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Hubble takes gigantic image of the Triangulum Galaxy (spacetelescope.org)
186 points by _Microft 67 days ago | hide | past | web | favorite | 56 comments

One of the largest known stars is VY Canis Majoris. It's in the Milky Way and has a diameter of 2 billion km. Triangulum is ~2.7m ly (2.5e19m) away. That's an angular diameter of 2e9/2.5e19 or roughly 1e-10 radians. Imagine a star this size in that galaxy.

A red blood cell has a diameter of ~8e-6m. Pick up a mechanical pencil sitting around you with a .5mm lead in it. Study the tip of that pencil and imagine 60 little red platelets lined up in a row fitting across its width. Now pick up one of those little red blood cells and stick it to a window. Then get in your car and drive 8e-6m/1e-10 or 86km away and look back across the (flat, obviously) Earth to that window. As Triangulum rises over the horizon on the other side of our window, that little red blood cell would fully eclipse our remote twin of VY Canis Majoris.

I don't know if I'm seeing individual stars in that zoomable image, but if I am I don't understand how we make optics that good. (Caveat, Hubble's resolution is rated at ~2.4e-7 radians, so it's likely that those blobs aren't stars, or my math is off somewhere.)

You absolutely are seeing individual stars in the Triangulum Galaxy (M33) in the zoomable image. The brighter "blobs" are a combination of foreground stars (i.e., stars in our own galaxy) and star clusters in M33, but the fainter points are almost all stars in M33 (a few may faint foreground stars or background galaxies and quasars).

This is no more mysterious than the fact that you can see individual (nearby, bright) stars with your naked eye when you go outside at night. The angular resolving power of your eyes (or of small telescopes) compared to the angular diameters of even nearby stars is terrible; nonetheless, you can still see them.

The key thing to understand is that objects which are too small to resolve will be blurred by the telescope/lens/eye (plus atmospheric turbulence if you're not in space) into a point spread function [https://en.wikipedia.org/wiki/Point_spread_function]. The light from each star is thus spread out over multiple pixels in a pattern which is basically the same for all the stars, varying only in total brightness and thus detectability; for really bright stars, the light in this pattern can be traced out over a significant fraction of the entire image. This includes features like the diffraction spikes; we don't see these for stars in M33 only because they're too faint to register.

This, exactly. And it should be noted that even with a theoretical ideal PSF, point sources would still have non-zero-sized images simply because the recording surface (film, retina, CMOS sensor) has discrete sensor elements. A point source will always be at least a single "pixel" wide in the image, it just needs to be bright enough to stand out against whatever else is projected onto that pixel.

If I'm not mistaken, ideal optics would focus a point source onto a single pixel. A blurred image would spread that point source over more pixels and a sharpening filter might be able to not only fix the blur, but locate the point source within the that single pixel. Yes? No?

I'm imagining this being similar to how adding the right kind of noise and oversampling can tease out smaller signals than one might think possible in a noise free sampling system.

Am I off base here?

Optics don't know about pixels. To a first approximation, they are continuous. Whatever discretization happens after that, it's not relevant to the optics in any way. A sharp lens can greatly outresolve a low-resolution sensor; the bottleneck is the sensor in that case. Conversely, a high-resolution sensor may not be of much use if the lens is soft - but in this case you can at least in theory use deconvolution to recover detail if you know a good approximation of the PSF of the lens. This is not uncommon when working with scientific instruments, actually, but pretty rare in the case of consumer photography equipment.

No, because a pixel is not a point. Even if not-perfect optics blur the point a bit, it can still be one pixel or less. Ideal optics might be able to focus the light much smaller than one pixel, but that won't change the resulting image because the pixel sensor will pick up the same amount of light.

"A Pixel Is Not A Little Square"


And elementary photosensitive units (whether silver halide grains on photographic film, individual sensor elements in a CMOS or CCD image sensor, or photoreceptive cells in a mammalian retina) are not points. They have a nonzero area and turn photons into signal irrespective of where they hit. That article, while very informative, is not relevant. Sensels are not pixels.

Its worth mentioning that VY Canis Majoris has a mass of "only" 17 Solar masses (+-8). This means it has a very low density.

"Despite the mass and very large size, VY CMa has an average density of 5.33 to 8.38 mg/m3 (0.00000533 to 0.00000838 kg/m3), it is over 100,000 times less dense than Earth's atmosphere at sea level (1.2 kg/m3). " [0]

Often these huge but low density objects are incorrectly rendered like our Sun with a "hard" surface. I think it's a pity because they are even weirder and more fascinating objects than that.

[0] https://en.wikipedia.org/wiki/VY_Canis_Majoris

The Mote in God's Eye had an interesting description of a star like that, including a starship going fairly deep into it and mentioning that the transition was barely noticable.

How is that possible? And how can that be determined? I'm both intrigued and in disbelief. Does it not collapse due to internal pressure from fusion reactions? Is that just heat, or radiation pressure too? Have I just answered my own questions?

