Sk y C an dy #7
Spaghetti in the Sky
HOO palette image © Stéphane Vetter (Nuits sacrées).
How does empty space get so full of the stuff we see in our eyepieces?
Looking at the emptiness between stars is the inverse of looking at the sky on a cloudy day. Beyond our housebound window some clouds are crisp-edged, dense, bubbly masses of cumulus. Ice-crystal cirrus can be sleek as horse tails or matted messes made by winds. Featureless stratus veils everything with dull. Even seemingly clear sky that looks lucid is actually tremulous with dust too minute to see until the sun sets and we enjoy glories of red. The visual band which constrains our spectrum makes up for its narrowness by giving us vividness as lovely as art.
Earthly clouds may look very different, but their shapes have four things in common: molecules, dust, wind, and temperature. The laws that govern the way these four interact are called fluid mechanics.
“Empty” space is actually beclotted with cloud types called nebulae. Molecular clouds that make star clusters are like cumulus — bulbous, compact, opaque, dense. Space has more varieties of filamentary cirrus than we see on Earth — indeed, magnet-made filaments are more common than gravity-made blobs. The space equivalent of featureless stratus is less common because there are so many self-gravitating objects that ingather gas and dust. Turbulent shocks try to corrupt. Magnetic fields try to cohere.
Yet galaxies have one cloud type we don’t have on Earth vacuities, or low-density bubbles that can preserve their emptiness for remarkable lengths of time. The Sun is gliding through one of these. The Local Bubble’s gas density is a tenth of the gas outside the bubble — lucky for us, because the placidity within the Local Bubble encourages the type of watery cinderella conditions required for life to occur.
This begs the question, “Why is there a Local Bubble?”
The bubble is actually a vacuity with a boundary, not a bubble. Inside it, negative pressure is greater than the gas outside. The cavity is rimmed with celestial neighbours like the Local Interstellar Cloud (which contains the Solar System), the neighboring G-Cloud, the Ursa Major moving group (the closest stellar moving group) and the Hyades (the nearest open cluster). The Bubble is about 1000 cubic light years in volume, a shape that is defined by its atomic hydrogen density of about 0.05 atoms/cm3 (the Milky Way ISM averages 1.0 atoms/cm3), and one third that of the Local Interstellar Cloud (0.3 atoms/cm3).
But how can a vacuity exist if it is surrounded by so much density? We find this hard to comprehend from the inside, so let us find an object that behaves like the Local Bubble which we can see from the outside. The above constrains point to a likely candidate: a still-expanding supernova shell.
Simeis 147 is one of these. Unfortunately Sim 147 is a difficult visual object. Its four brightest patches are 24 MPSAS. The generally agreed lowest limit for human eyes is 25 MPSAS. Hence the aspiring enthusiast chasing a glimmer needs a large aperture, super wide-field eyepiece, and Bortle 1 skies. My best scope is a 20-cm f/4 Intes Mak. I observe from high desert skies in which there is not a single human-originated light anywhere. Yet I have never caught even a hint of Sim 147.
Although Sim 147 is a toughie for we who have eyeballs, it is much easier in the gamma ray, H-alpha, and millimeter bands because of its searing radiation and swept-up dust; and the radio continuum band which traces magnetically propagated synchrotron radiation. S147 has enjoyed a bit of recent professional attention because of several studies in the X-ray band — https://arxiv.org/abs/2401.17312 and https://arxiv.org/abs/2401.17261. (These two papers are important because they use X-ray data from Rosita, the only wide-field X-ray telescope in space.)
Sim 147 presents a conundrum. Its measured H-alpha kinetic velocities suggest an expansion age of ~40,000 years, but its 160 light-year physical span across space points to an age of 150,000 years — if the canonical values of the original detonation energy of >1051 erg (1044 joules) and the 0.5 atoms/cm3 density of the surrounding medium are assumed. The ancestry problem is solved if the progenitor star detonated in an alreadyexisting low-density bubble. Vacuities are a natural consequence of several supernovae going off in a rapid-collapse star cluster. The speed of sound of the interstellar medium (ISM) is about 10 km/s. A supernova’s Mach 10–100 shock waves expel the local gas and lightweight peripheral stars, leaving a vacuity. However, there is a problem with this theory: Sim 147 does not have a moving cluster around it. A cluster hot enough to spawn multiple O star supernovae should also have an initial mass function retinue of B and lower-mass stars. Where are they? Astronomers solved one problem only to create another. What a good excuse for another funding cycle!
