Astrochemistry basicsHow we detect elements and molecular
compounds in space Begin
Background photograph - NASA -
http://hubblesite.org/gallery/album/galaxy_collection/pr1999041a/
Whoa!! I dont 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 conceptsSpectral 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 ofRadio 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 theAtom)
Molecular Spectroscopy
Molecules of a compound have rotation energy. When they change
rotation energy states, they emit a photon at a specific wavelength
(). 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 its 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 worlds 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 SpaceName (Popular Name) Suphur Monoxide
MethanolFormaldehyde Carbon Oxide SulfideSilicon Monoxide
Formula Wavelength Frequency (Chemical ( - in cm) (MHz) formula)
SO CH3OH H2C18O OCS SiO SiC4 NH3 8.28 8.29 6.83 8.22 7.056.98
Found In:
36202 36169 4388 36488 42519 42944 18017
Orion Sagittarius Sagittarius Taurus Sagittarius(Var.
Supergiant)
Silicon Tetracarbide Ammonia
TaurusGiant
16.65
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.