Learn Hardcore Indonesian: Di dalam planetarium, lampu dimatikan sehingga bintang di langit buatan terlihat jelas. - Inside the planetarium, the lights are turned off so the stars in the artificial sky look clear.

di planetarium = at the planetarium / in the planetarium (more general)
dalam planetarium = inside the planetarium (without di, sounds a bit like a phrase inside a longer structure)
di dalam planetarium = inside the planetarium (clearly a full prepositional phrase)

In everyday speech, di planetarium would usually be fine, because it’s understood you mean “inside” if you’re talking about lamps and stars.
di dalam just adds a bit of emphasis that we’re inside the building, not just at the location in general.

Why is lampu dimatikan passive instead of something like mereka mematikan lampu?

lampu dimatikan uses the passive di- form:

lampu = the lights (or light)
dimatikan = are / were turned off

Indonesian often prefers passive when:

The object is more important than the doer, or
The doer is unknown or not relevant.

Here, we care that the lights were turned off, not who turned them off.
If you wanted an active form, you could say:

Petugas mematikan lampu. — The staff turned off the lights.

But with no subject, mematikan lampu would sound incomplete. The passive lampu dimatikan works well with the subject omitted.

Is lampu singular or plural here? How do I say “lights” vs “light(s)”?

By default, Indonesian nouns like lampu are number-neutral: they can mean light or lights, depending on context.

In this sentence, lampu dimatikan is naturally understood as “the lights were turned off”, because a planetarium typically has multiple lights.

If you really want to show plural, you can use:

lampu-lampu — lights (formal / written)
banyak lampu — many lights
semua lampu — all the lights

But normally, lampu alone is enough; Indonesian relies on context, not plural endings.

What exactly does sehingga mean here, and how is it different from supaya, agar, or jadi?

In this sentence:

sehingga = so that / so … that / as a result

lampu dimatikan sehingga bintang … terlihat jelas
= the lights were turned off, so that / with the result that the stars … were clearly visible.

Nuances:

sehingga
- Focuses on the result of an action
- Often quite neutral and common in writing / careful speech
supaya / agar
- Means in order that / so that (with purpose)
- Focus on intention / purpose, not just result
- e.g. Lampu dimatikan supaya bintang terlihat jelas.
  → Emphasizes: They turned off the lights *in order to make the stars visible.*
jadi
- Means so / therefore (more casual)
- e.g. Lampu dimatikan, jadi bintang terlihat jelas.
  → More conversational, like “The lights were turned off, so the stars were visible.”

Here, sehingga nicely shows a cause–result relationship in a neutral, slightly formal style.

How should I understand the phrase bintang di langit buatan? What is modifying what?

Breakdown:

bintang = stars
di = in / on / at
langit = sky
buatan = artificial, man-made (literally: “made”)

Structure (left to right):

langit buatan = artificial sky
- buatan acts like an adjective, but comes after the noun.
bintang di langit buatan = stars in the artificial sky
- bintang is modified by (di langit buatan).

So the phrase means: the stars that are in the artificial sky (i.e., the dome projection in a planetarium).

Why does buatan come after langit, not before like in English (“artificial sky”)?

In Indonesian, most modifiers come after the noun, unlike English.

langit buatan
- langit (noun) + buatan (modifier)
- literally “sky artificial” → artificial sky

Other examples:

baju merah — red shirt
rumah besar — big house
mobil baru — new car

So bintang di langit buatan follows the normal pattern: noun + modifier(s).

What is the function of terlihat in terlihat jelas? Why not just jelas?

terlihat comes from the prefix ter- + root lihat (see), and here it means to be visible / to appear.

terlihat = is / are visible, can be seen
jelas = clear(ly)

So:

terlihat jelas = clearly visible / can be seen clearly

If you used only jelas, it would mean “clear”, but not necessarily that it can be seen:

bintang jelas — sounds like “the stars are clear/obvious” (unnatural here)

Better options:

bintang terlihat jelas — the stars are clearly visible
bintang kelihatan jelas — same meaning, more colloquial
bintang terlihat dengan jelas — clearly visible (more explicit, a bit more formal)

Can I say kelihatan jelas instead of terlihat jelas? Is there a difference?

Yes, you can.

terlihat jelas
- Slightly more formal / neutral
- Common in writing and careful speech
kelihatan jelas
- More colloquial / everyday
- Very common in spoken Indonesian

Meaning is basically the same: clearly visible.

Examples:

Bintang di langit buatan kelihatan jelas.
Bintang di langit buatan terlihat jelas.

Both are correct; choose based on formality and style.

Why is there a comma after planetarium?

The phrase Di dalam planetarium is an introductory adverbial phrase indicating location (“Inside the planetarium”). In Indonesian writing style, it’s common (and stylistically good) to put a comma after such an introductory phrase:

Di dalam planetarium, lampu dimatikan …
= Inside the planetarium, the lights were turned off …

Without the comma, it’s still understandable, but the comma makes the structure clearer and more natural in standard written Indonesian.

Could the sentence be simplified in casual speech? How might people actually say this?

In everyday spoken Indonesian, people might make it shorter and more casual, for example:

Di planetarium, lampunya dimatiin jadi bintang-bintang di langit buatan kelihatan jelas.

Changes:

di planetarium instead of di dalam planetarium
lampunya — the lights (with -nya: “the/its”)
dimatiin — colloquial form of dimatikan
jadi instead of sehingga
bintang-bintang — explicit plural for “stars”
kelihatan instead of terlihat

The original sentence is more neutral/formal; the version above sounds more like everyday conversation.