Transcript Astrochemistry Basics - MIT Haystack Observatory

Astrochemistry basics
How we detect elements and
molecular compounds in space
Begin

Background photograph - NASA - http://hubblesite.org/gallery/album/galaxy_collection/pr1999041a/
Whoa!! I don’t understand
the term Astrochemistry???

Astrochemistry is the (relatively) new branch of
Astronomy that deals with how/where elements
and compounds come from that are found in:
• The Milky Way (our very own Galaxy) learn more
• Nebulae (clouds of gas within our galaxy) learn
more
• Other galaxies learn more
• Any other place in the universe INCLUDING our
Solar System
Some useful concepts
Spectral lines
Absorption / Emission lines
Radio Telescopes
Detection
We detect these chemicals in
space by at least two methods:


Atomic Spectroscopy – Useful for
Visible, Infrared, and Ultraviolet Light
Molecular Spectroscopy – Useful for
Radio Waves, and Microwaves (type of
Radio Wave),
Atomic Spectroscopy

Atoms of a specific
element emit a
specific wavelength
of light when an
electron undergoes
a transition
between energy
levels (Bohr model of the
Atom)
Molecular Spectroscopy


Molecules of a compound
have rotation energy.
When they change
rotation energy states,
they emit a photon at a
specific wavelength (λ).
Different molecules emit
photons of unique
wavelengths for that
rotation energy state.

Diatomic molecule rotation
Absorption / Emission
lines



Electrons making an energy transition is
the key to either emission of light or radio
waves.
Absorption is the process where energy is
taken in by an electron to change it’s
energy level to a “non-rest” state.
Emission is the process where energy is
given off by the electron as it returns to
its “rest” state.
Absorption Spectra


Happens when waves
of an electromagnetic
wave (shown in visible
light) passes through a
cool gas. The gas
“robs” (absorbs) photons
at specific
wavelengths to
energize its electrons.
Result – Specific
wavelengths “drop
out” of the otherwise
continuous spectrum.
Absorption Spectra Examples:

The absorption of these specific wavelengths of light
prove the presence of Hydrogen gas in the outer
atmosphere of the star.
Absorption Spectra Examples:
Yup, another example…

Psst!, This is how we identify Spectral types!!!
Emission Spectra


Happens when
photons are created
by an energized gas.
The gas “creates”
(emits) photons at
wavelengths
corresponding to the
energy change of its
electrons.
Result – Specific
wavelengths of light
are created where
there was no light
before.
Emission lines of specific gasses
Tools of the Radio Astronomer


Radio Telescopes – Similar in
many ways to Optical
telescopes
Focus configurations:
• A. Prime Focus – radio receiver
at the prime focus
• B. Cassegrain – secondary
convex reflector at prime focus

How do you make a radio
telescope (almost) the size of
the Earth??
• Ans: VLBA
Very Long Baseline Array



What is a VLBA??
Answer: a Very Long Baseline Array –
A grouping of identical radio telescopes at
different places on the earth that act as one
large radio telescope
Combining the signals from more
than one telescope is called
interferometry. (learn more)
Telescopes of the VLBA

The National Science Foundation's VLBA
(learn more) is a system of ten identical
radio-telescopes controlled from a
common headquarters in New Mexico, and
working together as a single instrument.
The radio signals received by each
individual antenna contribute part of the
information used to produce images of
celestial objects with hundreds of times
more detail than Hubble Space Telescope
images.
Telescopes of the
Very Long Baseline Array (VLBA)

NRAO / AUI / NSF
Radio Telescopes

How big is the world’s largest Radio
Telescope, and where is it?
How Big: > 100 m (> 300 ft.)
Where: Green Bank, West Virginia
NRAO / AUI / NSF
Beginnings

In the ‘50s & ’60s, Radio telescopes first
discovered the “signature” line of
Hydrogen at 21 cm wavelength between
stars.
• The hydrogen gas was found (in abundance) in the
“empty” space between stars.
• Note: Domestic microwave ovens use a signal of
~ 12 cm wavelength to cook food

This gas was “dark” in optical wavelengths
but “bright” in radio wavelengths.
Keeping score

What molecules have been discovered and
at which wavelengths?
Ans: Well over 100 molecules of compounds
have been found in Interstellar space.

Here are just a few of them showing
where they were discovered and at the
corresponding “signature” frequencies and
wavelengths
Familiar Compounds in
Space
Name
(Popular
Name)
Formula Wavelength Frequency
(Chemical ( λ - in cm) (MHz)
formula)
Found In:
Suphur
Monoxide
SO
8.28
36202
Orion
Methanol
CH3OH
8.29
36169
Sagittarius
Formaldehyde
H2C18O
6.83
4388
Sagittarius
Carbon Oxide
Sulfide
OCS
8.22
36488
Taurus
Silicon Monoxide
SiO
7.05
42519
Sagittarius
(Var. Supergiant)
Silicon
Tetracarbide
Ammonia
SiC4
NH3
6.98
42944
16.65
18017
Taurus
Giant
Molecular
Cloud
How can we find Molecules
with a Radio Telescope

Check out this website. It relates many compounds found
in the universe and at which frequency they are detected.
http://physics.nist.gov/cgi-bin/micro/table5/start.pl
Hints:
• Start the frequency range at about 1200 Hz. You can type in
the upper frequency range at about 44,000 Hz.
• Check the “Name” field at the bottom. This will add the
familiar chemical formula name to the table which the web site
creates for you.