Photons from stars and carbon atoms from burning fuels

[mohdfasieh]

QUESTION:

1: Stars are known to flicker.Estimate the number of photons entering the eye of an observer when he looks at the star of first apparent visual magnitude.Such a star produces flux on the surface of the Earth of 10(exp)-6
lumens/m(exp)2. One lumen is 0.00a6 watts.Why do stars flicker ?not the plannets.

2:Calculate the number of carbon atoms released on burning petrol (octane) and natural gas (methane) to produce the same amount of energy of one joule.1 kg of petrol produces about 44 x 10(exp)6 of energy wheras 1 kg of natural gas would release 55 x 10(exp)6 Joules.What is the ratio?From the point of view of preserving environment,which is better to use?

 

[chroot]

The pupil has a diameter of about 7 mm on a very dark night, so its collecting area is about 49 square millimeters. All you need to do to solve (1) is to perform a division.

All you need to do to solve (2) is to do a division.

Please show your work if you'd like additional help.

- Warren

 

[mezarashi]

You won't be needing a genius to solve these.
1. As chroot has said, you have the power/area. You know the area of your pupil, area times power/area = power. Convert lumens into watts.

You'll need to know the frequency of the photons. E = nhf where n is the number of photons, h is Planck's constatn, and f is frequency. Stars flicker purely due to atmospheric disturbances. It is more apparent for stars since they tend to be more faint. You don't see the sun flicker now you do ;)

 

[Haelfix]

"Why do stars flicker ?not the plannets."

Refraction in the atmosphere. Actually planets can flicker too if you have very turbulent air, say in a hurricane. Planets though are a lot bigger (in terms of intensity per surface area of photons that you receive) so the molecules in the atmosphere cannot distort the full image as easily. Incidentally the stars can sometimes look like theyre changing color as well as flickering, again an optical property of the atmosphere.