Natural Sources of Radio
RET 2013
MIT HAYSTACK OBSERVATORY
Learning Objectives
NGSS Performance Expectations
—  Develop and use a model of two objects interacting through
electric or magnetic fields to illustrate the forces between
objects and the changes in energy of the objects due to the
interaction.
Force = ma …acceleration of a charge is the primary mechanism for EM
radiation emission
÷  We will investigate the nature of those forces leading to emision.
÷
—  Evaluate the validity and reliability of claims in
published materials of the effects that different
frequencies of electromagnetic radiation have
when absorbed by matter..
Lecture Outline
—  Types of emission
—  Thermal emission
¡  Background,
¡  Blackbody radiation
¡  Emission spectra analysis
—  Non-thermal emission
Lecture 1: Overview
Thermal Emission
—  Radiative Transfer process overview
—  Foundation of Thermal Emission
¡
Kinetic molecular theory
—  Types of thermal emission
¡  Blackbody Emission
¡  Free-Free emission
¡  Spectral Line emission
÷  Atomic
÷  Molecular
Radia&ve Source Processes Blackbody emission Free-­‐free radia&on Spectral line emission Cyclotron and Synchrotron radia&on Radia&ve Transfer processes Observed light Any process that will accelerate a charged par&cles will produce EM radia&on ˃  This could be a free electron traveling through the vaccum of space and being affected by a magne&c field and thus accelerated ˃  It could be a bound electron or proton and the mo&on associated with thermal energy is causing quick accelera&ons associated with that mo&on. •  The Kine&c molecular theory states all maKer is made of &ny par&cles in constant mo&on o
o
The constant mo&on will generate EM radia&on We call this type of emission, thermal emission The type of radia.on tells us something about the source Thermal emission Blackbody radia&on Spectral line emission Free-­‐free radia&on Non-­‐Thermal Cyclotron emission synchrotron emission MASERs •
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•
•
•
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All macroscopic (everyday) objects emit EM radia&on at all &mes!! (if T > 0 K) explaina&on: The Kine&c Molecular Theory,KMT »  all maKer is made up of &ny par&cles (atoms, molecules, sub-­‐atomic par&cles) in constant mo&on. Temperature is a direct measure of average kine&c energy of all microscopic par&cles. Velocity vector Distribu&on of the # of par&cles at each level of kine&c energy # of molecules Atom or molecule T Average Kine&c Energy Wein's Law »  Wavelength of peak emmission 𝜆∝​1/𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ˃  Wavelength of peak emission is inversely propor&onal to the Temperature. ˃  Higher Temp == lower 𝜆 (blue) ˃  Lower Temp == Higher 𝜆 (red) »  Recall that the EM spectrum ranges from frequencies of 1 cycle per second (1 Hz) to »  Stephan's Law ˃  The power output from the surface of a blackbody radiator is propor&al to the Temperature to the 4th power 𝑃∝ 𝜎​𝑇↑4 »  The KMT represents par&cles as moving at a distribu&on of Kine&c energy »  Accelera&ng charges create EM waves, The different accelera&ons produce different frequencies »  A blackbody spectrum represents the distribu&on of EM radia&on and changes with temperature »  Link to Starter Ac&vity: ˃  Imagine each student traveling randomly and they were carrying a flashlight that changed color depending on their speed. An observer from distance would see a combina&on of all the different colors represented by the different speeds. If put through a simple spectrometer or prism it would produce a spectrum. That’s the blackbody spectrum. Lecture 2: Spectral line analysis
—  Wave nature of light
—  Particle nature of light
—  Spectroscopy for absorption and emission processes
Spectral Line emission (spectroscopy)
¡
Radiation can be examined with a simple spectrometer
Interaction principle
—  The way that atoms and molecules absorb and emit
radiation can tell us something about their nature or
identity.
—  Demo: Hydrogen emission
Continuous spectrum
Absorption spectrum
Emission spectrum
Electron moved
from ground state to
elevated state.
