Transcript Stellar Classification - Solar Physics and Space Weather

Stellar Classification
Lab 4
Classification of Stars
• Based on spectral characteristics
• This gives information about temperature
in a different way
• Absorption lines can be observed only for
a certain range of temperatures
• The range involved shows atomic energy
levels which have been populated
So it is complicated…..
• Difference in stars is not just their
chemical make up but their surface
temperature AND size
• Spectra of two stars with same
temperature but different sizes is not the
same
• Also, larger star will have higher luminosity
Spectral Types
• Spectral type of a star gives information about
temperature, luminosity, and color
• From this information, the distance, mass,
surrounding environment, and past history of the
star can be deduced
• Spectral classification is basic to evolution of
stars
• An early schema (from the 19th century) ranked
stars from A to P, which is the origin of the
currently used spectral classes.
Note!
• While these descriptions of stellar colors
are traditional in astronomy, they really
describe the light after it has been
scattered by the atmosphere
• The Sun is not in fact a yellow star, but
has essentially the color temperature of a
black body of 5780 K
Standard Classes
Temperature
O 30,000 - 60,000 K Blue stars
B 10,000 - 30,000 K Blue-white stars
A 7,500 - 10,000 K White stars
F 6,000 - 7,500 K Yellow-white stars
G 5,000 - 6,000 K Yellow stars (like the Sun)
K 3,500 - 5,000K Yellow-orange stars
M < 3,500 K
Red stars
Spectral Types
• Class O stars are very hot and very
luminous, being strongly blue in color
• These stars have prominent ionized and
neutral helium lines and only weak
hydrogen lines
• Class O stars emit most of their radiation
in ultra-violet
• Naos (in Puppis) shines with a power
close to a million times solar
Class B
• Class B stars are again extremely luminous
• Rigel (in Orion) is a prominent B class blue supergiant
• Their spectra have neutral helium and moderate
hydrogen lines
• As O and B stars are so powerful, they live for a very
short time and tend to cluster together in OB1
associations, which are associated with giant molecular
clouds
• The Orion OB1 association is an entire spiral arm of our
Galaxy and contains all the constellation of Orion.
Class A
• Class A stars are amongst the more
common naked eye stars
• Deneb in Cygnus is another star of
formidable power, while Sirius is also an A
class star, but not nearly as powerful
• As with all class A stars, they are white.
• Many white dwarfs are also A.
• They have strong hydrogen lines and also
ionized metals.
Class F
• Class F stars are still quite powerful but
they tend to be main sequence stars, such
as Fomalhaut in Pisces Australis.
• Their spectra is characterized by the
weaker hydrogen lines and ionized metals,
their color is white with a slight tinge of
yellow.
Class G
• Our Sun is of this class.
• They have even weaker hydrogen lines
than F but along with the ionized metals,
they have neutral metals.
• Supergiant stars often swing between O or
B (blue) and K or M (red).
• While they do this, they do not stay for
long in the G classification as it is an
unstable place
Class K
• Class K are orange stars which are slightly
cooler than our Sun.
• Some K stars are giants and supergiants,
such as Arcturus, while others like Alpha
Centauri B are main sequence stars.
• They have extremely weak hydrogen lines,
if at all, and mostly neutral metals.
Class M
• Class M has the most number of stars
• All red dwarfs are Class M
• More than 90% of stars are red dwarfs, such as Proxima
Centauri.
• M is also host to most giants and some supergiants such
as Antares and Betelgeuse.
• The spectrum of an M star shows lines belonging to
molecules and neutral metals but hydrogen is usually
absent.
• Titanium oxide can be strong in M stars.
• The red color is deceptive, and is due to the dimness of
the star.
• An equally hot object like a halogen lamp (3000˚ K)
which is white hot, appears red at a few km away
Other Spectral Types
• W: Up to 70,000˚K - Wolf-Rayet stars
• L: 1,500 - 2,000˚K - Stars with masses insufficient to run
the regular hydrogen fusion process (brown dwarfs).Also
contain lithium which is rapidly destroyed in hotter stars.
• T: 1,000˚K - Cooler brown dwarfs with methane in the
spectrum.
• C: Carbon stars.
– R: Formerly a class on its own representing the
carbon star equivalent of Class K stars
– N: Formerly a class on its own representing the
carbon star equivalent of Class M stars
• S: Similar to Class M stars, but with zirconium oxide
replacing the regular titanium oxide.
• D: White dwarfs
Odd Arrangement of Letters
• The reason for the odd arrangement of letters is
historical
• When people first started taking spectra of stars, they
noticed that stars had very different hydrogen spectral
lines strengths
• So they classified stars based on the strength of the
hydrogen Balmer series lines from A (strongest) to Q
(weakest)
• Then other lines of neutral and ionized species then
came into play (H&K lines of calcium, sodium D lines
etc)
• Later it was found that some of the classes were actually
duplicates and so were removed
Divisions and subdivisions
• It was only much later that it was
discovered that the strength of the
hydrogen line was connected with the
surface temperature of the star.
• These classes are further subdivided by
numbers (0-9)
• A0 denotes the hottest stars in the A class
and A9 denotes the coolest ones
• The sun is classified as G2.
Energies in Electron Volts
• Room temperature thermal energy of a molecule:
0.04 eV
• Visible light photons:
1.5-3.5 eV
• Energy for the dissociation of an NaCl molecule into Na+
and Cl- ions:
4.2 eV
• Ionization energy of atomic hydrogen:
13.6 eV
• Approximate energy of an electron striking a color
television screen:
20,000 eV
• High energy diagnostic medical x-ray photons:
200,000 eV
• I electron volt = 1 eV = 1.6x10-19 joules
Review of Basic Units
• A joule is a unit of energy.
• Four joules is the amount of energy
needed to raise the temperature of a gram
of water by 1 degree Celsius
• 4 joules ~ 1 calorie
• A calorie is also a measure of energy
• 1 calorie = 4.186 joules.
Joules and eV
• Another way of visualizing the joule is
the work required to lift a mass of
about 102 g (like a small apple) for
one meter under the earth's gravity
• One joule is also the work required to
move an electric charge of 1 coulomb
through an electrical potential
difference of 1 volt