Our_Universe_lecture_3.pdf

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Our Universe
Lecture 3
Stars
The Luminosities of Stars
Stellar distances can be determined via
parallax
– the larger the distance, the
smaller the parallax angle,
π
From geometry
D(pc) = 1 /
π
(in parsecs)
The distance
then gives
Absolute
Luminosity
L
=
l
×
r
2
(or Absolute
Magnitude)
The Temperatures of Stars
In order of temperature (hot to cool), the spectral sequence of stars is
O-B-A-F-G-K-M. The traditional mnemonic is
Oh
Be
A
Fine
Girl
Kiss
Me.
(Recently, types
L
and
T
have been added to the cool end.)
The H-R Diagram
Some stars are not on
the main sequence.
Some are very cool,
but also very bright.
Since cool objects
don’t emit much light,
these stars must be
huge. They are
red
giants.
Some stars are faint,
but very hot. These
must therefore be very
small – they are
white
dwarf
stars.
The Sizes of Stars
The sizes of stars can be
anywhere from 0.01
R
☼
to
1000
R
☼
!

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