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Category Archives: 13. The death of stars: recycling

An update for Section 13.3f (pp. 352-355) in Pasachoff & Filippenko, The Cosmos, 4th ed:
In what many think is the most important step in astronomy since Galileo first turned a telescope on the heavens in 1609, ripples in space-time known as gravitational waves were detected on September 14, 2015, and reported to the world at large on February 11, 2016. Scientists gathered all over in auditoriums at the National Press Club in Washington, at Caltech, at MIT, in Moscow, and in many other places for the epochal event. “Ladies and gentlemen, we have detected gravitational waves,” said the executive director of the LIGO Laboratory. “We did it!”

The observational existence of gravitational radiation had been established by the changing period of the Hulse-Taylor “double pulsar” at a rate that matched the energy loss expected from the emission of gravitational waves (Section 13.3f, pp. 352-354), but the gravitational waves themselves – distortions of space-time – had not been detected directly until the Advanced LIGO (Laser Interferometer Gravitational Wave Detector) picked up a signal in its engineering run on September 14, 2015.  The signal, which could be heard as an upwards “chirp” in the audio range, resulted from two black holes 1.3 billion light years away, each containing about 30 solar masses, spiraling rapidly into each other and merging to become a single black hole with almost the same total mass but with 3 solar masses of material converted into energy in the form of gravitational waves.

Advanced LIGO has about 3 times the sensitivity of the earlier LIGO, and this event was within that advance of sensitivity. A further gain of 3x is expected with Advanced LIGO in the future.

Scientific papers were published February 11, 2016, in the Physical Review Letters and in the Astrophysical Journal Letters.

A few days later, I [JMP] attended a two-hour evening session at Caltech’s main auditorium to hear the principals of the LIGO project speaking, and I had the feeling that it was just like being at a meeting of the Lincei Academy in Florence in 1610 to hear Galileo speak about his new discoveries with the [optical] telescope.

There are many links to discussions and animations available. Here are some of them:

Popular and academic press:
New York Times coverage (includes movie); also NYT Opinion piece and an NYT Editorial a few days later, justifying scientific inquiry; a congressman’s letter, and sound bites from scientists.
Nature coverage (with explanatory graphics)
Science magazine in depth (requires subscription to access full text)
New Yorker article (with an account of LIGO’s inception and development)
Physics Today comparison of gravitational waves and sound waves
The Wall Street Journal and Michio Kaku’s desciption.

Societies and organizations:
American Physical Society Viewpoint
The Kavli Foundation Scientific spotlight
Caltech (press releasesillustrations, movies and animations)
NSF press release
AAPT resources on gravitational waves
STFC (UK) press release
Cornell Chronicle and media statement
University of Texas Rio Grande Valley press release
ESA congratulations
CSIRO (Australia) news release

LIGO websites:
LIGO labs (Observatories: Livingston | Hanford); Advanced LIGO; LIGO Scientific Collaboration; LIGO Partner Experiments and Collaborations






To commemorate October as American Archive Month, six new images have been released from the Chandra Data Archive. The archive houses the data from Chandra’s observations, making them available for ongoing and future studies.



The objects are: (top row, l-r) W44, a supernova remnant; SN 1987A, the remnant of a bright nearby supernova; Kes 79, a super nova remnant; (bottom row, l-r) MS 0735.6+7421, an erupting galaxy cluster; 3C295, a galaxy cluster within a superheated gas cloud; the Guitar Nebula, a pulsar.

Further details about these images and objects, and many more, may be found on the Chandra website.

From a HST press release, September 24, 2015:

A stunning new set of images from Hubble’s Wide Field Camera 3 capture the scattered stellar remains in spectacular new detail and reveal its expansion over the years since HST last captured them, in 1997.

Credit: NASA, ESA, Hubble Heritage Team

Credit: NASA, ESA, Hubble Heritage Team

Deriving its name from its delicate, draped filamentary structures, the beautiful Veil Nebula is one of the best-known supernova remnants. It formed from the violent death of a star twenty times the mass of the Sun that exploded about 8000 years ago. Located roughly 2100 light-years from Earth in the constellation of Cygnus (The Swan), this brightly coloured cloud of glowing debris spans approximately 110 light-years.

Astronomers suspect that before the Veil Nebula’s source star exploded it expelled a strong stellar wind. This wind blew a large cavity into the surrounding interstellar gas. As the shock wave from the supernova expands outwards, it encounters the walls of this cavity — and forms the nebula’s distinctive structures. Bright filaments are produced as the shock wave interacts with a relatively dense cavity wall, whilst fainter structures are generated by regions nearly devoid of material. The Veil Nebula’s colorful appearance is generated by variations in the temperatures and densities of the chemical elements present; they do not represent the real colors of the nebula.

Links: Full press release and description; images for download and video.

From a press release of the European Space Agency:

Over the past week, ESA’s Integral satellite has been observing an exceptional outburst of high-energy light produced by a black hole that is devouring material from its stellar companion.


Credit and copyright: ESA/ATG medialab

X-rays and gamma rays point to some of the most extreme phenomena in the Universe, such as stellar explosions, powerful outbursts and black holes feasting on their surroundings. In contrast to the peaceful view of the night sky we see with our eyes, the high-energy sky is a dynamic light show, from flickering sources that change their brightness dramatically in a few minutes to others that vary on timescales spanning years or even decades.

On 15 June 2015, a long-time acquaintance of X-ray and gamma ray astronomers made its comeback to the cosmic stage: V404 Cygni, a system comprising a black hole and a star orbiting one another. It is located in our Milky Way galaxy, almost 8000 light-years away in the constellation Cygnus, the Swan. In this type of binary system, material flows from the star towards the black hole and gathers in a disc, where it is heated up, shining brightly at optical, ultraviolet and X-ray wavelengths before spiralling into the black hole.

