Stars were originally classified according to their spectra (strength = width of their
absorption lines) either from a purely observational perspective or with an
incomplete understanding (i.e., educated guess) of how their spectra depend on their
physical properties. Today, we understand that the (optical) spectra of stars depend
on their (photospheric) chemical composition (abundance of different elements),
effective temperature, gas pressure (surface gravity), and also phenomena occurring
above the photosphere such as stellar winds or chromospheric/coronal activity. The
Harvard scheme classifies stars according to their effective temperatures. The
Morgan-Keenan scheme extends the Harvard classification to further classify stars
according to their luminosities.
Learning Objectives

Spectral Lines in Solar and Stellar Spectra
What does the strength of spectral lines indicate?

Classifying Stellar Spectra
Secchi classes
Harvard classes
Morgan-Keenan classification

Physics of the Formation of Stellar Spectral Lines
Excitation of atoms
Ionization of atoms
Widths of spectral lines
Learning Objectives

Spectral Lines in Solar and Stellar Spectra
What does the strength of spectral lines indicate?

Classifying Stellar Spectra
Secchi classes
Harvard classes
Morgan-Keenan classification

Physics of the Formation of Stellar Spectral Lines
Excitation of atoms
Ionization of atoms
Widths of spectral lines
Solar Absorption Lines

Recall that, in 1802, the English chemist and physicist
William Hyde Wollaston passed sunlight through a prism
(like Newton and many others had done before him) and
noticed for the first time a number of dark spectral lines
superimposed on the continuous spectrum of the Sun.

By 1814, the German optician Joseph von Fraunhofer
had cataloged 475 of these dark lines (today called
Fraunhofer lines) in the solar spectrum. He labeled the William Hyde Wollaston, 1766strongest lines A to K, and weaker lines with lower-case
1857
letters. Fraunhofer determined that the wavelength of
one prominent dark line corresponds to the wavelength of
yellow light emitted when salt is sprinkled in a flame.
Today, we know that this dark line is produced by the
sodium atom, and is in fact a doublet but was spectrally
unresolved at the time.
Joseph von Fraunhofer,
1787-1826
Solar Absorption Lines

Note that some of the spectral lines (e.g., O2, H2O) are produced by absorption in
the Earth’s atmosphere (such lines are called telluric lines).
Solar Absorption Lines

Among the most prominent absorption lines are the sodium (Na) doublet, the first
atomic species to be identified on the Sun.
Solar Absorption Lines

Among the strongest absorption lines are iron (Fe) lines.
Solar Absorption Lines

The singly-ionized calcium (Ca II) H and K lines also are particular prominent in
the solar spectrum (Ca = Ca I, Ca+ = Ca II, Ca++ = Ca III, etc.).
Solar Absorption Lines

Of course, the hydrogen (H) Balmer lines also are prominent.
Solar Absorption Lines

Of course, the hydrogen (H) Balmer lines also are prominent.
Solar Absorption Lines

Given that absorption lines of Na, Fe, and Ca are roughly as prominent as H, does
this mean that Na, Fe, and Ca are roughly as abundant as H on the Sun?
Cosmic Abundance of the Elements in the Solar System
Atoms of Element per 106 atoms of Silicon (Si)
Solar Absorption Lines

Given that absorption lines of Na, Fe, and Ca are roughly as prominent as H, does
this mean that Na, Fe, and Ca are roughly as abundant as H on the Sun? No
Cosmic Abundance of the Elements in the Solar System
Atoms of Element per 106 atoms of Silicon (Si)
Solar Absorption Lines

What then does the strength of absorption lines in the solar spectrum indicate?
Cosmic Abundance of the Elements in the Solar System
Atoms of Element per 106 atoms of Silicon (Si)
Solar Absorption Lines

What then does the strength of absorption lines in the solar spectrum indicate? In
(smaller) part abundance, and in (greater) part …
Cosmic Abundance of the Elements in the Solar System
Atoms of Element per 106 atoms of Silicon (Si)
Stellar Absorption Lines

Notice of that the strength of Balmer absorption lines change with the effective
temperatures of stars. What do we need to understand to predict the relative
strength of Balmer absorption lines across different stars?
Stellar Absorption Lines

