Transcript Astrochemistry Basics - MIT Haystack Observatory
Astrochemistry basics
How we detect elements and molecular compounds in space
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
We detect these chemicals in space by at least two methods:
Useful for Visible, Infrared, and Ultraviolet Light
– 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.
is the process where energy is taken in by an electron to change it’s energy level to a “non-rest” state.
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) cool gas. The gas “robs passes through a ” (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 images.
) 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
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
Name (Popular Name) Suphur Monoxide Methanol Formaldehyde
Familiar Compounds in
Formula (Chemical formula) SO
Space
Wavelength ( λ - in cm) 8.28 Frequency (MHz) 36202 Found In: Orion CH 3 OH H 2 C 18 O 8.29
6.83
36169 4388 Sagittarius Sagittarius Carbon Oxide Sulfide Silicon Monoxide Silicon Tetracarbide Ammonia OCS SiO SiC 4 NH 3 8.22
7.05
6.98
16.65
36488 42519 42944 18017 Taurus Sagittarius (Var. Supergiant) 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.