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Category Archives: 12. How the stars shine: cosmic furnaces


The American Association of Physics Teachers have collated a set of educational resources on “Neutrinos: Teaching the science behind the 2015 Nobel Prize in Physics” (see The Cosmos, Section 12.7, pp. 322-325).

Find out more on their website.



The 2015 Nobel Prize in Physics was given on October 6 to Takaaki Kajita of the Superkamiokande experiment and to Arthur McDonald of the Sudbury Neutrino Observatory.  The work at both those sites is thoroughly discussed in The Cosmos (see Section 12.7, p. 322-325) about the solar-neutrino experiment.


By showing definitively that a mix of the three types of neutrinos reaches the Earth, combining the knowledge that only electron-neutrinos leave the Sun shows that neutrinos change in type en route.  Only if neutrinos have mass can such changes take place, so the discovery is a major challenge to the Standard Model of particle physics.

This is the third Nobel Prize for neutrino research.  Half the 1995 Nobel Prize went Fred Reines for the discovery of neutrinos in an atomic-reactor beam (his co-discoverer, Clyde Cowan, having died before the prize was given, making him ineligible).  Half the 2002 Nobel Prize went to Ray Davis, who ran the chlorine version of the neutrino experiment at the Homestake Mine, and Masatoshi Koshiba, who was in charge of Kamiokande (the Neutrino Detection Experiment [NDE] in the Kamioka mine in Japan). John Bahcall from the Institute for Advanced Study, who had done the bulk of the theoretical work involved, was omitted from the prize, unfortunately (again, as the prize is not awarded posthumously).

Links: 2015 Nobel Prize in Physics at the Royal Swedish Academy of Sciences; Dennis Overbye’s analysis for the NY Times (including a discussion of the next investigations via The Deep Underground Neutrino Experiment, DUNE).

From a press release of the European Southern Observatory (ESO), September 23, 2015:

A new image of the rose-colored star forming region Messier 17 was captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. It is one of the sharpest images showing the entire nebula and not only reveals its full size but also retains fine detail throughout the cosmic landscape of gas clouds, dust and newborn stars.

Credit: ESO

Credit: ESO

Although officially known as Messier 17, its nicknames include: the Omega Nebula, the Swan Nebula, the Checkmark Nebula, the Horseshoe Nebula and the Lobster Nebula. M17 is located about 5500 light-years from Earth near the plane of the Milky Way and in the constellation of Sagittarius. The object spans a big section of the sky — its gas and dust clouds measure about 15 light-years across. This material is fueling the birth of new stars and the wide field of view of the new picture reveals many stars in front of, in, or behind M17.

The nebula appears as a complex red structure with some graduation to pink. Its coloring is a signature of glowing hydrogen gas. The short-lived blue stars that recently formed in Messier 17 emit enough ultraviolet light to heat up surrounding gas to the extent that it begins to glow brightly. In the central region the colors are lighter, and some parts appear white. This white color is real — it arises as a result of mixing the light from the hottest gas with the starlight reflected by dust. Throughout this rosy glow, the nebula shows a web of darker regions of dust that obscure the light. This obscuring material is also glowing and — although these areas are dark in this visible-light image — they look bright when observed using infrared cameras.

Links: full ESO press release, including further images and movies of M17.

From an article in Physics World by Ken Croswell, August 29, 2014:

Physicists working on the Borexino experiment in Italy have successfully detected neutrinos from the main nuclear reaction that powers the Sun. The number of neutrinos observed by the international team agrees with theoretical predictions, suggesting that scientists do understand what is going on inside our star. (See Section 12.7, p. 322.)

Credit: Borexino Collaboration

Credit: Borexino Collaboration

Each second, the Sun converts 600 million tons of hydrogen into helium, and 99% of the energy generated arises from the so-called proton–proton chain. And 99.76% of the time, this chain starts when two protons form deuterium (hydrogen-2) by coming close enough together that one becomes a neutron, emitting a positron and a low-energy neutrino. It is this low-energy neutrino that physicists have now detected. Once this reaction occurs, two more quickly follow: a proton converts the newly minted deuterium into helium-3, which in most cases joins another helium-3 nucleus to yield helium-4 and two protons.

Neutrinos normally pass through matter unimpeded and are therefore very difficult to detect. However, the neutrinos from this reaction in the Sun are especially elusive because of their low energy. The measurement therefore took scientists by surprise.

