# Mechanics of Supernovae

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Where does the energy from a supernova come from?

My understanding is that the iron core collapses into a large ball of neutrons - is that effectively a star-sized nuclear explosion? What is the reaction product called?

Why does the effect go supercritical, instead of finding some equilibrium rate of burn? I've read discussion that the reaction energy does not appear to be enough to explain the energy seen in a supernova? How big is the gap?

The energy derives from gravitational potential energy. The core of a bit more than a solar mass collapses from the size of the Earth to a 10km radius. Some of the gravitational energy (a tiny percentage) released is transferred to the overlying envelope and blasts it into space. Further energisation takes place due to radioactive decay.

A ball of neutrons is not a star-sized nuclear explosion ?

I don't understand what you mean by reaction product? The proto-neutron star consists mainly of neutrons(!) and neutron-rich nuclei.

The collapse occurs because electron degeneracy pressure is circumvented by neutronisation (inverse beta decay), which removes electrons from the gas. At that point, the collapse occurs on a freefall time of $<1$ second. There is no "burn-rate" as such; just a collapse and then a "core bounce" as the equation of state hardens because of neutron degeneracy pressure and the strong nuclear force between closely packed neutrons.

The "observed" energy (i.e. the electromagnetic output and ejecta kinetic energy) is only of order 1% of the energy released from the gravitational collapse. Most of it emerges in the form of unseen neutrinos. So there is no energy gap and plenty of energy to power what is observed.

## Supernova

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Supernova, plural supernovae or supernovas, any of a class of violently exploding stars whose luminosity after eruption suddenly increases many millions of times its normal level.

The term supernova is derived from nova (Latin: “new”), the name for another type of exploding star. Supernovae resemble novae in several respects. Both are characterized by a tremendous, rapid brightening lasting for a few weeks, followed by a slow dimming. Spectroscopically, they show blue-shifted emission lines, which imply that hot gases are blown outward. But a supernova explosion, unlike a nova outburst, is a cataclysmic event for a star, one that essentially ends its active (i.e., energy-generating) lifetime. When a star “goes supernova,” considerable amounts of its matter, equaling the material of several Suns, may be blasted into space with such a burst of energy as to enable the exploding star to outshine its entire home galaxy.

Supernovae explosions release not only tremendous amounts of radio waves and X-rays but also cosmic rays. Some gamma-ray bursts have been associated with supernovae. Supernovae also release many of the heavier elements that make up the components of the solar system, including Earth, into the interstellar medium. Spectral analyses show that abundances of the heavier elements are greater than normal, indicating that these elements do indeed form during the course of the explosion. The shell of a supernova remnant continues to expand until, at a very advanced stage, it dissolves into the interstellar medium.

## The Copernican revolution

Before the 16th century, Earth was commonly thought to be at the centre of the solar system, with all other celestial objects revolving around it. This is known as the geocentric model. This theory, however, did not match some confusing observations made by astronomers, such as the path of planets that appeared to move backwards on their orbits.

When we observe, from Earth, the planets around the Sun, they do not always appear to be moving in one direction in our sky. Sometimes they appear to loop backwards for short periods of time. This is called retrograde motion and is one of the key pieces of evidence that the Sun lies at the centre of the solar system and all the planets revolve around it.

In 1543, Polish astronomer Nicolaus Copernicus proposed a heliocentric model of the solar system in which the planets orbit the Sun. This model explained the unusual path of planets that astronomers had observed. The new theory was one of many revolutionary ideas about astronomy that emerged during the Renaissance period.

The work of astronomers Tycho Brahe and Johannes Kepler led to an accurate description of planetary motions and laid the foundation for Isaac Newton's theory of gravitation. This progress dramatically improved humanity's understanding of the universe. Their observations and investigations were strengthened by the invention of the telescope in the early 17th century. Italian astronomer Galileo Galilei popularized the use of telescopes to study and discover celestial objects, including Jupiter's four biggest moons. In his honour, they are known as the Galilean moons.

## Supernovae death reveals link to stars' birth

Credit: Shutterstock

It was previously thought that molecules and dust would be completely obliterated by the tremendous explosions of supernovae. Yet, for the first time, scientists have discovered that this is not actually the case.

A group of scientists, including those funded under the European Research Council (ERC) financed projects SNDUST and COSMICDUST, have identified two previously undetected molecules formylium (HCO+) and sulphur monoxide (SO), found in the cooling aftermath of Supernova 1987A. Having originally exploded in February 1987, Supernova 1987A is located 163,000 light years away in the Large Magellanic Cloud a satellite galaxy of our own Milky Way galaxy.

