What causes the apparent angular length of cometary tails? How to distinguish a comet without a tail from

What causes the apparent angular length of cometary tails? How to distinguish a comet without a tail from an ordinary nebula when observing?

The true size of the tail depends on the comet’s distance from the Sun, the apparent size of the tail also depends on the comet’s distance from the Earth. The comet moves against the background of fixed stars.

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