Van Gogh painted The Starry Night in June 1889, having mentioned the morning star in a letter to his brother Theo written around the 2nd of June 1889: "This morning I saw the country from my window a long time before sunrise, with nothing but the morning star, which looked very big." The morning star (actually the planet Venus, not a star) is generally taken to be the large white one painted just left of the center of the painting.
Earlier letters of Van Gogh's also mention the stars and night sky, and his desire to paint them:
"When shall I ever get round to doing the starry sky, that picture which is always in my mind?" (Letter to Emile Bernard, c.18 June 1888)
"As for the starry sky, I keep hoping very much to paint it, and perhaps I will one of these days" (Letter to Theo van Gogh, c.26 September 1888).
"At present I absolutely want to paint a starry sky. It often seems to me that night is still more richly colored than the day; having hues of the most intense violets, blues, and greens. If only you pay attention to it you will see that certain stars are lemon-yellow, others pink or a green, blue and forget-me-not brilliance. ... it is obvious that putting little white dots on the blue-black is not enough to paint a starry sky." (Letter to Wilhelmina van Gogh, 16 September 1888)
Van Gogh Painting Project
Van Gogh and Expressionism