Astronomy - Astronomical Observation

The light you see, the infrared (literally, "below red") radiation that warms you, the ultraviolet (literally, "beyond violet") rays that sunburn you, the radio waves that carry your music and television signals and the x-rays used to diagnose medical conditions, are all forms of electromagnetic radiation. They are all waves: periodic functions of space and time. The length of a spatial period is called the wavelength (denoted l), and the inverse of the temporal period is called the frequency (denoted n, and measured in cycles per second, or Hertz, abbreviated "Hz"). All of these waves propagate at the speed of light ("c"), and for our purposes, obey the simple dispersion relation
c = wavelength * frequency
This Java applet illustrates some of these wave characteristics.

All of these types of electromagnetic radiation consist of changing electric and magnetic fields, and differ only by their wavelength (or frequency):

radiationln (Hz)energy (eV)source
radio> 1 m< 3 * 108< 1.24 * 10-6low-energy atomic or molecular motions
microwave> .1 mm< 3 * 1012< .0124rigid molecular motions
infrared> 7000 Angstroms< 4.3 * 1014< 1.78molecular bond motions
visible light> 4000 Angstroms< 7.5 * 1014< 3.1atomic electron transitions
ultraviolet> 50 Angstroms< 6 * 1016< 248atomic electron transitions
x-rays> .03 Angstroms< 1020< 414 Kelectron transitions in heavy atoms
gamma rays< .03 Angstroms> 1020> 414 Knuclear decays

An Angstrom is 10-10 meters. An eV (electron-volt) is 1.602 * 10-19 Joules (a 60 Watt light bulb uses 60 Joules of energy every second).

The energy column corresponds to the energy of a single photon: the smallest amount of radiation possible. The energy (in eV) of a photon of frequency n is
energyeV = h * frequency / e
where h is Planck's Constant (6.626 * 10-34 Js) and e is the number of Joules in an eV.

Electromagnetic radiation from the near-infrared (close to the visible band) to x-rays are produced when an atomic electron moves from one energy level to a lower one. This Java applet provides 3-D images of Hydrogen atoms or Hydride ions (Hydrogen atoms with an extra electron) with various electron configurations. Infrared radiation of lower energy results from internal molecular vibrations (in these examples, of triatomic molecules).

Almost all that we know about the universe comes from these attributes of electromagnetic radiation:

The exceptions: cosmic rays, neutrinos and gravitational waves (see below).

We often measure the spectrum of an object: the distribution of the intensity (energy per unit area per unit time) of the electromagnetic radiation we observe as a function of wavelength.

The Spectrum Viewer will allow you to analyze spectra of several stars in detail. (We will come back to this applet often.)

(View Cosmos DVD 6, episode 10, dramatization of red shift discoveries at Mt. Wilson.)

Modern Astronomical Instruments

As we have discussed, we can only directly measure the angle between two objects, not their distances from us. But if we know their distance from us, we can compute the distance between the two objects. More usually, we are measuring the size of a single object from the angle it subtends:
size = distance * angular diameterradians
where a radian is 180/p degrees. Here are some examples of variation of angular diameter with distance:

By setting the angular diameter in the equation above to the resolution of a telescope, we can compute the size of the smallest object it can see at any given distance. The resolution is determined by diffraction effects (see below) and depends on the size of the optics (usually a mirror) and the wavelength being observed:

angular resolutionarcsec = .0025 * wavelengthAngstroms / mirror diametercm
For instance, the Hubble Space Telescope (below) has a mirror diameter of 2.4 meters, or 240 cm. At optical wavelengths (for instance, 5600 Angstroms), its angular resolution would be
.0025 * 5600 / 240 = .0583 arc seconds,

times p / (180 * 3600) radians in an arc second = 2.828 * 10-7 radians.