The dust factory of a very young supernova remnant

The lead author of the study published in the journal Monthly Notices of the Royal Astronomical Society, Dr. Mikako Matsuura, from Cardiff University's School of Physics and Astronomy said, 'This is the first time that we've found these species of molecules within supernovae, which questions our long held assumptions that these explosions destroy all molecules and dust that are present within a star.' Accompanying these newly identified molecules were compounds such as carbon monoxide (CO) and silicon oxide (SiO) which had already previously been detected.

Finding these unexpected molecules opens up the possibility that the explosive death of stars creates clouds of leftover gas which cool down to below 200°C, resulting in the various synthesised heavy elements starting to harbour molecules, creating what has been dubbed a 'dust factory'. As Dr. Matsuura goes on to explain, 'What is most surprising is that this factory of rich molecules is usually found in conditions where stars are born. The deaths of massive stars may therefore lead to the birth of a new generation.'

As new stars are created from the heavier elements scattered during explosions, this work opens up the prospect of better understanding the composition of these nascent stars by analysing their source.

A spectacular celestial farewell

The mechanics of supernovae are relatively well understood. When massive stars come to the end of their stellar evolution, they essentially run out of fuel, with not enough heat and energy remaining to counteract the force of their own gravity. Consequently, the outer regions of the star crash down on the core with formidable force, triggering the spectacular explosion and leaving what looks to be a new bright star behind, before it fades away.

Ever since its discovery over 30 years ago, astronomers have faced hurdles in the quest to study Supernova 1987A, especially when it comes to investigating its innermost core. This research was conducted using the Atacama Large Millimeter/submillimeter Array (ALMA) which enabled the team to explore in remarkable detail. As the facility with its 66 antennae is able to observe wavelengths in the millimetres – situated between infrared and radio light in the electromagnetic spectrum – it can penetrate the dust and gas clouds of the supernova. This ability enabled it to expose the newly formed molecules.

To expand on their current findings, the team are planning to continue using ALMA to ascertain the prevalence of HCO+ and SO molecules, as well as further explore for hitherto undetected molecules.

M. Matsuura et al. ALMA spectral survey of Supernova 1987A – molecular inventory, chemistry, dynamics and explosive nucleosynthesis, Monthly Notices of the Royal Astronomical Society (2017). DOI: 10.1093/mnras/stx830

## Course Listing

Below is a description of all courses offered by the Astronomy Department. Several courses are only offered every other semester or every other year, so check the current semester’s offerings.

All Astronomy courses can be used towards satisfying the Natural Sciences area requirement. ASTR 1210 and 1220 cover complementary subject matter. Each is complete in itself, and a student may elect to take either ASTR 1210 or ASTR 1220, or both concurrently.

#### ASTR 1210: Introduction to the Sky and the Solar System (3 credits)

Primarily for non-science majors.

The night sky. Brief history of astronomy through Newton. The properties of the sun, earth, moon, planets, meteors and comets. The origin and evolution of the solar system. Life in the universe. Recent results from space missions and ground-based telescopes.

#### ASTR 1220: Introduction to the Stars, Galaxies, and the Universe (3 credits)

Primarily for non-science majors.

Stars, star formation and evolution. Light, atoms, and modern observing technologies. The origin of the chemical elements. Supernovae, pulsars, neutron stars, and black holes. The structure and evolution of our galaxy. The nature of other galaxies. Active galaxies and quasars. The expanding universe, cosmology, the big bang, and the early universe.

#### ASTR 1230: Introduction to Astronomical Observation (3 credits)

Primarily for non-science majors.

An independent laboratory class in which students work individually or in small groups on observational projects. Extensive use is made of binoculars, 6-inch through 10-inch telescopes, and photographic equipment at the department’s student observatory. In addition, some projects use computers to simulate observations taken with much larger telescopes. Projects focus on the study of constellations, planets, stars, nebulae, and galaxies. Class work is done predominantly at night.

#### ASTR 1250: Alien Worlds (3 credits)

Alien worlds orbiting other stars were the subject of speculation going back to ancient times, and were first detected in the 1990s. Today, thousands of extrasolar planets are known and show a remarkable diversity compared to our own solar system. This introductory astronomy course for non-science majors discusses the known exoplanets: how they are discovered, their orbits, physical properties, formation, evolution and fate.

