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AZgazer
2004-Jun-03, 05:54 AM
Astronomy Noob Warning! This is probably something everyone but me knows, but I couldn't find the answer so here goes... 8-[

Space.com (http://www.space.com/scienceastronomy/aas_faintest_040531.html) has a picture of the faintest Galaxy (Andromeda IX). It is 100,000 x dimmer than the Milky Way. This made me wonder, with an object that faint what method would an Astronomer use to determine that it is a Galaxy and not a really bright star that is very far away?

It would just seem to me with an object that faint determining anything about it would be pretty hard to do. Thanks in advance. :)

Kullat Nunu
2004-Jun-03, 06:07 AM
Astronomy Noob Warning! This is probably something everyone but me knows, but I couldn't find the answer so here goes... 8-[

Space.com (http://www.space.com/scienceastronomy/aas_faintest_040531.html) has a picture of the faintest Galaxy (Andromeda IX). It is 100,000 x dimmer than the Milky Way. This made me wonder, with an object that faint what method would an Astronomer use to determine that it is a Galaxy and not a really bright star that is very far away?

Look at the picture on that page. The galaxy is those faint bluish stars in the center of the photo. (All bright stars in the image are in the Milky Way.) It is not a point of light or even a smudge. Any galaxy this close resolves into stars when viewed with a large telescope.


It would just seem to me with an object that faint determining anything about it would be pretty hard to do. Thanks in advance. :)

It really is. Especially when the object covers a large area. Only way to find it is to measure those stars.

AZgazer
2004-Jun-03, 06:12 AM
Thanks, I mistook the faintness. The impression I had was one of the singular points was the Galaxy, hence how do we know what it is if it's that faint...

Trying to imagine anything 100,00x dimmer than anything is hard for me to do, lol. I guess I still need big ol' circles or an arrow on the pic. *sigh*

ngc3314
2004-Jun-03, 02:14 PM
Thanks, I mistook the faintness. The impression I had was one of the singular points was the Galaxy, hence how do we know what it is if it's that faint...

Trying to imagine anything 100,00x dimmer than anything is hard for me to do, lol. I guess I still need big ol' circles or an arrow on the pic. *sigh*

Actually 100,000 times isn't all that much between astronomical objects.
Five magnitudes (the ratio between the brightness of, say, Vega, and the faintest stars I can ever see from my house) is a factor 100 in apparent brightness, so 100,000 times is 12.5 magnitudes. That's the ratio between the Full Moon and Sirius, or Vega and the brightest quasar, or Venus and the dimmest stars typical binoculars will show.

I'm still impressed that the ratio of apparent brightness between Vega and the dimmest objects in the last couple of Hubble deep fields is greater than the Sun and Vega.

AZgazer
2004-Jun-04, 04:01 AM
Actually 100,000 times isn't all that much between astronomical objects. Five magnitudes (the ratio between the brightness of, say, Vega, and the faintest stars I can ever see from my house) is a factor 100 in apparent brightness, so 100,000 times is 12.5 magnitudes. That's the ratio between the Full Moon and Sirius, or Vega and the brightest quasar, or Venus and the dimmest stars typical binoculars will show.

I'm still impressed that the ratio of apparent brightness between Vega and the dimmest objects in the last couple of Hubble deep fields is greater than the Sun and Vega.

Magnitude doesn't make it seem that significant to me. 100,000x just seems like such a difference. It's one of those things that you know what the units represent, but when I think about it in it's simplified form, it starts to become mind boggling again. :o