La constelación está en el hemisferio celeste.

Because estar is normally used for location.

• La constelación está en... = the constellation is located in ...
• Ser would usually be used for identity, classification, or essential characteristics.

So here, the sentence is talking about where the constellation is, not what it is.

These are the definite articles the in Spanish, and they must match the grammatical gender of the noun:

• la constelación → feminine singular
• el hemisferio → masculine singular

A useful clue is that many nouns ending in -ción are feminine, so constelación is feminine.

The accent mark shows where the stress goes:

• constelación → stress on the final syllable: con-ste-la-CIÓN

Without the written accent, Spanish spelling rules would make you stress it differently. The accent is there to mark the correct pronunciation.

The accent in está shows that the stress falls on the second syllable:

• es-TÁ

It also helps distinguish it in writing from other forms or words. In this case, it is the third-person singular form of estar:

• está = is

Here, en means in.

So:

• está en el hemisferio celeste = is in the celestial hemisphere

Spanish en is very common and can also mean on, at, or in, depending on context.

In Spanish, adjectives often come after the noun.

So:

• hemisferio celeste = celestial hemisphere

This is the normal order for a descriptive adjective in Spanish. English usually puts the adjective before the noun, but Spanish often does the opposite.

Celeste is a normal Spanish adjective meaning celestial, of the sky/heavens, or sometimes sky-blue, depending on context.

Here, because it modifies hemisferio, it clearly means something like celestial or sky-related, not the colour.

Spanish and English do not always use the exact same adjective form even when the meaning is similar.

It does agree, but celeste is one of those adjectives that has the same form for masculine and feminine in the singular.

Compare:

• el hemisferio celeste
• la esfera celeste

Both use celeste.

If the noun were plural, then the adjective would become plural too:

• los hemisferios celestes

So the adjective agrees in number, and in gender where applicable, but this particular adjective has the same singular form for both masculine and feminine.

No, that would sound wrong in standard Spanish.

For location, you need estar:

• La constelación está en el hemisferio celeste. ✅

Using ser with en in this way is not how Spanish normally expresses physical or spatial location.

Yes, but the original order is the most neutral and standard:

• La constelación está en el hemisferio celeste.

You could rearrange it for emphasis in some contexts, for example:

• En el hemisferio celeste está la constelación.

But that sounds more marked or literary. For learners, the original sentence is the best model.

It is a fairly standard astronomical expression.

• hemisferio = hemisphere
• celeste = celestial / sky-related

Together, hemisferio celeste refers to a hemisphere of the sky, as used in astronomy. So yes, it sounds technical and natural in that context.

A Spain Spanish pronunciation would be approximately:

La constelación está en el hemisferio celeste

Some key points:

• c before e in constelación, hemisferio, and celeste is pronounced with the th sound of think in most of Spain.
• ción is pronounced with stress on -ción.
• está is stressed on the last syllable.

A rough guide for peninsular Spanish might be:

la kon-ste-la-THYON es-TA en el e-mis-FE-rio the-LES-te

In much of Latin America, those c sounds would be pronounced like s instead.

Normally, no.

You would usually say:

• La constelación
• el hemisferio celeste

Spanish uses definite articles more often than English does, and in this sentence they sound fully natural and expected. Omitting them would make the sentence sound incomplete or unnatural in normal usage.