It's important to note that the average density is low, it doesn't mean it's uniformly that dense. I cannot speak for this star in particular, but a lot of variable stars expand and contract so fast that material actually bounces off of it sending out a shell that may or may not re-coalesce or continue going off into space. The shell continues to glow due to the heat it already had, and the heat it receives from the main part of the star.

Their atmosphere can be thought of as 'hot near-vacuum'.

As long as there is enough light collected objects should be visible, even when in theory they would cover only a fraction of a pixel. We can rather see different stars because there's vast space between them which means they will not blend/blur into each other.

Imagine you have a camera 2.7m ly away from a star that is 2 billion km across, and it's the only significant light source in that direction.

How many pixels would you expect the image of the star to be? It can't be less than 1. You'd still see the individual star even though it is a lot smaller than 1 pixel. Imperfect optics mean it will likely appear larger than 1 pixel.

Conversely, how many stars are contributing to that pixel?

Easily. Which focal length/field of view lens is on that camera?

I would have thought all these stars and galaxies are at optical-infinity?

True, and focal length is in fact defined (or at least thought of) under the assumption of parallel light rays coming in from an optically infinite distance. https://en.wikipedia.org/wiki/Focal_length

Yes? The sensor is not at optical infinity however, and the focal length determines the field of view you get. Quite clearly, one pixel of a camera with 0.001 degree angular resolution and the one of 10 degree FOV give you very different results.

2 billion km = 2e12m, not 2e9

The fact that individual stars can be seen doesn't necessarily mean the telescope has a resolution equal to the angular size of this star. A point source will be apparent even if it isn't resolved. Otherwise we could never see stars with our naked eye.

I was wondering about this as well, haven't ever found a good answer.


Only addresses visible stars, which are all in our Galaxy. The thing is Andromeda actually covers more of the night sky than the moon, so I kinda wonder if an instrument could resolve individual stars. I don't know a whole lot about that other than Airy discs, which would suggest no, stars are too close to each other in angular terms.

There are quite a lot of star clusters in M33/Triangulum (e.g. https://arxiv.org/abs/1402.3029), so some of the bright dots may not be individual stars. There are stars which are bright enough to dominate the light within a PSF (point spread function, or resolved region size). They will, of course, be physically much smaller than that size.

In the article, the image is stated to be 665 megapixels and the width of the galaxy is given as 60,000 light years.

Under the simplifying assumptions that the image is square and encompasses the exact galaxy width, that's about 22 trillion km (or ~11,000 VY Canis Majoris) per pixel. So I guess we're not seeing individual stars (edit: in the target galaxy) - or my math is also off somewhere.

Those are definitely individual stars. They might between us and the Triangulum galaxy.

Well the stated resolution of Hubble is about 3 orders of magnitude smaller than necessary to resolve VY per my likely flawed calculation, so we're roughly within an order of magnitude of each other.

Either that or all those stars are uuuuuge

It doesn't matter if the star is smaller than the pixel. You can see them individually as long as you can resolve the distance between them.

The ones with diffraction spikes are foreground stars. The others are in the Triangulum galaxy.

You only need to resolve the distance between stars, not the stars themselves.

Those blobs are individual stars, but not necessarily stars in Triangulum...

Platelets aren't red. Still very interesting.

Lol i thought there was something wrong with it when i wrote it down.

Platlets are about a tenth of the size of erythrocytes. Sometimes up to one fifth the size, for big ones.

Horizons on earth start at maybe 25 or 30 miles away.

Cars are about 10 feet across.

But you’re saying an 8 (maybe 6) micrometer erythrocyte, less than 10 feet away (on a car window), can eclipse a half a millimeter graphite pencil lead at a distance of 86Km (53 miles)?

Also, a meter is a million micrometers. A millimeter is a thousand micrometers. You would need 100 copies of a 10 micrometer object, to span a millimeter. Fifty, to span half.

Are you saying VY Canis Majoris is the erythrocyte to Triangulum’s pencil lead? As it eclipses said object at 53 miles?

Or are you saying the erythrocyte is fixed onto a house window, and an erythrocyte can occlude the direct observation of an actual star in the sky (the biggest one we know of), even at 53 miles, which is over, beyond and even twice past any terrestrial horizon, and would need to be on a window in a skyscraper taller than anything ever built?

EDIT: Sorry, the Burj Kalifa is 2,700 feet tall, so its spire would peek over the horizon, even at distances up to around 60 miles. So, I need to update my mental trivia model. We actually have built something that big, but it only happened about ten years ago.


> Or are you saying the erythrocyte is fixed onto a house window, and an erythrocyte can occlude the direct observation of an actual star in the sky (the biggest one we know of), even at 53 miles, which is over, beyond and even twice past any terrestrial horizon, and would need to be on a window in a skyscraper taller than anything ever built?

Yes. Also, I'm pretty sure your objections here are handled by "(flat, obviously)". The point is to make an analogy to sizes and distances on a human scale, not to make a point about the geometry of Earth.

The pencil lead and the car are distractions, that lead the reader to confuse possible relative size analogies, with frames of direct observation.