Hubble, bubble, toil and trouble
The Earth sits in a 1,000-light-year-wide void amid thousands of young stars. Most of those stars are general field stars that occupied this region of the Orion Spur for multi-millions of years. About 14 million years ago an estimated 15 stars in a young cluster went supernova in a relatively swift sequence of a few hundred thousand years. The result was a roughly spherical bubble that expanded so furiously that it sucked up about 90 percent of the gas inside and deposited it on the inner surface of the bubble. The bubble has been expanding ever since, but also slowing down. Today its surface velocity is about 4 km/sec. That is only about Mach 4. When the bubble slows into subsonic speeds it will evaporate into the general gas/dust field of the Orion Spur.
About 7 million years ago the blast wave began compressing gas in its way, initiating shock-induced or “triggered” star cluster formation. Today seven well-known star-forming regions mold across the surface of the billowing gas.
This summary generalizes the complex interaction of gas and dust dynamics, magnetic fields, thermal dissipation, and the mechanics of turbulence which are set into motion by supernovae. If we were to distance ourselves the 3000 light years between us and Sim 147, our Local Bubble might look quite a bit like Sim 147. Nor are we alone in our Bubble — a much younger supernova-triggered bubble named the Perseus-Taurus shell is replicating the same process. It already has two very young cluster just now forming on its surface. See Catherine Zucker’s 3D interactive graphic in the web link below.
The presence of the Sun near the center of the Bubble is sheer luck. The Sun wandered into the Bubble by chance some five million years ago and today basks in placid vacuity. But for that fortuitous moment of timing and stellar wanderlust, we might not be here to tell the tale.
See this YouTube for an animation of the Local Bubble’s original SN blast and the shock-shell’s expansion after: https://youtu.be/08UlpJBt5Ic.
See Catherine Zucker’s interactive 3D animation here: https://faun.rc.fas.harvard.edu/czucker/ Paper_Figures/Interactive_Figure1.html
The composition of a deep sky object dictates which narrowband filters will produce the most useful science data. The choice depends in part on our foreknowledge of the object under scrutiny. Supernova remnants are known for their extreme thermal energy, shock effects, and free electron abundance (which implies magnetic fields). SNRs are abundant in hydrogen and oxygen that were hurled from the progenitor’s thick envelope into space in the form of a supersonic shell which quickly fragmented into smaller turbulence bubbles. Hence a good filter choice for the astro-imager would be hydrogen-alpha and doubly ionized oxygen III. Hα falls in the red part of the spectrum, and OIII in the blue-green part of the spectrum. Narrowband red-green-blue filters make it possible to image in light polluted skies. When recombined they produce a white-light image cleansed of telluric (ground-based) pollution such as sodium-vapor (yellow) or LED light (blue).
According to Martin Heigan, who produced this montage, “Imaging a vast multi-panel mosaic with only two narrowband filters saves time. I chose to use the HOO Palette. The OIII blue channel is simply mapped to the green channel as well, leaving the red and blue colors unchanged. I like to call this enhanced color, as the color is not false but rather boosted via integration time.”
Click on https://www.wikiwand.com/en/Local_Bubble and https://youtu.be/08UlpJBt5Ic for an in-depth look.
Why is Simeis 147 so hard to see?
Simeis 147 is commonly called The Spaghetti Nebula. In loftier circles it is SNR G180.0-01.7, Shajn 147, or Sharpless 2-240. In HII it looks like the cook was overly fond of red sauce. In Hα the noodles are soap bubbles in the sunset. In OIII, the rims of nacreous shells.
Sim 147 is a supernova remnant (SNR) directly astern of the Milky Way as the sun rudders into the Sagittarian Sea. Viewed from our perspective on the disc plane it is barely two degrees south of the Galactic plane. Viewed from above it straddles Auriga and Taurus near the Galactic S polar axis. It does not appear on amateur astronomer star maps older than 20 years because it is one of most difficult extended objects to spot. It is also huge — three full degrees on the long axis. Hence Sim 147 has not been much of an amateur target till the availability of large-aperture short focal length telescopes and superwide eyepieces. Only four small pockets rise above the 25 mags per sq. arcsec (25 MPSAS) threshold, the feeblest luminance most human eyes can see. The images below show the approximate locations. How do we prepare for the search?
