Transcript Hertzsprung-Russell Diagram

Hertzsprung-Russell
Diagram
The Classification of Stars
Why Classify?
Use of spectral lines.
1880-1920
Spectral Lines
Mercury spectral lines.
Spectral Lines
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Represent electron “jumps”
Specific to each element
Used to identify elements present
Universal
Spectral Lines
Most stars have very similar spectral
lines indicating the same elements, but
the intensity of the lines vary.
The various intensities are caused by the
energy (temperature) available for the
jumps to occur.
Classification Based on Spectra
Early astronomers (1880-1920) grouped
the stars according to the intensity of the
hydrogen spectral line. The star with the
brightest hydrogen line was classified as
a group A star. The stars were grouped
according to their intensity from A to P.
Modern Atomic Theory
The evolution of the modern atomic
theory in the 1920’s allowed for an
explanation of the spectral lines. The
classification system was re-organized
to coincide with the temperature required
to produce the spectral lines.
The H-R Diagram
The H-R Diagram is a graph showing the
relationship between the surface
temperature of a star and the luminosity of
the star.
Modern Classification Order
The original letter classification system
was retained for purely historical reasons,
but the letters were rearranged. In order
of decreasing temperatures, the stellar
classification is now:
O, B, A, F, G, K, M. Or an easy way to
remember them….
Stellar Classification
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O oh
B be
A a
F fine
G girl
K kiss
M me
New Classification Requires L
Class Stars
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Officially
Bill
Always
Felt
Guilty
Kissing
Monica
Lewinsky
Stellar Classification
Each letter classification is further
subdivided into ten divisions, 0-9.
Our sun is a class G2 star, slightly hotter
than a G3, but slightly cooler than a G1.
Spectral
Class
O
Surface
Temperature
(K)
30,000
B
20,000
A
10,000
F
7,000
G
6,000
K
4,000
M
3,000
Prominent Absorption Lines
Ionized helium strong; multiply ionized
heavy elements; hydrogen faint
Neutral helium moderate; singly ionized
heavy elements; hydrogen moderate
Neutral helium very faint; singly ionized
heavy elements; hydrogen strong
Singly ionized heavy elements; neutral
metals, hydrogen moderate
Singly ionized heavy elements, neutral
metals; hydrogen relatively faint
Singly ionized heavy elements; neutral
metals strong; hydrogen faint
Neutral atoms strong; molecules
moderate; hydrogen very faint
Familiar Examples
Rigel (B8)
Vega (A0), Sirius
(A1)
Canopus (F0)
Sun (G2), Alpha
Centauri (G2)
Arcturus (K2),
Aldebaran (K5)
Betelgeuse (M2)
Barnard’s Star
(M5)
Working Independently
The Danish astronomer Ejnar Hertzsprung
and the American, Henry Norris Russell,
devised a method of graphing the stars
based on luminosity on the vertical axis
and temperature on the horizontal axis. The
temperature is read from high to low for
historical reasons.
A plot of stars
within 5 pc of
the Sun.
Diagonal lines
are constant
stellar radius.
An H—R
diagram for the
100 brightest
stars in the sky.
Such a plot is
biased in favor of
the most
luminous stars.
Absolute Magnitude
The absolute magnitude is the apparent
magnitude we would measure if the star
was located at a standard distance of
10 pc from us.
Absolute magnitude is equivalent to
luminosity.
The Main Sequence
Most stars seem to fall into a continuous
band from the lower right to the upper
left.
Cool stars tend to be faint, and hot stars
tend to be bright. This band is called the
main sequence. Stars spend the greatest
portion of their lives on the main sequence.
The Main Sequence
Surface temperatures of main sequence
stars range from 3000 K (Class M) to
over 30,000 K (Class O) This is a factor
of 10.
The Main Sequence
The stars on the main sequence have a
range in luminosity of about 100 million!
L is proportional to R^2T^4
The Main Sequence
At one end, the stars are big, hot and
bright. Due to their color and size they are
called blue giants, and the very largest are
blue supergiants.
At the other end they are small, cool and
dim and are known as red dwarfs. The sun
is right in the middle.
Spectroscopic Parallax
Knowledge of star's apparent brightness
and distance allows us to determine its
luminosity using the inverse-square law.
But we can also turn the problem
around.
Spectroscopic Parallax
If we somehow knew a star's luminosity
and then measured its apparent brightness,
the inverse-square law would give us its
distance from Earth. For a star, the trick is
to find an independent measure of the
luminosity without knowing the distance.
The H—R diagram can provide just that.
Spectroscopic Parallax
A star’s spectrum allows us to identify
it’s temperature and luminosity based
on location on the main sequence. (It has
to be assumed the main sequence is a line
for this process to work.)
Spectroscopic Parallax
If we then measure the amount of energy
hitting the earth (energy flux) we can use
the inverse square law to determine the
distance to the star.
Confidence in our measurements.
The Principle of Mediocrity
Virtually every statement made in this
presentation rests squarely on the
premise that the laws of physics, as we
know them here on Earth, apply
everywhere else too, without
modification and without exception.
Spectroscopic Parallax
The process just discussed to find the
distance to distant stars is called
spectroscopic parallax.
The Error of Our Ways
The main sequence is not really a line.
The sun fits on the main sequence when
it has a luminosity of 0.5 to 1.5 the actual
luminosity. The reason is due to the age
and composition of the star.
The Error of Our Ways
This correlates to an error in distance of
just about 25%. That is a fairly big
error, but better than no estimate at all.
Luminosity Class
But stars can be a giant or a dwarf and
have the same luminosity as a star on the
main sequence. In this case, the width of
the spectral lines is used to determine the
luminosity, and gives a quite valid answer.
Stellar Luminosity Classes
CLASS
DESCRIPTION
Ia
Bright supergiants
Ib
Supergiants
II
Bright giants
III
Giants
IV
Subgiants
V
Main-sequence stars/dwarfs
Stellar
luminosity
classes in the
H—R diagram
Fundamental Properties
Mass and composition are fundamental
properties of any star. They are set once
and for all at the time of a star's birth. They
even determine the eventual death process
of that star.
More than any
other factor,
mass determines
the star’s
characteristics.
The H-R Diagram
Like all good scientific instruments, the
H-R Diagram provides astronomers a
method for determining the age, distance
composition, size, and relative motion of
a star just by knowing how to look.