Luminosity classes
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Luminosity Classes. And Variable Stars. Luminosity Classes. The width of the absorption lines in a star’s spectrum indicates its density. The thinner the line the less the density. Supergiants & Giants are the least dense.

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Luminosity Classes

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Luminosity classes

Luminosity Classes

And

Variable Stars


Luminosity classes1

Luminosity Classes

The width of the absorption lines in a star’s spectrum indicates its density. The thinner the line the less the density.

Supergiants & Giants are the least dense.

In general the less dense a star is the more luminous it will be (because it has more surface area).

Luminosity and the thickness of the absorption lines are combined to group stars into Lumniosity Classes.

Luminosity Classes are combined with spectral class to describe Stars. The Sun is Class V so …

The Sun is a “G2 V” star.


Luminosity class chart

Luminosity Class Chart

I aBrighter Supergiants Rigel, Betelgeuse

I b Dimmer SupergiantsPolaris

II Bright GiantsMintaka

III Ordinary GiantsArcturus

IVSubgiantsAchenar

For classes I – IV luminosity and luminosity class (density) are inversely related (I’s are brightest)

V Main Sequence StarsSirius, the Sun

Main-sequence stars vary in luminosity but their density is about the same.

For main sequence stars: Mass determines luminosity


Variable stars

Variable Stars

The Luminosity of stars change over time

Some Stars change luminosity significantly and cyclically .

They get noticeably dimmer, then brighter, then dimmer again.

These are called Variable Stars.

The change in luminosity is due to a change in size.

(Though temperature changes too.)


Types of variables

Types of Variables

The period of the change determine the type:

Long Period Variables have a period of 100s of days. They are usually Giant stars.

Cepheid Variables have a period of 1 to 100 days.

Their period is related to their luminosity

so

they make good “Standard Candles”

and are used to find stellar distances… Polaris is one!


Luminosity classes

RR Lyrae Variables have periods of a day or less. They are smaller and dimmer than Cepheids

and their luminosities are all nearly the same making them good “standard candles” for finding stellar distances

Irregular Variables have inconsistent periods and are usually very young and very old stars.


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