Warmer-uppers
The best preparation a hobbyist can make to tackle Sim 147 is to log a lot of dwarf galaxies in one’s observations book. Dwarfs share Sim 147’s qualities of very faint threshold, low central luminosity, gradual density gradient, and star bedazzled background confusion.
A “confirmed” sighting in these conditions means three unquestionable glimpses repeated two times in a given night over three different nights. Suitable warmup candidates are the Ursa Minor Dwarf, the Draco Dwarf, Fornax Dwarf and 4 of its globulars; the Phoenix Dwarf, Sculptor, WLM, IC 1613, Leo X, Leo T, and for patient observers, the Pegasus Dwarf and Andromeda VI. Palomar 2 in Auriga makes a nice side-trip, but faint GCs aren’t as helpful as dwarfs because their round glow is easier to spot than an extended lozenge when using averted vision. (Remember the phrase “insufficient luminosity threshold” when presenting the case for a larger scope to the Household Finance Director.)
The big picture
Simeis 147 is a middle-age supernova remnant ≈40,000 years old. Some of S147’s filaments have a different chemical composition on one side than the filaments on the opposite side. That’s a clue to its origin. There is also a velocity gradient favoring the upper-left filaments but not the lower right. A velocity gradient in an extended thin nebula suggests an angular momentum excess after a binary decouple.
flow in CBT1 Abell 85
Sim 147’s original progenitor was a binary O system whose components were so close to each other that they exchanged material via Roche lobe transfer. Similarmass Type O binaries closer than 1 AU (the Earth-Sun distance) exchange surface gases via their Roche lobes. When one of the pair goes supernova, the drastic and sudden loss of mass frees the second star to wander off as an O runaway star. The runaway’s new transverse velocity is its original orbital velocity minus the momentum transfer losses donated to the overall association as the runaway departed.
The shock wave from a binary supernova blasts across the surviving O star, stripping material from the its surface and shallow envelope layers. That accounts for Sim 147’s metallicity anomaly. Roche lobe gas is hydrogen-rich while envelope gas is helium-rich. Excess helium in S147’s remnant implies envelope stripping from the surviving O star. With these clues in hand, astronomers went O runaway hunting, looking for Sim 147’s companion. It was a brief chase. A high-velocity >13 M⦿ star was located 4 arcmin away.
Sim 147’s nebulosity is ±3° diameter. The SN remnant is an estimated 3000 (±350) light-years distant — about half again further out toward the Perseus arm than the young open star clusters M35-36-37-38 in Auriga and Gemini. That puts it in the interarm region between the Orion Spur and Perseus Arm.
Lopsided metallicity gradients are tricky to quantify because of their large surface areas and low gas densities. Aladin Lite multiband images reveal the old supernova remnant PSR J0538+2817 (visible mainly in XMM Newton Xray) is also located well off-center in the direction of the most rapid filament expansion. This suggests bubble breakout (see example above) in which the expansion velocity of a Sim 147 gas parcel exceeded the binding energy of its bubble surface and broke out as champagne flow. “Champagne flow” occurs when ionized hydrogen gas accrues enough energy to burst out of a constraining shell and spews into the galactic medium. That is one way the enriched metals within an SNR escape to enrich the galaxy itself.
The progenitor star’s explosion left a 9 RPM neutron star packed so densely that it is 2.3 times the density of an atomic nucleus. The neutron star’s magnetic field is so powerful it could tear off your belt buckle from the distance of the Sun. Heaven help you if you have those old mercuryamalgam tooth fillings. Not for nothing are pulsars, black holes, and magnetars called “exotics” in the astrophysics literature.
Today Sim 147 is a roughly oblate “ear-shape” shell swiss-cheesed with undulating filamentary loops. The leading edges glow because they are expanding supersonically through a low-density interstellar medium, which induces both compression and friction heating. By now Sim 147 has dissipated so much energy that its compressive shells no longer trigger star formation. Sim 147 is fated to sigh out of existence so slowly no one would ever suspect the monster it once was.
