In shortened form, a hymn/carol, often used at weddings in Germany.
There are several short versions used in church hymn books;
the complete hymn consists of seven verses, composed by Philippus Nicolai and published in 1599.
Below is a translation of the first verse, as used in J. S. Bach Cantata BWV 1.

Wir schö:n leuchtet der Morgenstern
(How Beautifully Shines the Morning Star)

How beautifully shines the morning star
full of grace and truth from the Lord,
The sweet root of Jesse!
You son of David from the line of Jacob,
My king and my bridegroom,
Have taken possession of my heart,
(You who are) lovely, friendly,
Beautiful and glorious, great and honourable,
rich in gifts,
Lofty and exalted in splendour!

Complete German text and English translation given in the Bach Cantatas Website.
http://www.bach-cantatas.com/Texts/Chorale015-Eng3.htm

Versions on youtube.

Some hymnals use the abbreviated translation of William Mercer, 1855, 1859. The musical score at www.smallchurchmusic.com.

How Bright Appears the Morning Star
1
How bright appears the morning star,
With mercy beaming from afar!
The host of Heav'n rejoices!
O righteous branch! O Jesse's Rod!
Thou Son of Man and Son of God!
We too will lift our voices
Jesus! Jesus! Holy, holy! yet most lowly!
Draw Thou near us:
Great Emmanuel! stoop and hear us!