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THE IB SAYS
THE IB SAYS
Understandings: Cepheid variables
Applications and skills: Describing the reason for the variation of Cepheid; Determining distance using data on Cepheid variable
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What are Cepheid variables?
What are Cepheid variables?
Cepheid variables are stars whose luminosity varies with time. The period of their variation is very closely linked to their luminosity. If we have correctly identified the type of Cepheid variable, and we measure their period, then we know their luminosity. Once we know the luminosity of an object then we can measure the brightness and, therefore, determine the distance as we have previously seen. This relationship was first determined by Henrietta Swan Leavitt in 1908. She was able to establish this relationship as the stars she initially observed are close enough for their distance and luminosity to be determined by other means.
Why do Cepheid variables vary?
Why do Cepheid variables vary?
Classical Cepheid variables (there are several varieties) are massive or supermassive stars with a mass 4 - 20 times that of the sun and up to 50 000 or more times the luminosity. This allows them to be visible at large distances from Earth - as far as other galaxies. Cepheid variables are not main sequence stars - they have moved off the main sequence and lie in a region of the Hertzsprung-Russell diagram called the instability strip.
Cepheid variables have doubly ionised helium in their outer layers. This is relatively opaque. When the star is dim and small this layer absorbs heat from the star and expands. As it expands it becomes more transparent, doesn't absorb as much heat and stops expanding. Gravity then causes it to shrink. This process repeats. The whole process is known as the kappa-mechanism.
HR diagram showing various variable star regions in the instability strip, including classical δ Cepheids. By Rursus - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4997794
By Vedran V - Own work, CC0, https://commons.wikimedia.org/w/index.php?curid=31045125
How do we determine luminosity from the period?
How do we determine luminosity from the period?
Once we have determined the appropriate type of Cepheid variable we measure the period (in days) and use an equation or graph to determine absolute magnitude and / or luminosity.
Once you know the luminosity you use the methods we discussed previously.
Resources from textbooks
Resources from textbooks
Oxford Physics: pp 653 - 654 including a page of worked examples
Hamper HL (2014): p 547 including an exercise
Relevant questions from Dot Point Physics (E. Astrophysics)
Relevant questions from Dot Point Physics (E. Astrophysics)
Page 219 - 222 at least one question discussed apparent / absolute magnitude, but others don't DO THESE QUESTIONS
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