Download Astrophysics

3. We know the stars by their light


Radio and microwave
Visible, IR and UV

The spectrum of a star tells us:





It’s temperature
It’s luminosity
It’s distance
It’s size
And more.
The visible spectrum of the Sun
Stellar temperatures



We have already seen that the colour of a star
enables us to find its temperature…
provided the colour is not altered as it passes
through interstellar dust!
Fortunately there are other ways to find the
temperature from the spectrum…



By examining the temperature and spectra of nearby
stars, astrophysicists found that there were other
indicators of temperature
Certain spectral lines appeared consistently at certain
temperatures and disappeared at others
Different lines appear with different degrees of
ionisation – which results from different temperatures

It became possible to classify stars by the intensity of
certain lines in the spectrum

Stellar spectra are classified according to a
system which ranks them in order of surface
temperature (the letters were re-ordered from an
older system):
hot O B A F G K M cool (L T)

Oh Be A Fine Girl (Guy) Kiss Me . . .
The Hertzsprung-Russell diagram
brings order to this mass of information




It plots stars
according to their
temperature and
luminosity
A definite pattern
emerges:
Bright stars are bluer
- that is, hotter.
90% of stars are on
the ‘Main Sequence’






If a main sequence star looks blue it must be a bright
star -- so if it looks dim it must be a very long way away
A red star is not as bright, so if it looks bright it must be
relatively close. For example:
Sirius and Alpha Centauri are similar in apparent
brightness but Sirius is bluish while A.Cent. is yellowish
So A.Cent must be
relatively close
Sirius is at 2.6 pc
A.Cent is at 1.3 pc.

Binary stars give away their mass from their period and
distance apart.





This enables us to study the relationship between
the mass and the other properties of stars
It is found that there is a simple massluminosity relationship for main sequence stars
The luminosity increases with the cube of the
mass (this is consistent with other clues about the
size, density and mass) – big is brighter (much!)
Big bright stars are burning ferociously and don’t
last long!
This tells us about the nuclear processes
occurring within stars and hints at their lifetimes.


Clusters of stars,
all born about the
same time, enable
us to study the life
cycle of stars.
For example, the
Pleiades…
The birth of a cluster

The Pleiades cluster – otherwise known as the seven
sisters – is a ‘young’ cluster.



All stars begin on the main sequence.
Brighter stars ‘die young’ and become giants
Dimmer stars are very long lived.



Some stars end their
lives spectacularly!
They implode
producing such
enormous
temperatures that the
higher elements are
formed
Which is why we are
here talking about
them!