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The science dealing with objects and phenomena located beyond Earth. In particular, this applies to observations and data. At its core, astronomy is the physically informed cataloging and classifying of the contents of the universe in order to better understand what is out there.

Astronomy is a natural science that deals with the study of celestial objects (such as stars, planets, comets, nebulae, star clusters and galaxies) and phenomena that originate outside the atmosphere of Earth (such as cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.

(source: Wikipedia)

In more detail, can take two slightly different connotations. Broadly speaking, it is any scientific study of anything beyond Earth. However, when contrasted with , it is often taken to imply the observational side of such pursuits. This includes such topics as surveying galaxies, obtaining stellar spectra, identifying exoplanets, and measuring the cosmic microwave background. The collection, reduction, and analysis of this data, as well as the inferences drawn, are therefore core parts of proper. The physics of how this data is obtained (i.e. with telescopes) also falls under this umbrella, though might be more appropriate in cases regarding actual implementation details for how various instruments are used to gather such data.

While astronomy questions are on-topic on Physics Stack Exchange, you might want to check our sister site Astronomy Stack Exchange and see if your question isn't answered there already.