Transcript Luminosity Classes

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.
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
Ia
Brighter Supergiants
Rigel, Betelgeuse
Ib
Dimmer Supergiants
Polaris
II
Bright Giants
Mintaka
III
Ordinary Giants
Arcturus
IV
Subgiants
Achenar
V Main Sequence Stars
Sirius, the Sun
For the first 4 classes luminosity and luminosity class
(density) are directly related
For main-sequence stars vary in luminosity but their density is
about the same.
For main sequence stars: Mass determines luminosity
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
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!
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.