Vemos una estrella fugaz a simple vista.

Because ver means to see, while mirar means to look at.

• vemos = we see
• miramos = we look at

In this sentence, the idea is that the shooting star is visible, so ver is the natural verb.

• Vemos una estrella fugaz... = We see a shooting star...
• Miramos una estrella fugaz... would mean We look at a shooting star, which is a slightly different idea.

Vemos is the present tense of ver.

It can mean:

• we see
• sometimes we are seeing, depending on context

Here it is a simple present: we see.

Conjugation of ver in the present:

• yo veo
• tú ves
• él/ella/usted ve
• nosotros/nosotras vemos
• vosotros/vosotras veis
• ellos/ellas/ustedes ven

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

• vemos already tells you it means we see

So:

• Vemos una estrella fugaz... = perfectly natural
• Nosotros vemos una estrella fugaz... = also correct, but more emphatic

You would add nosotros only if you want contrast or emphasis, for example:

• Ellos no la ven, pero nosotros sí la vemos.

Because estrella is a feminine singular noun, so it takes:

• the feminine article una
• feminine adjectives if needed

So:

• una estrella
• not un estrella

Also, fugaz here describes estrella. The adjective fugaz has the same form for masculine and feminine in the singular:

• un momento fugaz
• una estrella fugaz

Literally, it means fleeting star or passing star.

But in normal usage, una estrella fugaz is the standard Spanish expression for a shooting star.

So even though the literal words may sound unusual in English, the whole phrase should be learned as one expression:

• estrella fugaz = shooting star

A simple vista is a fixed expression meaning:

• with the naked eye
• by plain sight
• without using instruments

So:

• Vemos una estrella fugaz a simple vista. = We see a shooting star with the naked eye.

It is idiomatic, so it is best to learn it as a whole phrase rather than word by word.

Because a simple vista is an established Spanish expression. It is just the way Spanish says with the naked eye.

You should treat it as a set phrase:

• visible a simple vista
• se ve a simple vista
• a simple vista parece pequeño

Using con or adding la would usually sound unnatural here.

Not exactly in a word-for-word way.

In a simple vista, simple contributes to the idea of plain, unaided sight. The whole phrase means with the naked eye, not literally at simple sight in natural English.

So this is a good example of why translating each individual word can be misleading. The full expression matters more than the parts.

This is a very normal Spanish word order:

• Vemos = verb
• una estrella fugaz = direct object
• a simple vista = adverbial phrase

So the structure is basically: Verb + object + extra information

You could move a simple vista for emphasis in some contexts:

• A simple vista, vemos una estrella fugaz.

But the original version is the most straightforward and neutral.

It mainly describes the seeing, so it relates to the verb vemos.

In other words, it tells you how or by what means the star is seen:

• We see it with the naked eye

So the meaning is not that the star itself is a simple-vista star, but that it is visible a simple vista.

It can often be understood as either, depending on context.

The Spanish present tense is flexible. It can express:

• something happening now
• a habitual action
• a general truth

So Vemos una estrella fugaz a simple vista could mean:

• We can see a shooting star with the naked eye (general fact)
• We are seeing a shooting star with the naked eye (right now), if the situation makes that clear

Context decides which reading is intended.

Yes.

Singular:

• una estrella fugaz

Plural:

• unas estrellas fugaces

Notice that fugaz changes to fugaces in the plural.

Examples:

• Vemos una estrella fugaz.
• Vemos varias estrellas fugaces.

Yes, it is grammatical and natural. The phrase a simple vista is very common in Spain.

That said, in everyday conversation, a speaker might also say things like:

• Podemos ver una estrella fugaz a simple vista.
• Se puede ver una estrella fugaz a simple vista.

These versions can sound a little more natural if the meaning is a general fact rather than a specific event. But your original sentence is perfectly correct.