A hypocycloid for which the diameter of the fixed circle is four times the diameter of the rolling circle.

An astroid looks like a rhombus with sides slightly curved in towards the center.

To plot astroid use the following parametric equations:
x = a * cos(Q) ^ 3
y = a * sin(Q) ^ 3

In case you're curious, the area will be equal to 3Pi*a^2/8. (You can verify that by integrating the aforemetioned equation from 0 to Pi/2 and then multiplying by 4 (it's symmetric)).

To plot in MapleV use the following command:
plot([4*cos(x)^3,4*sin(x)^3,x=0..2*Pi]);

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