Classification of stars AQA unit 5

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Astrophysics Notes
Classification of stars
Classification by luminosity
Relation between brightness and apparent magnitude.
Sun luminosity = 3.85 × 10 26 Watts.
Most luminous stars in the galaxy are over a million times more luminous than the sun.
Apparent brightness is different to luminosity.
Luminosity
Total energy in form of EM radiation emitted per second.
Brightness
How bright a star appears in the sky.
The brightness of a star depends on two things
Its luminosity and its distance away from us
Brightest stars will be close and have a high luminosity
Stars can be classified by luminosity
120BC
Greek astronomer = Hipparchus
Produced a catalogue of 1000 stars and relative brightness
o 1 = brightest stars
o 6 = dimmest = can be seen with a naked eye on a good night.
Known as the apparent mag scale
o Brightness of star from earth
Apparent Magnitude
Relation between luminosity and apparent magnitude
CCD and higher resolved photographs allow more quantative approach to Apparent mag.
Branch of astrology = "Photonomy"
1700's
o William Herschel
1st mag star delivers 100 times more light to earth than a sixth mag star
1856
o Norman Pogson
Produced quantative scale of apparent mag
With difference of 5 magnitudes a ratio of 100 in the light intensity was
received.
Scale is logarithmic.
An equal interval in brightness is actually equal ratios of light
intensity received.
Increase of 1 in apparent mag corresponds with an increases in
intensity received by 2.51, i.e the 5th of a root of 100.
o Thus a 4th mag star is 2.51 times brighter than a 5th mag star,
but 6.31 times brighter than a 6th mag star.

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The apparent mag, M, is related to intensity, I, by the following
equation.
The intensity, I, of an abject that we observe is the power received
from it per unit on earth
o Effective brightness of an object.
Mag difference between stars measure the relative brightness of
the stars.
With the faintest stars having an apparent mag of 6, this scale allows
the brightest stars to have negative apparent mag, the brightest
star having an apparent mag of -26.…read more

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The luminosity of a star is the total energy emited per second and is related to the
temp of the star and its surface area.
o This law tells us that, if two stars have the same black body temp (same spectral
class), the star with the brighter apparent mag has a larger diameter.
Consider two stars of black body temps T1 and T2 .…read more

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Will produce more of light at the blue/violet end of the spectrum.
Shows how much energy a black body radiates at different wavelengths, which wavelengths
it radiates with most intensity and total amount of energy radiated.
All black body curves have same shape
Hotter stars emit more energy at all wavelengths and the peak shifts to shorter wavelengths
Use of wien's displacement law to estimate black body temp of sources.…read more

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If the energy has been emitted from a point or sphere then it obeys the inverse
square law.
Use of Stephan's law to estimate area needed for sources to have same power output as the sun.
o The star Sirius B has a surface area of 4.1×1013 m2 and produces a black body
spectrum with a peak wavelength of 115nm. The intensity of the light from Sirius B
when it reaches Earth is 1.12 × 10-11Wm-2.…read more

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Description of main classes:
Lines in emission and absorption spectra occur because electrons in an atom can only exist in
certain, well defined, energy levels.
o in atomic hydrogen, the electron is usually in ground state (n=1), but there are lots of
energy levels (n=2 to n=infinity. ­ called excitation levels.) that the electron could
exist in if the electron was given enough energy.
The wavelengths corresponding to the visual bit of hydrogen's spectrum are caused by
electrons moving to higher energy levels (n=2).…read more

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B Blue 11000 He, H Rigel, `be'
-2500 spica
0
A Blue-whi 7500- H Sirius, `a'
te 11000 (stongest), Vega
ionised
metals
F White 6000- Ionised Conopus, `fine'
7500 metals procyon
G Yellow 5000- Ionised Capella, `girl'
white 6000 and the sun
neutral
metals
K Orange 3500- Neutral Arcturus, `kiss'
5000 metals Aldebaron
M Red <3500 Neutral Betelgeus `me'
atoms, TiO e, Antares…read more

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Special types
o R= similar to K stars
o N = spectra similar to M
o S = similar to M stars.
Temp related to absorption spectra limited to hydrogen Balmer absorption lines: need for atoms to
be in n=2 state.
For a hydrogen absorption line to occur in the visible part og a stars spectrum, electrons in
the hydrogen already need to be in the n=2 state.
o Happed at high temps.…read more

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Reveals presence of 60 chem elements in the sun.
The hertzburg-Russell diagram
General shape: main sequence, drawfs and giants.
Axis scales range from -15 to 10 (absolute mag) and 50000 K to 2500 K (temp) or O B A F G K M (spec
class)
Stellar evolution: path of a star similar to our sun on the H-R diagram from formation to white drawf.…read more

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Main sequence:
o Stable phase
Hydrogen fusing into helium
o 90% of stars in main sequence.
Other 10% either giants or dwarfs.
Red giants:
o Moved off main sequence
o Fusion reactions occurring including hydrogen to helium
o High luminosity
o Low surface temp
Told this by Stephan's law
White dwarf:
o Stars at end of lives
o Fusion reactions have stopped and they are cooling down.…read more

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