#### ASTR 1260: Threats from Outer Space (3 credits)

Primarily for non-science majors.

This introductory astronomy course for non-science majors deals with harmful, or potentially harmful, astronomical phenomena such as asteroid/comet impacts, supernovae, gamma ray bursts, solar storms, cosmic rays, black holes, galaxy collisions, and the end of the universe. Physical principles will be used to evaluate the dangers involved.

#### ASTR 1270: Unsolved Mysteries in the Universe (3 credits)

Primarily for non-science majors.

The universe is full of deep mysteries that astronomers are far from understanding. This course is designed to help students understand the limitations of our knowledge, and why finding solutions to these mysteries is so difficult. A number of though provoking topics will be covered, including: the beginning and end of the universe, black holes, extraterrestrial life, dark matter, and dark energy.

#### ASTR 1280: The Origins of Almost Everything (3 credits)

Primarily for non-science majors.

From ancient Babylon to modern cosmology, nearly every culture on Earth has stories and myths of creation. It is a universal human desire to understand where we came. In this introductory astronomy class for non-science majors, students will explore the origins of the Universe, structure and galaxies, stars, planets and life. The course will use the content to illustrate the nature of science and scientific inquiry.

#### ASTR 1290: Black Holes (3 credits)

Primarily for non-science majors.

Black holes are stellar remnants that are so dense that nothing, not even light, can escape their gravitational pull. Nevertheless, systems that are thought to contain black holes are among the brightest sources in the universe. In this introductory course, aimed primarily at non-science majors, students will learn the key concepts of the theory of relativity, explore the nature of black holes, and study their astrophysical importance. We will also discuss how astronomers' views of black holes evolved from broad skepticism to wide acceptance in the face of mounting observational evidence for their existence.

#### ASTR 1500, 1510: Seminar (1 credits)

A seminar designed primarily for first and second year students, taught on a voluntary basis by a faculty member. Topics vary.

#### ASTR 1610: Introduction to Astronomical Research for Potential Astronomy and Astronomy-Physics Majors (1 credits)

Intended primarily for first and second year declared and prospective Astronomy/Physics and Astronomy Majors.

Astronomy faculty members will describe various research projects. The goal is to acquaint students with the both the subject matter and the required physical, mathematical, and computational background of contemporary astronomy research. Potential long term undergraduate research projects will be emphasized.

#### ASTR 2110: Introduction to Astrophysics I (3 credits)

A thorough discussion of the basic concepts and methods of solar system, stellar, galactic, and extragalactic astronomy with emphasis on physical interpretation. Recent research developments such as black holes, pulsars, quasars, and new solar system observations from the space program.

#### ASTR 2120: Introduction to Astrophysics II (3 credits)

A thorough discussion of the basic concepts and methods of solar system, stellar, galactic, and extragalactic astronomy with emphasis on physical interpretation. Recent research developments such as black holes, pulsars, quasars, and new solar system observations from the space program.

#### ASTR 3130: Observational Astronomy (4 credits)

A laboratory course dealing with basic observational techniques in astronomy. Students make use of observational facilities at McCormick Observatory and at Fan Mountain Observatory. Classes generally meet at night.

#### ASTR 3140: Observational Radio Astronomy (4 credits)

An introduction to the tools, techniques, and science of radio astronomy. Discussion includes fundamentals of measuring radio signals, radiometers, antennas, and interferometers, supplemented by illustrative labs radio emission mechanisms and simple radiative transfer radio emission from the Sun and planets, stars, galactic and extragalactic sources, and the cosmic microwave background.

#### ASTR 3340: Teaching Astronomy (3 credits)

A seminar-style class offered primarily for non-majors planning to teach science or looking to improve their ability to communicate science effectively. In addition to astronomy contect, students will learn effective concept-based astronomy lessons.

#### ASTR 3410: Archaeo-Astronomy (3 credits)

A discussion of prescientific astronomy, including Mayan, Babylonian, and ancient Chinese astronomy and the significance of relics such as Stonehenge. The usefulness of ancient records in the study of current astrophysical problems, such as supernova outbursts, is also discussed. The course uses current literature from several disciplines including astronomy, archaeology, and anthropology.

#### ASTR 3420: Life Beyond the Earth (3 credits)

The possibility of the existence of intelligent extraterrestrial life methods and desirability of interstellar communication prospects for humanity’s colonization of space, interaction of space colonies and the search for other civilizations.