Absorption
UV Photon Electron falls down to ground state again A photon is emiKed equal in energy to the difference between ground state and excited state. Emission Each transi&on from higher to lower state emits a photon of a certain energy and therefore wavelength »  The emission spectra of an element provides a fingerprint that allows scien&sts to deduce its presence from the observa&on of the specta ˃  Analogy: Bar code »  Detec&ng composi&on ˃  The composi&on of an object is determined by matching its spectral lines with laboratory spectra of known atoms and molecules »  Link to Unit Starter: ˃  What if every element and molecule has a specific set of seats available on the bleachers: +  You would only see a specific # of emission lines as electrons move up and down into them? ˃  That’s exactly how atoms and molecules work. ˃  They have a fingerprint that is their absorp&on/emision spectrum that is unique to that element if you look for the transi&ons that should set it apart from all the others. ˃  The cataloguing of these transi&on loca&ons and energies in the lab has helped scien&sts find many atomic and molecular species in the night sky remotely. -­‐ Both the proton and the electron are going to have an individual spin The spin of both can therefore be in the same direc&on (aligned) or in opposite direc&ons (an&-­‐
aligned) Because of quantum mechanics, it turns out when the spins are aligned, the hydrogen is higher in energy •
•
hKp://upload.wikimedia.org/wikipedia/commons/thumb/e/e1/HydrogenLineParallel.svg/500px-­‐HydrogenLineParallel.svg.png -­‐ Even though the Aligned version is higher in energy, its electron s&ll exists in the S orbital Instead, the aligned version compared to the an&-­‐aligned version has hyperfine structure •
-­‐ It is possible for hydrogen to jump from its higher energy aligned state to the lower energy an&-­‐aligned state Very unlikely to happen: •
o
o
probability of 2.9×10−15 s−1
&me it takes for a single isolated H atom to undergo this transi&on is ~ 10,000,000 yrs •  When it does happen, it releases a specific wavelength of light... o
Care to guess what that wavelength is? -­‐ The energy gap between the hyperfine structures directly corresponds to the 21-­‐cm wavelength (1420.405... MHz) •
•
This wavelength was predicted by Jan Oort and Hendrick C. van de Hulst in 1944 Discovered by Edward Mills
Purcell and Harold Irving Ewen
in 1951
hKp://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Green_Banks_-­‐_Ewen-­‐
Purcell_Horn_Antenna.jpg/321px-­‐Green_Banks_-­‐_Ewen-­‐Purcell_Horn_Antenna.jpg •
•
hKp://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320px-­‐NGC_6384_HST.jpg hKp://upload.wikimedia.org/wikipedia/commons/thumb/4/43/ESO-­‐VLT-­‐Laser-­‐phot-­‐33a-­‐07.jpg/320px-­‐ESO-­‐VLT-­‐
Laser-­‐phot-­‐33a-­‐07.jpg -­‐ So what's the point? What can be done with this informa&on? First use of this was in 1952 where the first maps of neutral hydrogen in our galaxy were made These maps using the doppler shiq of the 1420 MHz spectral line revealed the spiral structure of our galaxy »  So we have seen that if maKer is moving in any way, charged par&cles are being accelerated »  If charges are being accelerated EMR photons are being produced »  The power and spectral distribu&on of those photons depends on The Temperature of the material. »  Therefore: We can detect the temperature of materials in space by analyzing the light coming to us on earth.! Lecture 3
NON-THERMAL EMISSION
AND OTHER WEIRDNESS
This energy distribution
can be modelled very
accurately. Everything
resembling this shape is
called THERMAL
radiation.
Remember that the temperature of an
object can be inferred from the peak
wavelength of the blackbody spectrum.
λ~1/T
Comparison of
Thermal vs.
Non-Thermal
radiation
Non-thermal
Thermal
Think of intensity as the
number of photons
In thermal radiation,
most photons are at the
peak frequency, thus
you can relate that to
the Temperature
(average kinetic energy)
Non-thermal
Thermal
In non-thermal you
can’t do that ……
Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring 2014. Minnesota State University, 6 Jan. 2014.
Direct observations leading to new insights
—  Particle physics studies the properties of the
fundamental particles of matter.
—  Uses very high energy
—  Alows us to discover how particles behave at these
high energies.
—  Non-thermal emission processes were discovered in
this way.
