ASTR 511 (O'Connell) Lecture Notes


Milky Way from CTIO

Southern Milky Way from CTIO


A. Definition

B. Uniqueness


Solar SED  
PLASMAS (to 105K)

Composite QSO  

===> 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.

12/97 GRB

First optical ID of a GRB.
Click for info.


  • 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)
  • Variability
  • Polarimetry


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)

  • 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 (optical/radio spectroscopy)
  • 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, 1995).
  • High redshift evolution of galaxies: "Butcher-Oemler effect" & "faint blue galaxies" (deep optical imaging)
      HST 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.

Counterexamples: theory-driven discoveries

  • 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)
  • Helioseismology
  • 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 successful.

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.

    What's meaningful? What's not? What's real vs. what's noise? How big are systematic errors? What's interesting? What's right?

        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 diagram?
        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?

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 & gravitational lenses.


Most groundbreaking discoveries are enabled by NEW observational capabilities.

  • Local example: High Electron Mobility Transistor detectors from the NRAO CDL enabled the current generation of CMB microwave experiments

Key technology milestones for UVOIR astronomy:

  • 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

Wilson 100-in

Mt. Wilson 100-in. Discovered external
galaxies, expanding universe.

UVOIR telescope size: determines ultimate sensitivity

  • Diameter doubling time ~45 years

  • Largest scopes now 8-10m diam

      Collecting area of 10-m is 4×106 that of the dark-adapted eye

  • 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 billion threshold.

Growth in TelDia
Other key developments:

  • 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)


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 signal-to-noise issue:

  • 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

B. Typical SNR's in Astronomy:

  • 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

C. The Magnitude System: a traditional way of describing brightnesses in astronomy but one which is confusing for newcomers. See Lectures 2 and 14.

D. Backgrounds

  • 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:

      UVOIR: 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.


A. EM Wavelength Coverage

  • 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.

B. Point Source Sensitivity

    Faintest UVOIR point source detected:

    • 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

    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.

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 telescopes.)

  • 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 < 1 µ)

  • 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 Å)

  • Physical analysis is done at moderate-to-high resolution (0.01-10 Å)

  • 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" (Arp 1965):

    • 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.



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.

Related pages:


  • 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.

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Last modified 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.