Download Simultaneous Multiple Wavelength Observation

Simultaneous Multiple Wavelength Astronomical Observation
Dr. Hoi Fung CHAU
Department of Physics, The University of Hong Kong
A Short Quiz
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)
Alabama"
Images provided by Prof. Bill Keel, University of
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 1010 to 1012 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 1.2 x 107 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.
Electromagnetic Waves
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 3 x 10 8
ms-1 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 10-9 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 10-10 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.
Can we investigate astronomical objects with EM waves 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.
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 10-4 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.
Gamma-ray Burst
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 10-12 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 expanded version of an article that the author has written for the Hong
Kong Space Museum.]
[About the author: Dr. Hoi Fung CHAU received his BSc and PhD degrees from the
University of Hong Kong. After postdoctoral work at the University of Illinois and the
Institute for Advanced Study in Princeton USA, he joined the University of Hong Kong
in 1996. He is now an Assistant Professor in the Department of Physics at HKU. His
research interests include astrophysics, quantum computations, and complex systems.]
Keywords:
Astronomy, Electromagnetic waves, EM waves spectrum, Gamma-Ray bursts
Related Topics in the syllabus:
Optical wavelengths/frequencies, EM waves spectrum, nuclear fusion energy (stars),
gamma-ray.
Extensions (from the syllabus):
Bring out ideas: on the importance of observations of emitted EM wave spectrum in the
advancement of astronomy and astrophysics, scientists can design detectors for
observations at different frequency ranges, different basic concepts in different sections
(waves, EM, atomic physics) of the school syllabus are indeed useful in doing research.
Warm-up discussion:
o
What are the wavelength and frequency of waves? For EM waves, how
are they related?
The distance between two neighbouring similar points (e.g. two
neighbouring crests) in a wave is called wavelength. Frequency is the
number of complete oscillations performed by a particle or the disturbance
in a medium per unit time. The relation between the frequency and the
period
is
. The unit of frequency is Hertz (Hz),
EM waves, the relationship is
o
. In
, where c is the speed of light.
Approximately, what is the wavelength of each of the different types of
EM wave?
o
In the articles, different frequency ranges, e.g. radio wave, infrared, farinfrared, X-ray, optical, ultra-violet, gamma-ray, are mentioned. List them
out according to their frequencies (wavelengths).
Wavelength: Radio wave > far-infrared > infrared > optical > ultra-violet
> X-ray > gamma-ray.
Frequency: Radio wave < far-infrared < infrared < optical < ultra-violet <
X-ray < gamma-ray.
o
What is the wavelength range of visible light which is between the
ultraviolet and infrared radiation?
o
How far is one light year?
Points for further discussion:
o
The nearest star to our solar system is about 4.2 light years away from us.
The typical speed of our current space probes is about 100 km/s. How long
does it take to our current space probes to reach the nearest star?
o
Try to view our world through glasses of different colors. Describe your
observation.
o
We can only see optical light. So how come we can take pictures (b) to (e)
in other wavelength bands? [One can introduce the concepts of different
light detectors such as the G-M tube as an X-ray detector as well as image
processing by computer or simply ask the students to look up information
on different detectors and how they work.]
o
How can we measure the temperature of hot objects, e.g. something that is
5000 K?
o
What makes we feel that something is “hot”? Why do we feel warm near a
heater?
o
If an object keeps on emitting EM waves, how can the object at room
temperature stay at a constant temperature instead of cooling down?
o
Where does the energy of a star come from?
Nuclear Reaction (Nuclear fusion)
o
What is blackbody radiation?
o
Can you point out the usage of ultraviolet radiation, X – ray and infrared
radiation other than the observation of the universe?
o
Why is the frequency of the emitted radiation increased as the object
becomes hotter?
Activities:
o
Visit Hong Kong Space Museum.
- Can you find out the biography Hubble Space Telescope?
o
Infrared Ear Thermometer is widely used in schools after SARS
suspension period. Briefly discuss the working principle of the Infrared
Ear Thermometer.
Related web sites:
o
NASA
This site contains most of the latest information on the outer space. The
2nd and 3rd site contain latest information and photos about the
