Answer: Pressure and gravity
The stars are formed by the dust and accumulation of gases present in the space. It typically formed by the fusion of the gases such as hydrogen, helium in the space. The pressure of the inner core of the star and the pressure exerted by the gases present outside the star influence the shape of the star. The celestial bodies of the universe like stars, planets and others are acted upon by the gravitational attraction. This is responsible for keeping all the gaseous molecules present in the star in shape.