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星 contains the radical 日 meaning sun/day. This makes sense from a modern
understanding, as it is well known that the sun was a star. However, it
appears this has only been known since the 19th century, suggesting that the
connection is not so intuitive.

How did the 日 radical come to be used here? Was there an understanding that
the sun was the same/similar to a star, or was there another reason for this?
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kanji etymology history radicals
Does the etymology of 星 suggest the japanese/chinese knew the sun was a star?
No, because「星」was not the original character for the word meaning _star_.

* * *

「星」was originally written「晶」:

[![!\[enter image description
here](https://i.stack.imgur.com/uCM38.png)](https://i.stack.imgur.com/uCM38.png)

These are [oracle bone
script](https://en.wikipedia.org/wiki/Oracle_bone_script) samples, and by that
stage stars were already characterised as being **more numerous and smaller
than the sun and moon** , hence the appearance.「晶」now means
_sparkling/crystal/radiant_ , and this is a semantic extension from the
original meaning _twinkling stars_.

Later on, a sound hint「生」was added (note, the common _On'yomi_ for
both「星」and「生」, which are both **しょう** and **せい** , are identical).

[![enter image description
here](https://i.stack.imgur.com/KEptd.png)](https://i.stack.imgur.com/KEptd.png)

The number of stars depicted ranged from 2-5, but eventually settled on to 3:

[![enter image description
here](https://i.stack.imgur.com/NJVnP.png)](https://i.stack.imgur.com/NJVnP.png)

_The right hand side is the modern representation of the form on the left._

A [Warring States](https://en.wikipedia.org/wiki/Warring_States_period)
innovation simplified it to one star,

[![enter image description
here](https://i.stack.imgur.com/0dKXd.png)](https://i.stack.imgur.com/0dKXd.png)

and this is what we're left with today.

To clarify, there was a clear recognition that「星」and「日」were both celestial
bodies shining in the sky, as seen by the structural composition of「晶」,
but「星」is also the same term given to planets and comets, and the
philosophical/cultural/religious significance of the sun itself, seen in words
like「太陽」(literally _Grand[Yang](https://en.wikipedia.org/wiki/Yin_and_yang)_,
by far the more pervasive word for _sun_ in Chinese), is not accorded to「星」.

* * *

## References

  * 裘錫圭《文字學概要》
  * 季旭昇《說文新證》
  * [小學堂](http://xiaoxue.iis.sinica.edu.tw/)
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A comment rather than an answer; the assertion that "the sun was a star [...]
has only been known since the 19th century" is not maintainable. Quote
[Wikipedia on Stars](https://en.wikipedia.org/wiki/Star) (emphasis mine):

> In 1584, Giordano Bruno suggested that the stars were like the Sun, and may
> have other planets, possibly even Earth-like, in orbit around them, an idea
> that had been suggested earlier by the ancient Greek philosophers,
> Democritus and Epicurus, and by medieval Islamic cosmologists such as Fakhr
> al-Din al-Razi. By the following century, **the idea of the stars being the
> same as the Sun was reaching a consensus among astronomers.**

Also note that supernovae were a known phenomenon to the Chinese of 2000 years
ago:

> In spite of the apparent immutability of the heavens, Chinese astronomers
> were aware that new stars could appear. In 185 AD, they were the first to
> observe and write about a supernova, now known as the SN 185. The brightest
> stellar event in recorded history was the SN 1006 supernova, which was
> observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan
> and several Chinese astronomers.

This gives a wholenother time horizon to the history of concepts than the
notion of '19th century' would suggest.

I otherwise second what @drooze writes, and just want to add that the really
interesting question is, why didn't the ancient Chinese develop a simple sign
for 'star' when they did so for e.g. 云 cloud, 日 sun, 月 moon, 天 sky?—The
Egyptians certainly did have two distinct signs, viz. () sun (with rays) vs
star.—Given the plethora of simple signs for things of importance in the
historical record and the demonstrable early Chinese interest in
meteorological and celestial phenomena, shouldn't one expect a simple graph
for 'star' right next to 日 and 月?
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