In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000 kelvins (spectral type A). Balmer series|Role in astronomy

At 10,000 K the hydrogen is a plasma, I suppose. How can we talk about Balmer lines if the electrons are not bonded anymore to the nucleus?

The Balmer series is characterized by the electron transitioning from n ≥ 3 to n = 2, where n refers to the radial quantum number or principal quantum number of the electron. Balmer series | Overview.