Looking at Saturn at closest approach (8.004 AU, or 1.197 * 1012 m), the Hubble could resolve an object whose
size = 1.197 * 1012 * 2.828 * 10-7 = 338.6 km wide.
The light gathering power (literally, how much electromagnetic radiation the instrument can gather in a given amount of time) is proportional to the square of the mirror radius.

There are severe limits to which types of electromagnetic radiation can be observed from the Earth's surface. The Earth's atmosphere is transparent to radio waves with wavelengths between about 10 meters and 1 cm, and to infrared and visible wavelengths from 105 Angstroms to the near ultraviolet (around 2900 Angstroms). And as we will see if we get any clear evenings, turbulence in the atmosphere blurs point-like sources into seeing discs; the degree of the effect is referred to as "good" or "poor" seeing. For these reasons, many modern observatories are based in space.

Some of the premier telescopes in use today are:

Portfolio Exercise 3: Find 3 additional telescopes in 3 different regions of the electromagnetic spectrum. For each, research the range of wavelengths it is sensitive to, and identify its resolution. What is the smallest object it could discern at 1 parsec? 1000 parsecs? 1 million parsecs?

Interferometry

Our first Java applet can also illustrate how individual waves can reinforce and cancel each other out. Choose the black and blue wave modes to be the same and vary the blue phase angle from 0 (reinforcement) to p (cancellation).

A second Java applet illustrates how two traveling waves can interfere with each other.

This Java applet illustrates diffraction patterns caused by two slit-like sources.

Color Filtering and Compositing

This graph shows the Johnson-Cousins photometric system (Standard Photometric Systems, Michael S. Bessell, Annual Review of Astronomy and Astrophysics 43 293-336 (2005)). Also denoted the "ubvri" system, it breaks the electromagnetic spectrum into five bands which correspond to the wavelengths passed by filters in the near ultraviolet ("u"), blue ("b"), "visual" ("v"), red ("r") and near infrared ("i") ranges.

This Java applet shows how red, green and blue images can be composited back into a multicolor image:

You need a Java-capable browser to be able to use the applets. If they do not work with your Windows system, download the Java VM (Virtual Machine) for your version of Windows at the download section at java.sun.com.

In this image of M1 (the Crab Nebula), optical wavelengths are shown in green and dark blue, infrared is shown in red and x-ray is shown in light blue:

(source).

In this false-color image of gas pillars in M16 (the Eagle Nebula),

emissions from Sulfur (S+, 6716 and 6731 A) were captured in black and white, then colored red;
emissions from Hydrogen (6563 A) were captured in black and white, then colored green; and
emissions from Oxygen (O++, 4363 and 5007 A) were captured in black and white, then colored blue,
and the three colored images were composited as in the Java applet above:

(source)

This has become a commonly-used color palette for images of nebulae. Here is a true-color optical image.

The red color of the nebula arises from Hydrogen emissions at 6563 Angstroms (called "H-Alpha"). These emissions are powered by ultraviolet radiation from hot stars, whose winds help to create the shapes and shock waves characteristic of so many nebulae. Light from blue nebulae, in contrast, is reflected from dust grains which, like our atmosphere, scatter blue light more efficiently than other colors. M42 (The Orion Nebula) and M20 (The Trifid Nebula) provide good examples of both types, as well as lanes of molecular dust.
Here is an example of contrast enhancement: the original image, and the same image with the midtone contrast stretched.

Images in multiple electromagnetic radiation bands

NGC 7293 (the Helix Nebula) in
visible (source) 4000-7000 A
infrared (source) R = 240000 A, G = 58000-80000 A, B = 36000-45000 A
ultraviolet (source) 1350-2800 A
x-ray (source) 6-100 A
M31 (the galaxy in Andromeda) is shown here in
visible (source) 4000-7000 A
infrared (source) R = 80000 A, B = 36000 A (star light subtracted)
ultraviolet (source) R = 1750-2800 A, B = 1350-1750 A
x-ray (source) R = 12.4-24.8 A, G = 6.2-12.4 A, B = 3.1-6.2 A
radio (source) 21 cm (2100000000 A)
NGC 5128 (Centaurus A) seen in
visible4000-7000 A
infrared36000-80000 A
radio20 cm (2000000000 A)
x-rayR = 12.4-24.8 A, G = 9.3-12.4 A, B = 6.2-9.3 A
(source, source)

M 104 seen in

visible4000-7000 A
infraredR = 80000 A, Orange = 58000 A, G = 45000 A, B = 36000 A
x-rayOrange = 8.3-41.4 A, B = 1.8-8.3 A
(source).

