ASTR 511 (O'Connell) Lecture Notes
1: INTRODUCTION TO
Southern Milky Way from CTIO
I. WHY UVOIR?
UVOIR = the "UV, Optical, Near-Infrared" region of EM spectrum
- Shortest wavelength: 912 Å (91.2 nm) --- the Lyman edge of H I;
the interstellar medium is opaque for hundreds of
Å below this point
- Longest wavelength: ~3 µ
(3000 nm) --- serious H2O absorption in Earth's
atmosphere above this point.
- Best developed instrumentation; best understood astrophysically
- Highest density of astrophysical information
- Provides prime diagnostics on the two most
important physical tracers:
===> UVOIR observations/identifications are almost always
prerequisites to a thorough understanding of cosmic
sources in other EM bands.
- Stars are the basic building blocks of the universe (even if not the
most important by mass)
- For instance, stars establish fundamental cosmic age
and distance scales
- Ages: e.g. star cluster turnoff temperatures/luminosities
- Distances: e.g. Cepheid variations, supernovae
- Example: Gamma Ray Burst sources first were detected in 1970's,
but only physically interpreted after optical ID's in 1997
showed they were associated with distant galaxies.
First optical ID of a GRB.
Click for info.
II. KINDS OF UVOIR OBSERVATIONS
- Imaging: distribution of EM energy on celestial sphere
- Astrometry : a sub-class of imaging: precision
measures of positions & motions
- Spectral Energy Distributions (SEDs): distribution of EM
energy with frequency
- Photometry (low resolution)
- Spectroscopy (higher resolution)
III. PRIORITY OF OBSERVATIONS IN ASTRONOMY
Astronomy is driven more by new observational discoveries than
by fundamental interpretive insights (i.e. theory). The
development of astrophysics has been shaped by observations in
about 3/4 of the instances.
Few important astronomical discoveries were predicted; many
were actually accidental
Examples: (the technique/original motivation is given in parentheses)
Counterexamples: theory-driven discoveries
- Uranus (visual telescopic sky-scan)
- Expanding universe (faint galaxy spectroscopic survey)
- Pulsars (radio scintillation observations)
- Supermassive black holes/AGN's (radio surveys, optical spectroscopic surveys)
Although nuclear activity had been recognized since
the 1940's (Seyfert), its prevalence and significance was not understood until
radio observations in the 1950's-60's, especially of the compact
Quasi Stellar Objects.
- Large scale structure (redshift surveys aimed at measuring galaxy luminosity function)
- Dark matter in spiral galaxies: flat rotation curves
- X-ray emitting gas in clusters of galaxies (early X-ray surveys)
- Gamma ray bursts (military satellites looking for clandestine nuclear tests)
- Extra-solar planets (optical spectroscopic monitoring)
There was a general expectation that these existed,
based on the Copernican Principle, for example. But theoreticians
predicted that massive planets could exist only at large distances
from parent stars, implying 5-year or longer survey periods. The
first planetary-sized bodies orbiting another star were found
accidentally by radio observations of a pulsar (Wolszczan & Frail,
1992). More normal exoplanets were first securely identified through
optical Doppler-shift surveys of bright stars (Mayor & Queloz,
- High redshift
evolution of galaxies: "Butcher-Oemler effect" & "faint blue galaxies"
(deep optical imaging)
contributions in this area---e.g. the Hubble Deep Field (1996)---were
actually hindered by theoretical prejudice that distant galaxies would
be too faint to detect. The HDF deep pencil-beam survey
was delayed by 5 years.
The priority of observations means that all astronomers, observers
or not, must know how to interpret and critically evaluate
them and must stay alert for the new opportunities they present.
- Neptune (Leverrier, Adams predictions from Newtonian dynamics)
- General relativistic distortion of space-time near Sun (Eddington expedition, 1919)
- 21 cm line of HI (Van de Hulst 1944; Ewen & Purcell 1951)
- Cosmic microwave background
Predicted in 1948 by Gamow, Alpher, & Herman.
Actual discovery by Penzias & Wilson 16 years later was accidental, but a second team led
by Dicke was
preparing a deliberate search and would have been
What's meaningful? What's not? What's real vs. what's noise?
How big are systematic errors?
What's interesting? What's right?
NB: The champion "discoverer" of the 20th century
was Fritz Zwicky. He discovered dark
matter; inaugurated research on supernovae & clusters of galaxies; and predicted neutron stars &
Example 1: what is this? how was it made?
what do the colors mean?
Example 2: what does this diagram test? what
important physical implications?
Example 3: what causes the scatter in this
Example 4: is there a statistically meaningful
result here? what is it?
Example 5: classic example of systematic error
Example 6: discovery of the year or statistical fluke?
IV. HISTORICAL LESSON: TECHNOLOGY DRIVES DISCOVERY
Most groundbreaking discoveries are enabled by NEW observational
Key technology milestones for UVOIR astronomy:
- Local example: High Electron Mobility Transistor detectors from
the NRAO CDL enabled the current generation of CMB microwave
- 17th century: telescopes
- 19th century: spectroscopy, photography, quality lens making,
large structure engineering
- 20th century: large mirror fabrication, electronic
detectors, digital computers, space astronomy
- Since 1980: array detectors
Mt. Wilson 100-in. Discovered external
UVOIR telescope size: determines ultimate sensitivity
Other key developments:
- Diameter doubling time ~45 years
- Largest scopes now 8-10m diam
Collecting area of 10-m is 4×106 that of the
- In planning: 15-m to 40-m
- For a given technology, cost scales as
Cost is roughly proportional to mass. Even using new technologies,
next generation of large ground-based telescopes will cross the $1
- Sky surveys
- First: Hipparchus, 130 BC. Thousands since.