From these types the synchrotron radiation
seemed to fit the models for non-thermal sources
—  The non-thermal emission properties were used to
model the spectra of quasars and other radio
sources.
—  The spectra of these could be explained with the
models
Synchrotron Radiation
—  First discovered in a Bell Laboratory particle
accelerator called a ‘synchrotron’ (1947)
—  The power law distribution was very different from
the Maxwellian-Planck distribution in that it
increased with higher frequency
—  High energy sources could then be detected by this
unusual spectral feature especially at x-ray and
gamma-ray bands.
Examples of Astrophysical Synchrotron
Radiation
The bluish region in the
center of the crab
nebula is caused by
synchrotron radiation
The bluish jet from M87 is
emerging from the AGN core
Case Study: Blazars
(yes, that is an actual group of objects in astronomy)
—  In 1963 Maarten Schmidt discovered quasars using
radio wave measurements
Quasars – Quasi-star radio sources
¡  Quasars are:
¡
÷  Very
distant (100s of billion LY)
÷  Very bright (about the same amount of light as our entire galaxy)
÷  Highly Variable (changing in periods of days to years)
¡
This was a discovery that confirmed the big bang cosmological
model over the static universe model.
Blazars cont.
—  Blazars are radio “quiet” but have red shifts similar
to quasars and are therefor very distant.
¡
Blazars are originally named BL Lac objects from observations
of the star BL lacertae
Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy 101 Spring 2014. Minnesota State University, 6 Jan. 2014.
Other Sources of Non-Thermal (Synchrotron)
Radiation: MASERS
—  Microwave
Amplification by
Stimulated
Emission of
Radiation
Emissions from a
particular transition
are used as a pump
for sustained emission
from other molecules
¡  Added together the
radiation becomes
amplified
¡
Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL
MASERS IN ONSALA 1." The Astrophysical Journal 716.1 (2010): 106-13. Web.
MASERs cont.
Requirements for interstellar MASERs
—  Low density
Less than 104 cm-3
¡  This is very difficult to achieve in the Lab but is very high
density for interstellar media
¡
—  But high gain
¡  Lots of particles in the path along the line of site
—  Therefore, we need large regions in space to form
masers
¡
1014 cm3
Summary of Non-Thermal Sources
—  Non-Thermal sources have a different
energy distribution function.
¡
Basically everything that doesn’t look like this
is non-thermal
—  Synchrotron radiation observed in particle
accelerators explains the spectra of distant
quasars
—  Observations of non-thermal radiation has
lead to important discoveries of Active
Galactic Nuclei (AGN)
1.  "Astronomy: A Beginner's Guide to the Universe" 7th ed. Chaisson, E.; McMillan, S. Pearson Educa&on inc. 2013 p.503 2.  hKp://physics.nist.gov/cgi-­‐bin/cuu/Value?me|
search_for=electron+mass 3.  “Outer Space is not Empty: A Teaching Unit in Astrochemistry”. RET 2004 Haystack Observatory MIT. Wesley Johnson and Roy Riegel. 4.
5.
6.
7.
8.
9.
10.
Course: ASTR 122: Birth, Life and Death of Stars hKp://jersey.uoregon.edu/~imamura/122/astro.122.html hKp://www.pbs.org/wgbh/aso/tryit/radio/indext.html
hKp://galileo.phys.virginia.edu/classes/241L/emwaves/emwaves.htm
hKp://www.astro.utu.fi/~cflynn/astroII/l4.html hKp://scienceworld.wolfram.com/physics/BrightnessTemperature.html Eskridge, Paul. "Ac&ve Galac&c Nuclei." Notes for Week 14 Astronomy 101 Spring 2014. Minnesota State University, 6 Jan. 2014. Web. 24 July 2014. <hKp://frigg.physastro.mnsu.edu/~eskridge/astr101/
week14.html>. Fish, Vincent L., and Loránt O. Sjouwerman. "GLOBAL VERY LONG BASELINE INTERFEROMETRY OBSERVATIONS OF THE 6.0 GHz HYDROXYL MASERS IN ONSALA 1." The Astrophysical Journal 716.1 (2010): 106-­‐13. Web.