Stellar classification: what we can learn from stars
This video deals with stellar classification.
Before going into the merits, however, it is good to say something about spectroscopy,
as it is the basis of the same stellar classification.
Spectroscopy is a very powerful method of investigation of the structure of matter which
is based on the analysis of the decomposition of the light emitted by it into its fundamental
wavelengths, that is, on the analysis of the electromagnetic spectrum.
In astrophysics it is essential to obtain information on the chemical composition and
physical conditions of stars.
But what is the electromagnetic spectrum?
It is the set of different types of electromagnetic radiation, each with a very specific wavelength,
frequency and energy.
It consists of: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma
rays.
Radio waves are electromagnetic radiation characterized by large wavelengths and low
frequencies, while on the opposite side of the spectrum we find gamma rays with very
small wavelengths and very high frequency.
Our eyes are sensitive to only a small part of the electromagnetic spectrum: the visible
band, characterized by wavelengths between 400 nanometers (blue / violet light) and 700
nanometers (red light).
To obtain a spectrum of a star, the basic idea is to pass the star light through a dispersive
element such as a prism or a diffraction grating.
In this way the incident light beam is broken down into component wavelengths, ranging from
those corresponding to radio waves to those corresponding to gamma rays.
For the purposes of stellar classification, the part of the spectrum that interests us
most is the one that falls in the visible band.
The spectrum is closely associated with the way in which the intensity of the radiation
emitted by a source depends on its wavelength.
This relationship between the intensity of the emitted radiation and its wavelength can
be studied by assuming that the star is comparable to a black body: that is, an ideal body capable
of absorbing all the electromagnetic radiation, both in the visible and in the other regions
of the spectrum.
The term black body derives from the circumstance that, when illuminated with light of any wavelength,
it absorbs it all.
By plotting the intensity of the emitted radiation as a function of the wavelength, we obtain
a series of bell curves, known as Planck curves, each of which represents the continuous spectrum
of the star.
The peaks of these curves are not coincident, but fall in a specific region characterized
by a certain wavelength.
In practice, each peak corresponds to a very specific set of wavelengths at which the star
emits.
For example, for the Sun the Planck curve has a peak in the yellow - green part of the
spectrum: this means that most of the Sun's emission falls into yellow, and that is why
the sun appears to us in this color.
If, on the other hand, we were to derive the Planck curve for a star like Rigel, its peak
would be shifted towards blue: it follows that the peak of the emission for Rigel falls
into the blue.
A similar argument can be made for any star.
But since the stars are surrounded by a colder atmosphere, the continuous spectrum will "lack"
some colors, corresponding to as many wavelengths: these wavelengths are selectively absorbed
by the same stellar atmosphere, producing an absorption spectrum that is superimposes
on the continuous spectrum.
We can somewhat imagine an absorption spectrum as a sort of barcode, in which each dark line
corresponds to a certain wavelength of the continuous spectrum that is absorbed by the
chemical elements present in the star's atmosphere.
Since each chemical element present in the stellar atmosphere is different, it will absorb
a particular wavelength.
It then becomes possible to chemically analyze the star's atmosphere.
The profile and intensity of the lines also make us aware of the relative abundance of
the elements present in the source, while the displacement of their wavelengths provides
information on the motion of the source itself with respect to the observer through the study
of the effect.
Doppler.
Based on these considerations, between 1862 and 1868 the Italian Father Angelo Secchi
drew up a first stellar classification, based on the spectra of more than 4000 stars:
• I - almost continuous spectrum with few lines, the most intense of which are due to
hydrogen (0 stars the lines of the Balmer series of hydrogen show their maximum.
The K line of ionized calcium grows over the whole class.
• II - spectrum similar to the solar one, characterized, next to lines weaker than hydrogen,
by a large quantity of metallic lines (sodium, calcium, iron) and by many other thin ones
(yellow or orange stars such as Sun, Capella);
• III - spectrum with many thin lines and also with very wide and shaded lines called
"bands", with traces of metals and hydrocarbons (orange or red stars such as Betelgeuse, Antares,
Mira Ceti);
• IV - spectrum with bands due to carbon, but without metals.
Secchi's classification, initially welcomed with great favor, was then subjected to revision,
but it was also a very important initial step, since it was the first to highlight that the
behavior of the spectral lines (number, position and intensity) depends on the color of the
star.
Over time, spectroscopes were increasingly improved and by the end of the nineteenth
century the details visible in stellar spectra were so rich that they required an adequate
classification for new discoveries.
The observatory of Harvard College at that time became the center of a monumental classification
activity which, by its director, Edward Pickering, and his collaborators, led in 1890 to a new
complex classification into 16 classes, distinguished from as many letters of the alphabet.
Later (1901) one of Pickering's collaborators, Annie Cannon, revisited and considerably simplified
the classification previously carried out bringing it to the familiar and still (albeit
with some modifications) adopted 7-letter classification, OBAFGKM.
Famous is the nursery rhyme invented to remember the order of letters and classes (Oh Be A
Fine Girl Kiss Me).
And what rhyme would you invent to remember the order of the spectral classes?
Write it in the comments!
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Miss Cannon's classification was a true masterpiece of simplicity and efficiency, as the order
of the classes was at the same time a chromatic order, that is, of temperature, and of complexity
of the spectra and lines.
It then passes from white, blue and white stars of very high temperature and relatively
simple spectra, with few lines, to yellow and then red stars, with a lower surface temperature
and more complex spectra, with many lines or with molecular absorption bands.
