Radio astronomy has changed. For years it studied relatively rare sources, which emit mostly non-thermal radiation across the entire electromagnetic spectrum, i.e. radio quasars and radio galaxies. Now, it is reaching such faint flux densities that it detects mainly star-forming galaxies and the more common radio-quiet active galactic nuclei. These sources make up the bulk of the extragalactic sky, which has been studied for decades in the infrared, optical, and X-ray bands. I follow the transformation of radio astronomy by reviewing the main components of the radio sky at the bright and faint ends, the issue of their proper classification, their number counts, luminosity functions, and evolution. The overall “big picture” astrophysical implications of these results, and their relevance for a number of hot topics in extragalactic astronomy, are also discussed. The future prospects of the faint radio sky are very bright, as we will soon be flooded with survey data. This review should be useful to all extragalactic astronomers, irrespective of their favourite electromagnetic band(s), and even stellar astronomers might find it somewhat gratifying.

中文翻译：

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微弱的射电天空：射电天文学成为主流

射电天文学发生了变化。多年来，它研究了相对稀有的源，这些源主要在整个电磁波谱中发射非热辐射，即射电类星体和射电星系。现在，它达到了如此微弱的通量密度，以至于它主要探测到恒星形成星系和更常见的无线电静默活动星系核。这些来源构成了银河系外天空的大部分，几十年来一直在红外、光学和 X 射线波段进行研究。我通过回顾射电天空在亮端和暗端的主要成分、它们的正确分类问题、它们的数量计数、光度函数和演化来跟踪射电天文学的转变。这些结果的总体“大局”天体物理学意义，以及它们与河外天文学中许多热门话题的相关性，也在讨论中。微弱的射电天空的未来前景非常光明，因为我们很快就会被调查数据所淹没。这篇评论应该对所有河外天文学家都有用，不管他们最喜欢的电磁波段是什么，甚至恒星天文学家也可能会觉得有些欣慰。