Dr. Hoi Fung
CHAU
Department of Physics, The University of Hong Kong
Let’s have a short quiz. (Take a look at the pictures.)
Do you know what are the following five astronomical objects
in the pictures? (Sorry, you will not win a million dollars
even if you can answer it correctly. Nevertheless, please
feel free to call up your friends for help.)
(a)
(b)
(c)
(d)
(e)
(Images provided by Prof.
Bill Keel, University of Alabama)
If you are an astronomy fan, you probably recognize that
picture (a) is a galaxy.
In other word, the object in picture (a) is made up of about
to
stars that are bounded together by the attractive gravitational
force between massive objects. The shape of this galaxy looks
like a whirlpool, and astronomers called it a spiral galaxy.
But I guess most of you will not have any ideas on what kinds
of object pictures (b) to (e) are. Would you like to take
a guess?
Time is up! The answer is that pictures (b)
to (e) are galaxies, too. Believe it or not, the most surprising
fact is that all the five pictures you have just seen are
taken from the same galaxy. In fact, all the five pictures
you have just seen are taken from the spiral galaxy with the
name M81 located at a distance about light
years away from us. (One light year is
the distance traveled in one year with the speed of light
in vacuum.) I am not joking. The five pictures of the same
galaxy look so different because they are taken at different
wavelengths of the electromagnetic spectrum, namely, in (a)
optical, (b) ultraviolet, (c) X-ray, (d) infrared and (e)
radio wave frequencies, respectively.
Optical light, as well as radio waves, belongs to a family
of transverse wave called electromagnetic waves (EM waves).
EM waves are nothing but oscillations of electric and magnetic
fields. All EM waves travel at the speed of light of approximately
in vacuum. Different EM waves have different wavelengths and
hence different frequencies. Red light, for example, has a
wavelength of about 700 nm (nm is the shorthand for nanometer
or
m) while blue light has a wavelength of about 400 nm. In addition
to optical EM waves that we can see, there are EM waves that
our eyes cannot see. For instance, the radio waves we use
to transmit radio signals are EM waves with wavelength ranges
from several centimeters to several meters; while the X-rays
used in hospitals are EM waves with wavelengths of about
m. Light sensitive cells in our retina are only sensitive
to EM waves in a small wavelength window between about 400
nm to 700 nm. Thus, our naked eyes can only see objects that
are emitting EM waves in this small wavelength window, known
as the optical window.
(Figure1 The Electromagnetic Spectrum)
All objects are continuously emitting and absorbing EM radiation.
The same object looks so different at different wavelengths
simply because the object emits, reflects or absorbs light
differently at different wavelengths. In an astronomical object,
many different physical processes are going on. These processes
may lead to production of ¡§light¡¨ at different wavelengths.
Here, the word ¡§light¡¨ is used not only for EM waves in the
optical frequency range, but instead for EM waves in all frequency
ranges. For example, light emits from the surface of a typical
star falls mainly in the optical window. This is why we can
see our Sun in daytime and countless number of stars at night.
Actually, the spectrum of a hot object, such as the surface
of our Sun, depends on its temperature. If the temperature
of the object is around 3000 K, most of its radiation is emitted
as infrared radiation while a tiny fraction of its radiation
is emitted as red light. Thus, a hot iron bar at 3000 K appears
dull red. In contrast, a 6000 K object such as the surface
of our Sun emits most of its radiation in the form of yellow
light. In summary, the rule of thumb is that the hotter the
object, the emitted radiation will be of higher frequency.
Besides temperature, many other factors could also affect
the emission and absorption of EM waves. For example, electrical
charges moving under the influence of a strong magnetic field
may lead to the emission of radio waves. Since physical conditions
differ from stars to stars in a galaxy, the appearance of
an astronomical object such as M81 depends dramatically on
the window of wavelengths we choose to observe. With this
in mind, it is therefore not surprising that the pictures
(a) to (e), which look so different, are in fact pictures
of the same object.
(Metal bar appears red color at 3000K,
yellow color at 6000K and blue color at 7000K.)