#### ASTR 3460: Development of Modern Astronomy (3 credits)

The 20th Century saw a revolution in our study of the origin and evolution of the universe. It was a dynamic period with the opening of the electromagnetic spectrum and the transition to "Big Science." This course is a survey of the development of modern astrophysics, with an emphasis on the second half of the 20th Century.

#### ASTR 3470: Science and Controversy in Astronomy (3 credits)

A critical evaluation of controversial topics in science and pseudo-science from the astronomer’s perspective. The methods of science and the nature of scientific evidence with their implications for unresolved astrophysical problems extraterrestrial life UFO’s alien abductions X-files astrology, etc.

#### ASTR 3480: Introduction to Cosmology (3 credits)

A descriptive introduction to the study of the ultimate structure and evolution of the universe. Covers the history of cosmological speculation, the nature of the galaxies, a qualitative introduction to relativity theory and the nature of space-time, black holes, models of the universe (big bang, steady-state, etc.) and methods of testing them, history of the universe.

#### ASTR 3880: Planetary Astronomy (3 credits)

The goal of this course is to understand the origins and evolution of bodies in the solar system. The observations of atmospheres and surfaces of planetary bodies by ground-based and orbiting telescopes and by spacecraft will be described. The principal topics will be the interpretation of remote sensing data for atmospheres and surfaces of planetary bodies, the chemistry and dynamics of planetary atmospheres, the interactions of these atmospheres with the surfaces and with the local plasma, and the role of meteorite and comet impacts on surfaces of planetary bodies.

#### ASTR 4140: Research Methods in Astrophysics (3 credits)

Primarily for astronomy/astrophysics majors. Students will be exposed to a research methods-intensive set of mini projects,with emphasis on current active areas of astrophysics research. The goal is to prepare students for research in astrophysics. Topics will include databases and database manipulation, astronomical surveys, statistics, space observatories and observation planning, intro to numerical simulations, and proposal writing.

#### ASTR 4440: The Nature of Discovery in Astronomy (3 credits)

This course examines the development of astronomy from about 1950 to the present. Initially, we review the historical development of modern astronomy — how the emphasis on the research frontiers shifted over time as new ideas and instruments developed. We discuss the nature of scientific creativity and the conditions which encourage or discourage scientific and astronomical discovery. This leads us to analyse the conditions in Universities and research organizations which promote scientific research, and how well they have succeeded. Questions of how to judge success in these matters will arise.

Although some background in astronomy would be advantageous for the seminar, it is not necessary since we will explain the basic scientific questions which are discussed.

#### ASTR 4810: Astrophysics (3 credits)

Basic concepts in mechanics, statistical physics, atomic and nuclear structure, and radiative transfer are developed and applied to selected fundamental problems in the areas of stellar structure, stellar atmospheres, the interstellar medium and extragalactic astrophysics.

#### ASTR 4993: Tutorial (3 credits)

A study of a topic of special interest to the student under individual supervision by a faculty member. May be repeated once for credit.

## Dr. Daniel Erenso

A general introduction to astronomy through an overview of planets, stars, systems of stars, and the overall structure of the universe. Topics will be discussed by answering questions such as "How do you weigh stars?" and "Will the universe die?" TBR Common Course: ASTR 1030

Prerequisite or corequisite: ASTR 1030. Introduction to observational astronomy through laboratory exercises and outdoor observing activities. Topics include telescopes, the analysis of starlight, and observations of stars and planets. TBR Common Course: ASTR 1032

Prerequisite: MATH 1710, MATH 1730, MATH 1810, and MATH 1920. Comprehensive study of the solar system including models of solar and planetary formation. Analysis of the chemical makeup and physical nature of the Sun, planets, moons, and comets using mathematics and the scientific method. Focus on planetary interiors, surfaces, atmospheres, solar-planetary interactions, and solar system evolution. Discussion of spacecraft missions, future solar system exploration, and possibilities of extraterrestrial life.

Prerequisite: MATH 1710, MATH 1730, MATH 1810, and MATH 1920. A comprehensive study of stellar, galactic, and cosmological astronomy. Analyzes the basic theories of stellar and galactic formation and evolution using mathematics and the scientific method. Includes the cataclysmic topics of supernovae, neutron stars, pulsars, and black holes as well as the nature of galaxies including the Milky Way galaxy, active galaxies and quasars, and the formation and evolution of our universe, the big bang theory, and the possibility of other life in the universe.

