Due to penance (tapas), the lords Sun and the Moon are engaged in the welfare of the worlds. The stars and the planets too shine by virtue of their penance.
न चास्ति तत्सुखं लोके यद्विना तपसा किल।। तपसैव सुखं सर्वमिति वेदविदो विदुः ।। ५० ।।
There is no happiness in the world that can be gained without penance. The knowers of the Vedas understand that all happiness is attained through penance alone.