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Notes p.56
The Expanding Universe
4. Temperature Considerations
Additional reading: Higher Physics for CfE, p.77-80.
The radiation emitted by stellar objects can give
us information about the temperature of the
objects.
When heated, at first objects emit invisible
infrared .
On further heating they glow … red
Hotter again we get … orange
And even hotter gives … yellow
Then …
then
In fact, hundreds of years ago, potters recognised
that the colour their materials glowed in the kiln
gave them an actual measurement of the
temperature of the kiln:
Dark Red
Cherry Red
Orange
Yellow
White
… 550oC
… 750oC
… 900oC
… 1000oC
… 1200oC
…
…
…
…
…
823 K
1023 K
1173 K
1273 K
1473 K
Black-Body Radiation
Very hot solids and liquids usually emit continuous
spectra of radiation as opposed to the line spectra
we detect from gases.
Intensity (Wm-2)
l
solids & liquids
gases
A surface that can emit and absorb all wavelengths
of the em spectrum is called a black-body. Its
continuous spectrum is called black-body
radiation.
A black-body spectrum can be identified by the
following shape characteristics:
1.
2.
3.
A bell shape that rises steeply, at shorter
wavelengths, to a peak intensity, then falls
off gently at longer temperatures.
As the object temperature increases, the
overall shape remains the same, but the peak
wavelength shifts to a shorter value.
As the object temperature increases, the
intensity for all emitted wavelengths,
increases.
Typical Black Body Spectra
Determine the
peak
wavelength for
each of the
temperatures
in the diagram.
Peak
Temperature wavelength
(K)
(nm)
800
3500
700
4000
650
4500
5000
580
5500
510
intensity
a)
intensity
b) Stars behave quite
like “black
bodies”.
Describe what
happens to the peak
wavelength emitted
by a star, as its
temperature
increases.
For higher temperatures, the peak wavelength
shifts to a lower value.
Categorising Stars – Additional Info
The Hertzsprung -Russell (H-R) Diagram is a
graph that plots stars color (spectral type or
surface temperature) vs. its luminosity
(intrinsic brightness or absolute magnitude).
On it, astronomers plot stars' color,
temperature, luminosity, spectral type, and
evolutionary stage. This diagram shows that
there are 3 very different types of stars:
Most stars, including the sun, are "main
sequence stars," fueled by nuclear fusion
converting hydrogen into helium. For these
stars, the hotter they are, the brighter. These
stars are in the most stable part of their
existence; this stage generally lasts for about
5 billion years.
As stars begin to die, they become giants and
supergiants (above the main sequence). These
stars have depleted their hydrogen supply and
are very old. The core contracts as the outer
layers expand. These stars will eventually
explode (becoming a planetary nebula or
supernova, depending on their mass) and then
become white dwarfs, neutron stars, or black
holes (again depending on their mass).
Smaller stars (like our Sun) eventually become
faint white dwarfs (hot, white, dim stars) that
are below the main sequence. These hot,
shrinking stars have depleted their nuclear
fuels and will eventually become cold, dark,
black dwarfs.
Big Bang Expansion
Complete Problems from Tutorial IV
The Big Bang Theory
Q. 1
Answers