Reminds me of Gigapixels of Andromeda [4K]: https://www.youtube.com/watch?v=udAL48P5NJU (listening with sound is a plus). Now imagine there are at least as many galaxies as dots of stars in these images from Hubble...

What is that bright blob/nebula thing in the upper right of the zoomable image? That is really pretty. Also, if that's closer than the galaxy how come we can't resolve individual stars in the blue haze?

NGC 604 — a gigantic gas cloud in the Triangulum Galaxy

It's really fun to zoom around.

Anybody know what the story is with the lens flare on some of the bigger/brighter objects? For some reason I expected that a Hubble image would be beyond such things.

Caused by diffraction around structural supports for the optics: https://en.wikipedia.org/wiki/Diffraction_spike

EDIT - for Hubble specifically, here’s an exploded view: https://www.spacetelescope.org/images/exploded_view1/

The spikes are caused by the arms holding the secondary mirror.

I was having trouble making sense of the figures in the article: "three million light-years", "40 billion stars"...

So I visited Scale of the Universe [1] to try to get a grip on things. Wonderful site (and music!), but I think it had the opposite of the desired effect. I am now completely numb to any notion of trailing zeros.

[1] http://scaleofuniverse.com/

Your comment reminded me of the Total Perspective Vortex - "the TPV is the only known means of crushing a man's soul" :)

[1] https://hitchhikers.fandom.com/wiki/Total_Perspective_Vortex

I love these kinds of things so much. It makes the universe feel so massive and vibrant with infinite potential. The sea of stars that make up that galaxy is absolutely staggering.

Suppose you were on an earthlike planet orbiting some star near the center of it. Do you suppose the "night sky" would be about as bright as day due to the number of stars?

Yep, space is just IMMENSE, and this reminds me of the reason why I like exploration in Elite Dangerous so much - a very beautifully rendered space game. With a highly realistic space model, it even gets darker as you reach the galaxy edges.

Apologies in advance to the real scientists here for my naivete, however: As we know there's billions of galaxies too - it makes me feel like everything is just a big Mandelbrot set. We've picked galaxies on the top end, and atoms on the other, but is that really where it ends, or is it just the edges of our ability to perceive?

To my amateurish understanding, it does go smaller than atoms, with subatomic particles like protons which themselves are made of quarks. Beyond that string theorists would say that there would be strings and that's it. Others might say some kind of quantum foam.

Going up, galaxies make up groups which combine into superclusters that form the cosmic web of the observable universe.

So for all intents and purposes, we can essentially say the universe and the various scales are infinite because it may as well be for humans.

"Do you suppose the "night sky" would be about as bright as day due to the number of stars?" --> I seriously doubt it. Not an astronomer but: 1) Remember that you are looking at a 2D projection of a 3D galaxy. There's a lot more room between each star in 3D. 2) Hubble photos use massive amounts of time lapse to create detail[1]. The picture is being misinterpreted by your brain's intuitive sense of brightness in a typical night sky photo. The density of the galaxy seems fairly typical, it's just a really detailed photo. 3) Look up Olber's Paradox[2].

[1] https://astronomy.stackexchange.com/questions/21261/how-does...

[2] https://en.wikipedia.org/wiki/Olbers%27_paradox

Some might mostly know this Messier-33, or M33 as an object that may be found with binoculars or a small scope.

I have seen that bright star forming region in telescopes and it always blows me away to think it is a nebula in another galaxy. The closest thing we have to a large star forming region is the Orion nebula but the one there is so big that if it were where the Orion nebula is, our solar system would be inside it.

My God, it's full of stars!

Judging from the downvotes you should maybe add where this quote comes from...

Zoom all the way in [1] all the way in to get some feeling of the huge number of stars. Then think about every one supporting its own civilization at a scale greater than ours. An awesome thought.

I sure hope interstellar travel is something that intelligent civilizations do, our galaxy is full of them, and the Fermi paradox is because Earth is in some sort of nature preserve situation, not because of a great filter we are yet to encounter.

[1] https://www.spacetelescope.org/images/heic1901a/zoomable/

Does anyone know if bubble images are “true color”?

Don't know specifically about this one but many aren't. The Pillars Of Creation isn't[1]. Also we humans can never see most of space in "true color" with our own eyes, even in a telescope, because the brightness of the incoming light is so low that our eyes are detecting the photons via color-insensitive rods rather than cones. Only a time-lapse photo can bring out the real colors.

[1] http://hubblesite.org/image/351/news_release/1995-44 -- "The color image is constructed from three separate images taken in the light of emission from different types of atoms. Red shows emission from singly-ionized sulfur atoms. Green shows emission from hydrogen. Blue shows light emitted by doubly- ionized oxygen atoms."

This one [-7] is visible-light and doesn't look too different, though. The infrared one [2.4] looks very cool, too.

[-7] http://hubblesite.org/image/3471/news_release/2015-01

[2.4] http://hubblesite.org/image/3475/news_release/2015-01

The first image [-7] is basically the same as the classic WFPC2 image jzl was referring to -- i.e., using the same mapping of emission-line filters to R, G, and B -- just using a newer camera.

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