SHOBANA.N.S

QUEEN MARY’S COLLEGE FOR WOMEN1

What is a spectra?

What is an absorption spectrum ?

Can we see what stars are made up of ?

Can we measure how hot they are ?

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For the other two questions the answer is :

Yes we can !!! Using spectral classification of the stars.

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Surface temperature of the star is associated with specific spectral classification.

The spectral classification includes 7 main types:

O, B, A, F, G, K, M.

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This is called “Morgan-Keenan spectral classification”

The classes, listed from hottest to coldestClass Temperature Star Color

O 30,000 – 60,000 °K Blue

B 10,000 – 30,000 °K Blue

A 7,500 - 10,000 °K White

F 6,000 - 7,500 °K White (yellowish)

G 5,000 - 6,000 °K Yellow (like the Sun)

K 3,500 - 5,000 °K Orange

M 2,000 - 3,500 °K Red7

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Notice that hottest stars are blue, while coldest stars are red.

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Naos (in the constellation Puppis)

These have prominent ionized andneutral helium lines and only weakhydrogen lines.

Class O stars emit most of theirradiation in ultra-violet.

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Class B stars are again very luminous

Rigel (in the great constellation Orion) is a prominent B class blue supergiant.

Their spectra have neutral helium and moderate hydrogen lines.

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Class A stars are amongst the more common naked eye stars.

Deneb in Cygnus is another very powerful star.

Sirius, that appears the brightest star as seen from Earth, is also an A class star.

As with all class A stars, they are white. Many white dwarfs are also A. They have strong hydrogen lines and also ionized metals.

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Class F stars are still quite powerful..

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.

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Class G stars are probably the most well known for the reason that our Sun is of this class.

They have even weaker hydrogen lines than F and have neutral helium lines but along with the ionized metals, they have neutral metals.

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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 smaller.

They have extremely weak hydrogen lines, if they are present at all, and mostly neutral metals.

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Class M is the most common class by the number of stars.

All red dwarfs, such Proxima Centauri, the closest star to our Solar System

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.

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M stars may be dwarf stars or supergiant stars, and A stars can be white dwarfs or white giants as well.

This can be understood through the Hertzsprung-Russell diagram, that is very important in astrophysics and relates temperature and spectral classification of stars with their luminosity and size.

A number of other spectral types have been taken into use for rare types of stars: these are W, L, T, S, and C (that includes R and N).

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Hertzsprung-Russell diagram

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