Astronomical Spectroscopy
Lab #2: “Introduction to Spectroscopy”This lab teaches how to obtain spectra and conduct basic calibration of spectroscopic data.
• Due is Nov 6 (start early!!!) • No class on Oct 14 and Nov 4 • Group-led discussion on Oct 21 and 18
Two experimental steps:(1) using the in-lab spectrometer (lab tutorial
sessions on Oct 7 & 8);(2) using the campus telescope spectrometer (late-
night telescope sessions between 4th and 5th
week of October)
Short Introduction to Lab #2
What is spectroscopy and why is it important?
How does spectroscopy work?
What is the equipment the we need for spectroscopy?
How do we reduce spectroscopic data? Specifically how do we know the wavelengths of the photons (= wavelength calibration)?
Astronomical Spectroscopy
They are very different … Let’s talk about the difference.
Image (Photometry) Spectrum (Spectroscopy)
Astronomical Spectroscopy
Integrated Light Over Wavelength Range
Image (Photometry) Spectrum (Spectroscopy)
Dispersed Light Over the Wavelength
Astronomical Spectroscopy
What is the best real life example of spectrum?
Astronomical Spectroscopy
Rainbow is a dispersed light (= spectrum) of sunlight by the Earth’s atmosphere (acting as a prism)!
Astronomical Spectroscopy
Solar spectrum looks like this – why?
Astronomical Spectroscopy
Solar spectrum looks like this – why?
Blackbody-like Continuum Emission (T 6000K)
Line Emission from different atoms (e.g., H, He, C, etc)
Astronomical Spectroscopy
The peak wavelength increases as the temperature decreases!
Blackbody Radiation
Astronomical Spectroscopy
What is the best real life example of line emission?
Astronomical Spectroscopy
Astronomical Spectroscopy
Neon Light!
Astronomical Spectroscopy
Astronomical Spectroscopy
Neon has many bright emission lines in the visible band.
Wavelength ( )
Astronomical Spectroscopy
Spectroscopy tells us:
(1) Continuum shape (or temperature if the continuum is blackbody radiation)
(2) Chemical elements/composition for line emission/absorption and physical conditions of the elements
(3) Velocity of the source ( how?)
Astronomical Spectroscopy
What do we need to get spectra?
(1) light sources (e.g., stars, galaxies, The Sun, bulb, etc)
(2) ?
(3) detector (e.g., CCD)
Astronomical Spectroscopy
What do we need to get spectra?
(1) light sources (e.g., stars, galaxies, The Sun, bulb, etc)
(2) dispersing elements (like what?)
(3) detector (e.g., CCD)
Astronomical Spectroscopy
What do we need to get spectra?
(1) light sources (e.g., stars, galaxies, The Sun, bulb, etc)
(2) dispersing element (prism, grating, etc)
(3) detector (e.g., CCD)
Astronomical Spectroscopy
Dispersion of Light by Diffraction Grating
After a dispersion element (e.g., prism, grating), photons of different wavelengths (= colors) travel different paths!
Astronomical Spectroscopy
“Photons of different wavelengths arrive at different positions (= pixels) on the detector (e.g., CCD)”
So you can tell the wavelengths of the photons.
Source
Detector
(Grating, Prism, etc)
2
1
Simple Spectrograph Configuration
Astronomical Spectroscopy
How do we know the wavelengths of the photons obtained in spectroscopy?
All we have is digitized intensities of detector pixels.
Astronomical Spectroscopy
How do we know the wavelengths of the photons obtained in spectroscopy?
Take a spectrum of a known line source (e.g., Ne lamp) with the same instrument.
Establish a relation (“wavelength mapping solution”) between the wavelengths and detector pixels using the wavelengths and detector pixel positions of the known lines. The more line you use, you can obtain the more reliable solution.
Apply the wavelength solution to your spectra.
Astronomical SpectroscopyNeon Calibration Lamp Spectrum
Obtaining a wavelength mapping solution btw. the wavelengths of the calibration lines (e.g. Ne lines) and detector pixel positions using linear least-square fitting is an essential component of Lab #2.
Astronomical Spectroscopy
1 (Detector Pixels) 2048
Example: Wavelength calibrated spectrum
All right, let’s look (astronomical) spectroscopy a bit more.
Stellar Spectrum
Example solar spectrum obtained in previous AST325/326
Does this make
sense to you?
Spectrum: Intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission
Continuum occurs at all wavelengths: e.g., Bλ(T) as below.
Spectrum: Intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission
Line emission occurs at specific wavelength: λ = hc/(E2-E1) E = h = h/c
How are they (left vs. right) different?
Radiation: Continuum & Line Emission
Continuum emission: three types are known
Radiation: Continuum & Line Emission
Continuum emission:
Blackbody Radiation (e.g., stellar radiation)
Synchrotron Radiation (electrons around B field, e.g., accelerators, pulsars)
Thermal Free-Free Radiation (= Bremsstrahlung, e.g., ionized gas around hot stars)
Radiation: Continuum & Line Emission
Line emission:
Atomic Transition (e.g., H I lines)
Molecular Transition (e.g., OH, CO lines)
Solid-State Feature (e.g., aerosol, …)
Line Emission/Absorption: e.g., Hydrogen (H)
H series (mostly in the visible bands)
e.g. H transition: n = 3 → 2 transition at 656.3 nm.
R: Rydberg constant for hydrogen.
Observed Stellar Spectrum = Continuum + Line Transition (mostly absorption)
Spectrum: intensity of radiation as a function of wavelength (“dispersed light”)Continuum (e.g., blackbody radiation) and Line Emission
Galaxy Spectra: examples
Galaxy: numerous stars and gas clouds
Mixture of continua, absorption, and emission lines
Astronomical Spectroscopy
Example Spectrum of a Dusty Galaxy: Complex spectral features
Spectrum provides information for chemical composition!
Star: let’s assume it to be a pure continuum source
Gas cloudContinuum source through gas cloud
Colored bars: gas cloud emission lines
Continuum source
Assumptions:
[1] Star is a pure continuum source.
[2] Gas cloud has no continuum emission.
White bars: gas cloud absorption lines
Depending on the relative positions of the sources and observers, spectra appear differently.
Let’s understand formation of absorption lines.
The Sun radiates continuum emission close to a blackbody radiation of 6000 K. Many elements at its photosphere absorb the continuum emission to create the numerous absorption line features.
Spectra give critical information about the source (e.g., temperature, density, composition, etc)
Stellar Spectra Stars are often classified to be “OBAFGKM”
type depending on their surface temperature.
Finally, spectrum gives velocity information. How?
Photons wavelength changes depending on the relative motion of the source. By measuring the wavelength offset, you can calculate the velocity of the source.
Spectrum gives velocity information.
Doppler Shift
Spectrum gives velocity information.
Doppler Shift
: Offset btw. the measured and intrinsic wavelength.So by measuring the wavelength offset (= Doppler shift), we can obtain the velocity of the source.