The Borexino detector is a large sphere containing a benzene-like liquid that is located deep beneath a mountain at the Gran Sasso National Laboratory to shield the experiment from cosmic rays. Occasionally, a neutrino will collide with an electron in the liquid and the recoiling electron will create a flash of ultraviolet light that can then be detected.

Links: the full Physics World article; Borexino website.

As described in The Cosmos, 4ed, Section 12.7, the solar-neutrino and other experiments have been detecting subsidiary nuclear interactions, but could not reach the energy range of the most fundamental process that fuels the sun and stars like it–the interaction of two protons. Scientists of the Borexino project in Italy has announced that they have finally detected this fundamental process. Here is an abridged version of their press release of August 27, 2014:

Scientists working on the neutrino experiment in the Italian National Institute for Nuclear Physics (INFN) Gran Sasso Laboratories have managed to measure the energy of our star in real time: the energy released today at the center of the Sun is exactly the same as that produced 100,000 years ago. For the first time in the history of scientific investigation of our star, solar energy has been measured at the very moment of its generation. The study was published on August 28, 2014, in the journal Nature.

INFN Borexino infographic

Credit: INFN/Borexino experiment

Borexino has managed to measure the Sun’s energy in real-time, detecting the neutrinos produced by nuclear reactions inside the solar mass: these particles take only a few seconds to escape from it and eight minutes to reach us. Previous measurements of solar energy, on the other hand, have always taken place on radiation (photons) which currently illuminate and heat the Earth and which refer to the same nuclear reactions, but which took place over a hundred thousand years ago: this, in fact, is the time it takes, on average, for the energy to travel through the dense solar matter and reach its surface. The comparison between the neutrino measurement now published by Borexino and the previous measurements concerning the emission of radiant energy from the Sun shows that solar activity has not changed in the last one hundred thousand years.

The Borexino detector, installed in the INFN underground Laboratories of Gran Sasso, has managed to measure the flux of neutrinos produced inside the Sun in the fusion reaction of two hydrogen nuclei to form a deuterium nucleus: this is the seed reaction of the nuclear fusion cycle which produces about 99% of the solar energy. Up until now, Borexino had managed to measure the neutrinos from nuclear reactions that were part of the chain originated by this reaction or belonging to secondary chains, which contribute significantly less to the generation of solar energy, but which were central to the discovery of certain crucial physical properties of this “ephemeral” elementary particle, the neutrino.

Links: Full INFN press release on; Borexino homepage.

An article in ScienceNews reports that data from the IceCube experiment under Antarctic ice have shown that the highest energy neutrinos they detect come from all directions, indicating that they are probably at cosmological distances (see Section 12.7c, pp. 324-325). The results were first announced at the American Physical Society’s meeting in April 2014.

Neutrinos open a window into the very distant and high-energy Universe that is extremely difficult to access by any other means. This is because neutrinos, unlike every other subatomic particle, are electrically neutral and rarely interact with matter. By detecting these particles and charting the directions they come from, scientists aim to identify the sources of neutrinos: star-forming galaxies, supermassive black holes or perhaps some as-yet unknown violent objects. These sources can accelerate neutrinos and other subatomic particles to energies far greater than any human-made machine could achieve.

Credit: Sven Lidstrom, IceCube/NSF

IceCube was specifically built to aid in this quest. For three years, strings of sensors stretching as deep as 2.5 kilometers below the surface of an Antarctic glacier have detected subtle flashes of light created when neutrinos and other particles collide with atoms. Last year, IceCube researchers identified 28 high-energy neutrinos from all directions that are almost certainly from outside the Solar System. The researchers have since found nine more, including the highest energy neutrino ever detected.

To complement this painstaking search for the highest energy neutrinos, Christopher Weaver, an IceCube physicist at the University of Wisconsin-Madison, decided to cast a wider net for the larger population of slightly lower-energy astronomical neutrinos. His approach relied on selecting particles that fell from the skies of the Northern Hemisphere, whizzed through Earth’s interior and arrived at IceCube from below. Only neutrinos, and not other particles that often trigger IceCube’s sensors from above, can make it through Earth’s dense crust and core. He also limited his search to detections at a specific energy – about 100 trillion electron volts – so that the number of neutrinos from space wouldn’t be dwarfed by the amount of neutrinos produced in the atmosphere. (IceCube’s sensors can’t distinguish between the two.)