What do we need to understand to predict the relative strength of Balmer
absorption lines across different stars? How the number of hydrogen atoms
excited to the n = 2 level depends on the gas temperature.
Solar and Stellar Absorption Lines

What then does the strength of absorption lines in the solar and stellar spectra
indicate? In (smaller) part abundance, and in (greater) part how atoms are excited
and ionized to produce the observed spectral lines.
Solar and Stellar Absorption Lines

Why are solar/stellar lines seen in absorption rather than emission?
Kirchhoff’s laws

Why are solar/stellar lines seen in absorption rather than emission? Because the
outer layer of the Sun/stars (where the absorption lines are produced) is cooler
than the inner layer where the light is produced (photosphere).
Structure of the Sun

Why are solar/stellar lines seen in absorption rather than emission? Because the
outer layer of the Sun/stars (where the absorption lines are produced) is cooler
than the inner layer where the light is produced (photosphere).
Structure of the Sun

On the Sun, the temperature drops with
height from the interior to the
photosphere until increasing in the
chromosphere and corona.
Learning Objectives

Spectral Lines in Solar and Stellar Spectra
What does the strength of spectral lines indicate?

Classifying Stellar Spectra
Secchi classes
Harvard classes
Morgan-Keenan classification

Physics of the Formation of Stellar Spectral Lines
Excitation of atoms
Ionization of atoms
Widths of spectral lines
Classifying Stellar Spectra

Suppose that you were asked to classify
stars according to their spectra, with
little understanding of how the strength
of their different absorption lines depend
on stellar properties.

What would you do?
Classifying Stellar Spectra

Suppose that you were asked to classify
stars according to their spectra, with
little understanding of how the strength
of their different absorption lines depend
on stellar properties.

What would you do?

Presumably you would start by:
- looking for strong spectral lines
common to all stars
- identify the elements that produce
these lines
Classifying Stellar Spectra

Among the strongest and common lines
are produced by hydrogen, the most
common element in the Universe (75%
Hα
of all baryons).
Near-IR
Optical
Ultraviolet
Hβ
Hγ Hδ Hε
Classifying Stellar Spectra

Among the strongest and common lines
are produced by helium, the second most
common element in the Universe (23%
of all baryons).
Classifying Stellar Spectra

Among other relatively strong and
common lines are the most abundant
metals (elements other than hydrogen
and helium).

Cosmic abundance
of the elements:
The Secchi Classes


During 1860s and 1870s, Father Pietro Angelo Secchi, Director
of the Observatory of the Roman College, made the first
classifications of stars based on their spectra (from his collection
of about 4000 stellar spectrograms):
- Class I stars exhibit prominent hydrogen Balmer lines
- Class II stars exhibit calcium and sodium lines
- Class III stars exhibit broad and complex bands of lines
- Class IV stars show prominent carbon lines
- Class V stars show lines in emission
Pietro Angelo Secchi,
1818-1878
The Secchi classes have been superseded and largely forgotten.
Recall that:
1802: Wollaston discovered absorption lines in sunlight
1814: Fraunhofer cataloged 475 lines in sunlight, and identified the strongest line as being
produced by calcium
1860s: Foundations of spectroscopy established by Bunsen and Kirchhoff
1880s: Wavelengths of Balmer lines at optical wavelengths precisely determined
The Secchi Classes

The Secchi classes:
- Class I stars exhibit prominent hydrogen Balmer lines
- Class II stars exhibit calcium and sodium lines
- Class III stars exhibit broad and complex bands of lines
- Class IV stars show prominent carbon lines
- Class V stars show lines in emission
The Harvard Classes

The modern classification of stellar spectral types dates back to Edward C.
Pickering of the Harvard College Observatory and his assistants, Williamina P.
Fleming, Antonia Maury, and most importantly Annie Jump Cannon.

