This is because appositives give more information.

这是因为同位语会提供更多的信息。

It uses an appositive to explain the meaning of the word " constellation" .

它用同位语来解释" constellation" 一词的意思。

An appositive is a noun or noun phrase that comes after another noun, where the second defines the first.

同位语是跟在另一个名词之后的名词或名词短语，第二个名词定义了第一个名词。

The sentence uses a comma followed by the word " or" to introduce the appositive " groups of stars that form shapes in the sky."

这个句子使用了逗号，后面跟“或”这个单词，来引介同位语“在天空中组成形状的群星”。

In a sentence, a noun can play the role of a subject, an indirect object, a direct object, a subject compliment, an object compliment, and appositive or adjective.

在一个句子中，名词可以充当主语，间接宾语，直接宾语，主语补语，宾语补语和同位语或形容词。

But note that sometimes appositives do not use the signal word " or." And be careful -- commas have many purposes in written texts and they do not always signal appositives.

但请注意，有时同位语不使用“或”这个信号词，而且要小心，逗号在书面语中有很多用途，并不总是表示同位语。