Transcript MS PowerPoint - National Schools` Observatory

Stellar Spectroscopy
National Schools’ Observatory
Introduction
 Spectroscopy is a branch of science that is concerned with the investigation and
measurement of the spectrum of light produced when matter interacts with
electromagnetic radiation.
 In astronomy, spectroscopy is a very useful tool because telescopes can easily
measure the spectra of far away objects, such as stars and galaxies.
 The following activity will focus on stellar spectroscopy (the spectra of stars)
and how those spectra can be used to classify stars.
National Schools’ Observatory
Introduction
 We measure the spectra of luminous
objects by attaching a special instrument
to a telescope, known as a spectrograph.
 Within the spectrograph, light is passed
through a series of prisms and mirrors in
order to split the light into its different
component wavelengths (its spectrum).
 The intensity of those different wavelength components is then measured to
create a spectra. The spectra can then be investigated in detail to discover
useful information about the target. Rainbows are the spectra of our Sun.
 The Liverpool Telescope (LT) has its own spectrograph – FRODOspec (Fibre
fed Robotic Dual-beam Optical Spectrograph), and the following activity uses
real data taken by the LT using FRODOspec.
National Schools’ Observatory
FRODOspec
Light collected by the telescope enters
the FRODOspec instrument and is split
into two beams:
 Longer wavelength light is sent down
the ‘red arm’.
 Shorter wavelength light is sent down
the ‘blue arm’.
 The light from each arm is then split as
it passes through a prism, and a
detector captures the resulting
spectrum on a CCD device.
National Schools’ Observatory
The above image shows the inside of FRODOspec
during its development.
FRODOspec
The image to the right shows the raw
spectrum of the star BD-02-5059.
 Each of the vertical lines are repeats
(or orders) of the same spectrum.
 You can see a line feature (circled
red) running across those orders.
 In this activity you will be provided
with a 2-D spectrum (wavelength
versus intensity) that has been
extracted from the raw data.
National Schools’ Observatory
Types of Spectra
In general, there are two types of spectra that can be easily identified, EMISSION
spectra and ABSORPTION spectra.
An example of an emission spectra:
An example of an absorption spectra:
National Schools’ Observatory
Emission Spectra
Emission spectra occur when electromagnetic radiation originates from a cloud of
relatively hot (compared to its surroundings) gas.

Due to the temperature of the gas, the atoms within it will possess lots of energy, and
some of their electrons will occupy excited states around the nucleus.

These electrons will de-excite (loose energy and fall to a lower energy level), with the lost
energy being emitted as a photon at a specific wavelength.

Such processes cause bright lines in a spectra – Emission lines.
National Schools’ Observatory
Absorption Spectra
Absorption spectra occur when electromagnetic radiation from a background
star passes through a relatively cold gas.

Radiation at specific wavelengths from the star interacts with (is absorbed by) atoms in the cold
gas, causing their electrons to gain energy and enter excited states.

These electrons quickly de-excite and emit photons at the same wavelengths. However, the
direction of the emitted light is random and this leads to the appearance of dark lines (or
missing light) in the resulting spectra, corresponding to the wavelengths that were absorbed by
the gas. These lines are known as absorption lines.

Astronomers most frequently use absorption spectra to study stars.
National Schools’ Observatory
Absorption Spectra
With careful analysis of the absorption lines, astronomers
can learn many things about as star, including:
 Temperature
 Rotational velocity
 Radial velocity (motion of the star towards or away)
 Density
 Chemical composition
 Metallicity (how many heavy elements stars contain)
Image of the Pleiades
Star Cluster
Some astronomers liken the spectrum of a star to a very basic DNA test that
allows them to determine what kind of star the light originated from.
National Schools’ Observatory
Chemical Composition
 Certain elements and molecules
preferentially absorb and emit light at
different wavelengths.
 It is possible to deduce from spectra
which elements are causing lines at
particular wavelengths, and hence
determine the chemical composition of
Above: Emission spectra of different elements.
the object being observed.
 Analysis of a spectrum of the Sun led to
the discovery of Helium, even before it
was discovered on Earth!
National Schools’ Observatory
Blackbody Radiation
Before investigating real spectra, we need to talk about Blackbody radiation.

All stars are made up of a hot, dense interior and a cooler outer layer, or atmosphere.

The interior of the star causes a continuous spectrum, known as a blackbody spectrum.

If there was no absorption or emission from the surrounding atmosphere, a star’s
spectrum would be close to a pure blackbody spectra.

The blackbody curve (see next slide) represents the spread of intensity of the radiation
being emitted over all possible frequencies.

A metal glowing red hot in a furnace is an example of blackbody radiation.

The shape of the blackbody spectrum depends on the temperature of the body.
National Schools’ Observatory
Blackbody Radiation
Warmer objects have blackbody spectra that peak at shorter wavelengths, whereas
cooler objects have blackbody spectra that peak at longer wavelengths.
National Schools’ Observatory
Blackbody Radiation
In reality the measured spectra is never
a perfectly smooth blackbody (BB) due
to absorption and emission features.
 The dips in the curve (where it drops
below the BB spectra) represent
ABSORPTION lines.
 The peaks (where it rises above the
BB spectra) represent EMISSION
lines.
National Schools’ Observatory
An example of a spectra showing absorption and emission
features (Sloan Digital Sky Survey).
Wien’s Law
 In order to classify a star, it is important for an astronomer to know the temperature
of that star.
 Stars range in temperature from about 2000K to 40,000K.
 Using Wien’s law, it is possible to calculate the surface temperature, or ‘effective’
temperature, if we know the wavelength its blackbody curve peaks at.
Teff 
W
max
 where W is Wien’s constant and = 2.898×10−3 m·K.
As an example, our Sun’s blackbody spectrum peaks at a wavelength of around 500 x 10 -9
m, giving an effective temperature of about 5800K.
National Schools’ Observatory
The Doppler Effect

One important effect, the Doppler Effect, occurs due to an apparent shift in the wavelength
and frequency of a wave due to relative motion between the source and the observer.