This classification system is sometimes referred to as the Harvard system, familiar
to all: Oh Be A Fine Girl/Guy Kiss Me (coined by Cannon).
Edward C. Pickering,
1846-1919
Williamina P.
Fleming, 1857-1911
Antonia Maury,
1866-1952
Annie Jump Cannon,
1863-1941
The Harvard Classes

In the 1880s, Pickering began a survey of stellar spectra at the Harvard College
Observatory using the objective-prism method. This method uses a prism placed
in front of the telescope to disperse light, and has the advantage of simultaneous
spectral measurements of multiple astronomical sources with wide wavelength
coverage but low spectral resolution.
Illustration of spectra
taken with an objective prism
The Harvard Classes

In the 1880s, Pickering began a survey of stellar spectra at the Harvard College
Observatory using the objective-prism method. This method uses a prism placed
in front of the telescope to disperse light, and has the advantage of simultaneous
spectral measurements of multiple astronomical sources with wide wavelength
coverage but low spectral resolution.
Actual spectra
taken with an objective prism
Illustration of spectra
taken with an objective prism
The Harvard Classes

A first result of this work was the Draper Catalogue of Stellar Spectra, published
in 1890. This work was later extended to become what is known today as the
Henry Draper Catalogue, where stars are labeled according to their number in this
catalogue (e.g., Betelgeuse is HD 39801).

Henry Draper was a pioneer of astrophotography
(his father made the first photograph of the Moon
through a telescope). In 1872, Draper succeeded
in taking the first photograph of a stellar spectrum
that showed absorption lines. Upon his untimely
death, his widow funded the Henry Draper Medal
for outstanding contributions to astrophysics, and
an endowment used to finance the compilation of
the Henry Draper catalog.
Henry Draper,
1837-1882
The Harvard Classes

In the 1890s, Fleming labeled stellar spectra with capital letters according to the
width of their hydrogen absorption lines, beginning with the letter A for the
broadest lines. She divided the Secchi classes I to IV into more specific classes,
given letters from A to N. Also, the letters O, P and Q were used, O for stars
whose spectra consisted mainly of bright lines (today recognized as Wolf-Rayet
stars), P for planetary nebulae, and Q for stars not fitting into any other class.
Hδ
Hγ
Hβ
Hα
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines?

The stellar spectra were recorded
on photographic plates. An
example of the spectrum of six
individual stars as recorded on
photographic plates is shown here.

Obviously, it is difficult to quantify
the intensity of absorption lines.
On the other hand, it is relatively
easy to quantify the width of
absorption lines.

What determines the width of
stellar absorption lines?
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines?

The stellar spectra were recorded
on photographic plates. An
example of the spectrum of six
individual stars as recorded on
photographic plates is shown here.

Obviously, it is difficult to quantify
the intensity of absorption lines.
On the other hand, it is relatively
easy to quantify the width of
absorption lines.

What determines the width of
stellar absorption lines?
- natural broadening
- Doppler (thermal) broadening
- pressure broadening
The Harvard Classes

The sizes of stars on a photographic plate/CCD image depends on the brightness
of the star.
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines?
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines?
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines?
The Harvard Classes

Why did Fleming use the width rather than the intensity of spectral lines as a
measure of the strength of spectral lines? The width reflects the absorption depth
of a spectral line.
The Harvard Classes

In 1897, Maury (Henry Draper’s niece) developed a much more complex
classification scheme she was using to study the widths of spectral lines. Maury
rearranged her classes in a way that would be equivalent to placing Fleming’s B
class before the A class.
Hδ
Hγ
Hβ
Hα
The Harvard Classes

In 1901, Cannon negotiated a compromise and based on her classification scheme
dropped all letters apart from O, B, A, G, K, and M, and placed B class before A,
and O class before B. She also subdivided each class into 10 subclasses from 0 to
9 (e.g., the Sun is G2).
Hδ
Hγ
Hβ
Hα
The Harvard Classes

In 1901, Cannon negotiated a compromise and based on her classification scheme
dropped all letters apart from O, B, A, G, K, and M, and placed B class before A,
and O class before B. She also subdivided each class into 10 subclasses from 0 to
9 (e.g., the Sun is G2).

