars must be apart for them to be isolated from each other. Since this galaxy has very sparse regions (as between the core and ring), even a single bright star has a good chance of being resolved (detected).

You made me pay a lot of attention to the region between the core and the ring now, Chris. There are a number of rather faint yellow dots there, which I take to be globular clusters due to their sheer number and the uniformity of their color and brightness. There are, however, also two much brighter spots, one white at 6 o'clock, one bluish at 2 o'clock. I can't believe they are stars, since they are much brighter than the faint yellow dots that I take to be globular clusters. Are they clusters too, but younger and brighter? They do seem to be extended.

Ann

[Chris Peterson]

Quite a few of them? I'm not sure which ones you mean. The central bulge of stars is probably the best place to look since the dust is cleared out but I assumed even the solitary looking ones in that were clusters of stars. When I process Hubble images I always have a hard time distinguishing the single pixel specks from noise, cosmic rays, or actual stars.

Actually, the central bulge is the worst place to look, because that's where the insufficient resolution is a problem. The place to look is where there aren't many stars- between the bulge and the ring. There are a number of star-like objects in there which I don't think are clusters.

Noise and cosmic rays can be distinguished from stars because only the latter are present in each color channel. Since the objects I'm seeing are white, yellow, or orange, they aren't single channel artifacts.

[geckzilla]

Some of those might be single stars. Or they might be multi-star systems. I don't understand how you can say with any certainty that any of them are single stars. Is it even possible to look at just one and make that determination? I still wonder if the dots around the central nebula are something like globular clusters or if they are massive stars. They seem blurry so I'm guessing they are clusters. Also, there are some single pixel anomalies that do happen across channels. I'm not saying I see any of them here but I'd have to go get the FITS files and spend a lot more time on it to try to make that argument. Usually those are stark white, though.

[rstevenson]

... Looking at the image, it appears that quite a few individual stars within the galaxy are resolved.

I suspect not, at least not in this partcular image.

Here is a screen capture of a portion of this image, just inside the outer ring at about the 3 o'clock position, shown at 500% magnification.

I suppose any of those individual pixels could be stars. So how big is a pixel in the image?

Hoag's object is about 100,000 light years across. Conveniently, this APOD shows Hoag's Object as about 1000 pixels wide, or a scale of about 1 pixel = 100 light years. (It's closer to 1100 pixels wide actually, but let's keep the math head-doable.) Our Sun is about 4.7 light seconds wide, for comparison. VY Canis Majoris, the largest star we know of, is about 1425 times larger than the Sun, or a little less than 2 light hours wide, still nowhere near large enough to be seen in this image if it were in Hoag's Object.

Note that I'm not saying we can't see stars in other galaxies using our best telescopes. Just not in this image.

Rob

[Chris Peterson]

Some of those might be single stars. Or they might be multi-star systems. I don't understand how you can say with any certainty that any of them are single stars.

I'm not saying it with certainty. I simply suspect that some of the brighter, star-colored spots in that region are, in fact, stars. There's no reason they couldn't be. I'm not sure what the expected distribution of globular clusters is in the case of a ring galaxy like this.

Is it even possible to look at just one and make that determination? I still wonder if the dots around the central nebula are something like globular clusters or if they are massive stars. They seem blurry so I'm guessing they are clusters.

At that distance, I don't think clusters would be optically resolved as anything other than point sources, the same as stars. So geometrically, there is no way to distinguish the two. That could only be done by other methods, such as spectroscopically or photometrically. In either case, diffraction will make the objects larger than single pixels, and will make them "fuzzy".

Also, there are some single pixel anomalies that do happen across channels. I'm not saying I see any of them here but I'd have to go get the FITS files and spend a lot more time on it to try to make that argument. Usually those are stark white, though.

Right. Hot, warm, cool, and cold pixels are always present. Warm and cool pixels are usually eliminated by calibration, but hot and cold ones may not be. If they aren't taken care of by some sort of bad pixel mapping, they usually show up as either white or black dots. They can usually be distinguished from actual signal because they are only one pixel in extent. That can't be seen in JPEG images, but is usually not hard to see in the released TIFFs, and of course in the original FITS files.

[Chris Peterson]

I suppose any of those individual pixels could be stars. So how big is a pixel in the image?

