# PHOEBE 2.2 Documentation, components and semi-major axis definitions

I'm looking at PHOEBE 2.2 Documentation, specifically at a tutorial about constraints

What are Constraints?

Constraints live in their own context of the Bundle, and many are created by default - either when you add a component or when you set the system hierarchy.

Let's look at all the existing constraints for our binary system...

and below that:

To see what all of these constraints do, see the 'Built-in Constraints' section below.

For now let's look at a single constraint by accessing a ConstraintParameter.

b['constraint']['primary']['mass']

<ConstraintParameter: {mass@primary@component} = (39.478418 * ({sma@binary@component} ** 3.000000)) / ((({period@binary@component} ** 2.000000) * (1.000000 + {q@binary@component})) * 2942.206217504418873431859537959099) (solar units) => 0.9988131358058302 solMass>

Here we see the equation used to derive the mass of the primary star from its orbit, as well as the current value

If we access the Parameter that it is constraining we can see that it is automatically kept up-to-date.

Could you explain to me the difference between the following please?

• the semi-major axis of a component about the center of mass of its parent orbit
• the semi-major axis of the parent orbit

The best would be a figure.

• It's not clear to me what the difference is that you're looking for. One thing to keep in mind is that Kepler's laws ignore the center of mass between the two bodies (barycenter), only the center of mass of the individual bodies. When you say center of mass of orbit, that suggests barycenter to me and that's not Kepler or you might mean the center of the ellipse, which is Kepler. Hyperphysics has a pretty good diagram of this stuff which might help: hyperphysics.phy-astr.gsu.edu/hbase/kepler.html#:~:text= Mar 13 '20 at 8:00
• I don't understand the question. Mar 13 '20 at 9:01