Q&A: Our Solar System

Q:
I have read that gravitational attraction between objects is dependent on the masses of the objects. With the Sun losing mass all the time due to radiation, is it possible that one day Earth will escape the Sun's gravitational pull?

A:
You are right, the Sun is losing mass via radiation, although compared to the enormous mass of the Sun, the amount lost is quite small. Which means it would take a very long time for the Sun to lose enough mass so that the Earth would not be held in gravitational orbit.

Long before this time, the Sun will run out of nuclear fuel. The Sun burns hydrogen (its fuel), fusing it into helium. This process releases heat and light and supports the Sun against gravitational collapse. When the hydrogen is almost used up, the core is mainly made of helium, and a shell of burning hydrogen moves outward towards the surface of the Sun. The core begins to collapse under gravitational pressure while the outer regions are pushed outward by the burning hydrogen shell. This is called the Red Giant phase and we believe it will be another 5 billion years before the Sun reaches this stage. The Sun will be brighter and cooler and will expand outward, likely engulfing Mercury, Venus, and Earth. Hopefully we will be somewhere else by this time!

Stars with cores of at least half the mass of the Sun will eventually become hot enough (as they collapse) to ignite the helium. Once the helium runs out, another Red Giant phase begins. After this phase the Sun will collapse again but never have enough gravitational pressure to ignite the core. It will then end its life as a small white dwarf star, slowly cooling off for the rest of its life. Other more massive stars can go through this burning expanding then collapsing cycle many times, but our Sun will stop after helium burning.

For an x-ray image of the Sun (NOT taken by Chandra, the Sun is too bright in x-rays for Chandra's sensitive mirrors) please see:
http://chandra.harvard.edu/xray_sources/solar_system.html
which includes Chandra images of other solar system objects.