Hoag's object is about 100,000 light years across. Conveniently, this APOD shows Hoag's Object as about 1000 pixels wide, or a scale of about 1 pixel = 100 light years. (It's closer to 1100 pixels wide actually, but let's keep the math head-doable.) Our Sun is about 4.7 light seconds wide, for comparison. VY Canis Majoris, the largest star we know of, is about 1425 times larger than the Sun, or a little less than 2 light hours wide, still nowhere near large enough to be seen in this image if it were in Hoag's Object.

Note that I'm not saying we can't see stars in other galaxies using our best telescopes. Just not in this image.

The pixel scale isn't important, since we can't spatially resolve stars in any other galaxy in any case. The pixel scale is only important because it defines how far apart two bright stars must be to be resolved as separate point sources. As long as we meet that condition, the only thing that determines whether we can detect individual stars in an image is the intrinsic brightness of those stars, and the S/N of the camera. Even a tiny amateur telescope can resolve supernovas (which are nothing more than very bright stellar sources) in galaxies much farther away than Hoag's Object.

Consider a typical luminous star, with an absolute magnitude of -11. Hoag's Object is 600 million ly away, or 184 million parsecs. That means from Earth, the luminous star will have an apparent magnitude of 25. That is very easily detected by Hubble, which has a limiting magnitude around 31. So I think it is completely possible that we are seeing individual stars in the sparse sections of this galaxy where luminous ones are far enough apart that they don't blend into the background.

[geckzilla]

In any case, I think that gonzo may have actually been referring to the galactic bulge as a red star. If I had realized that yesterday I could have more clearly stated that the central bulge is millions or billions of stars and not a singular entity.

[neufer]

In any case, I think that gonzo may have actually been referring to the galactic bulge as a red star. If I had realized that yesterday I could have more clearly stated that the central bulge is millions or billions of stars and not a singular entity.

• "Gonzo" was pulling your leg.

[Sum Buddy]

Friend, this cosmos is chock full of coincidences, lucky breaks and things that are downright weird. I am in it and that's extremely unprobable. If you really want a coincidence, how about a huge great rock slamming into the Ocean just when it was most needed to help the little furries grow into Hominids? Or Sol just happening to be in a quiet part of the galaxy for long enough for humans to get born? (Ooooh, anthropic principle alert.)
This cosmos is itself unlikely, or maybe it's absolutely certain. Whatever, it's certainly fun and it is decorated with some truly gorgeous special effects. Ring galaxies are among the loveliest.

If the Universe is destined to expand forever until it dies and becomes dark, wouldn't the Universe spend most of its time in that state? I mean, it could be in that state infinitely longer than it is in its living, moving, bright state. Coincidentally, we happen to exist in this tiny slice of time. That's my ultimate coincidental conundrum. (Something isn't right with it.)

Two days and 25 posts late but I still have to say it - Comedic Axiom #1: Timing is Everything. That should be proof enough that the universe has a sense of humor.

[geckzilla]

In any case, I think that gonzo may have actually been referring to the galactic bulge as a red star. If I had realized that yesterday I could have more clearly stated that the central bulge is millions or billions of stars and not a singular entity.

• "Gonzo" was pulling your leg.

Despite all of my cynicism I'm still rather gullible.

[rstevenson]

The pixel scale isn't important, since we can't spatially resolve stars in any other galaxy in any case. The pixel scale is only important because it defines how far apart two bright stars must be to be resolved as separate point sources. As long as we meet that condition, the only thing that determines whether we can detect individual stars in an image is the intrinsic brightness of those stars, and the S/N of the camera. Even a tiny amateur telescope can resolve supernovas (which are nothing more than very bright stellar sources) in galaxies much farther away than Hoag's Object.

Consider a typical luminous star, with an absolute magnitude of -11. Hoag's Object is 600 million ly away, or 184 million parsecs. That means from Earth, the luminous star will have an apparent magnitude of 25. That is very easily detected by Hubble, which has a limiting magnitude around 31. So I think it is completely possible that we are seeing individual stars in the sparse sections of this galaxy where luminous ones are far enough apart that they don't blend into the background.

Thanks Chris, that clarifies things greatly. My brain pixels are now much brighter, though no larger.

Rob

[ta152h0]

high tech leg pulling is an art

[Beyond]

high tech leg pulling is an art

Yes... we have one of those here.