Ellen Ruby, age 12, of Falmouth, Maine, for her question:

Is the sun really heavier than all the planets?

Our starry sun is about 700 times heavier than the rest of the Solar System. But to grasp this comparison, we must borrow a tricky word from the astrophysicist. These experts consider astronomical weights in terms of mass, or the amount of matter packed into an object.

Earthlings think of their home in the heavens as a whopping whale of a world, but compared with other celestial objects it is merely a medium sized planet belonging to a medium sized solar system. On this scale, the only whopper in our system is the starry sun at the center of ail the revolving planets. Some of the planets have no moons, others have two or more, and one has at least a dozen. The spaces between the planetary orbits are occupied by a host of swooping comets, swarms of meteors and lots of dusty debris. All these items are captives of the sun's mighty gravity and all of them should be included in our comparison with the weight of the sun.

Suppose we could perform the impossible and weigh the entire Solar System on a pair of celestial scales. Let's say that the staggering weight is 100 zillion tons. Now let's remove the whopping sun from our impossible scales. The planets and all the remaining items of the Solar System would weigh about one seventh part of one zillion tons. If we weighed the sun separately, we would find its weight to be 99   6/7 zil¬lion tons, or about 700 times more than all its planets and other captive material.

An astrophysicist, however, would consider our measuring to be a very amateur job. Astrophysicists like to use the concept of mass rather than weight in dealing with celestial bodies because the weight of an object changes with the pull of gravity while its mass, which is defined to be the amount of matter packed into its size, does not. But even astrophysicists estimate mass in tons, the same tons we use to weigh freight carloads. They estimate the earth's mass to be about 6.6 sextillion tons. This is the figure 66 followed by 20 zeros.

Using the earth's mass as one unit, we can compare it with the masses of other members of the Solar System. Mercury and Venus, Mars and perhaps Pluto have smaller masses. The other planets are greater, and the mass of giant Jupiter is more than 318 times greater than the mass of the earth. The mass of the sun is 331,950 times greater than our earth mass unit. On our impossible scales, we would need 331,950 earths to equal its weight. Or we could balance the weight of the sun with about 700 copies of the rest of the Solar System. This means that all but 1/7th of one per cent of the entire matter in our Solar System is packed inside the blazing sun.

Although the sun is one and one third million times larger than the earth, it is only one third of a million times more massive. This is because its mass, the matter packed into its volume, has only 1/4 the density of the earth's. Because it is so huge, the sun's total mass is 700 times greater than the rest of the Solar System, but if we look at the amount of matter crammed into each cubic foot, the sun is far from being the densest member of the system.