For example, we experience the Doppler Effect when an ambulance drives past with its
sirens on:

As the ambulance (the source) approaches you (the observer), the sound wave is
compressed (bunched up) and the resulting sound is higher pitched (shorter λ)

Once the ambulance has passed, the wave is stretched and the pitch sounds lower
(longer λ)
Shorter
wavelength
National Schools’ Observatory
Longer
wavelength
The Doppler Effect

The same effect is observed with light, when a star (the source) is moving towards or away from
observers located on the Earth.

We can detect this motion in the spectrum of the star.

If the star is moving AWAY from the Earth, the light waves are stretched out and the whole
spectrum experiences a shift towards the red end of the spectrum – a red shift.

If the star is moving TOWARDS the Earth, the light waves are bunched up and the spectrum
experiences a shift towards the blue end of the spectrum – a blue shift.
Shorter
wavelength
National Schools’ Observatory
Longer
wavelength
Spectral Lines

If there were no external factors at play, the absorption spectrum of a star would show very
thin spectral line features at the exact wavelengths corresponding to amount of energy
transferred to the atom when one of its electrons has been excited to a higher energy level.

In reality, there are a number of different factors that act to spread out or blur the spectral
lines across a wider range of wavelengths, a process called spectral line broadening.
National Schools’ Observatory
Spectral Line Broadening
The images below are of the same absorption line feature, in this case due to
hydrogen, as it appears in the spectra of three different stars.
Spectral broadening can be caused by a number of different factors, including:
 Temperature (Doppler Broadening)
 Stellar Rotation
 Pressure
National Schools’ Observatory
Doppler (Thermal)Broadening
The Doppler broadening of spectral lines is the major factor is stellar spectra.

Atoms in a gas move around at different speeds and in different directions .

This movement can cause light to be Doppler shifted as an atom in motion emits a photon,
and because some of the atoms will be moving away from the observer and some moving
towards, the overall effect will contain both blue-shift and red-shift components.

In a hot gas, the atoms possess more energy and thus move around more quickly than in a
cooler gas. This means that Doppler broadening is more significant in hot stars.
Hot Star
Cool Star
National Schools’ Observatory
Rotational Velocity
 Stars are not just static objects as they
appear in the night sky.
 They are made up of a fluid mass of
gas and plasma (hot gas) that rotates
around a common axis. The Sun
rotates about once every 24.5 days.
 When we obtain the spectra of distant
stars, however, the light is gathered
from across the entire disc of the star
and not from just one point.
National Schools’ Observatory
Rotational Velocity
 This means that some of the distant
star is rotating towards us and some
away.
 The parts rotating TOWARDS the
observer are blue-shifted.
 The parts rotating AWAY from the
observer are red-shifted.
 The net effect is that the star’s
spectral lines appear to be smeared
out, or rotationally broadened.
National Schools’ Observatory
Spectral Classification
 It is possible to use the absorption spectra of individual stars to classify them
into distinct groups.
 Astronomers classify them by temperature, into groups “OBAFGKM”, with
group O containing the hottest and M containing the coolest.
Note that there is a direct
correlation between
temperature and mass.
National Schools’ Observatory
Spectral Classification
 These groups are further divided into sub-groups numbered from 0 to 9, where
0 is the hottest; for example, B1 is hotter than B3, and F2 is hotter than F9.
 The Sun has a surface temperature of 5800K, putting it in the G2 classification.
National Schools’ Observatory
Activity
Now you know a little more about stellar spectroscopy, you will soon get the chance to look at some
real spectra from the Liverpool Telescope! Your teacher will supply you with an Excel file containing
the 2-D spectral data for 9 stars, each under its own separate tab or page of the worksheet. You are
required to plot the spectra for each of these stars.

Plot the number of counts against the wavelength for each star. You should produce spectra
similar to the one seen below (don’t forget to label your axes appropriately).

The shape of each spectrum will vary due to the spectral class of the star.
Intensity
The nine spectra cover many of the different classes of stars.
Wavelength
(Angstroms)
National Schools’ Observatory
Activity

Once you have plotted the nine spectra, you can compare them to see how the different stellar classifications vary. Things to
look out for are the width and depth of particular spectral lines.

The Hydrogen line at 6563 angstroms (1 Angstrom is 10-10m) is a good absorption line for spotting any differences.

Hotter stars will have a wider absorption lines due to Doppler broadening.

Cooler stars tend to have more erratic spectra than hotter stars.

Some spectral lines will appear in some stars, but not in others.
Intensity
Make notes of your observations !
Wavelength
(Angstroms)
National Schools’ Observatory
Activity
Once you’ve had a good look at the spectra of the 9 known star types, it’s time to use this
knowledge to classify stars of an unknown star type.
Your teacher will now supply you with another Excel file containing the spectra for two
stars that you will have to plot and then classify.
Using the information provided in this presentation, and by making comparisons to the
spectra you have already plotted, try to classify the mystery stars.
Good luck!
p.s. If you are really stuck, your teacher will have the answer.
National Schools’ Observatory