Use rectangle() and your x and y centroid which you say you already have:
[rows, columns, numberOfColorChannels] = size(mask);
imshow(mask);
axis on;
hold on
% Specify half height of the rectangle.
halfHeight = 200; % Whatever you want.
% Plot rectangle in upper half.
rectangle('Position',[1, yCentroid - halfHeight, columns, 2*halfHeight], ...
'LineWidth', 3, 'EdgeColor', 'b');
% Plot star in middle
plot(xCentroid, yCentroid, 'b*', 'MarkerSize', 30)
hold off
