Astro News
- Recent Updates of Astro News
- Active Mercury(07/09)
- Hubble Status Report: Directly Observes a Planet Orbiting Another Star(01/09)
- A Non-trivial Answer to a Trivial Astronomical Question-The Origin Of Absolute Magnitude(07/08)
- Assault by a Black Hole(04/08)
- New Lakes Discovered on Titan(01/08)
- ¡§Deviant Behaviour¡¨ in the Solar System(10/07)
- Cosmic Ripples - Cosmic Microwave Background - CMB(07/07)
- Interplanetary Superhighway(04/07)
- Is Pluto a Planet?(01/07)
- Breathing Moonrocks(10/06)
- My Thoughts on the Theory of Relativity, Quantum Mechanics, Superstring Theory and Dark Matter(07/06)
- Space-time Vortex(04/06)
- Radio Astronomy(01/06)
- Neutrino Astronomy(10/05)
- The Active Earth(07/05)
- What is Dark Energy?(04/05)
- The Mysterious Black Holes(01/05)
- Intermediate-Mass Black Holes And Quasisoft X-Ray Sources(10/04)
- Time Travel: From a Scientific Approach(07/04)
- What is Astrobiology?(04/04)
- Black Hole: From Fantasy To Reality (II)(01/04)
- Black Hole: From Fantasy To Reality (I)(10/03)
- From The Oldest Light In The Universe To The Fate Of The Universe(7/03)
- The Cosmic HERO(4/03)
- Quaoar - the Tenth Member of the Solar System?(1/03)
- The First Chinese Telescope in Space(10/02)
- Diamonds and Other Stardust(7/02)
- Supermassive Black Hole in Andromeda Galaxy(4/02)
- Detection of Solar Neutrinos(1/02)
- Simultaneous Multiple Wavwlength Observation(10/01)
- Celestial Distance(7/01)
- Solar-Terrestrial Relations(7/00)
- Fundamental Particles in Astronomy(4/00)
- The Solar Maximum in 2000(1/00)
- Hubble Constant(10/99)
- New Findings on Cosmology(7/99)
- Strange Stars(4/99)
- How Strong Stellar Magnetic Field Can Be?(1/99)

Important notices

Have you ever wondered how it is possible for physicists to dig out so much information about distant stars from the feeble starlight we receive on Earth? For hundreds of years, astronomy has been built upon the starlight emitted from the surfaces of stars. We have no way to peer into their interior (fig 1 and 2). So when physicists tell us that stars generate light and heat from nuclear fusion reactions inside the stars, what evidences do they have?


Fig 1 : We are unable to peer into the heart of the Sun with its light though it is so close to us. Other than light, the Sun is also emitting neutrinos which penetrate through the whole Sun. The photo was taken by the writer.


Astronomers have two ways to "see through" a star: by detecting the vibration of stars and the neutrinos created in nuclear reactions. Due to the limited space, here we will only cover the latter - neutrino astronomy.

Neutrino is an elementary particle. It is electrically neutral and has only a small mass. They only take part in weak interactions and gravity. Since they interact only very weakly with matters, they could pass through almost anything unimpeded. On average for every one million solar neutrinos that pass through the Earth, only one will interact with the Earth. The whole Earth seems to be "transparent" to them! Most of the neutrinos emitted from the Sun's core can pass through the whole Sun and spread in the vast realm of the universe. The Sun produces about 2¡Ñ1038 neutrinos per second. In the time you are reading this article, over 1015 neutrinos have already streamed through your body!

Fig 2 : The spectrun is one of the main tools in astronomy, but ir only tells what is happenign on the stellar surface. The photo was taken by Mr Lee Wing Kit, a student of Dept of Physics, The Chinese University of Hong Kong

How does the Sun produce neutrinos? When a star is at the main sequence stage, its interior continuously fuses four hydrogen nuclei (i.e. protons) into one helium nucleus. A proton carries a positive charge and repels each other. When two protons come close together, one of them will transform via weak force into a neutron (p¡÷n¡Ïe¡Ï¡Ï£o)and release energy. Positrons and neutrinos are produced during the process, and are the telltale trace of the nuclear fusion reaction inside the Sun.

Another source of neutrinos is from dying stars. The star will become unstable when it uses up its nuclear fuel inside. Stars which are at least a few times more massive than the Sun will experience violent explosions and become supernovae (fig 3). The neutrinos resulted from various kinds of nuclear reactions during the explosion take away a huge amount of energy, even more than that by light and heat, and have a significant impact on the mechanism of supernova. After a supernova explosion, a small but compact star, called neutron star1 which is also a strong source of neutrinos, will be left behind. Emission of neutrinos will cool off the neutron star at a rate that is sensitive to the properties of the stellar matter. Therefore we might be able to get a glimpse of the secret of these little stars through the neutrinos they emit. 

As to explosion, the most violent one ever happened about 13.7 billion years ago - the Big Bang. At that time, a huge number of neutrinos was made and filled the whole universe. It is estimated that the Big Bang contributed about 300 neutrinos per cm3 today throughout the universe. They may play an important role in cosmological processes such as structure formation and the development of matter-antimatter asymmetry in the universe. If we could detect these "cosmic background neutrinos", we might be able to know what the universe was like when it was less than one second old. What an exciting possibility this would be!

Neutrino physics is a hot topic in current research in fundamental science. Students and teachers in the Physics Departments of both The Chinese University of Hong Kong and The University of Hong Kong are actively taking part in neutrino physics research, including theoretical neutrino astrophysics and experiments2. With further advance in technology, neutrinos will become a new tool for human to explore the universe. The development of neutrino astronomy will open up a window to new discoveries, as well as new puzzles.

Fig 3 : The supernova remnant N63A is believed to be left behind after the explosion of a star with 50 solar mass. The photo was taken by American astronomers. Y. H. Chu and R. M. Williams using Hubble Space Telescope
Photo Credit: NASA/STScI, Y.-H. Chu and R. M. Williams, UIUC.


1 Neutron star is a star that is largely made up of neutrons. Some recent studies suggest that a number of neutron stars may be composed of quarks instead which are known as strange stars.
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