31 Variable Stars

pptx

School

Sam Houston State University *

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Course

MISC

Subject

Astronomy

Date

Oct 30, 2023

Type

pptx

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Uploaded by BrigadierFlyMaster937

Variable Stars After completing this lesson, you should be able to answer the following questions: – What are Variable stars? – What are the main types of variable stars? How do they differ? – Where on an HR diagram do we find variable stars? – Will our Sun ever evolve into a variable star? – What relationship did Henrietta Leavitt discover for variable stars? – How does this make them good distance indicators? – What series of steps must one go through to measure their distances? – For what range can their distances be reliably measured?

Pulsating Variable Stars Within our galaxy, there are single stars whose luminosities vary with time. They are called pulsating variable stars . The light from these stars varies periodically because of changes in the star’s opacity (how gas blocks the passage of light through it). If the gas in the outer part of the star has a low opacity, the star gets brighter because radiation can escape easily; this causes the gas pressure to decrease, and the star responds by shrinking its size. However, when the opacity is high, the star gets fainter because the radiation is trapped; so the gas pressure increases, and the star gets bigger. This variability occurs when stars pass through the “ instability strip ” in the HR Diagram. The pulsation period is the time between two adjacent pulses. There are two main types of pulsating variable stars: • RR Lyrae • Cepheids

Period-Luminosity Relation Henrietta Leavitt studied variable stars in a nearby galaxy called the Small Magellanic Cloud and discovered that the average luminosity of a pulsating variable star was related to its pulsation period. This is called the Period-Luminosity (or P-L) Relation . She got this result by measuring the apparent brightness of each star, then she converted it to a luminosity by assuming that the stars in that galaxy are approximately the same distance away from us. She discovered that RR Lyrae stars pulsate rapidly (more than once a day) with a constant luminosity. However, the Cepheids pulsate more slowly. Either and can be used as distance indicators . Here is the procedure that we use today: 1. Observe the light variation of the star for a long time and measure the period of pulsation. 2. Measure the average brightness, b, of the star. 3. Use the P-L relation to find the star’s luminosity, L. 4. Use b and L to calculate the distance from the Inverse Square Law. Astronomers have used this P-L relation to find the distances to other variable stars in nearby galaxies out to roughly 25 Mpc.

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Definitions  Instability strip – a portion of the Hertzsprung-Russell diagram occupied by pulsating stars  Opacity – the measure of impenetrability to electromagnetic or other kinds of radiation  Pulsating variable stars – stars whose radius alternately expands and contracts as part of their natural evolutionary ageing processes, resulting in a repetitively changing luminosity over time