Individualized intensive study of a specific topic in astronomy or astrophysics not normally covered in the standard undergraduate physics and astronomy curriculum. Arrangements must be made with an approved faculty member prior to registration.

Prerequisite: PHYS 2021 or PHYS 2121 and MATH 1910 with C or better. Modern astronomical knowledge and techniques using classical and modern physical principles. Possible topics include star formation, black holes and neutron stars, galaxy structure and evolution, formation of planetary systems, and large-scale structure of the universe.

Prerequisites: PHYS 2021 or PHYS 2120 or consent of instructor. Principles and techniques of astronomical data acquisition and reduction. Possible research topics involve photometry, spectroscopy, astronomical applications of electronic detectors, and computer modeling.

Prerequisites: PHYS 3100 and PHYS 3150 or approval of department chair. In-depth, organized study of a contemporary topic of interest not normally covered in the undergraduate physics and astronomy curriculum. Possible topics include planetary geology, radio astronomy, stellar atmospheres or interiors, space physics, pulsating stars, dark matter and energy, galactic evolution, and general relativity and cosmology.

Prerequisite: Consent of instructor. Independent study of a selected research problem in astronomy. Includes experimental and/or theoretical investigation of an important yet unexplored problem or experimental design. Includes literature research and experimental design/problem formulation and execution resulting in oral and written presentation of results suitable for submission/presentation to a suitable journal/conference. One hour lecture and significant additional time working with research mentor.

Prerequisites: ASTR 4850 and consent of department chair. Focuses on a specific research/experimental design problem chosen with the consent of the thesis committee and with the potential for original discovery or for creative development of a tool, technique, or instrumentation applicable to scientific research. Independent pursuit of research objectives outlined in a research proposal results in a written thesis, the approval of which will include an oral defense. One hour lecture and independent writing of thesis.

## To Find Life in Our Galaxy, Follow the Phosphorus

New research into the composition of supernova remnants suggests phosphorus might be isolated in parts of the galaxy&mdashand phosphorus is a requirement for life as we know it.

(Image: A laser guide star cast on the night sky from the William Herschel Telescope at the Roque de los Muchachos Observatory on the island of La Palma in the Canary Islands.)

When astronomers look for parts of the galaxy that could contain life, they generally search for elements like oxygen and carbon. But another element essential to life could be the key to finding systems in the Milky Way that have the right conditions for living organisms.

"Phosphorus is one of the six elements on which biology depends," Jane Greaves, an astronomer at Cardiff University in Wales, told Popular Mechanics in an email. "The others are carbon, hydrogen, nitrogen, oxygen and sulphur. Without phosphorus, there would be no adenosine triphosphate (ATP), which is the molecule cells use to transfer energy."

Phosphorus is relatively rare in the universe, the rarest of the six elements required for life as we know it. It is created in trace amounts in some stars' natural evolution, but the majority of the universe's phosphorus is fused in supernovae. The element, atomic number 16, only accounts for about 0.0007 percent of all matter.

Greaves and fellow Cardiff astronomer Phil Cigan are presenting new research at the European Week of Astronomy and Space Science in Liverpool that compares the amount of phosphorus in the stellar dust of two supernova remnants&mdashCassiopeia A (Cas A) in the constellation Cassiopeia, and the Crab Nebula in the constellation Taurus. The early results suggest that the Crab Nebula contains significantly less phosphorus than Cas A.

The discrepancy comes as a surprise, as computer models suggested the two collections of stellar dust, created by the same type of supernova, should contain similar amounts of phosphorus. Understanding this difference could help us understand how levels of this crucial element are distributed across the stars.

"Cas A and the Crab Nebula are Core Collapse Supernovae, where the middle of the star implodes and then rebounds very fast, expelling the new elements made," says Greaves. "My guess is that Cas A had more reactions that made phosphorus because the star was more massive or denser, but that's just a guess so far."

If unknown processes cause some stellar explosions to produce more phosphorus than others, then life could be isolated to phosphorus-rich areas of the galaxy. At this point, however, only Cas A and the Crab Nebula have been studied with telescope spectroscopy to determine their chemical compositions. "As far as I know, phosphorus has not been looked for in any other supernova, of any type," says Greaves.

