- Apoorv Umang
ASTRONOMICAL
SPECTROSCOPY

Spectroscopy

Spectroscopy
 Spectroscopy is a technique in which the visible
light that comes from objects (like stars and
nebulae) is examined to determine the object's
composition, temperature, motion, and
density.
A representation of the spectrum of a compound

 When something is hot enough to glow (like a star), it
gives us information about what it is made of, because
different substances give off a different spectrum of
light when they vaporize. Each substance produces a
unique spectrum, almost like a fingerprint.
 In addition, different cool gases will absorb different
wavelengths of light and generate a signature spectrum
with dark lines at characteristic places. Because of
this, we can determine the composition of gases by
observing light that has passed through them.
The spectrum of Hydrogen gas

Astronomical Spectroscopy

Astronomical Spectroscopy
 It is the use of spectroscopy in astronomy.
 In this, spectroscopy is used to derive many properties of
stars and galaxies, such as their composition and
movement.
 Astronomical
spectroscopy
began with Isaac
Newton's initial
observations of
the light of the
Sun, dispersed by
a prism.
Dispersion of light by a prism

 However, when the spectrum was closely examined, the
rainbow was found to be interrupted by hundreds of tiny
dark lines (called Fraunhofer lines). These lines showed
that some wavelengths are being absorbed by gases in
the outer atmosphere of the Sun, and from this, we can
determine which elements are in the Sun's atmosphere.
Extremely high resolution spectrum of the Sun

Discovery of Helium
 In 1868, Sir Norman Lockyer
observed strong yellow lines in
the solar spectrum which had
never been seen in laboratory
experiments. He deduced that
they must be due to an unknown Norman Lockyer spectroscope
element, which he called
helium, from the Greek helios
(sun)
 It was only 25 years later (in the
1890s) that Helium was detected
on earth. Helium in Solar Prominences

Spectral types
 The spectral type of stars is a system of classification of stars
based on the stars' spectra that correlate with each star's
surface temperature and color.
 Stars range from blue
and hot to red and cool.
The seven spectral
types are:
O, B, A, F, G, K, and M
(from hottest to
coolest). Each of these
letters is divided into 10
numerical classes, from
hotter to cooler:
The sun’s spectral type is G2 0, 1, 2, 3, 4, 5, 6, 7, 8, an
d 9.

Nebulae
 In earlier times, the word ‘nebula’ was used to describe
any fuzzy patch of light that didn’t look like a star.
 However, when their spectra were studied, it was found
that many of these nebulae, such as the Andromeda
Nebula, had spectra that looked similar to stellar
spectra, and these turned out to be galaxies.

 Others, such as the Cat’s Eye Nebula, had very different spectra –
consisting of a few strong emission lines rather than the
continuous spectrum seen in the sun.
 These lines did not
correspond to any element
seen on Earth, and
astronomers suggested
them to be from a new
element nebulium.
However, later studies
showed that because of the
extremely rarified vacuum
found in nebulae, atoms
The Cat’s Eye Nebula behaved
differently, leading to the
strange spectrum.

Galaxies
 Galactic spectroscopy has led to many fundamental
discoveries. Edwin Hubble discovered in the 1920s
that, apart from the nearest ones (those in what is known
as the Local Group), all galaxies are receding from the
Earth. The further away a galaxy, the faster it is receding.
 This was the first indication that the universe originated
from a single point, in a Big Bang.

Planets and Asteroids
 Planets and asteroids shine only by
reflecting the light of their parent
star.
 The reflected light contains
absorption bands due to minerals in
the rocks present for rocky bodies, or
due to the elements and molecules
present in the atmospheres of the
Gas giants.

 Asteroids can be classified into three main
types, according to their spectra: the C-types are made
of carbonaceous material, S-types consist mainly of
silicates, and M-types are 'metallic'. C- and S-type
asteroids are the most common.

Quasars
 The distant nature of quasars were discovered in the
early 1960's, when their spectral lines were noted to be
substantially-shifted redder than they should normally
be.
 This redshift was attributed to the recession (speeding
away) of quasars from us. Thus, it was concluded that
quasars were the most distant objects known to us.

Thank You