APOD: IC 1795: The Fishhead Nebula (2024 May 01)

[APOD Robot]

IC 1795: The Fishhead Nebula

Explanation: To some, this nebula looks like the head of a fish. However, this colorful cosmic portrait really features glowing gas and obscuring dust clouds in IC 1795, a star forming region in the northern constellation Cassiopeia. The nebula's colors were created by adopting the Hubble color palette for mapping narrowband emissions from oxygen, hydrogen, and sulfur atoms to blue, green and red colors, and further blending the data with images of the region recorded through broadband filters. Not far on the sky from the famous Double Star Cluster in Perseus, IC 1795 is itself located next to IC 1805, the Heart Nebula, as part of a complex of star forming regions that lie at the edge of a large molecular cloud. Located just over 6,000 light-years away, the larger star forming complex sprawls along the Perseus spiral arm of our Milky Way Galaxy. At that distance, IC 1795 would span about 70 light-years across.

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[Christian G.]

The APOD link showing a fish head bears little resemblance, here's another one, colour and shape alike!

(I usually consider such resemblances distractions, but this time it happens to be a favorite fish of mine! - the giant trevally)

[zforgetaboutit]

"At that distance, IC 1795 would span about 70 light-years across."

I don't understand how a width, expressed here in absolute units, is related to distance.

[Chris Peterson]

"At that distance, IC 1795 would span about 70 light-years across."

I don't understand how a width, expressed here in absolute units, is related to distance.

I believe the number is incorrect... but probably because they've identified the nebula incorrectly. IC 1795 is just the "head" part of the nebula, not the entire structure seen here. In which case, this image shows it spanning a quarter of a degree, which means that at a distance of 6000 ly, it is about 25 ly across.

Left to right, the image spans 41.4 arcmin, which represents a width of 72 ly (which is probably where the number in the caption came from).

[Guest]

I think, regardless of the observer's distance to the object, its width, in light-years, is static.

I.E.

Q:"How tall is Snowman?"
A: 2 meters

Q: I've doubled my viewing distance from Snowman; now how tall is Snowman?
A: 2 meters

[Chris Peterson]

I think, regardless of the observer's distance to the object, its width, in light-years, is static.

I.E.

Q:"How tall is Snowman?"
A: 2 meters

Q: I've doubled my viewing distance from Snowman; now how tall is Snowman?
A: 2 meters

But in order to determine that the snowman is 2 meters high, we need to know our distance from it and the angle it subtends at that distance. Likewise for the nebula. If our estimate for the distance is incorrect (which is often the case in astronomy) than our estimate of size is, as well. The caption is merely making the simple calculation, based on an assumption about distance.

[Holger Nielsen]

"At that distance, IC 1795 would span about 70 light-years across."

I don't understand how a width, expressed here in absolute units, is related to distance.

Look at the illustration below:

A distant more or less (!) spherical object with a radius of R will at a distance of d span an angle of 2a, wherw a is half the angular diameter. From the values given in the discription we have R = 35 la and d = 6000 la, so with a little trigonometry (sin(a) = R/d) we get a = 0.334° = 20′. The angular diameter would then be 40′.
Perhaps R was found starting with this value and working the other way round.
If d is doubled, then a would be (very nearly) halved, R being unchanged.

[roses]

Your last post mentions spanning an angle, which is reasonable. The article didn't - it mentioned an absolutewidth.

I think the author should re-state the width as an angular diameter, or relative to something else E.G. Moon radius, not light-years.

[Chris Peterson]

Your last post mentions spanning an angle, which is reasonable. The article didn't - it mentioned an absolutewidth.

I think the author should re-state the width as an angular diameter, or relative to something else E.G. Moon radius, not light-years.

The absolute distance (which is a useful thing to know) is calculated from the angular size of the object. The majority of APOD images of deep sky objects present a physical (absolute) size based upon subtended angle and estimated distance.

[JWP456]

"The absolute distance (which is a useful thing to know) is calculated from the angular size of the object. The majority of APOD images of deep sky objects present a physical (absolute) size based upon subtended angle and estimated distance".
Yes, the descriptions usually give the angular diameter (as seen from earth, of course) and estimated distance, from which the estimated diameter of the object may be calculated as the tangent of the given angle times the distance. Thus in this case tan 0.668 x 6000 = 70. While determining the diameter in light years by multiplying the distance in light years by half the subtended angle of the object and then doubling the result is more mathematically precise, I think the distances to objects like this nebula are not in most cases known with sufficient accuracy to justify that step.

