[Conan the Librarian]

Odd thing about this is they don't mention delta cepheid variables. All local galaxies can be measured fairly reliably by using them (they are the milepost of astronomy). Why bother with anything else?

(Or did they imply it in the part about absolute vs apparent magnitude?)

[PatrickHenry]

Odd thing about this is they don't mention delta cepheid variables. All local galaxies can be measured fairly reliably by using them (they are the milepost of astronomy). Why bother with anything else? (Or did they imply it in the part about absolute vs apparent magnitude?)

An excellent question. Upon reading it, I assumed that they had to be using Cepheids, but the article says:

They studied two of the brightest stars in M33, which are part of a binary system, meaning that the stars orbit each other. As seen from Earth, one star eclipses the other every five days.

They measured the mass of the stars, which told them how bright those stars would appear if they were nearby. But the stars actually appear dimmer because they are far away. The difference between the intrinsic brightness and the apparent brightness told them how far away the stars were -- in a single calculation.

They appear to have used a different method, starting with mass to indicate what brightness should be. I'm not up on that method, but it seems to complement the Cepheid variable method -- if you have a handy pair of binaries that reveal their mass. I need to read up on this.

[PatrickHenry]

A bit of Googling informs me that where we can observe eclipsing binaries, we can determine their mass (well, the smart guys can). Then, knowing their spectra, we presume to know how bright that kind of star is, so by observing their apparent brightness, and applying the inverse square law -- ta da! -- we know the distance. Very neat. But it seems that although binaries are common, it's not all that common to find them positioned just right so we see one eclipsing the other.