Download Slide 1 - Beverley High School

(i) What, in the atmosphere, is responsible for absorbing infrared radiation?
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(ii) The spectrum of light from a star can be used to determine its temperature.
Explain why
this absorption can lead to errors in the value.
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(i) water vapour 1 (or carbon dioxide)
(ii) longer wavelengths absorbed 1
shifts peak of graph to shorter wavelengths 1
star appears hotter [or reference to appropriate equation] 1
(iii) What is responsible for [ultra Violet] absorption?
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(iv) What effect can this absorption have on the measured temperature of a
star? Explain your
answer.
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(iii) ozone !
(iv) lower temperature, hits peak (λmax) to longer wavelengths !
λmaxT = constant !
From the Specification
PRINCIPLES OF THE USE OF STELLAR SPECTRAL CLASSES
Description of the main classes, O B A F G K M
Temperature required: need for excitation
Helium absorption (O): need for higher temperature
Hydrogen Balmer absorption lines (B, A): need for atoms in 2 = n state
Metals absorption (F, G): occurs at lower temperature
Molecular bands (K, M): occur at lowest temperature
Absolute
Magnitude
When absolute magnitude is plotted
against spectral class most stars are
found to lie in a narrow band. These are
called the main sequence stars
Very hot large
blue stars
-10
-5
0
+5
Very cool
dim red
stars
+10
+15
O
B
A
F
Spectral type
G
K
M
Here the diagram is replotted showing the power output of the
star against temperature
Mass of a star and the HR diagram
• The position of a star on the main
sequence depends entirely on its mass.
Low mass stars are found at the left and
high mass stars at the right.
• The mass of a star controls its radius, its
temperature, its life expectancy and its
eventual fate.
Points to note on HR diagram
• At the top left of the main sequence the stars have very
large radius and high surface temperatures. This is what
makes them highly luminous.
• They are using up their hydrogen fuel tremendously
quickly. So they are extremely short lived (c10 000 000
years)
• These high mass stars finish their lives in massive
supernova explosions
• At the bottom right the stars are cool. These low mass
stars are very long lived as they use their fuel so slowly.
Very low mass M stars live many billions of years and
will simply run out of fuel without dramatic events.
Surface temperature K
40 000
10 000
7 000
5 000
3 000
Red giant
stars
Red giants have large
radius and therefore are
very luminous ( therefore
have high absolute
magnitude). However they
are much cooler than would
be expected for stars of
large radius.
White dwarf
stars
White dwarfs are high
temperature stars but
their small radius (some
around the size of Earth)
is abnormal for such high
temperatures
Surface temperature K
40 000
10 000
7 000
5 000
3 000
Red giant
stars
These classes of stars
off the main sequence
have finished burning
hydrogen as their fuel.
Red giants are “using
helium and later
heavier atoms as fuels.
Stars here are
reaching the end of
their lives
White dwarf
stars
White dwarf stars are
“remnants” They are
essentially the central
cores of what were main
sequence stars like the
Sun
Red giants
• Stars of around solar mass will become
red giants
• Red giants are stars which are excessively
luminous for their temperature.
• This is because they have large radius.
• They are stars that have ceased burning
hydrogen as a fuel.
• Their outer layers have expanded and
cooled.
White dwarfs
• Red giants are unable to hold onto their
outer layers
• This material is lost into outer space
• The dense core of the dead star is
exposed. This is a WHITE dwarf star.
Absolute
Magnitude
Very hot large
blue stars
-10
The Fate of our Sun
Red giants
-5
0
Sun today
+5
White
dwarfs
+10
Very cool
dim red
stars
+15
O
B
A
F
Spectral type
G
K
M