That left Weaver with about 35,000 neutrinos, at least some of which began their journeys beyond the Solar System. He tracked the directions they came from and found no evidence of clustering in any particular parts of the sky – a finding that confirmed previous analyses and suggests that no local source is primarily responsible for the population of neutrinos whizzing by Earth. As IceCube continues to collect more data, scientists hope these two independent neutrino search methods will converge on trends in the neutrinos’ direction of arrival. It’s an exciting time in neutrino astrophysics.

Links: ScienceNews article

The Astronomy Picture of the Day (APOD) on February 4, 2014, shows star birth in action: a bipolar particle beam is seen, forming what we call a Herbig-Haro object, named for astronomers George Herbig and Guillermo Haro (see Section 12.1b, pp. 314-316).

Credit: Hubble Legacy Archive, NASA, ESA – Processing: Judy Schmidt

The powerful jet likely contains electrons and protons moving hundreds of kilometers per second. The above image was taken by the Hubble Space Telescope in infrared light in order to better understand turbulent star forming regions known as Young Stellar Objects (YSOs). Frequently when a star forms, a disk of dust and gas circles the YSO causing a powerful central jets to appear. In this case, the energetic jets are creating, at each end, Herbig-Haro object 24 (HH 24), as they slam into the surrounding interstellar gas. The entire star forming region lies about 1,500 light years distant in the Orion B molecular cloud complex. Due to their rarity, jets like that forming HH 24 are estimated to last only a few thousand years.

Links: APOD, February 4, 2014.

The Sloan Digital Sky Survey (SDSS) is one of the most ambitious and influential surveys in the history of astronomy. Over eight years of operations it has obtained deep, multi-color images covering more than a quarter of the sky and created 3-dimensional maps containing more than 930,000 galaxies and more than 120,000 quasars.

Credit: Sloan Digital Sky Survey

The education team at SDSS have prepared a variety of astronomical resources, interactive tools, and science projects, for teachers and educators to use. They aim to show us the beauty of the Universe, and share with us their excitement as they build the largest map in the history of the world!

SkyServer‘s tools allow you to access all publicly available data from the Sloan Digital Sky Survey. It offers access to many different types of data, but most users will usually focus on four types: images, spectra, photometric data, and spectroscopic data. See their ‘Getting Started‘ page for more details.

Their projects pages come in both Basic (suitable for high-school and Astronomy 101-level students) and Advanced (for students with a deeper understanding of astronomy) levels.  There are also ideas for extended independent research projects.

Instructor guides are also available.

A massive telescope buried in the Antarctic ice has detected 28 record-breaking, extremely high-energy neutrinos – elementary particles that likely originate far beyond our Solar System. (See Sections 12.7c and 13.2g)

The achievement, which comes nearly 25 years after the pioneering idea of detecting neutrinos in ice, provides the first solid evidence for astrophysical neutrinos from cosmic accelerators and has been hailed as the dawn of a new age of astronomy. The team of researchers that detected the neutrinos with the IceCube Neutrino Observatory in Antarctica published a paper describing the detections on November 22, 2013, in the journal Science.

Credit: IceCube Collaboration

The neutrinos had energies greater than 1,000,000,000,000,000 electron volts, or 1 peta-electron volt (PeV). Two of these neutrinos had energies many thousands of times higher than the highest-energy neutrino that any man-made particle accelerator has ever produced. (1 joule of energy = 6.2419 × 1018 eV.)

While not telling scientists what the cosmic accelerators are or where they’re located, the IceCube results do provide scientists with a compass that can help guide them to the answers. Unlike other cosmic particles, neutrinos are electrically neutral and nearly massless, so that they travel through space in a straight line from their point of origin, passing through virtually everything in their path without being deflected by interstellar masses and magnetic fields.

Credit: Jamie Yang, IceCube Collaboration

The IceCube observatory consists of over 5,000 basketball-sized light detectors called Digital Optical Modules (DOMs). These are suspended along 86 strings that are embedded in a cubic kilometer of clear ice starting 1.5 kilometers beneath the Antarctic surface. Out of the trillions of neutrinos that pass through the ice each day, a couple of hundred will collide with oxygen nuclei, yielding the blue light of Cherenkov radiation that IceCube’s DOMs detect.

Links: LBNL press release; U. Wisconsin press release (with image gallery); Penn State press release (with movie).