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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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.
I a Brighter Supergiants Rigel, Betelgeuse
I b Dimmer Supergiants Polaris
II Bright Giants Mintaka
III Ordinary Giants Arcturus
IV Subgiants Achenar
For classes I – IV luminosity and luminosity class (density) are inversely related (I’s are brightest)
V Main Sequence Stars Sirius, the Sun
Main-sequence stars vary in luminosity but their density is about the same.
For main sequence stars: Mass determines luminosity
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.)
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
they make good “Standard Candles”
and are used to find stellar distances… Polaris is one!
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.