Magnitude 9.0 vs. 9.1

[jakob]

I'm puzzled about difference in star visibility.
I oserved Acrux and clearly saw a Star about 10 mins away. I think it's SAO 251902 Mag 9.0. But I see no trace of the nearby SAO 251912 Mag 9.1.

Is there such a difference in brightness of 0.1?

I was using 105 Mak at 147x from Kellyville on 6/2/06 11pm.

Any explanations? :confuse2:

[Lester]

G'day Jakob,

Each magnitude is approximately 2.5 times fainter. So 0.1= 0.25 times fainter or one quarter fainter.

The only reasons I can think of as to why you cannot see the star

1. You are looking in the wrong area
2. The star is a variable star, and may be at a much fainter magnitude when you looked.
:astron:

[janoskiss]

It does not work like that. It is a logarithmic scale.

+1 magnitude = (2.5) times the brightness
+2 magnitude = (2.5 x 2.5) = 2.5^2 = 6.25 times the brightness
+3 magnitude = (2.5 x 2.5 x 2.5) = 2.5^3 = 15.625 times the brightness
+N magnitude = 2.5^N times the brightness
+0.1 magnitude = 2.5^0.1 = 1.096 times the brightness

So we are talking about 10% difference in the amount of light per surface area on the retina. Barely noticeable for most people.

[jakob]

Thanks for explanation. Magnitude is not reason not to srr star if my chart and info is correct. I will check.

[jakob]

I think I found the reason for not seeing 9.1 Mag, SAO 251912 is a spectral type B9 and SAO 251902 is type A0.
I'm don't now what this difference means but it is probably the cause of the difference in visibility.:confused:

[astroron]

SAO 251912 is seven minutes 19 arc seconds east of SAO 251902, which is a fair distance. You would not notice the .01 mag difference in the two stars
It is not a double star, just a star in the same field. astroron

[stinky]

Could you clear something up for me? The eyes sensitivity is also logarthmic so do I "Percieve" a differnce of 2 in magnitude as 5 times different or 6.25 time brighter?

[janoskiss]

Please don't ask me how your brain works. :prey:

[jakob]

I was looking in the wrong place!

I was not paying enough atention to the scale and FoV when comparing to the Star chart.
I can see clarley both stars. What confused me was SAO 251 903 which is only 1min 30sec away from Acrux and I thought it was SAO 251 902.

I'm sorry for asking stupid questions.:doh:

[astroron]

Jakob, not stupid, by making mistakes , then seeing where we went wrong helps make us better observers. cheers astroron;) :thumbsup: :stargaze: