Question about cepheid variable stars....

[tenbyfifty]

Been meaning to post this for a while but kept forgetting.

My understanding of cepheid variable stars is as follows:

there's a direct relationship between the period over which the star's brightness changes and its actual luminosity

and therefore by comparing it to the apparent brightness of the star you can figure out how far away it is.

So how do you know that for 2 stars with the same period of luminosity, the same distance from Earth, that the apparent brightness is not being affected by different amounts of gas and dust in the intervening space therefore giving different values of apparent brightness and hence distance?

[lunator]

TBF

You are on the right lines

Cepheid absolute magnitude is directly related to their period of variation. So if 2 Cepheids have the same period but one is much fainter then your can assume that the fainter one is further away.

As for extinction, the professional astronomers have a pretty good idea of the level of extintion/reddening that happens and can take this into account.

Cheers

Ian

[tenbyfifty]

As for extinction, the professional astronomers have a pretty good idea of the level of extintion/reddening that happens and can take this into account.

I suppose apparent magnitude could be also measured using other wavelengths eg infra red which

penetrate dust and gas more easily?

[lunator]

TBF

I know the pros use a JHK system where they measures at 3 wavelengths. One is in the near infrared

Cheers

ian

[Astroman]

There's just a bit more going on with cepheids that allows them to be used as distance markers, or "standard candles", but TBF's and Ian's assesments are correct enough for here. Part of the relationship also includes the character of the light curve itself. The light curve changes in a known way also, depending on the mass of the cepheid as well. For example, higher mass stars rise in magnitude slower than lower mass stars, stay bright longer and decrease slower, too. This is another way they can determine the absolute brightness and therefore distance. Galactic extinction doesn't really enter into the picture for local stars much. The reason cepheids are used as distance markers is mainly to determine the distance to the galaxies that contain them, since most local distances are easily measured by paralax or spectral considerations. So, if you can see the cepheid in a distant galaxy, measure its light curve characteristics and period and all that, you can get a good idea as to its distance based on similar, local cepheids.

HTH.