Since most astronomical objects are so far away from us,
it is difficult, if not impossible, to make measurements on
the objects by actually taking samples from the object. Therefore,
analyzing their EM waves spectrum is by far the most important
way of investigating distant objects. With the advancement
of detector technology, astronomers can now exploit observations
in different wavelengths to study astronomical objects. For
example, the far infrared (of wavelengths around
m) image in picture (b) can be used to locate the regions
of M81 in which there is formation of new stars. It is because
the temperature of star formation region is hotter than typical
interstellar space and yet cooler than a typical star.
An even more aggressive way to investigate astronomical objects
is to observe an object in different windows of wavelengths
at the same time, a technique known as simultaneous multiple
wavelength observation. By doing so, astronomers are able
to deduce the sequence of physical processes or events in
or near the astronomical object. Thus, they put themselves
in a better position to infer the behavior of the observed
object and the underlying physics. Perhaps one of the most
striking examples is the confirmation of the origin of gamma-ray
burst. Gamma-rays are EM waves of wavelengths shorter than
m.
Gamma-ray bursts are sudden bursts of gamma-ray coming from
the sky. It was first discovered in the cold war era (1960¡¦s
to mid-80¡¦s) by gamma-ray detector satellites whose original
task was to monitor nuclear bomb tests carried out on Earth.
These bursts are found at a rate of about once every day and
they do not seem to repeat. In other words, it is seldom to
find a direction in the sky that bursts more than once. Because
of the non-repeating nature of the burst, astronomers had
a hard time understanding its origin. Different theories had
been proposed but it was not sure which of them is correct.
The situation changed completely in 28th February 1997. With
the (almost) simultaneous gamma-ray, X-ray (both detected
by the Italian-Dutch BeppoSAX satellite) and optical observations
(detected by the La Palma Astronomical Observatory in Argentina)
of a gamma-ray burst known as GRB970228, astronomers realized
that after a gamma-ray burst (detection at gamma-ray frequencies),
a star-like object in a distant galaxy along the direction
of the burst brightens up (detection at X-ray and optical
frequencies). This has led to the first direct confirmation
that the bursts are coming from a cosmological distance.
[This essay is an extended version of an article
that the author has written for the Hong Kong Space Museum.]
to 
stars that are bounded together by the attractive gravitational
force between massive objects. The shape of this galaxy looks
like a whirlpool, and astronomers called it a spiral galaxy.
But I guess most of you will not have any ideas on what kinds
of object pictures (b) to (e) are. Would you like to take
a guess? 
in vacuum. Different EM waves have different wavelengths and
hence different frequencies. Red light, for example, has a
wavelength of about 700 nm (nm is the shorthand for nanometer
or 
m) while blue light has a wavelength of about 400 nm. In addition
to optical EM waves that we can see, there are EM waves that
our eyes cannot see. For instance, the radio waves we use
to transmit radio signals are EM waves with wavelength ranges
from several centimeters to several meters; while the X-rays
used in hospitals are EM waves with wavelengths of about 
m. Light sensitive cells in our retina are only sensitive
to EM waves in a small wavelength window between about 400
nm to 700 nm. Thus, our naked eyes can only see objects that
are emitting EM waves in this small wavelength window, known
as the optical window.
m) image in picture (b) can be used to locate the regions
of M81 in which there is formation of new stars. It is because
the temperature of star formation region is hotter than typical
interstellar space and yet cooler than a typical star. 
m.
Gamma-ray bursts are sudden bursts of gamma-ray coming from
the sky. It was first discovered in the cold war era (1960¡¦s
to mid-80¡¦s) by gamma-ray detector satellites whose original
task was to monitor nuclear bomb tests carried out on Earth.
These bursts are found at a rate of about once every day and
they do not seem to repeat. In other words, it is seldom to
find a direction in the sky that bursts more than once. Because
of the non-repeating nature of the burst, astronomers had
a hard time understanding its origin. Different theories had
been proposed but it was not sure which of them is correct.
The situation changed completely in 28th February 1997. With
the (almost) simultaneous gamma-ray, X-ray (both detected
by the Italian-Dutch BeppoSAX satellite) and optical observations
(detected by the La Palma Astronomical Observatory in Argentina)
of a gamma-ray burst known as GRB970228, astronomers realized
that after a gamma-ray burst (detection at gamma-ray frequencies),
a star-like object in a distant galaxy along the direction
of the burst brightens up (detection at X-ray and optical
frequencies). This has led to the first direct confirmation
that the bursts are coming from a cosmological distance.