how do astronomers use cephid variables to measure distances in the universe?

Answer 1

The above isn't quite right!

All Cepheids doesn't have the same absolute magnitude, or luminosity. There is however a known relation between the period the Cepheid pulsates at, and its luminosity. By measuring the period you can therefor calculate its absolute magnitude, and hence use the distance modulus m-M=5log(d)-5, where m is the apparent magnitude (measured), M is the absolute magnitude (calculated from the period of the Cepheid) and d is the distance to the Cepheid in parsec (3,26 light-years)

Answer 2

Many years ago I could read a interesting book from Isaac Asimov ("Asimov's Guide to Science") that described the method for measuring large distances in Universe.

He commented that foundation for measuring those long distances are the common concept of paralax that is "the change of angular position of two stationary points relative to each other as seen by an observer, due to the motion of an observer. Simply put, it is the apparent shift of an object against a background due to a change in observer position."

You can see it doing a simple experiment, put a finger in front of your face then, close (for example) your right eye and see the finger against a blank screen then close the left eye and see the finger with the right eye. Did you see it? It appears that finger changed its actual position. Separation between two positions can give you, using simple trigonometry, the distance from you and the finger.

The same applies when measuring long diatances from Earth to other celestial bodies(in this case, the eyes is replaced by a powerful telescope and paralax movement by the Earth movement itself), but as long as distances are larger and larger, this method is overrun and it is needed another form of measuring.

It is when cepheids enters the scene. Regarding this point I could find this information in the famous www.wikipedia.org:


"The relationship between a Cepheid variable's luminosity and variability period is quite precise, and has been used as a standard candle for almost a century. This connection was discovered in 1912 by Henrietta Swan Leavitt. She measured the brightness of hundreds of Cepheid variables and discovered a distinct period-luminosity relationship. A three-day period Cepheid has a luminosity of about 800 times the Sun. A thirty-day period Cepheid is 10,000 times as bright as the Sun. The scale has been calibrated using nearby Cepheid stars, for which the distance was already known. This high luminosity, and the precision with which their distance can be estimated, makes Cepheid stars the ideal standard candle to measure the distance of clusters and external galaxies. Of course, a small error will be present because we do not know the precise location of the Cepheid variable within the cluster or galaxy. This error is typically small enough to be irrelevant in these kinds of measurements. Because of relatively high luminosity, Cepheid stars are visible from great distances. Edwin Hubble first identified some Cepheids in the Andromeda galaxy, thus proving its extragalactic nature (not known at that time). More recently, the Hubble Space Telescope succeeded in identifying some Cepheid stars in the Virgo cluster, at a distance of 60 million light years."

Hope it helps you!

I recommend you to read Isaac Asimov's Guide to Science (first chapter)

Good luck!

Answer 3

When viewed from a particular distance, Cepheid variables are all of a similar brightness. This is known as absolute magnitude (M).

On Earth, Cepheid variables are all at different distances and so some appear fainter than others. This brightness is known as apparent magnitude (m).

The distance can be worked out using the distance modulus equation:

m - M = - 2.5 log d - 5

where d is the distance.

Or, rearranged:

d = 10^(-(m-M+5)/2.5)

So because we know how bright they really are, and we know that as you move something away from us it gets dimmer, we can work out how far away it has to be in order for it to get that dim.

Cepheid variables can only be used to work out distances to things that contain them, such as globular clusters.

Answer 4

I agree with Jens F!!!!!!!!!!!