Transcript Natural Sources of Radio

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

Radiative Source
Processes
Blackbody emission
Free-free radiation
Spectral line emission
Cyclotron and
Synchrotron radiation
Radiative
Transfer
processes
Observed
light
Any process that will accelerate a charged particles will
produce EM radiation
•
˃ This could be a free electron traveling through the vaccum of space and being
affected by a magnetic field and thus accelerated
˃ It could be a bound electron or proton and the motion associated with
thermal energy is causing quick accelerations associated with that motion.
The Kinetic molecular theory states all matter is
made of tiny particles in constant motion
o
o
The constant motion will generate EM radiation
We call this type of emission, thermal emission
The type of radiation tells us something about the
source
Thermal emission
Blackbody radiation
Spectral line emission
Free-free radiation
Non-Thermal
Cyclotron emission
synchrotron emission
MASERs
•
•
•
•
•
•
All macroscopic (everyday) objects emit EM radiation at
all times!! (if T > 0 K)
explaination: The Kinetic Molecular Theory,KMT
» all matter is made up of tiny particles (atoms,
molecules, sub-atomic particles) in constant motion.
Temperature is a direct measure of average kinetic
energy of all microscopic particles.
Velocity vector
Distribution
of the # of
particles at
each level of
kinetic
energy
# of molecules
Atom or molecule
T
Average Kinetic Energy
Wein's Law
» Wavelength of peak
emmission
𝜆∝
1
𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
˃ Wavelength of peak emission
is inversely proportional 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
proportial to the Temperature to
the 4th power
𝑃 ∝ 𝜎𝑇 4
» The KMT represents
particles as moving at a
distribution of Kinetic
energy
» Accelerating charges
create EM waves, The
different accelerations
produce different
frequencies
» A blackbody spectrum
represents the
distribution of EM
radiation and changes
with temperature
» Link to Starter Activity:
˃ 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
combination 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 emitted equal in energy
to the difference between ground
state and excited state. Emission
Each transition 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 scientists to
deduce its presence from the
observation of the specta
˃ Analogy: Bar code
» Detecting composition
˃ The composition 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 absorption/emision spectrum that is
unique to that element if you look for the transitions that should set it apart
from all the others.
˃ The cataloguing of these transition locations and energies in the lab has
helped scientists 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
direction (aligned) or in opposite directions (antialigned)
Because of quantum mechanics, it turns out
when the spins are aligned, the hydrogen is
higher in energy
•
•
http://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 still exists in the S orbital
Instead, the aligned version compared to the
anti-aligned version has hyperfine structure
•
- It is possible for hydrogen to jump from its higher
energy aligned state to the lower energy anti-aligned
state
Very unlikely to happen:
•
o
o
•
probability of 2.9×10−15 s−1
time it takes for a single isolated H atom to undergo this transition 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
http://upload.wikimedia.org/wikipedia/commons/thumb/f/f7/Green_Banks_-_EwenPurcell_Horn_Antenna.jpg/321px-Green_Banks_-_Ewen-Purcell_Horn_Antenna.jpg
•
•
http://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/NGC_6384_HST.jpg/320px-NGC_6384_HST.jpg
http://upload.wikimedia.org/wikipedia/commons/thumb/4/43/ESO-VLT-Laser-phot-33a-07.jpg/320px-ESO-VLTLaser-phot-33a-07.jpg
- So what's the point? What can be done with this
information?
First use of this was in 1952 where the first maps
of neutral hydrogen in our galaxy were made
These maps using the doppler shift of the 1420
MHz spectral line revealed the spiral structure of
our galaxy
» So we have seen that if
matter is moving in any
way, charged particles are
being accelerated
» If charges are being
accelerated EMR photons
are being produced
» The power and spectral
distribution 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 Education
inc. 2013 p.503
2. http://physics.nist.gov/cgibin/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.
Course: ASTR 122: Birth, Life and Death of Stars
http://jersey.uoregon.edu/~imamura/122/astro.122.html
http://www.pbs.org/wgbh/aso/tryit/radio/indext.html
http://galileo.phys.virginia.edu/classes/241L/emwaves/emwaves.htm
http://www.astro.utu.fi/~cflynn/astroII/l4.html
http://scienceworld.wolfram.com/physics/BrightnessTemperature.html
Eskridge, Paul. "Active Galactic Nuclei." Notes for Week 14 Astronomy
101 Spring 2014. Minnesota State University, 6 Jan. 2014. Web. 24 July
2014.
<http://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.