3 syllables
"An astronaut" is the correct form, not "a astronaut." "an" is used instead of "a" whenever the word it modifies begins with a vowel.
Singular = astronaut, singular possessive = astronaut's. Plural = astronauts, plural possessive = astronauts'.
The astronaut's mass is the same on the moon but the gravitational force applied on the astronaut is weaker thus the astronaut appears to weigh less.
The astronaut's inertia is MORE on the moon.
3 syllables
The first syllable.
AS-tro-naut
Altogether is divided up into 4 syllables .
There are three syllables. As-tro-naut.
There are two syllables divided like this: pic-ture.
There are two syllables divided like this: daugh-ter.
Greeted is divided into syllables like so: greet-ed.
There are two syllables divided like so: sus-pect.
The word "hungry" is divided into two syllables: hun-gry.
There are 3 syllables. Syll-a-bles.
It has three syllables: lar-i-at.