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?

[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.

[janoskiss (Steve H)]

Originally Posted by Lester

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

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]

Originally Posted by 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.

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.

[astroron (Ron)]

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]

Originally Posted by 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.

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 (Steve H)]

Please don't ask me how your brain works.

[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.

[astroron (Ron)]

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