Astronomy for Kids

Online Astronomy Classes for Children

Astronomy Packet:  7
Assignment # 3

 


Apparent Magnitude/Absolute Magnitude:

If it is nighttime, look out your window.  If it is day, just imagine the sky in your head.  Are all the stars the same brightness?  No, of course not.  They are all different.  How bright a star looks depends on two things.  First, it depends on how bright the star really is, and secondly, it depends on how far away the star is.

A dim star might look bright to us on Earth if it is close.  At the same time, we may barely be able to see a very bright star if it is too far away.  Betelgeuse, which is one of the brightest stars in the sky, is 310 light years away.  There are thousands of closer stars in the sky that we do not see because they are too dim. 

I want you to imagine you are standing on a beach.  One mile away on the ocean there is a group of 10 fishing boats.  Each one of these boats has a flashlight on it.  From where you stand you will not be able to see any light from these flash lights.  Now imagine there is a lighthouse 20 miles away up the coast.  You can easily see the light from the lighthouse even though it is much further than the light from the fishing boats. 

Most of the stars we see in the sky are like the lighthouse.  They are very bright.  If every star shined as brightly as Betelgeuse, the nighttime sky would be almost as bright  as it is in the day.

How bright a star or any other celestial body looks in space is measured by something called Apparent Magnitude.  The dimmer an object is, the higher apparent magnitude it has.  The dimmest objects human eyes can see have an apparent magnitude of about 6.  Objects with an apparent magnitude higher than 6 are too dim for humans eyes to see.  Objects with an apparent magnitude of 1 are bright.  Some objects, like the Sun for example, are so bright they have a negative apparent magnitude.   The Sun's apparent magnitude is -26.7.

Scientists use what is called Absolute Magnitude to help them understand how bright a star really is.   The absolute magnitude of an object, such as a star, equals how bright that object would look if it were 10 parsecs, or 32.6 light years away from the Earth. 


Assignment # 3:

What is the difference between Apparent Magnitude and Absolute Magnitude?

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