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Does the mass of galaxies every change? Well, what are galaxies made of? They're made
of dark matter, dust, gas, and stars. Now the first three components do not change
their mass. But the stars do, because most stars radiate light due to the nuclear fusion
that happens in their core. So, how much mass in the stars is lost to
light? Let's go through a derivation. Let's take the sun as our average star, which
is a very good approximation. It can be shown that the sun will lose 0.34%
of its mass to light in its entire lifetime of 10 billion years.
Now, one sixth of the matter of galaxies is of the visible kind.
A tenth of this are stars. This means that roughly 0.005% of the entire mass of the galaxy,
or five parts every hundred thousand is lost to light in the entire history of the universe.
What does that mean in a tangible way? If for example you weigh one hundred and twenty
pounds you'd give a tenth of an ounce of your total mass to light, which corresponds to
a tablespoon of you!