How does cartes du ciel measure stuff?

[spurius]

Cartes du ciel is an alternative to stellarium. Stellarium uses Open GL 2 software. I dont have a clue what that is but I am aware that it does not work with the hardware I run on my laptop. Another laptop I use has the exact same software but with different hardware and it works just fine. This explains why I im not using the much better stellarium. Now according to Cartes du ciel:

Sirius has a "magnitude" of -1.42 but has a visual magnitude of -1.44. The difference is sod all but what is the difference in their definition?

Now lets go to the andromeda galaxy which is a naked eye object in dark enough skies. Its not a naked eye object where I am tho. So heres the specs: magnitude is 3.4 but its surface brightness is 13.5. Im assuming surface brightness is the magnitude of a specific point, where as the normal magnitude is the total amount of light given off by the entire galaxy, including the parts only visible by NASA stuff. However they say the human eye can see down to magnitude 6 but the surface brightness of an apparently naked eye object is given as 13.5? Uranus is mag 6 (or just brighter), and there is no way I can see that with the naked eye. In fact I need 200 mag to be able to see it as a planet but thats a size matter, not a brightness one.

M101 is impossible for me. Its magnitude is 7.9, or a little less dim than neptune. Surface brightness is 14.9. More like pluto. My money would be on that pluto is still actually brighter even with a similar mag (based on google search, couldnt find it Cartes du ciel). I am aware that nebulous objects like galaxies should be far harder to see (based on mag) because their light is spread out. But isnt this what "surface brightness" is supposed to represent?

M81 and m82 are visible to me. M81 is mag 6.9 and its surface brightness is 13,2. M82 is 8.4 and SB is 12.5. Both these are dimmer than M31 at a SB of 13.5. Someone wanna explain? The simple answer would be that cartes du ciel is wrong but just in case it isnt I am seeking the advice of you guys.

[acey]

There are various ways of measuring magnitude, hence you come across different quoted figures. Originally it was all done by eye, but a complication was that the eye detects different colours differently. Then it was done by measuring images on photographic plates, taken using filters in different colour bands. The V filter was regarded as being the one closest to what the eye sees, though it's not really an exact match. But it means that you can see different figures quoted for a magnitude, and you really need to know which filter was used, and what the data source was.

These are all measures of apparent magnitude. Surface brightness is in effect the average brightness over an area. If you move away from an object its apparent magnitude lessens but its surface brightness will stay the same. Surface brightness is measured in magnitudes per square arcminute, or magnitudes per square arcsecond. Deep sky objects that we can see have surface brightnesses ranging from about 15 down to about 24 mag per square arcsecond, with most galaxies being around 21 to 22. In mag per square arc minute those figures are about 6 down to about 15, with most galaxies being around 12 to 13. So in speaking of surface brightness it's important to remember what units are being used, and not to confuse the figure with apparent magnitude. Notice that to convert mag per square arcminiute to mag per square arcsecond you just add approximately 9 (the figure is actually 8.89).

Notice also that while there is no limit to the faintness of apparent magnitude (there are galaxies of magnitude 15, 20, 50, anything), there are bounds to the surface brightness of what can be seen by eye from Earth. This is because the sky itself has a surface brightness of about 22 mag per square arcsecond (at the very darkest sites on Earth), or much brighter at light polluted sites (e.g. 18 mag per square arcsecond). A telescope can increase the apparent magnitude of a star as seen at the eyepiece, but it can't increase the surface brightness of an extended object.

You quote the magnitude of M81 as 6.9. This means that when you look at M81 with the naked eye it is effectively a star of that apparent magnitude, meaning that at an extremely dark site it might be visible to the naked eye. Some people have managed it.

You quote the surface brightness of M81 as 13.2. That is obviously a figure in magnitude per square arcminute, and is a typical figure for a galaxy. But to arrive at that figure you need to integrate the total magnitude across the whole galaxy, and need to measure the size of the galaxy, but there is an issue about exactly how far any galaxy extends. Usually there is a convention of measuring it out to a certain level of faintness, but different levels will produce different surface brightness figures.

Maybe Stellarium and Cartes du Ciel got their figures from different catalogues. But if you want to get the most authoritative figures, try using the databases that professional astronomers use:

http://cdsportal.u-strasbg.fr/

For example, at the link above I typed in M81 then followed the link "More SIMBAD data". Going down the page to "fluxes" I got V-magnitude 6.94. Doing the same for "Sirius" gave me V-magnitude -1.47.

[KevUU]

Wow, that's really very thorough and helpful acey, thanks for taking the time

[Mr Spock]

As with any of these programs, the magnitude reported depends on which catalogue you have loaded.

[Double Kick Drum]

You quote the magnitude of M81 as 6.9. This means that when you look at M81 with the naked eye it is effectively a star of that apparent magnitude, meaning that at an extremely dark site it might be visible to the naked eye. Some people have managed it.

You quote the surface brightness of M81 as 13.2. That is obviously a figure in magnitude per square arcminute, and is a typical figure for a galaxy. But to arrive at that figure you need to integrate the total magnitude across the whole galaxy, and need to measure the size of the galaxy, but there is an issue about exactly how far any galaxy extends. Usually there is a convention of measuring it out to a certain level of faintness, but different levels will produce different surface brightness figures.

Acey's description is very thorough and hopefully helpful.

Practically, you can use both magnitude and surface brightness values to get an indicative idea of what is possible to see and how difficult it will be.

Galaxies with a surface brightness better than 13 are more tolerant of light pollution than say M101, which with its low surface brightness require decent skies to be seen and very dark skies to be seen well, despite its high over all magnitude.

I find that as a general rule of thumb, the dimmer the galaxy's magnitude, the higher the surface brightness will need to be for it to be seen. This is not an absolute rule, however. A galaxy with a smaller bright core may have the same s.br as a galaxy with a larger and less luminous core but will be easier to view.