Starship Asterisk*

My first post: not really an astronomy question, but a language question about something frequently given in APOD. As in 11/11/11, "The close-up field of view spans over 25,000 light-years at the estimated distance of M83". This carries the implication that the expanse shown is dependent on the object's distance from us. But wouldn't the span of such a photo be 25,000 light-years regardless of how far away we are? Beings twice as far away could take such a picture and it would still be 25,000 light-years in extent. Is this a norm of astronomer-speak that I just don't recognize, or am I missing something? Thanks.

clevecallison wrote:
My first post: not really an astronomy question, but a language question about something frequently given in APOD. As in 11/11/11, "The close-up field of view spans over 25,000 light-years at the estimated distance of M83". This carries the implication that the expanse shown is dependent on the object's distance from us. But wouldn't the span of such a photo be 25,000 light-years regardless of how far away we are? Beings twice as far away could take such a picture and it would still be 25,000 light-years in extent. Is this a norm of astronomer-speak that I just don't recognize, or am I missing something? Thanks.

Hi "clevecallison" ,

This image (like every image) has a definite and well defined angular dimension.

How that angular dimension translates into a physical dimension depends upon how far away is the subject in question.

Thanks, neufer. But since the angular dimension is nowhere specified, it seems that the phrase "at the estimated distance" adds no information to the sentence. Might as well say "this object is distant from earth." This is not an earth-shaking matter, but every time I read this in APOD I wonder.

clevecallison wrote:Thanks, neufer. But since the angular dimension is nowhere specified, it seems that the phrase "at the estimated distance" adds no information to the sentence. Might as well say "this object is distant from earth." This is not an earth-shaking matter, but every time I read this in APOD I wonder.

What they are saying is that the object (or the plane the object lies on, perpendicular to the direction we are looking) is 25,000 light years across, assuming some distance estimate for that object. If we later figure out the object is twice as far away, then it would span 50,000 light years.

In astronomy, distances are frequently known only to some rough approximation, so if the size of an object is estimated by its angular size, it is good to note the connection to distance.

Of course, in an image like this, it is the object plane that matters. Any stars in the foreground span a much smaller distance, and galaxies in the background span a much larger distance.

Any photo/telescope/other field of view covers an angle, not inherently a specific width. For a given field of view, the width increases with distance. The reason it's worded that way is because Dr. Nemiroff or Dr. Bonnell (whoever wrote today's caption) is speaking as an astronomer who thinks in terms of field of view: degrees, arcminutes, and arcseconds.

Astronomers think in field of view at least as often as they do in distance because field of view is something you can definitely know. It depends on your instrument. Actual size is something you derive from field of view, assuming you know the distance.

So to explicitly answer your question:

But wouldn't the span of such a photo be 25,000 light-years regardless of how far away we are? Beings twice as far away could take such a picture and it would still be 25,000 light-years in extent.

No. If the target was twice as far away, and the photo was taken with the same field of view, the span would 50,000 light years.

By the way, if you own a pair of binoculars, it probably has a label according to the same convention. My 10x50's say "Field of View at 1000m: 114m"


* Edit - oops. Cross posted with Chris.

neufer wrote:360px-Europe_2007_Disk_1_340.jpg

LOL

clevecallison wrote:But since the angular dimension is nowhere specified, it seems that the phrase "at the estimated distance" adds no information to the sentence. Might as well say "this object is distant from earth."

Like Chris mentions, it means the span is an estimate as well, and calculated from the estimated distance and the known angle/field of view. If the distance is less, then the span is less too. If the distance is more, then the span is more. (The distance to M83 is somewhere between 13 and 17 million lightyears, with a variation depending on measurement method.)

Thanks to Case, Chris, iamlucky13 and neufer for taking the time to answer this. I understand about angle/field of view. I think the issue must be that I was assuming (wrongly, it seems) that APOD editors crop photos in order to show certain features and eliminate other irrelevant ones. If what we see is the full image from the telescope(s) then the language I questioned makes more sense. Thank you.

clevecallison wrote:Thanks to Case, Chris, iamlucky13 and neufer for taking the time to answer this. I understand about angle/field of view. I think the issue must be that I was assuming (wrongly, it seems) that APOD editors crop photos in order to show certain features and eliminate other irrelevant ones. If what we see is the full image from the telescope(s) then the language I questioned makes more sense. Thank you.

