NORMAL STARS SPECTRA
Type A stars
Type A stars are well represented among constellations because their high temperature
(around 10000 K) makes dwarfs of this class over 50 times brighter than our sun.
Among the more representative main sequence dwarfs we find:
Vega (A0 V), Sirio (A1 V), Alcor (A5 V), Altair (A7 V)
Among the giants:
b Eridani (A3 III),
a Oph (A5 III),
g Bootis (A7 III)
Among the supergiants:
Deneb (A2 Ia) and
h Leonis (A2 Ib)
In pictures 1-3 spectra of Vega and Deneb are compared. Both spectra have been recorded
with the spectrograph described in the page
The biggest difference concerns the Balmer hydrogen lines that are broadened in main sequence dwarfs
while in giants they are not.
This effect is due to the high pressure at the surface of dwarfs that makes the encounter
between an hydrogen atom and a free electron more probable. The electric field of the free
electron perturbs the energy levels of hydrogen (Stark effect) thus broadening the lines.
As in the giants the density and the pressure are much lower, the broadening is weaker.
Picture 1: Spectrum of Vega (A0 V) and Deneb (A2 Iae) between 3700 e 4400 ┼ recorded with 2 ┼/pixel dispersion. The spectrum is not corrected for atmospheric transmission and spectral response of the spectrograph.|
Picture 2: Spectrum of Vega (A0 V) and Deneb (A2 Iae) between 4300 e 5100 ┼ recorded with 2 ┼/pixel dispersion. The spectrum is not corrected for atmospheric transmission and spectral response of the spectrograph.
Picture 3: Spectrum of Vega (A0 V) and Deneb (A2 Iae) between 5100 and 6600 ┼ recorded with 2.5 ┼/pixel dispersion. The spectrum is not corrected for atmospheric transmission and spectral response of the spectrograph. The red rectangle around H a in Deneb spectrum shows which part of the spectrum is enlarged at 0.45 ┼/pix dispersion in the lower left corner. This enlarged view of H a shows its complex structure and asymmetrical shape. This is due to emissions and absorptions sligtly Doppler shifted produced by the outflowing giant athmosphere.
Looking at the two spectra, we can see an other striking difference in the strength of ions lines
(and in particular Fe II, singly ionized iron) that are stronger in giants than in dwarfs.
Again, the reason for this is the low atmospheric density of giants that reduces the probabily
of ions recombination with more dilute free electrons.
In type A stars, Balmer hydrogen lines reach their maximum intensity.
An absorption in the Balmer series is produced when an electron jumps from the second orbit
(n=2 that is 10.2 eV from the fundamental state orbit n=1) to higher n orbits.
At lower temperatures (that is to say, for spectral classes F, G, K, M) the second orbit
has a low population respect to the fundamental one and thus Balmer lines are weak.
At the same time, lines that originates from the first orbit are all visible in the UV, and for this reason when you look at some late type star spectra (M type) it seems that these stars contain no hydrogen.
The same happens for the hottest stars, where hydrogen atoms having lost their only one electron
(ionisation energy for hydrogen is 13,6 eV) can no longer absorb light.
Paschen Hydrogen Series in the Near IR spectrum of Vega
Looking at the NIR part of the spectrum of Vega, that is accessible to modern CCD detectors, one can observe also the less known Paschen series of hydrogen (picture 4) that is very similar to the Balmer one except that it origins from electrons that jump from the 3rd orbit instead of jumping from the second one.
Paschen-alfa line is far in the IR at 1,87 mm and thus is not visible with CCDs but starting from Paschen-delta at 10050 ┼ (just on the red edge of CCD detector spectral range) and up to the end of the series at 8204 ┼ all the lines are visible.
Picture 4: NIR spectrum of Vega that shows the first part of the Paschen line series of Hydrogen merged with atmospheric absorptions. (4 min exposure, 2 ┼/pixel, 60 cm F/20 Cassegrain telescope)
Rotation speed of stars measured with Doppler broadening of the spectral absorption lines
Stellar rotation speed increases suddenly passing form late stellar classes (M, K, G) to the B type. As an example, our sun spins at 2 km/sec at the equator
while many A stars show rotation speeds that exceed 200 Km/sec.
In late type stars infact the deep convective layer generate a magnetic field that brakes rotation, while for early type stars this layer (and thus the magnetic field) is
absent and the rotation remain unchangend during the life of the star. This thery is confirmed by the observation that young stars in open clusters have alla the same rotation speed, independently from their spectral
Among A type stars we can however find many excepion to this rule. Magnetic A stars (Am), for example, are all very slow rotators even if the origin of their
magnetic field is not still understood.
The speed of rotation of a star ca be easily deduced from the Doppler broadening of the absorption lines. Rotation broadening cannot be
confused with other sources of line broadening because is perfectly symmetric and independent from polarisation of light.
Unfortunately Doppler effect allows us only to measure radial velocities and then a star that is viewed with rotation axis close to the
line of sight does not show line broadening. The velocities measured with Doppler shift are thus only the radial component Vsin(i)
of the rotation speed V, where i is the angle between rotation axis and line of sight. Doppler speeds must thus be considered only as
a lower limit to rotation speeds.
In the following example (Altair spectrum) can be observed that a common rotation speed of 220 Km/sec produce a 4-5 ┼ broadening of
the absorption lines that can be easily observed even at medium resolution (R=5000 for our spectrograph). Rotational broadening is thus
a much bigger effect than magnetic Zeeman spitting that achieve only a fraction of ┼ even for the strongest magnetic fields.
Among the A stars that show the highest Doppler broadening due to rotation speed we find z Aquilae that spins
at 350 Km/sec at the equator.
Picture 5: Spectrum of Altair compared with Cor Caroli. Cor Caroli is a magnetic A star thus has a very low rotation speed. The broadening of its lines is only due to spectrograph resolving power (dispersion 0.6 ┼/pixel).
On the contrary the absorption lines of Altair appear broadly widened. In the left corner of the picture we obtained a line profile for Altair summig up 5 different lines (to improve S/N ratio)
that were calibrated with V=0 in the middle of the line and then with V=cxDl/l for each pixel.
The averaged line is compared with the profile of the 4481 Mg II line extracted from Cor Caroli spectrum that results much thinner. The symmetric broadening
infact is due to the fast rotation of Altair that reaches 220 Km/sec at the equator. This high speed produces considerable centrifugal forces that
makes this star elliptically shaped.
Peculiar A class stars
Among A stars about 25% are peculiars. The main peculiar classes are metallic A stars and magnetic A stars.
These classes are detailed in the Peculiar Stars Page
Goto other spectral classes: