What Do Spectra Tell Us?
Most bright astronomical objects shine because they are hot. In such
a case, the continuum they emit tells us what the temperature is.
Here is a very rough guide.
||Planets, warm dust|
||The photosphere of Sun and other stars|
||The photosphere of very hot stars|
||The corona of the Sun|
||The coronae of active stars|
We can learn a lot more from the spectral lines than from the
* The chemical composition of stars
During the first half of the 19th century, scientists such as John
Herschel, Fox Talbot, and William Swan studied the spectra of different
chemical elements in flames. Gradually, the idea that each element
produces a set of characteristic emission lines has become established.
Each element has several prominent, and many lesser, emission lines
in a characteristic pattern. Sodium, for example, has two prominent
yellow lines (the so-called D lines) at 589.0 and 589.6 nm --- any sample
that contains sodium (such as table salt) can be easily recognized
using this pair of lines.
The studies of the Solar spectrum (Joseph Fraunhofer is the most famous,
and probably also the most important, early contributor to this field),
however, revealed absorption lines (dark lines against the brighter continuum).
The precise origin of these 'Fraunhofer lines' as we call them today
remained in doubt for many years, until Gustav Kirchhoff, in 1859,
announced that the same substance can either produce emission lines
(when a hot gas is emitting its own light) or absorption lines
(when a light from a brighter, and usually hotter, source is shone through it).
Now scientists had the means to determine the chemical composition of
stars through spectroscopy!
One of the most dramatic triumphs of astrophysical spectroscopy during the
19th century was the discovery of helium. An emission line at 587.6 nm
was first observed in the Solar corona during the eclipse of 1868 August
18th, although the precise wavelength was difficult to establish at the time
(due to the short observation using temporary set-ups of
instruments transported to Asia).
Two months later, Norman Lockyer used a clever technique and managed to
observe the Solar prominence without waiting for an eclipse. He noted
the precise wavelength (587.6 nm) of this line, and saw that no known
terrestrial elements had a line at this wavelength. He concluded that this
must be a newly discovered element, and called it 'helium'. Helium was
discovered on Earth eventually (1895) and showed the same 587.6 nm line.
Today, we know that helium is the second most abundant element in the
We also know today that the most abundant element is hydrogen. However,
this fact was not obvious at first. Many years of both observational and
theoretical works culminated in 1925, when Cecilia Payne published her
PhD thesis entitled 'Stellar Atmospheres'. (Footnote: this was the first ever
PhD awarded at Harvard; it was also praised as "undoubtedly the most
brilliant PhD thesis ever written in astronomy" nearly 40 years later.
She later turned to studies of variable stars, and coined the term
'cataclysmic variables'.) In this work, she utilized many excellent
spectra taken by Harvard observers, measured the intensities of 134
different lines from 18 different elements. She applied the up-to-date
theory of spectral line formation, and found that the chemical
compositions of stars were probably all similar, the temperature
being the important factor in creating their diverse appearances.
She was then able to estimate the abundances of 17 of the elements relative
to the 18th, silicon. Hydrogen appeared to be more than a million times
more abundant than silicon, a conclusion so unexpected that it took many
years to become widely accepted.
* The motion of stars and galaxies
In such an analysis of chemical abundances, the wavelength of each line
is treated as fixed. However, this is not true when the star is moving
toward us (the lines are observed at shorter wavelengths, or 'blueshifted,
compared to those measured in the laboratory) or moving away from us (observed at
longer wavelengths, or 'redshifted'). This is the phenomenon of
If the spectrum of a star is red or blue shifted, then you can use that
to infer its velocity along the line of sight. Such 'radial velocity'
studies have had at least three important applications in astrophysics.
The first is the study of binary star systems. The component stars in a binary
revolve around each other. You can measure the radial velocities for one
cycle (or more!) of the binary, then you can relate that back to the
gravitational pull using Newton's equations of motion (or their astrophysical
applications, Kepler's laws). If you have additional information, such as
from observations of eclipses (see Light Curve), then you can sometimes
measure the masses of the stars accurately. Eclipsing binaries in which
you can see the spectral lines of both stars have played a crucial role
in establishing the masses and the radii of different types of stars.
The second is the study of the structure of our Galaxy. Stars in the
Galaxy revolve around its center, just like planets revolve around the Sun.
It's more complicated, because the gravity is due to all the stars in
the Galaxy combined in this case. (In the Solar system, the Sun is such
a dominant source that you can ignore the pull of the planets --- more or
less). So, radial velocity studies of stars (binary or single) have played
a major role in establishing the shape of the Galaxy. It is still an active
field today: for example, one of the evidences for dark matter comes from
the study of the distribution of velocities at different distances from
the center of the Galaxy. Another exciting development is the radial velocity
studies of stars very near the Galactic center, which strongly suggest that
our Galaxy contains a massive black hole.
The third is the expansion of the Universe. Edwin Hubble established that
more distant galaxies tended to have more red-shifted spectra. Although
not predicted even by Einstein, such an expanding universe is a natural
solution for his general relativity theory. Today, for more distant galaxies,
the redshift is used as a primary indicator of their distances. The ratio of
the recession velocity to the distance is called the Hubble constant, and
the precise measurement of its value is one of the major goals of astrophysics
today, using such tools as the Hubble Space Telescope.