electromagnetic waves.
http://www.nasa.gov/home.html
http://imagers.gsfc.nasa.gov/ems/gamma.html
http://imagers.gsfc.nasa.gov/ems/waves3.html
o
NOVA Online Death Star
This program probes the deep mysteries of gamma-ray bursts, the most
powerful celestial explosions since the Big Bang.
http://www.pbs.org/wgbh/nova/gamma/
o
HubbleSite - News Center - News Archive
This site contains the latest news on astronomy.
http://hubblesite.org/newscenter/archive/
o
BeppoSAX MISSION HOME PAGE
The X-ray astronomy satellite BeppoSAX (Satellite per Astronomia X,
"Beppo" in honor of Giuseppe Occhialini), is a project of the Italian Space
Agency (ASI) with participation of the Netherlands Agency for Aerospace
Programs (NIVR). The main scientific characteristic of the BeppoSAX
mission is the wide spectral coverage, ranging from 0.1 to over 200 keV.
http://www.asdc.asi.it/bepposax/
o
News - Astronomers spot new Jupiter moons
University of Hawai'i astronomers yesterday announced the discovery of
seven new moons orbiting Jupiter, bringing to 47 the number of satellites
of the largest planet in the solar system.
http://the.honoluluadvertiser.com/article/
2003/Mar/06/ln/ln16a.html
o
BBC NEWS Science-Nature Distant galaxies break record
Astronomers have obtained the deepest near-infrared image of the sky
ever. The resulting image reveals extremely distant galaxies, which are
barely detected in the deepest optical images acquired with the HST.
http://news.bbc.co.uk/2/hi/science/nature/2566359.stm
o
New Scientist - End of Universe's 'dark age' spied by astronomers
The emergence of quasars at the end of the astronomical "dark age" that
enshrouded the early Universe has been identified by astronomers.
http://www.newscientist.com/news/news.jsp?id=ns99993255
o
New Scientist - “First high-energy image from black hole hunter”
A European spacecraft designed to reveal the secrets of black holes has
produced its first high-energy vision of the universe. The Integral gamma
ray observatory imaged a gamma ray burst emanating from a violent
cosmic event 5,000 million light years from Earth.
http://www.newscientist.com/news/news.jsp?id=ns99993201
o
New Scientist - “Black holes are double trouble for galaxy”
A European spacecraft designed to reveal the secrets of black holes has
produced its first high-energy vision of the universe. The Integral gamma
ray observatory imaged a gamma ray burst emanating from a violent
cosmic event 5,000 million light years from Earth.
http://www.newscientist.com/news/news.jsp?id=ns99993088
o
Sciscape 新聞 - 高解析的微波宇宙 (Chinese version only)
New data from a NASA probe located a million miles from Earth has
provided scientists with the information necessary to paint the most
precise picture yet of the early universe. The long-awaited images,
unveiled yesterday, support theories that posit that the universe underwent
a tremendous growth spurt shortly after the big bang.
http://www.sciscape.org/news_detail.php?news_id=1015
o
Sciscape 新聞 - 140 億年前的宇宙微波背景輻射(Chinese version only)
The DASI (Degree Angular Scale Interferometer) at the South Pole has
detected the CMB (cosmic microwave radiation) from the depth universe.
http://www.sciscape.org/news_detail.php?news_id=923
o
Sciscape 科景 - “天空之眼 - INTEGRAL gamma 射線觀測衛星”
(Chinese version only)
Astronomers will soon be seeing some of the most violent events in the
Universe in unprecedented detail, following today's launch of Integral, the
most sensitive gamma-ray telescope ever sent into space.
http://www.sciscape.org/news_detail.php?news_id=856
o
Astronomy Picture of the Day
Each day a different image or photograph of our fascinating universe is
featured, along with a brief explanation written by a professional
astronomer.
http://antwrp.gsfc.nasa.gov/apod/astropix.html
o
天文園主頁 (Chinese version only)
AstroWorld is explored by the department of Physics of the Chinese
University of Hong Kong. It provides information, photos, articles,
stargazing, and activities on astronomy.
http://www.phy.cuhk.edu.hk/astroworld/
o
香港太空館
Hong Kong Space Museum was the first local planetarium for the
popularization of astronomy and space science. This site provides a lot of
information related to stargazing, basic astronomy, astronomical news and
educational resources.
http://www.lcsd.gov.hk/CE/Museum/Space/index.htm
o
Hubble Space Telescope
This site provides all the information about the Hubble Space Telescope,
including astronomy resources and many useful linkages.
http://www.stsci.edu/hst/
o
紅外線的應用 (Chinese version only)
This site introduces one application of infra-red radiation--- Infrared Ear
Thermometer.
http://che.cycu.edu.tw/jamnet/course/C4%2D3.htm
o
台灣師大物理系 物理教學示範實驗教室
NTNU Virtual Physics Laboratory contains a lot of original physics java
applets, and many interesting teaching ideas of physics.
http://www.phy.ntnu.edu.tw/demolab/
Further Readings:

科學人：中文版，2002 年 8 月號，p.78-88，“星系傳奇”，講述星系演化的
研究和進程。

科學人：中文版，2002 年 5 月號，p.69-78，“宇宙天體普查”，講述以探測
背景輻射來研究天體的方法與發現。

科學人：中文版，2002 年 5 月號，p.79-81，“夜空有多暗”，講述發現背景
輻射的來龍去脈。