- Two most important in 20th century:
- Large format, 2-D array detectors are driving the current explosion in
imaging/spectroscopic sky surveys (e.g. 2dF, SDSS, 2MASS, and many
others, often with cutie-pie names)
- Classification systems (e.g. HD stellar spectral
classification, ca. 1890; Hubble galaxy classification, ca. 1920)
V. FLUX MEASUREMENTS IN ASTRONOMY
A. Signal-to-Noise Ratio
"Sensitivity"---i.e. the faintest source measurable---is not simply a matter
of the size of the photon collector.
It is instead a
B. Typical SNR's in Astronomy:
- SNR (or "S/N") = value measured / uncertainty in measure
- Depends on structure of source (point vs. extended), nature
of the luminous background & surroundings, foreground absorption, telescope &
instrument throughput, characteristics of detectors (quantum efficiency, noise)
- Fundamental limits are from photon statistics
- , where N is number of detected source photons
C. The Magnitude System: a traditional way of describing brightnesses
in astronomy but one which is confusing for newcomers.
See Lectures 2 and 14.
- Some things are known exactly (SNR is infinite)
- Sun is a star
- Only one star interior to Earth's orbit
- No new elements possible with fewer protons than Uranium
- High precision measures: e.g. length of the AU; period
of pulsars. SNR > 107.
- Measures of astronomical EM fluxes:
- Best precision: SNR ~ 1000 (0.1% error)
- Low by lab standards! Problems: difficulty of
instrument calibration in the "field"; faintness of interesting sources.
- Typical "good" measures: SNR ~ 20-30
- Threshold detections: SNR ~ 5-10
- Even when source fluxes are appreciable, detection can be inhibited
by luminous backgrounds, which reduce SNR.
These become important when:
(background flux)resol-element ~ (source flux)resol-element
- Diffuse backgrounds, examples:
artificial light pollution + Earth's atmosphere + ecliptic scattered
sunlight + scattered Galactic light
Far IR: interstellar "cirrus" = warm dust
Radio: Cosmic Microwave Background
- Discrete source backgrounds, e.g.:
Exclusion zone around bright stars caused by scattered light within instrument
Source "confusion" caused by diffractive blending of multiple faint sources,
e.g. in star clusters or for faint,
distant radio galaxies.
VI. LIMITS OF OBSERVATIONAL CAPABILITY
A. EM Wavelength Coverage
B. Point Source Sensitivity
- Rapid expansion since
1950: click on link for breadth of EM coverage
- The entire EM spectrum except for the extremes
is now accessible at some level to astronomers.
Faintest UVOIR point source detected:
NB: current optical detectors have ~ 100% quantum efficiency.
Therefore, we can't improve sensitivity via detector development. In
UV, IR there is room for detector improvement.
- Naked eye: 5-6 mag
- Galileo telescope (1610): 8-9 mag
- Palomar 5-m (1948): 21-22 mag (pg),
25-26 mag (CCD)
- Keck 10-m (1992): 27-28 mag
- HST (2.4-m in space, 1990): 29-31 mag
C. Spatial Resolution
- The fundamental limit is governed by diffraction in
telescopes and instruments: the minimum image diameter is given by:
where D is the diameter
of the optics aperture
- At 5500 Å,
- Inside Earth's atmosphere, turbulence strongly affects image diameter.
The resulting image blur & motion
is called "seeing", and typically yields:
... i.e. spatial resolution in most instances is governed by the atmosphere,
not the telescope. (Much effort is now aimed at turbulence control near
- Best UVOIR images: HST, ~ 0.06" (~ a quarter at 90 km)
- Best overall: VLBA (~ 0.001")---but
limited to very high surface brightness radio sources (rare)
- Ground-based (8-m) single-aperture "adaptive optics"
systems yield 0.05" over limited fields (in the near infra-red, but not for <
- Anticipated UVOIR interferometers: 0.001"
D. Spectral Resolution
- The theoretical maximum resolution is governed by diffraction in
optical components, but the practical limit is set by photon rates.
High resolution devices in astronomy are typically photon-starved (except
for observations of the Sun).
- Source identification, surveys, and classification are typically
done at low resolution (10-5000
- Physical analysis is done at moderate-to-high resolution
- Highest to date: ~ 0.001 Å
E. Other Properties : e.g. polarization, variability
F. Examples of Background-Induced Selection Effects
Galaxy surface-brightness selection, shown
in the "Arp Diagram"
- The diagram shows that identified galaxies occupy a relatively
small range of parameter space, bounded by the night sky surface
brightness on one side and the spatial resolution of survey telescopes
on the other.
- Example of a previously-concealed class of galaxies: "ultracompact dwarf
galaxies" (Drinkwater et al. 2003)
Brown dwarf companions to bright stars
Brown dwarf companions can be 103 to 106
times fainter than their primary stars. Scattered light from the
primary star inhibits searches. For detection by direct imaging, we
require a scattered light suppression technique. The same problem,
much worse, affects the search by direct imaging for Earthlike planets
in orbit around nearby stars.
VII. NON-EM CONVEYORS OF COSMIC INFORMATION
Most astrophysical information is derived
from the study of electromagnetic waves propagating over significant
distances. However, there are several niches where important
information is, or could be, conveyed by other means.
For a list, click here.
- LLM, Chapter 1
- Harwit, Cosmic Discovery [QB43.2.H37]
- "99 Things About the Last 100 Years of Astronomy," V. Trimble,
Mercury, Nov-Dec 99, p. 17.
September 2015 by rwo
Text copyright © 2000-2015 Robert W. O'Connell.
All rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 511 at the
University of Virginia.