Updated Wednesday, 21-Jan-2004 09:57:23 EST
Types of Astronomical Measurements
Remote sensing -- we can't visit most places in the universe (but we've
sent probes to almost all the planets and some of their moons)
Can use same techniques as for Earth-sensing (determine temp., chemical
Some astronomical objects of interest come to us -- solar wind, cosmic
Problems with Earth-based astronomy
Atmospheric blurring -- caused by changes in density and temperature
at various altitudes and locations
Atmospheric absorption -- caused by water vapor and other chemical
constituents (e.g., visible light is partly absorbed by various chemicals,
x-rays are totally absorbed by the atmosphere)
Light pollution -- artificial light (from cities) diffused through
the atmosphere interferes with astronomical observations
Gravity -- distorts the shape of very large mirrors or lenses used
- Problems with space-based astronomy
Cost -- remember, it costs about $10,000 per pound to put any kind
of satellite into orbit (and that doesn't include the cost to build
Added complexity -- need not only a telescope, but a satellite to
carry it, communications to send commands and receive data, etc.
Astrometry -- measure positions and distances of objects
Parallax shift -- observe apparent shift in position of a distant
object and use that information to determine how far away that object is
from Earth (a form of triangulation, uses trigonometry)
Not too accurate for very distant objects because the angular shift between
measurements is too small
Doppler shift -- uses shift in spectral pattern to determine how
fast the object is moving relative to Earth.
A simple example of Doppler shift is shown below. The observer sees three
stars that are all emitting yellow light. Because star #2 is not moving
relative to the observer, the observed light is yellow. Star #1 is moving
toward the observer, so between times when a wave peak is emitted, the
star has moved closer and therefore the distance between peaks (the wavelength,
shown as a black bar) is shortened. Shorter wavelengths correspond to blue
light. Conversely, Star #3 is moving away from the observer, so the wave
peaks are spread out and the longer wavelengths correspond to red light.
We say that stars moving away from us are red-shifted. The amount
of red-shift (elongation of the wavelength) is proportional to the star's
speed, and that is proportional to the star's distance from us.
The relationship between speed and wavelength shift is given by the Doppler
v = c(Dl/l)
where v = object's speed, c = speed of light = 3 x 108
m/s, Dl = shift in wavelength of a characteristic
peak in the object's spectrum, and l = wavelength
of the characteristic peak.
For example, if the normal spectrum of a star (i.e., if the star were not
moving) had a characteristic peak at l = 0.45
microns, but the observed spectrum from the actual star showed this characteristic
peak shifted to 0.4502 microns, then Dl = 0.0002
microns, and the star's speed relative to Earth would be
v = c(Dl/l) = 3 x 108
m/s (0.0002 microns/0.45 microns) = 133333 m/s.
(note that this star is moving away from Earth since the spectral
peak shifted to a longer wavelength).
As a result of the Big Bang (when the universe formed), the more distant
an object is from us, the faster it is moving away (in general). The relationship
between speed and distance is given by
Hubble's Law, v = Hd, where v = object's speed
relative to Earth, H = Hubble's constant, and d = object's
distance from Earth.
We now know Hubble's constant (newest data from May 1999) to be approximately 21 km/sec
for every 1 million light years of distance. A light year is the distance
that light travels in one year, about 9.5 x 1015 meters. This
gives H = 2.2 x 10-18/second. Thus, a star 1 million
light years away from Earth is moving away from us at 21000 m/sec, while
a star 6 million light years away is moving away from us at 126000 m/sec. (Note
that Hubble's constant is named for the astronomer Edwin Hubble, who discovered
this speed vs. distance relationship, and for whom the Hubble Space Telescope
is also named).
Combining Doppler shift measurements with Hubble's Law, we can estimate
the distance to a star. Using the example above, the Doppler shift
indicates that the star is moving at 133333 m/s. If we solve for
the distance in Hubble's Law:
d = v/H = (133333 m/s)/(2.2 x 10-18/second) = 6.06
x 1022 m. (or 6.4 million light years).
Brightness -- gives information about how much energy a star is generating
over a given period of time
Color -- information about temperature (Wien's Law)
Spectroscopy -- provides information about chemical composition
of a star, planet, moon
Some current space astronomy missions:
Hubble Space Telescope (HST)
Gamma Ray Observatory (GRO) -- re-entered Earth's atmosphere in 2000
- Swift -- gamma-ray burst detector and observer -- to be launched in Dec. 2003
- Cosmic Background Explorer (COBE)
(For information about the present locations of these satellites and when
you can see them from any particular location on Earth, try running the
Java applet JTrack.
Your Web browser must support Java in order to run this program.
Additional material contained in The Big Missions
Copyright © 1998, Robert G. Melton
Updated Wednesday, 21-Jan-2004 09:57:23 EST