M - M as the distance modulus, and a question about the distance ladder

[CPW]

TL;DR

I'm attempting to understand fully the distance ladder we use in astronomy to determine the distance to stars that are too far away for parallax to work.

I'm attempting to understand fully the distance ladder we use in astronomy to determine the distance to stars that are too far away for parallax to work. I understand we calibrate to a standard candle data of period vs luminosity for the cepheid variable stars in a group. Then from knowing the absolute magnitude M we can use the distance modulus equation involving the term m - M to figure out the absolute distance.

But am I understanding this correctly?
m = apparent magnitude as it the brightness of the star as it appears with the telescope
M = absolute magnitude, which is determined by... What?

Ok, that's the gap in my knowledge about this subject.
How do we know absolute brightness of a distant star?
And how does the period versus luminosity graph determine absolute magnitude? (Isn't that luminosity a relative measurement?)

Please forgive the nativity of my question.

 

[Vanadium 50]

M = absolute magnitude, which is determined by... What?

I understand we calibrate to a standard candle data of period vs luminosity for the cepheid variable stars in a group.

And there you go.

 

[lomidrevo]

Absolute magnitude is a measure of the luminosity, so there is a relationship between them, and you can easily compute one from the other. You just need to supply some star as a reference. For example, if you know luminosity and apparent magnitude of Sun, you can compute the apparent magnitude of any star (knowing its luminosity):
$M = M_{Sun} - 2.5 \log_{10} \left( \frac{L}{L_{Sun}} \right)$
See here for more details.

It is useful to realize that absolute magnitude (and luminosity) are somehow intrinsic properties of a star, whereas apparent magnitude (and radiant flux) depends on the distance between the star and the observer.

 

[CPW]

Thank you for the explanation.

 

[sophiecentaur]

There is a similar method which involves supernovae in distant galaxies, The time profile relates to the absolute magnitude and 'off you go' again.

 

[phyzguy]

Absoulte magnitude is defined to be the apparent magnitude that the star would have if it were exactly 10 parsecs away. As others have said, it is an intrinsic property of the star. To know the absolute magnitude of a distant star, we need to know how far away it is by some method. Did your questions get answered?

 

[CPW]

Hi phyzguy.

Yes. The gap in my knowledge about this topic is no longer there. I think I just needed to settle on the fact that the period - luminosity relationship of cephiad variable stars was a discovery. A revealing of a fact of nature, discovered by Henrietta Leavitt.

The comments and references the PF members sent to me were helpful too.
Understanding the idea of absolute vs relative measurements in my own work (radiation dosimetry) was helpful for me in understanding what astronomers do in establishing an absolute standard. And in agreeing on a definition.

This website was also helpful to me:
https://www.atnf.csiro.au/outreach/...photometry_magnitude.html#magnmagcalculations

So, luminosity and absolute mangnitude are the same thing.
So, the distance modulus equation and the standard candle of the cephiad variables are well established with scientific rigor. I never really doubted that.
And, if I had to explain this concept to my kids, I think now that I could.

Thanks!