It is common for the images to be cropped. That doesn't change anything about the concept. Cropping an image doesn't change the angular scale (that is, the angle subtended by a pixel). Of course, it changes the number of pixels, and therefore the total angle subtended. But when that is translated into a linear distance, that is taken into consideration. Images may be rescaled in size, as well, which changes the pixel scale. Again, though, this would be taken into consideration if a linear dimension was being reported based on angular units.

Different methods are used to estimate the distance to galaxies.
The most popular method is redshift. The universe is expanding at a rate that is quite well known, and as the universe is expanding, it carries the galaxies with it. The light that reaches us from a galaxy will be "stretched", or redshifted, by an amount that is proportional to how much the universe has expanded since the light that reaches us from this galaxy was emitted. This method usually works very well, but it can't be reliably used for a galaxy like M83, which is too nearby. Galaxies have their own specific movement independent of the expansion of the universe, and this specific movement causes a Doppler shift. For a galaxy that is far away the Doppler shift is negligible compared with the redshift, but for galaxies that are much closer the Doppler shift may be so large compared with the redshift that the redshift can't be realiably used to infer the distance. Here the specific motion of the galaxy itself may either amplify or counteract the Hubble redshift.
A method that works quite well for nearby galaxies is to search for Cepheid variables. These are variable stars whose period, which can be easily measured, is directly linked to the absolute luminosity of these stars. By measuring how fast or slowly a Cepheid variable is pulsating, you can infer how bright it is. And by comparing how bright the star appears to be, compared with how bright it is known to be, you can work out the distance. There may conceivably be a problem if the galaxy is very dusty and the Cepheids are dust-reddened in such a way that it is hard to know how bright they really are. However, this problem should be solved by photographing the Cepheid through several filters to work out how reddened it is.
A third method that works well is to find a supernova type Ia in the galaxy and measure how bright it gets. Supernovae type Ia are "standard candles", whose absolute peak luminosity is known. M83 is actually a supernova factory, which has produced six supernovae since 1923. However, the latest supernova seen in M83 was, interestingly in view of the Messier number of this galaxy, in '83 - in 1983, that is. That was 28 years ago, and due to the southerly declination of this galaxy, it is likely that the supernova wasn't well observed back then. After all, even the supernova that was seen in the Large Magellanic Cloud four years later, Supernova 1987A, wasn't as well observed as it might have been. http://en.wikipedia.org/wiki/Messier_83 wrote:

It was discovered by Ian Shelton and Oscar Duhalde at the Las Campanas Observatory in Chile on February 24, 1987, and within the same 24 hours independently by Albert Jones in New Zealand.

A bright naked-eye supernova shouldn't have to be independently discovered within the same 24 hours by anybody. In the 1980s, the astronomical community was not on the alert for new supernovae in the same way that it is today. As for the supernovae of M83, chances are that it is not really known if any of the supernovae in this galaxy were of type Ia. It is also possible that the supernovae weren't sufficiently well measured to have their peak brightness established without a doubt. Therefore it is likely that these supernovae can't be used as distance indicators.
There are other methods that can be used as distance indicators. As normal stars age, they will follow an evolutionary track that puts them in a position whose absolute luminosity is relatively well known, the so-called "red clump". In these color-magnitude diagrams, you can see stars located at the "red clump".

You can also search for stars that are located "at the tip of the red giant branch", the brightest red giants, whose luminosity is also relatively well known.

There are still more methods that can be used, such as searching for planetary nebulae and measuring how bright they appear to be, compared with how bright they may be assumed to be. But all in all, the distance to M83 is still "estimated". According to http://en.wikipedia.org/wiki/Messier_83, the estimated distance to M83 is 14.7 million light years.

Ann

Thanks for the post. I can't imagine it was really that far.