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As you can see this image http://i.stack.imgur.com/YOt8C.jpg and other galaxy images, the centers generally much brighter.

Why is that?

Is there a very big star? A very big gravitational field?

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Simply, the galaxies are thicker in the middle (that is, they bulge further above and below the ecliptic) and have a higher density of stars and gas towards the core.

There are exceptions to the 'brighter in the center' observation, such as this beauty from the Arp 147 pair imaged by the Chandra observatory, which is the result of a galactic collision:

enter image description here

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