The Distance Ladder

To calibrate this to make sure this really works you need some cephids whose distance you can measure using parallax so there's a few stars where it overlaps or few cephids that are close enough. And we can measure their distance using their period and we see that the two things agree which is why it's called the distance ladder because we have like a little bit of overlap. Then we assume the cephids and like other galaxies operate the same way and that way we can measure the distance to other galaxies where parallax doesn't work.