Estas necese, ke ni atendu ĝis nokto por vidi galaksion.

Because necese is being used in an impersonal expression: estas necese = it is necessary.

In Esperanto, when there is no real noun being described, an -e form is often used:

• Estas grave. = It is important.
• Estas utile. = It is useful.
• Estas necese. = It is necessary.

By contrast, necesa is an adjective and normally describes a noun:

• La atendo estas necesa. = The waiting is necessary.
• Necesa paŝo = a necessary step

A very common alternative is also:

• Necesas, ke ni atendu...

That means essentially the same thing.

Ke introduces a subordinate clause, much like English that.

So:

• Estas necese, ke ni atendu...
  = It is necessary that we wait...

In Esperanto, ke is normally required here. English often drops that, but Esperanto usually does not.

Compare:

• Mi scias, ke li venos. = I know that he will come.
• Estas klare, ke... = It is clear that...

Because after expressions of necessity, desire, command, intention, or purpose, Esperanto often uses the -u verb form.

Here, atendu does not mean a present-tense statement like we are waiting. It expresses something more like:

• we should wait
• we are to wait
• it is necessary that we wait

So:

• ni atendas = we are waiting / we wait
• ni atendu = let us wait / we should wait / that we wait

In this sentence, Estas necese, ke ni atendu..., the -u form is used because the sentence expresses necessity.

Yes.

A very natural alternative is:

• Estas necese atendi ĝis nokto por vidi galaksion.

That means the same basic thing: It is necessary to wait until night to see a galaxy.

The version with ke ni atendu explicitly includes the subject ni = we.

So the difference is mostly:

• Estas necese atendi... = necessary to wait
• Estas necese, ke ni atendu... = necessary that we wait

If you want to name who must do it, the ke + -u structure is very useful.

Ĝis means until, up to, or as far as.

So ĝis nokto means until night or until nighttime.

In time expressions like this, Esperanto often leaves out the article when speaking generally:

• de mateno ĝis vespero = from morning until evening
• ĝis nokto = until night

It is a general time point, not a specific named night.

Also, ĝis itself already shows the relationship, so no extra accusative ending is needed on nokto here.

Because Esperanto has no indefinite article.

English distinguishes:

• a galaxy
• the galaxy

Esperanto only has a definite article, la. If there is no la, the noun may often correspond to English a/an or just a bare noun, depending on context.

So:

• galaksio = a galaxy / galaxy
• la galaksio = the galaxy

That is why galaksion can naturally mean a galaxy here.

The -n marks the direct object.

The verb is vidi = to see, and the thing being seen is galaksion.

So:

• vidi galaksion = to see a galaxy
• Mi vidas stelon. = I see a star.
• Ni observas la ĉielon. = We observe the sky.

This -n ending is one of the most important features of Esperanto grammar.

Por + infinitive is the normal way to express purpose when the subject is understood:

• por vidi = in order to see / to see

So:

• atendu ĝis nokto por vidi galaksion
  = wait until night in order to see a galaxy

If you want to state a different or explicit subject in the purpose clause, then por ke + -u is often used:

• Ni silentis, por ke la infano dormu.
  = We stayed quiet so that the child would sleep.

In your sentence, por vidi is simpler because the one who waits and the one who sees are understood to be the same people.

Because in standard Esperanto punctuation, a subordinate clause introduced by ke is usually separated with a comma.

So:

• Estas necese, ke ni atendu...

This is very normal and standard in written Esperanto.

English punctuation is less regular here, but Esperanto often marks clause boundaries more clearly with commas.