- The labeled
atomic line
wavelengths are in
angstroms (Å).
- Helium is a
noble gas, and so has tightly bound
electrons.
Thus, exciting helium to
even the lowest-energy
excited states
(i.e., energy levels
of energy
above the ground state)
takes considerable energy
supplied by either or both of
heat energy
and
photon energy.
- The diagram shows that even the 1st
excited-state
energy level is
∼ 19.75 electron-volts (eV) above
the ground state.
Note that 19.75 eV means that high occupation of this
energy level
in
thermodynamic equilibrium conditions
requires a temperature of
>∼ 2*10**5 K.
The argument is as follows.
The fractional occupation of
an excited state
is of order the
Boltzmann factor exp[-E/(kT)],
where E is the energy above the
ground state,
T is temperature,
and
Boltzmann contant k = 8.617 3303(50)*10**(-5) eV/K ≅ 10**(-4) eV/K
(see NIST: CODATA:
Constants: Complete Listing).
So for the
Boltzmann factor
to be
>∼ exp(-1)
one must have E/(kT) <∼ 1, and so T >∼ 2*10**5 K.
- Most atoms have some much
lower energy
excited states
than He I.
Those excited states
are, in fact, more abundantly occupied in
stars with relatively
low (stellar atmosphere)
temperatures
than the excited states
of He I.
- Typically,
He I
absorption lines
are strong
for
B stars
with temperatures
∼ 11000--25000 K
(see UCL: The Classification of Stellar Spectra) where
the relative occupation of the
excited states is still
<< exp(-1), but evidentlly
still big enough for relatively strong
absorption lines.
Very weak He I
absorption lines
can be found in lower temperature stars
(e.g., the
Sun) if you look in detail
(see
Wikipedia; Fraunhofer lines: Naming).
- The most prominent
He I
absorption line
in the visible band
in stellar spectra
is usually the
He I 5876 Å line
(which is labeled by 5875 in the figure).
- The ultraviolet
absorption lines
arising from the ground state
are unobservable by
ground-based astronomy
because of the strong
opacity
of the Earth's atmosphere
in the ultraviolet
blueward of
∼ 3000 Å
(see Wikipedia:
Earth's atmosphere: Absorption).
-
As you go to hotter stars than
B2 stars,
He I
absorption lines
get weaker.
This is because as temperature increases,
the helium becomes
increasingly ionized
i.e., becomes increasingly singly-ionized
He II which exhibits
He II
spectral lines.
- For O stars,
He II
absorption lines
and/or emission lines
are the dominant
helium
spectral lines
(see UCL: The Classification of Stellar Spectra).
- Actually,
absorption lines
of any kind of
helium
are only ever strong in
stars
because helium
is so abundant---about 25 % by
mass fraction
of baryonic matter in the
observable universe.
- Note that non-thermal processes can lead to strong
helium
spectral lines
(absorption lines
or emission lines)
when low temperature would otherwise
preclude them.