[Chris Peterson]

"The absolute distance (which is a useful thing to know) is calculated from the angular size of the object. The majority of APOD images of deep sky objects present a physical (absolute) size based upon subtended angle and estimated distance".
Yes, the descriptions usually give the angular diameter (as seen from earth, of course) and estimated distance, from which the estimated diameter of the object may be calculated as the tangent of the given angle times the distance. Thus in this case tan 0.668 x 6000 = 70. While determining the diameter in light years by multiplying the distance in light years by half the subtended angle of the object and then doubling the result is more mathematically precise, I think the distances to objects like this nebula are not in most cases known with sufficient accuracy to justify that step.

It is not just the distance which frequently has large error bars. In most cases, we're looking at objects that don't even have well defined edges. And objects that are not perfectly round. So yeah... these kinds of numbers are really just to give us a reasonable sense of scale, not to be reliable out to 10 decimal places!

[dwhightowe]

Beautiful image except for the artificial diffraction spikes. If your imaging system does not naturally produce them, don't add them in post

[johnnydeep]

In context from a wider field of view image at one of the links in the text:

[VictorBorun]

I think, regardless of the observer's distance to the object, its width, in light-years, is static.

I.E.

Q:"How tall is Snowman?"
A: 2 meters

Q: I've doubled my viewing distance from Snowman; now how tall is Snowman?
A: 2 meters

But in order to determine that the snowman is 2 meters high, we need to know our distance from it and the angle it subtends at that distance. Likewise for the nebula. If our estimate for the distance is incorrect (which is often the case in astronomy) than our estimate of size is, as well. The caption is merely making the simple calculation, based on an assumption about distance.

sometimes we get lucky and there's a flash. Then we can watch the light echo moving across the dust and draw a 1 ly tick for every year of our watching.

The bad news is that there may be doubts on whereabouts of the dust. For example, the echoed flash can be from the foreground dust in front of the flash origin. For all we know those 1 ly ticks can refer to flashing rings of dust half way between us and the deep space Snowman

Sometimes we get lucky and the deep space Snowman has a binary star with measurable orbital period and velocities. Then we can calculate the masses and guess the powers emitted by the stars. Then the distance would be proportional to ((the power emitted there)/(the power we observe here))^2.

[johnnydeep]

Just wanted to note that there are two possible right angle triangles that could be used to determine the distance and angle, but at the large distances typically involved, they are almost the same. I had always assumed that we were using the smaller ɑ angle, but it's really the slightly larger angle b that we are seeing because of the tangent line. That is, we're not seeing through the object!

[AVAO]

In context from a wider field of view image at one of the links in the text:


fish head, heart, and soul nebulas.jpg

I still find the area very exciting even in IR.

Click to view full size image 1 or image 2

jac berne (SSDS/GLIMPSE)

Click to view full size image 1 or image 2

jac berne (SSDS/SPITZER)

[VictorBorun]

In context from a wider field of view image at one of the links in the text:


fish head, heart, and soul nebulas.jpg

I still find the area very exciting even in IR.

Click to view full size image 1 or image 2

jac berne (SSDS/GLIMPSE)

Click to view full size image 1 or image 2

jac berne (SSDS/SPITZER)

I wonder why does SSDS fish have a collar (just like APOD fish does) and neither GLIMPSE nor SPITZER fish do?

[AVAO]

In context from a wider field of view image at one of the links in the text:


fish head, heart, and soul nebulas.jpg

I still find the area very exciting even in IR.

Click to view full size image 1 or image 2

jac berne (SSDS/GLIMPSE)

Click to view full size image 1 or image 2

jac berne (SSDS/SPITZER)

I wonder why does SSDS fish have a collar (just like APOD fish does) and neither GLIMPSE nor SPITZER fish do?

Good point. Well I think that since both work in the IR range, the view changes from dark dust clouds to bright warm gas clouds which appear practically equally bright in the front and back areas. This causes the dark collar to dissolve.

[roses]

I think, regardless of the observer's distance to the object, its width, in light-years, is static.

I.E.

Q:"How tall is Snowman?"
A: 2 meters

Q: I've doubled my viewing distance from Snowman; now how tall is Snowman?
A: 2 meters

But in order to determine that the snowman is 2 meters high, we need to know our distance from it and the angle it subtends at that distance. Likewise for the nebula. ...

I was declaring the Snowman's height, not estimating.

I thank everybody who posted here. I enjoyed all posts

Having slept on it, I now realize I was hung up on the semantics on the declaration of the nebula's width, based on estimated distance.

Because no subtended angle was explicitly mentioned, my brain decided there were only 2 knowns in a 3-variable equation relating angle, distance and width, and the width estimate was premature.

Perhaps for brevity, angular widths are not mentioned every time in APOD size estimates. From now on, when not mentioned, I'll assume it's known.

[Chris Peterson]

I think, regardless of the observer's distance to the object, its width, in light-years, is static.

I.E.

Q:"How tall is Snowman?"
A: 2 meters

Q: I've doubled my viewing distance from Snowman; now how tall is Snowman?
A: 2 meters

But in order to determine that the snowman is 2 meters high, we need to know our distance from it and the angle it subtends at that distance. Likewise for the nebula. ...

