Observamos galaxias, nebulosas y cúmulos estelares.

Observamos is the 1st person plural form of observar (to observe), so it means we observe.

A useful extra point: observamos can also mean we observed in the preterite. For regular -ar verbs, the nosotros form is the same in the present and the preterite:

• observamos = we observe
• observamos = we observed

Usually, the context tells you which meaning is intended. In a general scientific statement like this, it is most naturally understood as the present.

Spanish often drops subject pronouns when they are not needed, because the verb ending already shows who the subject is.

• observamos already tells you the subject is we
• so nosotros is usually unnecessary

You could say Nosotros observamos galaxias..., but it would sound more emphatic, as if you were stressing we in particular.

In Spanish, plural nouns are often used without articles when speaking in a general way.

So:

• Observamos galaxias, nebulosas y cúmulos estelares = a general statement
• Observamos las galaxias, las nebulosas y los cúmulos estelares would sound more like specific ones already known from the context

English also sometimes does this with plural nouns, as in We observe galaxies.

Y means and.

Spanish normally uses y before the last item in a list:

• galaxias, nebulosas y cúmulos estelares

There is a small spelling/pronunciation rule: y changes to e before words beginning with an i sound:

• padres e hijos
• España e Italia

But here the next word is cúmulos, so y is correct.

In standard Spanish, the serial comma (Oxford comma) is usually not used.

So a list is normally written like this:

• galaxias, nebulosas y cúmulos estelares

This is standard punctuation in Spanish.

In Spanish, adjectives usually come after the noun they describe.

So:

• cúmulos estelares = stellar clusters

Here:

• cúmulos = clusters
• estelares = stellar

This noun + adjective order is the normal one in Spanish.

Because the adjective has to agree in number with the noun.

• cúmulo estelar = singular
• cúmulos estelares = plural

Since cúmulos is plural, estelares must also be plural.

Because nebulosa is already a complete noun by itself: a nebula.

So in the list:

• galaxias = galaxies
• nebulosas = nebulae / nebulas
• cúmulos estelares = stellar clusters

Only the last item is a noun + adjective combination. The first two are just plural nouns.

The singular forms are:

• galaxia → galaxias
• nebulosa → nebulosas
• cúmulo estelar → cúmulos estelares

This is a good example of regular plural formation in Spanish:

• words ending in a vowel usually add -s
• adjectives also become plural to match the noun

The accent mark shows where the stress falls:

• cúmulos → stress on the first syllable: CÚ-mu-los

Without the accent, Spanish stress rules would make you stress it differently.

The accent mark is part of the correct spelling and helps with pronunciation.

A simple pronunciation guide is:

ob-ser-VA-mos ga-LAK-syas, ne-bu-LO-sas ee KOO-mu-los es-te-LA-res

A few notes:

• v and b sound very similar in Spanish
• cúmulos is stressed on CÚ
• estelares is stressed on LA
• in Spain, pronunciation may vary slightly by region, but this guide works well for learners

This word order is the most natural and neutral:

• Observamos galaxias, nebulosas y cúmulos estelares.

You could change the order of the listed objects:

• Observamos nebulosas, galaxias y cúmulos estelares.

That would still be grammatical, but it might sound different in emphasis or style. The verb normally comes before the list here, and that is the most straightforward order.

Because the personal a in Spanish is mainly used with specific human beings (and sometimes animals treated as personal beings), not with things.

Here the objects are astronomical objects, not people:

• Observamos galaxias... ✔
• not Observamos a galaxias...

So no a is needed.

Yes. The Spanish present tense often has a broad use. It can mean:

• something happening now
• something done regularly
• a general fact

So Observamos galaxias, nebulosas y cúmulos estelares could mean:

• We are observing galaxies, nebulae, and stellar clusters (depending on context)
• We observe galaxies, nebulae, and stellar clusters
• We study/observe galaxies, nebulae, and stellar clusters as a general activity

The exact meaning depends on context.