R E T 2 0 1 3 M I T H A Y S T A C K O B S E R V A T O R Y

Natural Sources of Radio

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  Transfer  processes  

Blackbody  emission  

Free-­‐free  radia&on  

Spectral  line  emission  

Cyclotron  and  Synchrotron  radia&on  

Observed    light  

Radia&ve  Source  Processes  

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  The  constant  mo&on  will  generate  EM  radia&on    o  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  

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  

Atom  or  molecule  

#  of  m

olecules  

Average  Kine&c  Energy  

T  

Distribu&on  of  the  #  of  par&cles  at  each  level  of  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∝  𝜎𝑇↑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

UV  Photon  

Electron moved from ground state to elevated state. Absorption

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  probability of 2.9×10−15 s−1

o  &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  

-­‐  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  

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hKp://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320px-­‐NGC_6384_HST.jpg  

»  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.!  

N O N - T H E R M A L E M I S S I O N

A N D O T H E R W E I R D N E S S

Lecture 3

Remember that the temperature of an object can be inferred from the peak wavelength of the blackbody spectrum. λ~1/T

This energy distribution can be modelled very accurately. Everything resembling this shape is called THERMAL radiation.

Comparison of Thermal vs. Non-Thermal radiation

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)

In non-thermal you can’t do that ……

Thermal

Non-thermal

Non-thermal

Thermal

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.   Course:  ASTR  122:  Birth,  Life  and  Death  of  Stars  hKp://jersey.uoregon.edu/~imamura/122/astro.122.html  

5.    hKp://www.pbs.org/wgbh/aso/tryit/radio/indext.html    6.  hKp://galileo.phys.virginia.edu/classes/241L/emwaves/emwaves.htm

   7.  hKp://www.astro.utu.fi/~cflynn/astroII/l4.html    8.  hKp://scienceworld.wolfram.com/physics/BrightnessTemperature.html  9.  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>.  

10.  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.