Cannon’s and her coworker’s work was published between 1928-1924 in nine
volumes that became the Henry Draper catalog, listing nearly 230,000 stars.
(Cannon was the first woman to receive the Henry Draper Medal, awarded since
1886 by the National Academy of Sciences for outstanding achievement in
astronomical physics).
The Harvard Classes

In Cannon’s classification scheme, visible lines of ionized helium (He II) are
detectable in O stars. (An atom’s ionization stage is indicated by a Roman
numeral, where I is neutral, II is singly-ionized, III is doubly-ionized, etc.)
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

In Cannon’s classification scheme, visible lines of neutral helium (He I) is
strongest for B2 stars.
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

Energy diagram of helium and permitted transitions to the n = 1 and n = 2 states.
(In this rendering of the energy diagram, the ground state of parahelium is defined
to have an energy of 0 eV). How does the ionization energy of helium compare to
hydrogen?
Orthohelium
Parahelium
The Harvard Classes

How does the ionization energy of helium compare to hydrogen? Parahelium
higher, but orthohelium lower, ionization energy than that of hydrogen.
Orthohelium
Parahelium
The Harvard Classes

In Cannon’s classification scheme, hydrogen Balmer lines reach their maximum
intensity in the stars of spectra type A0.
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

In Cannon’s classification scheme, hydrogen Balmer lines reach their maximum
intensity in the stars of spectra type A0.
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

In Cannon’s classification scheme, visible lines of singly ionized calcium (Ca II)
are most intense for K0 stars. (Neutral Ca has 20 electrons.)
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

In Cannon’s classification scheme, broad (molecular) bands are prominent in M
stars.
Note that this photograph is a
negative, so that bright lines
correspond to absorption lines.
The Harvard Classes

A modern description of the
Harvard classification scheme.
The Harvard Classes

Today, we know that Cannon’s
classification scheme reflects the
effective temperatures of stars, with
spectral type O being the hottest and
M being the coolest stars.

Why, and how, do the strength of
absorption lines in stellar spectra
depend on stellar effective
temperatures?

We need to understand how the
excitation and ionization of atoms
depend on the gas temperature.

In Cannon’s days, only how the
excitation of atoms depend on the gas
temperature was understood.
Hγ
Hβ
Hα
The Morgan-Keenan Classification

In 1943, William W. Morgan and Phillip C. Keenan published the Atlas of Stellar
Spectra comprising 55 photographic prints showing how stellar spectra depend on
both stellar effective temperatures and luminosities. They extended the Harvard
classification system to include the dependence of stellar spectra on stellar
luminosities, giving rise to the present-day system for classifying stars.
The Morgan-Keenan Classification

Notice that, in the spectra shown below, all six stars have the same
effective temperature, yet the widths of their spectral lines (as
measured at their relatively diffuse line wings) are different.

These stars have different luminosities, and therefore different
sizes. Their different spectral linewidths (as measured at their line
wings) is caused by their different surface gas pressures.
Learning Objectives

Spectral Lines in Solar and Stellar Spectra
What does the strength of spectral lines indicate?

Classifying Stellar Spectra
Secchi classes
Harvard classes
Morgan-Keenan classification

Physics of the Formation of Stellar Spectral Lines
Excitation of atoms
Ionization of atoms
Widths of spectral lines
Formation of Spectral Lines in Stellar Atmospheres

Today, we know that the (optical) spectra of stars depend on their (photospheric):
- chemical composition (abundance of different elements)
- effective temperature
- gas pressure (surface gravity)
- presence of above-photospheric phenomena such as stellar winds and
chromospheric/coronal activity (beyond scope of this course)
The Harvard scheme classifies stars according to their effective temperatures. The
Morgan-Keenan scheme extends the Harvard classification to further classify stars
according to their luminosities.

To understand how the strengths of stellar spectral lines depend on effective
temperature, we need to know
- how atoms in stellar atmospheres are excited and ionized (through collisions)
- how the speeds of atoms are distributed, and the dependence of the speed
distribution with temperature
- how the excitation and ionization of atoms, and hence the strengths of their
spectral lines, depend on temperature
Formation of Spectral Lines in Stellar Atmospheres

Today, we know that the (optical) spectra of stars depend on their (photospheric):
- chemical composition (abundance of different elements)
- effective temperature
- gas pressure (surface gravity)
- presence of above-photospheric phenomena such as stellar winds and
chromospheric/coronal activity (beyond scope of this course)
The Harvard scheme classifies stars according to their effective temperatures. The
Morgan-Keenan scheme extends the Harvard classification to further classify stars
according to their luminosities.

To understand how the widths of the wings of stellar spectral lines depend on
luminosity, we need to know
- what processes affect the widths of the wings spectral lines (pressure
broadening)
- how the widths of the wings of spectral lines depend on gas pressure