View Full Version here: : ITN: Cepheid Pulsation Theory Confirmed!

25-11-2010, 06:59 AM
In the News:

Pulsating star mystery solved (http://www.physorg.com/news/2010-11-pulsating-star-mystery.html)

This is terrific news ! Provided they're reporting the end results, that is. ;)
A classic example of Astronomical measurements 'truing up' theories and adding weight to one of them !!
It also improves the precision of cosmic distances by a huge factor!

Real progress !! I'm thrilled, if it is accepted !


28-11-2010, 10:34 AM
Thanks, Craig

Very interesting. The name of the system OGLE-LMC-CEP0227 had me intrigued me. Lechery in astronomy? Turns out the system was detected as part of the Optical Gravitational Lensing Experiment (OGLE).

I assume they mean the mass has been calculated to an accuracy of 1%. I'm intrigued as to how they do this. Being in a binary system, the period of orbit would depend on the primary's mass but then they would have to know that as well.
I know that Cepheids are pulsating stars used as standard candles for measuring galactic and extragalactic distances. So we know the distance. A Cepheid's period of pulsation is connected to its luminosity. I wonder if the luminosity is used to determine mass as well?

Regards, Rob

28-11-2010, 11:19 AM
Hi Rob;

Interesting. I'm not sure, but it looks like they've used spectroscopic and photometric data modelling to calculate the masses of the Primary and Secondary.

I don't have access to their paper, but the Figures and Tables Index is here (http://www.nature.com/nature/journal/v468/n7323/fig_tab/nature09598_ft.html).

Table (http://www.nature.com/nature/journal/v468/n7323/fig_tab/nature09598_T1.html)1 gives the results.

From the abstract (http://www.nature.com/nature/journal/v468/n7323/pdf/nature09598.pdf), I'd say that they've confirmed the primary as a cepheid, used the standard candle method of accurately determining the mass of the primary. The eclipse with the secondary seems to then have been used to check the accuracies of the respective models.

Not sure though. Interesting, none-the-less.


28-11-2010, 12:16 PM
Yes, the article says that the smaller component is the Cepheid. From your Table 1, they are the same mass so they must be basing it on radius, which means the primary, which is of smaller radius but greater luminosity, is the Cepheid.

Regards, Rob

28-11-2010, 12:27 PM
I should confess that I think I misnamed this thread.

The actual news is that they've confirmed that the stellar pulsation theory predictions are spot on, whilst the prediction from stellar evolution theory is at odds with the new observations.

The more accurate mass measurement, I think comes about by applying precise sensing technologies to a newly discover binary Cepheid system (the latter of which, was a new discovery).

How about "Cepheid Pulsation Theory Confirmed" as a better title ?


28-11-2010, 12:40 PM
Craig, sounds good to me!


28-11-2010, 02:25 PM
For the mathematically minded ...
The total mass of the system M = 4*pi^2*r^3 / (G*T^2)
For the derivation look here ... http://outreach.atnf.csiro.au/education/senior/astrophysics/binary_mass.html

From Table 1 (see Craig's Post 3 below),
T = 309.673 days = 309.673*24*3600 s
r = 389.4 * radius of Sun = 389.4*6.96*10^8 metres
G = gravitational constant = 6.673*10^-11 m^3/kg/s^2
Mass of Sun = 1.989*10^30 kg

Substituting, gives M = 1.64488*10^31 kg ~ 8.27 * Sun.
Data indicates the two star's masses are equal,
so each star has mass 8.27/2 ~ 4.14 * Sun

Regards, Rob

28-11-2010, 04:45 PM
Just trying to figure out what you just did there, Rob.

Looks like the formula you quoted is derived from Keplerian & Newtonian Mechanics (which is cool).

I notice from your link:

"To date (August 2004) only one single star other than our Sun has had its mass accurately determined by a means unrelated to Kepler's laws. "

(They did it be lensing observations and parallax measurements by the HST).

Noteworthy, and a real eye-opener here, is that with all the sophisticated theories we have kicking around this business, we're only just pinning down the fundamental parameters - like mass !

The real ramification of this recent observation/derivation is that a cornerstone of everything, ie: the Stellar Evolution Theory, is in error by up to a whopping 20% when it comes to a fundamental parameter like stellar mass!!!

From my original post article:

This is truly an outstanding step forward !! I'm really chuffed about this !!
One can only wonder what the flow-through effect will be.

There is so much dependent upon Stellar Evolution Theory being accurate!
From Wiki:

Simply awesome !!

28-11-2010, 05:15 PM
We owe it all to a really smart woman Henrietta Leavitt who first worked out the luminosity period relationship of Cepheid variables. It was her work that inspired Hubble.

See here or Google her.


If she had not died prematurely she would have got the Nobel Prize in spite of the way she was treated because she was a woman.


29-11-2010, 08:39 AM
And of course that calculation has not taken into account the mass from any orbiting planets in the system :)

I still find it intriguing that people still quote such high precisions in their measurements solely based on theoretically modelled comparisons with our own solar system.....

29-11-2010, 09:15 AM
Interesting point there David.

I think all the mass, resulting in the eccentric orbits, is assumed to be concentrated in, and distributed between, the binary pairs.

And yet, wobble in the stars is used to deduce the presence of exoplanets, eh ?

Hmmm .. interesting good point.

Perhaps exoplanet mass could be assumed to be in the 1% error, which is still unaccounted for (?)


29-11-2010, 10:07 AM
Hi guys,

I would think that any planet(s) would be rather insignificant in the comparative mass and orbital calculations of the two stars. A Jupiter size planet 1/1000 the mass of our Sun would be about 1/8000 the mass of this system or 0.0125%. The masses for each star were quoted to an accuracy of 1.2% and 1.7%.

Regards, Rob

29-11-2010, 10:29 AM
Yes Rob.

This system, I think was discovered in the LMC (??).

I can see it may be possible for significant mass to exist elsewhere within the gravity influenced realm of the system. Not necessarily in orbiting exoplanets, either.

Mind you, this conversation is getting very 'speculative', now.


29-11-2010, 11:46 AM
It is all about perturbation theory. Two relatively massive stars orbited by planets can be considered as a two body problem. If there is a Fourier term due to planets on the Doppler measurements it would be a bonus and also put a lower limit on the accuracy of these measurements.
There was a time when speculations such as these only occurred in the hallowed halls of higher learning only by the senior students and teaching staff not interested amateurs.
Even the police are getting in on the act of using Doppler measurements as they ping you for doing 63km/hr in a 60km/hr.


29-11-2010, 01:24 PM

I thought that was a great cue for comment from some of our female members. I did wait.
Thanks for bringing that to our attention. Apparently 2008 was the 100th year anniversary of Leavitt's Law.
Progress on understanding our Universe seems awfully slow.

Regards, Rob