Learn Russian: Учёные наблюдают звёзды.

You would use the plural form все (all) and say: Учёные наблюдают все звёзды. Because звёзды is an inanimate direct object, both the noun and the pronoun все take the accusative case, which looks exactly like the nominative plural.

How would I say 'All scientists observe the stars'?

You would again use the plural form все, but this time it matches the subject of the sentence in the nominative case: Все учёные наблюдают звёзды.

Why does the word for scientists (учёные) have an adjective ending?

The word учёный originally comes from an adjective meaning 'learned' or 'taught'. Over time, it became used as a noun meaning 'scientist', but it still changes its endings exactly like a regular adjective. Here, it is in the nominative plural.

Is звёзды in the nominative or accusative case here?

It is in the accusative case because it is the direct object of the verb (the thing the scientists are observing). However, for inanimate nouns in the plural (things that are not people or animals), the accusative form is completely identical to the nominative form.