A Circle of Stars druid can transform into a starry form with a choice of three constellation including Dragon which helps concentration. If the druid is in Dragon constellation and then cast Polymorph on self, do the Dragon features remain?
Probably not but ask your DM
The Starry Form as a class feature is a game statistic, and is therefore removed by Polymorph, as Polymorph replaces the creature's statistics:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast.
This means that once polymorphed you for sure cannot actively use any class feature any more. The question what happens with ongoing effects from class features already activated however has no such clear and simple consensus:
The effects of a class feature by consensus will not persist, notably, for ongoing Rage, per inofficial guidance of Jeremy Crawford. But even for a feature like bladesong, there is no strong consensus on this.
There also is at least one exception, Arcane Ward, where the (inofficial) guidance states: Arcane Ward is an external effect that you activate. Its duration isn't dependent on your form., external because it is a side effect of the spell cast.
Spellcasting is also a class feature. By official ruling, ongoing spells cast are not shut down for a polymorphed creature, so they are be exempt from being counted as class features (otherwise you could not even uphold the polymorphed shape for this discussion). The official Sage Advice Compendium says:
Can you concentrate on a spell while transformed by polymorph? You can’t cast spells while you’re transformed by polymorph, but nothing in the spell prevents you from concentrating on a spell that you previously cast before being transformed.
Together these suggest that effects that come from casting spells persist under polymorph, while other, more direct effects of class features don't. But the only real support for effects of class features ending when the access to the feature ends comes from the Rage ruling of JC, and even that is inofficial.
Now, the Starry Form feature says explicitly that the form does not affect your game statistics, it is some kind of overlay effect.
While in your starry form, you retain your game statistics, but your body becomes luminous
Therefore, talk to your DM -- he might rule that it is OK to have the effect continue like that for Arcane Ward. Rules as written it probably would end.
P.S. Note that you still need to make concentration checks when taking damage in polymorphed form, or it will end, at which point your features would become available again and the starry form would return (if it has not timed out), even if you did not have it in polymorphed form.