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Rahman
2004-May-13, 10:00 PM
Anyone know what the visual magnitude of the pistol star is? I did the calculation last time and got something like mag 4 but since it lies 25 000 light years away and is obscurred by dust towards the centre of the milky way, what is its current visual magnitude.

ngc3314
2004-May-13, 10:16 PM
Anyone know what the visual magnitude of the pistol star is? I did the calculation last time and got something like mag 4 but since it lies 25 000 light years away and is obscurred by dust towards the centre of the milky way, what is its current visual magnitude.

The answer is probably "faint enough not to matter". A 1998 paper by Figer et al. (Astrophysical Journal 506, 384, titled simply "The Pistol Star"), estimates an extinction of 3.4 magnitudes in the K band (2.2 microns). For typical Milky Way dust, the visual V-band extinction is about 8.7 times that at K band, implying that we see the Pistol Star dimmed in V by 3.4 times 8.7 = 29.6 magnitudes. This very crude estimate fits with the typical values to the Galactic Center of 34 magnitudes of visual extinction. It's detected longward of 1.2 microns; with a V magnitude of something like 33, no wonder...

2004-May-13, 11:06 PM
Is that factor of 8.7 in the magnitude value, or the extinction factor? 8.7 in absorption is only about 2.3 mag or so, while 8.7 magnitudes is a factor of 3000 in absorption!

ngc3314
2004-May-13, 11:37 PM
Is that factor of 8.7 in the magnitude value, or the extinction factor? 8.7 in absorption is only about 2.3 mag or so, while 8.7 magnitudes is a factor of 3000 in absorption!

The extinction in magnitudes is that much greater (since it acts exponentially). The old working rule is that extinction in optical depth of in magnitudes (related by that factor 1.086) scales crudely as the inverse of wavelength, but you always need to set the starting value at some selected wavelength.

An easy way to see that it must be multiplicative in the magnitudes of extinction is to imagine stacking absorbing screens each with one magnitude of (say) K-band extinction, each of which would produce 8.7 magnitudes of (V)isual extinction. And that number 8.7 comes from a tabulation of A(V)/E(V-K) - A(V)/(A(V)-A(K)), thus already working in magnitudes. Now that I look at it, maybe that was eactly what your question was getting at.

On a side note related to my BABB handle - the "vertical" extinction through the center of a typical spiral is something like half a magnitude at K, a fairly big deal in V on average but not much commpared to what we have top content with looking through so much of the disk.

2004-May-14, 12:27 AM
What would be its magnitude if there was no intervening gas or dust?

Chip
2004-May-14, 04:43 AM
...On a side note related to my BABB handle - the "vertical" extinction through the center of a typical spiral is something like half a magnitude at K, a fairly big deal in V on average but not much commpared to what we have top content with looking through so much of the disk.

Hello NGC 3314 (http://antwrp.gsfc.nasa.gov/apod/ap000511.html), welcome to the BABB. How are things in Hydra?
:wink:
Actually, I was wondering if there is a list of overlapping (but not colliding) galaxies somewhere. NGC3314 is the only one I know of.

ngc3314
2004-May-14, 11:33 AM
...On a side note related to my BABB handle - the "vertical" extinction through the center of a typical spiral is something like half a magnitude at K, a fairly big deal in V on average but not much commpared to what we have top content with looking through so much of the disk.

Hello NGC 3314 (http://antwrp.gsfc.nasa.gov/apod/ap000511.html), welcome to the BABB. How are things in Hydra?
:wink:
Actually, I was wondering if there is a list of overlapping (but not colliding) galaxies somewhere. NGC3314 is the only one I know of.

Funny you should ask! Several of us spent some time looking for overlapping pairs about 10 years back. In the nearby Universe (the world as probed by NGC, UGC, etc.) NGC 3314 is the only central superposition of similar-apparent-size galaxies. There are several more if you go to half-overlapping or different sizes: AM 0500-620, AM 1316-241, AM 1318-432 =NGC 5090/1 (where the foreground spiral is in front of a radio galaxy), NGC 1275 and that odd gas-rich thing in front of it, UGC 2942/3. Adding spiral/spiral pairs, there are NGC 1738, ESO 0320-51, NGC 4567/8 in the Virgo cluster, AM 1311 (whatever its last name is), just maybe NGC 4647/9 also in Virgo, and NGC 450/UGC 807. There are also some interesting examples in the HDF-S STIS field, and a friend at STScI also grabbed me to show me a good one in the GOODS field.

Pictures can be seen at http://www.astr.ua.edu/pairs2.html
and http://www.astr.ua.edu/white/pairs/individual.html. More details from HST and (gasp) ISO observations are to be found at http://www.astr.ua.edu/keel/research/dust.html and (along with more detail about dust than anyone really wanted) http://www.astr.ua.edu/keel/galaxies/dust.html.

After all this, my personal home page might need to feature a portrait of me in silhouette...

Edited to add: How could I forget NGC 2207/IC 2163, shown in a very nice Hubble Heritage image at http://heritage.stsci.edu/1999/41/index.html? Also, the released picture of NGC 3314 pales in comparison to one that should be put together this summer (haahaahaahaa). There has been a 2-year HST ACS monitoring program to seek microlensing by whitedwarfs, neutron stars, etc. in the foreground galaxy. This should also turn up Cepheids in each galaxy. There are 60 orbits' worth of data (of which I believe that last was just taken), each orbit of which goes noticeably deeper than the stacked WFPC2 images in its Heritage release.

Looking at my list, I can now add NGC 6040 in the Hercules cluster. As soon as I can find my complete working list, there should be a few more. These exclude cases in which the overlapping piece is a tidal arm.