To better divide each single class, additional numbers have been introduced, from 0 to 9,
which divide each class into tenths.
For example, an A5 star indicates a star halfway between an A0 and an F0, while an A2 indicates
an A-class star that is two tenths cooler than an A0.
In fact, lower numbers indicate hottest stars within a given class.
The gigantic work culminated in the magnificent "Henry Draper Catalog" published between 1918
and 1924 by Pickering and Cannon which showed the classification made by the latter out
of 225,300 stars.
Here is the scheme: • O - presence of strong absorption lines
due to ionized helium.
There are absorption lines of ionized helium and hydrogen.
The stars belonging to this class have surface temperatures between 28000 and 35000 kelvins
(remember that 0 kelvins correspond to -273.15 ° C).
• B - hydrogen lines grow.
The three helium lines are at their maximum intensity in B2, they disappear completely
in B9.
The lines of ionized silicon, oxygen and magnesium are visible.
Surface temperatures range from 10,000 Kelvin to 28,000 Kelvin.
• A - in class A0 the lines of the Balmer series of hydrogen show their maximum.
The K line of ionized calcium grows over the whole class.
Surface temperatures are between 7500 Kelvin and 10000 Kelvin.
• F - hydrogen lines decrease, ionized calcium lines increase.
The first lines of metals appear.
Stars of this class have surface temperatures between 6000 Kelvin and 7500 Kelvin.
• G - is the class to which the Sun belongs (G2).
The lines of hydrogen are further decreasing.
The K line of the stock is strengthened.
Present numerous lines of neutral metals.
Surface temperatures range from 4900 Kelvin to 6000 Kelvin.
• K - the lines of metals grow while those of hydrogen disappear.
Surface temperatures are between 3500 Kelvin and 4900 Kelvin.
• M - corresponding to group III of Secchi.
The titanium oxide bands, faded towards red, grow for the whole class.
Surface temperatures are less than 3500 Kelvin.
Would you be able to indicate the name of a star belonging to each of the spectral classes?
Write it in the comments!
Later further modifications were made, and a parallel classification was added to the
main one.
Among the additions we remember the class S with characteristics similar to M and K
and with an abundance of zirconium oxide, the classes R and N, including stars with
a high carbon content, and the W group which some consider a variant of the O class, but
it creates a real class of its own: the very hot Wolf – Rayet stars belong to it, anfd
their spectra have large emission lines.
Here are the salient features of these additional classes:
• W - Wolf-Rayet stars characterized by wide bands of hydrogen and helium.
There are two subclasses, WC and Wn, which show evidence of carbon and nitrogen respectively.
• R - intermediate class between G and N. • N - stars corresponding to Secchi group
IV, with spectrum characterized by the bands of carbon compounds.
• S - characterized by the zirconium oxide bands.
However, this classification only considers the temperature of a star and says nothing
about their brightness or if you want their size or, even better, the width of certain
spectral lines.
For this reason, a parallel classification is also used.
In it, Roman numbers from I to V indicate, with the same spectral class, the brightness
and dimensions of the star.
Class I is also known as the "supergiants", very bright.
To class II belong luminous giants, to III giants, to IV sub-giants and to V dwarfs,
or main sequence stars.
Less frequently, VI (sub-dwarfs or small dwarfs) and VII (white dwarfs) are also used.
Sometimes the 0 (number) relating to hypergiants is also added, but the symbol creates some
confusion, since a star of class O (letter) can also become O0 if it is at maximum temperature.
They are also used under brightness classes.
The I splits into Ia and Ib to further divide the supergiants.
At this point, our Sun becomes a G2V, which translates into "yellow star, two tenths towards
orange, main sequence dwarf".
We also summarize the brightness classes: • Hypergiants - 0
• Supergiants - Ia, Ib • Luminous Giants - II
• Giants - III • Sub-giants - IV
• Dwarf or main sequence stars - V • Sub-dwarfs - VI
• White dwarfs – VII
Thanks to these two classifications it is possible to take a step forward: by inserting
in a Cartesian diagram the spectral classes (or temperatures) in abscissa and luminosity
(or the absolute magnitude or indirectly the diameter) in the ordinate, it is possible
to construct the Hertzsprung diagram - Russel, also known as HR diagram.
Ejnar Hertzsprung and Henry Norris Russell independently conceived it around 1910, plotting
the temperature of stars and their absolute magnitude.
This was obviously done only for objects whose spectrum and distance were available.
However, it was enough to understand from the outset the enormous potential of it: the
stars did not position themselves at random, but followed precise and clearly identifiable
lines and groupings.
The H-R diagram is an essential tool for understanding the evolution and physical characteristics
of individual stars and stellar agglomerates: open clusters, globular clusters and galaxies.
Thanks to the H-R diagram it is possible to compare the theoretical predictions of stellar
evolution models with observations to verify the accuracy of the former; determine the
age, chemical composition and distance of a stellar population; derive the history of
the star formation of an agglomeration of stars etc.
From a first examination of the HR diagram, one immediately observes how the stars tend
to position themselves in very distinct regions: the predominant evolutionary structure is
the diagonal starting from the upper left corner (where the most massive, hot and bright
stars are located) towards the lower right corner (where the less massive, cooler and
less bright stars are positioned), called the main sequence.
At the bottom left is the sequence of white dwarfs, while above the main sequence, to
the right, the red giants and supergiants are arranged.
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