I was declaring the Snowman's height, not estimating.

Well, yes... not really an option with most astronomical objects, though!

Because no subtended angle was explicitly mentioned, my brain decided there were only 2 knowns in a 3-variable equation relating angle, distance and width, and the width estimate was premature.

Perhaps for brevity, angular widths are not mentioned every time in APOD size estimates. From now on, when not mentioned, I'll assume it's known.

Angular sizes are usually only given when they are interesting in their own right for some reason (like if an object is the apparent size of the Moon). But they are always easily determined in any image with more than a handful of stars.

[johnnydeep]

But in order to determine that the snowman is 2 meters high, we need to know our distance from it and the angle it subtends at that distance. Likewise for the nebula. ...

I was declaring the Snowman's height, not estimating.

Well, yes... not really an option with most astronomical objects, though!

Because no subtended angle was explicitly mentioned, my brain decided there were only 2 knowns in a 3-variable equation relating angle, distance and width, and the width estimate was premature.

Perhaps for brevity, angular widths are not mentioned every time in APOD size estimates. From now on, when not mentioned, I'll assume it's known.

Angular sizes are usually only given when they are interesting in their own right for some reason (like if an object is the apparent size of the Moon). But they are always easily determined in any image with more than a handful of stars.

Why is that? Is it simply because if the star's identities are known, then we already know their angular separations?

[Chris Peterson]

I was declaring the Snowman's height, not estimating.

Well, yes... not really an option with most astronomical objects, though!

Because no subtended angle was explicitly mentioned, my brain decided there were only 2 knowns in a 3-variable equation relating angle, distance and width, and the width estimate was premature.

Perhaps for brevity, angular widths are not mentioned every time in APOD size estimates. From now on, when not mentioned, I'll assume it's known.

Angular sizes are usually only given when they are interesting in their own right for some reason (like if an object is the apparent size of the Moon). But they are always easily determined in any image with more than a handful of stars.

Why is that? Is it simply because if the star's identities are known, then we already know their angular separations?

Pretty much any image with stars in it can be solved with local or online software that matches physical stars in an image to a catalog. For instance, here's the Astrometry.net blind solution for this APOD. And below that, a much richer solution identifying many stars, this from PixInsight (not a blind solver, but using the Astrometry.net solution as a starting point).

Of course, from the imager's standpoint, the image scale is known simply from the telescope focal length and the size of the sensor.
_

[johnnydeep]

Well, yes... not really an option with most astronomical objects, though!


Angular sizes are usually only given when they are interesting in their own right for some reason (like if an object is the apparent size of the Moon). But they are always easily determined in any image with more than a handful of stars.

Why is that? Is it simply because if the star's identities are known, then we already know their angular separations?

Pretty much any image with stars in it can be solved with local or online software that matches physical stars in an image to a catalog. For instance, here's the Astrometry.net blind solution for this APOD. And below that, a much richer solution identifying many stars, this from PixInsight (not a blind solver, but using the Astrometry.net solution as a starting point).

Of course, from the imager's standpoint, the image scale is known simply from the telescope focal length and the size of the sensor.
_
Screenshot 2024-05-02 072406.jpg
new_image_Annotated.jpg

Ok, thanks. There’s that almost magical star “solver” again!

[VictorBorun]

I still find the area very exciting even in IR.

Click to view full size image 1 or image 2

jac berne (SSDS/GLIMPSE)

Click to view full size image 1 or image 2

jac berne (SSDS/SPITZER)

I wonder why does SSDS fish have a collar (just like APOD fish does) and neither GLIMPSE nor SPITZER fish do?

Good point. Well I think that since both work in the IR range, the view changes from dark dust clouds to bright warm gas clouds which appear practically equally bright in the front and back areas. This causes the dark collar to dissolve.

Here I mix IR-with-collar (as cyan+blue) and IR-without-collar (as yellow+red) to make the things look more 3d.
And hang the fitting fragment of APOD (Hubble colours).

...

Click to view full size image 1 or image 2

[AVAO]

I wonder why does SSDS fish have a collar (just like APOD fish does) and neither GLIMPSE nor SPITZER fish do?

Good point. Well I think that since both work in the IR range, the view changes from dark dust clouds to bright warm gas clouds which appear practically equally bright in the front and back areas. This causes the dark collar to dissolve.

Here I mix IR-with-collar (as cyan+blue) and IR-without-collar (as yellow+red) to make the things look more 3d.
And hang the fitting fragment of APOD (Hubble colours).

IC 1795 The Fishhead Nebula (2024 May 01)..jpgIC 1795 The Fishhead Nebula (2024 May 01).2.jpg

...

Click to view full size image 1 or image 2

Very cool. I am learning. Thanks!