Cosmic Ray Astronomy

Cosmic rays are not radiation at all: they are high-energy atomic nuclei, mostly protons. Their energies range from 1 GeV (109 electron volts) up to (rarely) 1020 eV. Those up to around 1015 eV are thought to originate from within our galaxy, while the more energetic cosmic rays are thought to originate outside of the galaxy. The energy density of cosmic rays in the galaxy is around 10 MeV per cm3, maintained largely by exploding supernovae. Those cosmic rays originating outside the galaxy come from within a radius of about 250 million light years. Above approximately 4 * 1019 eV, cosmic rays interact with the Cosmic Microwave Background Radiation; this limits the range of the most energetic cosmic rays.

When cosmic rays hit the atmosphere they produce a shower of particles. It is this shower which is detected by cosmic ray observatories such as the Pierre Auger Cosmic Ray Observatory in western Argentina. Their goal is not to produce an image of a cosmic ray source, but to simply identify the sources and measure the energies associated with cosmic rays. To do this, the Auger Observatory has constructed 1600 detectors covering an area of about 3000 km2. Events with energies over 1019 eV have a flux of about 1 per km2 per year.

Neutrino Astronomy

The purpose of neutrino astronomy is much the same as that of cosmic ray astronomy: to detect sources and measure energies. Neutrinos are electrically neutral, almost massless, and interact so weakly that the flux of solar neutrinos through the Earth of approximately 65 billion per cm3 passes through the Earth each second with only a handful of interactions. These facts makes this perhaps the most difficult astronomical undertaking besides the detection of gravitational waves.

The Kamiokande observatory in Japan has been instrumental in confirming our understanding about supernova explosions, and in determining that neutrinos indeed have mass. Its successor ("Super-Kamiokande") is comprised of 13027 photomultiplier tubes in 50000 tons of pure water. The tubes detect Chernekov Radiation: electromagnetic energy emitted by charged particles whose speed is greater than the speed of light in water (about 2.25 * 108 m/s). When a neutrino interacts with an atom in the water, an electron or muon is created whose track the tubes can measure. The instrument typically detects less than 14 solar neutrino events per day.

The latest high-energy neutrino observatory is currently under construction using 1 km3 of Antarctic ice: the IceCube Neutrino Observatory. It will focus on neutrinos entering the opposite side of the Earth so that the lower-energy neutrinos associated with cosmic ray showers will not be confused with those of extraterrestrial origin.

Gravitational Wave Astronomy

In 1974, Taylor and Hulse found a binary system of neutron stars, one of which is a pulsar. After two decades of observation, they determined that the change in the rate of spin of the pulsar matched the predictions of General Relativity for such a system emitting gravitational waves (source):

But no one has ever directly detected a gravitational wave. It is the purpose of LIGO - the Laser Interferometer Gravitational-wave Observatory - to change that.

LIGO consists of two L-shaped detectors, each 4 km long, one in Hanford, Washington, and the other in Livingston, Louisiana. In each, laser light travels repeatedly from one end to the other, reflected by mirrors. A passing gravitational wave will change the relative lengths of the two beams, and the change in the interference pattern will be registered by a photodetector. The fifth science run (recently completed) achieved a sensitivity of one part in 1021 from 70 Hz up, enabling detection of binary inspiral of 1.4 solar mass neutron stars at a distance of 12 Mpc. No news yet on what if anything they have seen.

©2008, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

Please send comments or suggestions to the author.