The team stresses that their research is preliminary and uses limited data. Phosphorus was detected in Cas A by a team of international astronomers in 2013. Graves and Cigan only recently used the William Herschel Telescope in the Canary Islands to study the infrared spectrum of the Crab Nebula, measuring the proportion of phosphorus and iron to compare to that of Cas A. Observations of the Crab Nebula were somewhat hindered by cloudy weather, however, and follow up research is needed to confirm that it is indeed lacking in the element P.

Another possibility is that the age difference between the two clouds of cosmic dust could explain the different amounts of phosphorus. The Crab Nebula was created by a supernova seen and documented from Earth by Chinese astronomers almost a thousand years ago, while the light from the supernova that created Cas A is thought to have reached Earth about 300 years ago, though no one is known to have observed it.

"It is possible that with the older event, the Crab Nebula, that some phosphorus has disappeared from gas and [formed] into solid material, something we hope to learn more about at this scientific meeting," says Greaves.

After being ejected from supernovae, phosphorus gasses coalesce and are trapped in rocky objects. These rocky, icy, and metal bodies clump together further to create rocky planets, which is how most of the phosphorus made it into Earth. However, the phosphorus that was first used in cells to transfer energy, and spark reproductive life, likely came after the planet formed and had large bodies of water, as meteorites bearing phosphorous crashed into the wet parts of the world.

To find where else in the galaxy the spark of life could occur, the trick might be to look for planetary systems that came from phosphorus-rich areas. The upcoming 6.5-meter James Webb Space Telescope, designed for infrared astronomy, should be particularly suited to measuring phosphorus in supernova remnants&mdashgasses that will ultimately form stars and planets.

"I'm very much looking forward to JWST, as this can potentially look for schreibersite [an iron-nickel mineral containing phosphorus] in discs around stars where new planets are forming, and it has a good wavelength range to look for this mineral we know occurs in meteorites," says Greaves.

With only two supernova remnants scanned for the element, and the capability to look for schreibersite in planetary systems coming online soon, the hunt for life-bearing phosphorus could just be getting started.

## Charting the expansion history of the universe with supernovae

Schematical representation of the expansion of the Universe over the course of its history. Credit:NAOJ

An international research team analyzed a database of more than 1000 supernova explosions and found that models for the expansion of the Universe best match the data when a new time dependent variation is introduced. If proven correct with future, higher-quality data from the Subaru Telescope and other observatories, these results could indicate still unknown physics working on the cosmic scale.

Edwin Hubble's observations over 90 years ago showing the expansion of the Universe remain a cornerstone of modern astrophysics. But when you get into the details of calculating how fast the Universe was expanding at different times in its history, scientists have difficulty getting theoretical models to match observations.

To solve this problem, a team led by Maria Dainotti (Assistant Professor at the National Astronomical Observatory of Japan and the Graduate University for Advanced Studies, SOKENDAI in Japan and an affiliated scientist at the Space Science Institute in the U.S.A.) analyzed a catalog of 1,048 supernovae that exploded at different times in the history of the Universe. The team found that the theoretical models can be made to match the observations if one of the constants used in the equations, appropriately called the Hubble constant, is allowed to vary with time.

There are several possible explanations for this apparent change in the Hubble constant. A likely but boring possibility is that observational biases exist in the data sample. To help correct for potential biases, astronomers are using Hyper Suprime-Cam on the Subaru Telescope to observe fainter supernovae over a wide area. Data from this instrument will increase the sample of observed supernovae in the early Universe and reduce the uncertainty in the data.

But if the current results hold-up under further investigation, if the Hubble constant is in fact changing, that opens the question of what is driving the change. Answering that question could require a new, or at least modified, version of astrophysics.

## Black dwarf supernovae: The last explosions in the Universe

Here's a happy thought: The Universe may end in a whimper and a bang. A lot of bangs.

Calculations done by an astrophysicist indicate that in the far future, the Universe will have sextillions of objects called black dwarfs, and that eventually they can explode like supernovae. In fact, they may represent the very last things the Universe can do.

But this won't happen for a long time. A very, very, very long time * . So long from now I'm having difficulty figuring out how to explain how long it'll be. I'll get to it — your brain will be stomped flat by it, I promise — but we need to talk a bit first about stars, and nuclear fusion, and matter.

Stars like the Sun release energy as they fuse hydrogen atoms into helium atoms in their cores. It's very much like the way a hydrogen bomb works, but on a massively larger scale the Sun outputs about the equivalent energy of one hundred billion one-megaton bombs. Every second.

