Why might a star show evidence of containing titanium oxide?

Question

...but no evidence of hydrogen, even if it is almost entirely composed of hydrogen?

Answer 1

In astronomy graduate school, there is a full-year course called "Stellar Atmospheres".  It's about radiative transfer in the very uppermost layers of stars.  In such a course, toward the end of the year, you would learn the answer to this question.

Basically, it's because (1) there is titanium oxide in the upper layers of the star and (2) because the conditions of radiative transfer are such that hydrogen lines don't appear.  Such a star would be old, metal rich, hydrogen poor, and cool.

Answer 2

It would be an old star that has burned most of its hydrogen fuel, and titanium oxide would be one of its producs. More specifically, they are observed in M-class stars, aka red dwarves and red giants.