I was wondering this myself as "Stern" is the object of the sentence which normally requires the akkusativ.
After a little googling, I came across the concept of predicate nouns.
A predicate noun is a noun that restates or redefines the subject, which it does through the use of
'sein' (to be),
'heißen' (to be called),
'werden' (will), and occasionally
'bleiben' (to stay).
Predicate nouns always take the nominativ case.
haben - akkusativ (regular noun)
- Ich habe einen Bruder.
sein - nominativ (predicate noun)
- Ich bin ein Bruder.
That would be fine for "That one is not a star", but not for "That is not a star".
Introducing something new with "that" always uses neuter singular, regardless of the gender and number of the thing(s) you are introducing.
Using the gendered and singular/plural forms are for identifying one or some out of a group of things that you had already spoken about.
Shouldn't it be ''Das ist kein ein Stern''?
kein is a bit like a combination of nicht + ein, i.e. like "not ... a" in English (before a countable singular noun) or "not ... any" (before an uncountable noun or a plural noun). It already has the "a" / "any" meaning built into it as well as the "not" meaning.
- Ich habe kein Buch. = I do not have a book.
- Ich habe kein Wasser. = I do not have any water.
- Ich habe keine Tiere. = I do not have any animals.
There is no word stern in German -- it is Stern: capitalised, since it's a noun.
And there's no reason to use the accusative case in this sentence.
"to be" is not a transitive verb that takes a direct object ("That is not a star" can't turn into "a star is not been by that", for example).
Use the nominative case after sein (to be), e.g. Das ist kein Stern.
in simple English: "to be" has nominative case on both sides.
in a few more words: it's not a transitive verb that takes an object. (One test: "I am eating an apple" can get turned into "An apple is being eaten by me" - the object becomes the subject of a passive clause. But you can't turn "That is an apple" into "An apple is being been by that" - it makes no sense because "an apple" is not an object of "is".)
Does the negation form of 'Kein' function as not only 'not' but also 'an article "a(masculine)"'?
Yes, pretty much -- kein is more or less like nicht ein, i.e. "not a" (masculine/neuter), and keine more or less like nicht eine, i.e. "not a" (feminine).
keine is also used in the plural, where English either has no article or uses "any", so Er isst keine Kekse can translate to "He doesn't eat cookies" or to "He doesn't eat any cookies".
Similarly with uncountable singular nouns: kein then translates to "not (any)" rather than "not a", e.g. Ich trinke kein Bier "I do not drink (any) beer".