If the force of gravity were reduced 50 percent, how would this change stars and the universe?

This is a complicated question because so many physical factors that describe stars and the universe are intertwined and rely on the value of 'G'.

The universe would be a different place because, all other things being equal, cosmic expansion would be faster which means far less helium produced when the universe was hotter and denser so the present universe would have started out with nearly pure hydrogen. There might even be less luminous matter produced during the Big Bang so stars and galaxies would be a bit rarer than today.

Stars would get much larger in mass given the speeds of the particles that they contain. The core energy of these stars would be lower because there would be less gravitational potential energy per gram of mass to cause central temperatures to become very large. The stars would be more distended because radiation pressure would support the overlying stellar mass to much larger distances.

For some systems such as planets orbiting stars, or stars orbiting each other, the objects can become unbound and the parts can actually fly away because at a given distance, the orbital speed of the objects would now exceed the escape velocity for the system.

The Earth orbits the Sun at a speed that is just under the escape velocity from the Sun at this distance by a factor of about 40%. If the force of gravity was halved, its speed would be exactly the escape speed. In fact, any body orbiting in a circular orbit would become unbound if the force of gravity was reduced by a factor of two!

Return to Dr. Odenwald's Gravity page at the Astronomy Cafe Blog.