While retagging, I get confused a lot as: "which is which". This is bugging me for a while...

For now, the tag wiki definition of astronomy -

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.

And, for astrophysics -

Astrophysics: Physics related to astronomical objects: stars, galaxies, nebulae, globular clusters, clusters and super-clusters of galaxies, white dwarf stars, neutron stars and pulsars, black holes, the inter-stellar medium, non-terrestrial planets, ring systems, MACHOs, the inter-galactic medium, the great voids, quasars, etc...

Looks like both the tags are having the same definition except that Astronomy is having additional objects like "planets" and Astrophysics is not... From the current definition of both tags, I see that is dealing with a lot of things than ...

Let's have a look at cosmology now...

The study of the large-scale structure, history, and future of the universe.

Sometimes, we could give this kind of definition from the last sentence of astronomy wiki def. Because, it covers the history and future with a single word evolution. So,
+ =

But, I don't like these tags to be inter-linked with each other. Each should have an unique definition. So, I've got three choices:

  • We can use astronomy for observational purposes by merging under and not the other way round. Now, Astrophysics and cosmology could be set free. I mean, all would've got different meanings.
  • If we wanna keep , then - we should necessarily merge the other two (astrophysics & cosmology) under astronomy.
  • This is the best choice I like. We redefine all the tags, or atleast astrophysics and astronomy and finally arrive at a definition for the

2 Answers 2


Historically, the astro community has not necessarily been clear on astronomy vs. astrophysics vs. cosmology, but from my perspective in the field, the trend that is emerging is this:

Astrophysics is the application of physical theory to celestial systems. For example, this includes:

  • Stellar evolution - what are the stages?
  • Neutron stars - what is the equation of state of such matter?
  • Nucleosynthesis - what processes make new elements?
  • Galactic dynamics - what forces act in spiral vs. elliptical galaxies?
  • Orbital mechanics - how does the Kozai mechanism cause planet migration?

Astronomy is more on the observational side of things - saying what is out there rather than speculating about how or why. This includes questions about

  • Stellar evolution - what is this "main sequence" thing?
  • Neutron stars - what detections of their radii do we have?
  • Nucleosynthesis - what cosmic abundances do we see?
  • Galactic dynamics - which is more common, spirals or ellipticals?
  • Orbital mechanics - why can't I see Saturn until the Spring?

Cosmology is the study of the "big questions" - how did the universe get to be like this, where will it end up, etc. There are different approaches to cosmology, though if you ask a cosmologist "what is cosmology" you are likely to only get one of the following as a response:

  • Particle physics/QFT/strings applied to the early universe at energy scales we can never test experimentally (as a high-energy physicist would say)
  • General relativity and the expansion (or contraction) of spacetime on the largest of scales (as a GR person would say)
  • Imprinting and growth of structure - CMB, halo occupation number, even galaxy formation at high redshifts (this from an astrophysicist)

In the end there can certainly be overlap, but some questions only fall into one category.

  • Questions about how to use a telescope are not or .
  • Questions about the neutrino-producing processes in core-collapse supernovae are better in than and certainly don't belong in .
  • Dark matter and energy often apply to (though questions about "can we ever see dark matter?" go to , and questions about "can dark matter scattering cool stellar cores?" go to ).

Finally, there's the perennially frustrating tag . Honestly, if you have the astronomy/astrophysics distinction in place as I've described, then pretty much all of astronomy is observational by definition. Certainly the astronomy/astrophysics distinction is more important to make (analogous to experiment/theory in most fields of physics) than astronomy/observational-astronomy. Therefore I propose either


  • Keep them, but only bother with the tag if the question is really, honestly, actually about getting your hands dirty with equipment and taking observations (either professionally or as an amateur I don't know) as opposed to just using the data from observations.

In any event, I very much agree that the tag descriptions can use some tweaking.

  • $\begingroup$ I quite like this proposal. And the summaries of the topics, which would make excellent tag wiki material IMO. $\endgroup$
    – David Z
    Commented Jan 17, 2013 at 6:16
  • $\begingroup$ Good thing that you gave a definition for all those tags... Wonderful :-) $\endgroup$ Commented Jan 25, 2013 at 12:31
  • 1
    $\begingroup$ @CrazyBuddy Well just two so far, but I'll keep working at it. $\endgroup$
    – user10851
    Commented Jan 25, 2013 at 16:59

It is perfectly fine to have tags which are subtags of another tag. The purpose of tags is to allow easy categorization, searching, and filtering of questions. For example, if a user is interested in , they should only have to favorite . If, on the other hand, they are interested in and nothing else, then they can favorite that tag. If was merged into , then someone wanting to favorite/search for questions will have to find all the "subtags" (, , , etc). That makes it much harder.

So, to me, the current system is OK.

  • $\begingroup$ I know that they are used for searching, etc. But why do we need duplicates then..? Why do we need the wiki def., etc..? If we define something, I think we must be sure that it's not already been defined :-) $\endgroup$ Commented Jan 15, 2013 at 12:29
  • $\begingroup$ @CrazyBuddy: They aren't duplicates. They either overlap or are subsets of each other. Which is fine. $\endgroup$ Commented Jan 15, 2013 at 12:32

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