Question

The “equilibrium” in hydrostatic equilibruim can be understood by imagining what would happen if we could somehow squeeze the Sun (add an outer layer to increase mass). Choose the correct sequence of events if we could right now add mass to the Sun: a) The core would heat up, leading to higher nuclear reaction rates, higher core pressures, and expansion of the core. b) The Sun would shrink to become a brown dwarf, with higher nuclear reations rates and lower surface temperature. c) The carbon inside the Sun would ignite, leading to rapid collapse of the core and then a supernova explosion. d) The dust inside the Sun would start appearing at the Sun’s surface, preventing light from escaping to create a much larger hotter version of the Sun. e) The size and temperature and nuclear reaction rates in the Sun would stay exactly the same.

Answer 1

A star is formed when a large amount of gas (mostly hydrogen) starts to collapse in on itself due to its gravitational attraction. As it contracts the atoms of the gas collide with each other more and more frequently and at greater and greater speeds the gas heats up. Eventually, the gas will be so hot that when the hydrogen atoms collide they no longer bounce off each other, but instead coalesce to form helium. The heat released in this reaction, which is like a controlled hydrogen bomb explosion, is what makes the star shine. This additional heat also increases the pressure of the gas until it is sufficient to balance the gravitational attraction,and the gas stops contracting. Stars will remain stable like this for a long time, with heat from the nuclear reactions balancing the gravitational attraction. Eventually, however, the star will run out of its hydrogen and other nuclear fuels. Paradoxically, the more fuel a star starts off with, the sooner it runs out. This is because the more massive the star is, the hotter it needs to be to balance its gravitational attraction. And the hotter it is, the faster it will use up its fuel. Our sun has probably got enough fuel for another five thousand million years or so, but more massive stars can use up their fuel in as little as one hundred million years, much less than the age of the universe. When a star runs out of fuel, it starts to cool off and so to contract.

So the correct sequence of events if we could right now add mass to the Sun: a) The core would heat up, leading to higher nuclear reaction rates, higher core pressures, and expansion of the core.