The V404 Cygni black hole system has not been this bright and active since 1989, when it was observed with the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.

Link: the full ESA press release.

From an article on

The High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located at 4000 m above sea level on the slopes of Mexico’s Volcán Sierra Negra, is the newest tool available to visualize the most energetic phenomena in the Universe, such as supernovae, neutron star collisions and active galactic nuclei.


Credit: HAWC Collaboration

In March 2015, construction was completed on HAWC’s 300th and final detector tank (each holding 200,000 liters of water), and the observatory will soon begin collecting data at full capacity.

It is a joint project between U.S. and Mexican scientists, with some participation from Polish and Costa Rican scientists.

Links: article; HAWC Observatory home.

Adapted from a press release of the Max Planck Institute for Extraterrestrial Physics, November 24, 2014:

Astronomers at the Max Planck Institute for Extraterrestrial Physics recently presented new observations of the gas cloud G2 in the Galactic Centre, which was originally discovered in 2011. These data are in remarkably good agreement with an on-going tidal disruption. As a complete surprise came the discovery that the orbit of G2 matches that of another gas cloud detected a decade ago, suggesting that G2 might actually be part of a much more extensive gas streamer. This would also match some of the proposed scenarios that try to explain the presence of G2. One such model is that G2 is originating from the wind from a massive star.

The gas cloud G2 is on a highly eccentric orbit around the Galactic Center. Observations in 2013 have shown that part of the gas cloud is already past its closest approach to the black hole, at a distance of roughly 20 light hours (a bit more than 20 billion kilometres).

The new, deep infrared observations with the SINFONI instrument at the VLT track the ongoing tidal disruption of the gas cloud by the powerful gravitational field. While the shape and path of the gas cloud agrees well with predictions from the models, so far there has been no significant enhanced high-energy emission, as one might have expected from the associated shock front.

Copyright and credit: Max Planck Institute for Extraterrestrial Physics

Copyright and credit: Max Planck Institute for Extraterrestrial Physics

However, a closer look into the data set led to a surprise. A decade ago, another gas cloud – now call G1 – was observed in the central region of our galaxy and it has a similar orbit. The researchers postulate that G1 and G2 might be clumps of the same gas streamer. G1 and G2 could be clumps in the wind ejected from of one of the massive disk stars in the vicinity. This could help to explain the missing X-ray emission from the gas cloud near the black hole (although the non-detection of such emission is not yet understood).

Links: MPE press release, including figures and detailed captions.

A series of images (Fig. 13-12, p. 338) shows the eruption of V838 Monocerotis.

Credit: NASA, ESA and H.E. Bond (STScI)

The text says:
“An especially peculiar, and still poorly understood, eruption was V838 Monocerotis. This object brightened by a large amount, but probably for a different physical reason than normal novae. It was surrounded by many shells of dust that were “lit up” by the nova outburst. These “light echoes” evolved with time, as shells at different distances from the nova were successively illuminated. A consensus is emerging that its outbursts were from a violent merger of the two components of a binary star.”
Prof. Howard Bond of Penn State University writes (October 2014):
This consensus has been gaining even more popularity. The “Rosetta Stone” was the eruption of V1309 Scorpii in 2008…. It was in a field in the Galactic bulge that had been imaged by the OGLE project for about a decade before the outburst, and it turned out that the progenitor was a close binary, and it was even shown that its period was getting shorter up until the eruption.
A recent paper by Kochanek et al. estimates that a stellar merger occurs in the Milky Way about once every 5 years, so these are actually pretty common events.

To celebrate the 15th anniversary of the launch of the Chandra X-ray Observatory’s launch, the Chandra X-ray Center at the Smithsonian Astrophysical Observatory has released four new images of supernova remnants in their press release of July 22, 2014.



Since its deployment on July 23, 1999, Chandra has helped revolutionize our understanding of the Universe through its unrivaled X-ray vision. One of NASA’s current “Great Observatories,” along with the Hubble Space Telescope and Spitzer Space Telescope, Chandra is specially designed to detect X-ray emission from hot and energetic regions of the universe.

With its superb sensitivity and resolution, Chandra has observed objects ranging from the closest planets and comets to the most distant known quasars. It has imaged the remains of exploded stars, or supernova remnants, observed the region around the supermassive black hole at the center of the Milky Way, and discovered black holes across the universe. Chandra also has made a major advance in the study of dark matter by tracing the separation of dark matter from normal matter in collisions between galaxy clusters. It is also contributing to research on the nature of dark energy.

The four new images of supernova remnants – the Crab Nebula, Tycho, G292.0+1.8, and 3C58 – are very hot and energetic and glow brightly in X-ray light, which allows Chandra to capture them in exquisite detail.

Links: the Chandra X-ray Center press release; images and further descriptions here.

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

Adapted from AIP Advances press release, March 18, 2014:

A powerful, new computer model provides fresh insight into the turbulent death throes of supernovae (see Section 13.2, p. 337).

Credit. W. D. Arnett, C. Meakin and M. Viallet/AIP Advances

The new model, developed by W. David Arnett (U. of Arizona) and colleagues, is the first to represent the start of a supernova collapse in three dimensions. It shows how the turbulent mixing of elements inside stars causes them to expand, contract, and spit out matter before they finally detonate. Arnett’s new model better matches what we observe in supernova remnants, with ejections of star material mixing with the material expelled during its final explosion.

The article, ‘Chaos and turbulent nucleosynthesis prior to a supernova explosion’ by David Arnett, Casey Meakin and Maxime Viallet is published in the journal AIP Advances.

Links: full AIP press release; the research article.