Eventually the hydrogen runs out. A lot of complicated things can happen then depending on how massive the star is, what's in it, and more. But for stars up to about 8 – 10 times the mass of the Sun the outer layers all blow away, exposing the core to space a core that has become a ball of material so compressed weird quantum mechanics rules come into play. It's still made up of atomic nuclei (like oxygen, magnesium, neon, and such) and electrons, but they're under incredible pressures, with the nuclei practically touching. We call such a material degenerate matter, and the object itself is called a white dwarf.

The nearest white dwarf to us, Sirius B, has the mass of the Sun but the size of the Earth. For comparison, the Sun is over 100 times wider than Earth. Credit: ESA and NASA

For stars like this, that's pretty much the end of the road. The kind of fusion process they enjoyed for billions of years — thermonuclear fusion, where (hugely simplified) the atomic nuclei are so hot they slam into each other and fuse — can't work any more. The white dwarf is born very hot, hundreds of thousands of degrees Celsius, but without an ongoing heat source it begins to cool.

That process takes billions of years. White dwarfs that formed in the early Universe are just now cool enough to be red hot, around 4,000° C.

But the Universe is young, only about 14 billion years old. Over very long periods of time, those white dwarfs will cool further. Eventually, they'll cool all the way down to just about absolute zero: -273°C. That will take trillions of years, if not quadrillions. Much much longer than the Universe has already existed.

But at that point the degenerate matter objects won't emit any light. They'll be dark, which is why we call them black dwarfs.

So is that it? Just black dwarfs sitting out there, frozen, forever?

Artwork depicting a black dwarf in the far-flung future a dead star that was once like the Sun. This is somewhat fanciful by the time black dwarfs exist all the stars in the Universe should be dead as well. Credit: Baperookamo / Wikimedia Commons / Creative Commons Attribution-Share Alike 4.0 International

Well, maybe not, and this is where things start to get weird (yes, I know, they're already weird, but just you wait a few paragraphs). Currently, physicists think that protons, one of the most basic of subatomic particles, can decay spontaneously. On average this takes a very long time. Experimental evidence has shown that the proton half-life may be at least 10 34 years. That's a trillion trillion times longer than the current age of the Universe.

If true, that means that the protons inside the atomic nuclei in the black dwarfs will decay. If they do, then after some amount of time, 10 35 or more years, the black dwarfs will… evaporate. Poof. Gone. At that point all that will be left are even denser neutron stars and black holes.

Artwork depicting the magnetic field surrounding a neutron star. Credit: Casey Reed / Penn State University

But proton decay, while predicted by current particle theory, hasn't yet been observed. What if protons don't decay? What happens to black dwarfs then?

That's where this new paper comes in. It turns out that there are other quantum mechanics effects that become important, like tunneling. Atomic nuclei are loaded with protons, which have a positive charge, so the nuclei repel each other. But they are very close together in the center of the black dwarf. Quantum mechanics says that particles can suddenly jump in space very small distances (that's the tunneling part, and of course it's far more complicated than my overly simple synopsis here), and if one nucleus jumps close enough to another, kablam! They fuse, form a heavier element nucleus, and release energy.

This is different than thermonuclear fusion, which needs lots of heat. This kind doesn't need heat at all, but it does need really high density, so it's called pycnonuclear fusion (pycno in ancient Greek means dense).

Over time, the nuclei inside the black dwarf fuse, very very slowly. The heat released is minimal, but the overall effect is that they get even denser. Also, like in normal stars, the nuclei that fuse create heavier nuclei, up to iron.

That's a problem. The effects holding the star up against its own intense gravity is degeneracy pressure between electrons. When you try to fuse iron it eats up electrons. If enough iron fuses the electrons go away, the support for the object goes with it, and it collapses.

Artwork of a core collapse hypernova, a super-supernova. Credit: NASA/Dana Berry/Skyworks Digital

This happens with normal stars too. They have to be pretty massive, more than 8–10 times the mass of the Sun (so the core is at least 1.5 or so times the Sun's mass). But for stars like those the core suddenly collapses, the nuclei smash together and form a ball of neutrons, what we call a neutron star. This also releases a lot of energy, creating a supernova.

This will happen with black dwarfs too! When enough iron builds up, they too will collapse and explode, leaving behind a neutron star.

But pycnonuclear fusion is an agonizingly slow process. How long will that take before the sudden collapse and kablooie?

