请给出一个关于宇宙中黑洞的简单解释，并且按照以下格式输出：用三个段落，最少3句话，最多5句话，全英文，不允许出现in, at, a, and, the. 黑洞 forms when massive star dies. Gravity pulls on star's material very strongly. Event horizon appears, trapping everything inside. 

Once inside, nothing escapes. Object falls toward singularity, where density becomes infinite. Even light cannot get out, making black hole invisible.

Physicists study black holes carefully. They want to know more about extreme gravity effects. Understanding black holes helps us learn about universe's secrets.
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Black hole forms when massive star dies. Gravity pulls on star's material very strongly. Event horizon appears, trapping everything inside.

Once inside, nothing escapes. Object falls toward singularity, where density becomes infinite. Even light cannot get out, making black hole invisible.

Physicists study black holes carefully. They want to know more about extreme gravity effects. Understanding black holes helps us learn about universe's secrets.
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