Yeah, I promised earlier that I'd explain this number. For the highest mass black dwarfs, which will collapse first, the average amount of time it takes is, well, 10 1,100 years.

That's 10 to the 1,100th power. Written out, it's a 1 followed by eleven hundred zeroes.

I… I don't have any analogies for how long that is. It's too huge a number to even have any kind of rational meaning to the pathetic globs of meat in or skulls.

I mean, seriously, here it is written out:

I mean, c'mon. 10^1100th power written out. Credit: Phil Plait

That's a lot of zeroes. Feel free to make sure I got the number right.

I tried to break it down into smaller units that make sense, but c'mon. One of the largest numbers we named is a googol, which is 10 100 , a one followed by 100 zeroes.

The number above is a googol 11 , a googol to the 11th power.

<sound of me running around in circles and making mewling noises>

And that's the black dwarfs that go first. The lowest mass ones take much longer.

That's not a typo. It's ten to the thirty-two-thousandth power. A one with 32,000 zeroes after it.

I'll note that this is for stars that start out more massive than the Sun. Stars like ours aren't massive enough to get the pycnonuclear fusion going — they don't have enough mass to squeeze the core into the density needed for it — so when they turn into black dwarfs, that's pretty much it. After that, nothing.

Assuming protons don't decay, I'll note again. They probably do, so perhaps this is all just playing with physics without an actual outcome we can see (not that we'll be around to anyway). Or maybe we're wrong about protons, and in that unimaginably distant future the Universe will consists of neutron stars, black holes, low mass black dwarfs like the Sun, and something like a sextillion black dwarfs that will one day collapse and explode.

A simulation of what a black hole with a disk of gas swirling around it would look, given the bizarre effects of its fierce gravity on the light from the disk. Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman

Black holes, I'll note, evaporate as well, and the last of those should go in less than a googol years. If so, then black dwarf supernovae may be the last energetic events the Universe can muster. After that, nothing. Heat death. Infinite cold for infinite time.

Oh hey, it gets worse. The Universe is expanding, but the part of it that we can see, the observable Universe, is actually shrinking. This has to do with dark energy and the accelerated expansion of the Universe, which I have explained elsewhere. But by the time the black dwarfs start to explode, the Universe we can see will have shrunk to the size of our own galaxy. Well, what's left of it by then. Odds are the black dwarfs will be scattered so far by then that we there won't even be one in our observable frame.

That's a rip-off. You'd think that waiting that long would have some payoff.

So why go through the motions to calculate all this? I actually think it's a good idea. For one thing, science is never wasted. It's possible this may all be right.

Also, the act of doing the calculation could yield interesting side results, things that have implications for the here-and-now that might be observable (like the decay of protons). There could be some tangible benefit.

But really, for my money, this act of spectacular imagination is what science is all about. Push the limits! Exceed the boundaries! Ask, "What's next? What happens after?" This expands our borders, pushes back at our limitations, and frees the brain — within the limits of the known physics and math — to pursue avenues otherwise undiscovered.

Seeking the truth can be a tough road, but it does lead to understanding, and there's beauty in that.

## Mechanics of Supernovae - Astronomy

Supernovae are exploding stars which are broadly classified into two main types depending on the type of star which explodes. The progenitors of a Type Ia supernova ( SNI a) is a white dwarf accreting matter from a companion, while the progenitors of core-collapse supernovae are massive stars at the end of their lives.

Supernovae are transient objects. They appear suddenly as a bright star (that can outshine an entire galaxy) at a random position in the sky, and fade relatively quickly never to be seen again. For this reason they are difficult objects to find and study, and astronomers have now established several supernova searches dedicated to locating new supernovae and obtaining rapid and extensive follow-up observations of these objects.

With such rapid follow-up, Type Ia supernovae have become one of the primary distance indicators in astronomy, helping to tie down the Hubble constant and revealing that the Universe is, in fact, accelerating.

Supernovae leave behind a supernova remnant, which are often beautiful objects that fade over tens of thousands of years before enriching the interstellar medium with a multitude of chemical elements.

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All material is © Swinburne University of Technology except where indicated.

## Watch the video: Mechanics Of A Side Split (July 2022).

1. Asif

You are similar to the expert)))

2. Brandan

Why does your resource have such a small number?

3. Ioseph

Huge human salvation!

4. Brahn

Of course, I apologize, but this answer does not suit me. Who else can suggest?