In a binary system, a pair of stars close enough to be inside their Roche limit become gravitationally unstable. The instability first affects surface layers, which detach from the stars and fall into their mutual Roche lobe. In a fully detached binary there is no exchange between the stars because the Roche lobes do not fill with gas. In semi-detached binary one star fills its Roche lobe and transfers mass to the denser star. In a contact binary both stars fill their Roche lobe and have a common envelope (CE). CE’s are gravitationally unstable and transfer mass from the less dense to the more dense star. This can lead to an oscillatory imbalance when mass transfer see-saws back and forth. In O and >B5 star binaries the dance ends when one of the stars goes supernova.
Champagne
Lustrous wallflowers
The WikiSky screen shot below shows the location of Sim 147’s supernova remnant, the pulsar PSR J0538+2817. Most pulsars are strong X-ray emitters. The radiation is not thermal energy from a hot star but rather synchrotron radiation emitted when relativistic electrons spiral around magnetic field lines. The emission direction propagates as a forward-pointing “beaming” cone of radiation around the field line (see ¶4 here). Pulsars have powerful poloidal magnetic fields. Their field lines erupt out of the poles instead of whipping around the equator in a toroidal donut ring (see next page). Around 70% of O stars are binaries at birth. In 2015 Dincel, Neuhauser et al searched the nearby field for a high-mass runaway star. The team found an early type candidate inside Sim 147’s shell structure. HD 37424 is a B0.5V-type star with a peculiar velocity of 74±8 km/s. Tracing its trajectory back via Monte Carlo simulations, the Dincel team calculated that HD 37424 was at the same position as Sim 147’s central pulsar PSR J0538+2817 some 40,000 years ago. This position is only 4 arcmin away from Sim 147’s geometrical center. Hence HD 37424 was the pre-supernova binary companion to the star that gave us the nebulae. Today HD 37424 is 1333 pc (4345 ly) from Earth.
HD 37424 may not end up as ill-fated as its erstwhile companion. It may have lost so much envelope mass that the reduced gravitational pressure on its core brings it below the mass threshold for a supersonic detonation. The alternative is mass loss via subsonic deflagration, which bleeds the star of excess gas gradually. The Dincel team calculated that the Sim 147 progenitor had a mass of >13 M⦿. The team then projected what the pre-supernova binary was like. They found via Roche lobe simulations that the two stars were within one AU of each other’s surfaces during the late stages before the progenitor went supernova. Pulsars emit feebly in all but a few wavebands, mainly radio, X-ray, and gamma. Simeis 147 is no exception. Its 147’s pulsar PSR J0538+2817 is as close to anonymous you can find in astronomy website searches. Aladin Lite (below) revealed emission only in the XMM Newton image set. Yet in that band the star blazed in the 9.3 arcmin field of ESA / SSC XMM-Newton X-ray images at Red=0.5-1 Kev, Green=1-2 Kev, Blue=2-4.5 Kev. HD 37424 also emits in the 1.4 GHz radio band at 1.432 mJy. The Landoldt V extinction here is 3.86 magnitudes, which is less an issue here than it is with visual observers, but of great interest to SNR physicists keen to know what exactly they are seeing.
PSR J0538+2817 in the XMM Newton 9.3 arcmin field. The star does not present an image in any of the other Aladin Lite bands shown on the partial list to the left.
Magnetic attraction is romantic, but not with a magnetar.
Of the 30 known Galactic magnetars, eight are in supernova remnants. PSR J0538+2817 in Simeis 147 is one of them.
Close O binaries put on quite a tit-for-tat mass exchange via Roche-lobe transfer.
The initially larger star donates gas to the smaller one until it swells to larger than the first star. The gas flow then reverses until the newly smaller star becomes the larger one again.
The cosmic doe-si-doe goes on until one of them detonates. When that happens to two very close stars, the supernova blasts away up to half of the binary’s total mass, sending its former companion off to wander the heavens in splendid solitude. Don’t get in its way.
Magnetar poloidal field lines are so dense that they interact with each other at 1015 times the field strength of the Earth’s 25 gauss.
The Sim 147 shell is expanding at 80 120 km s 1. Using GAIA parallax data, the pulsar traces the original super-nova to 1.47 kpc (4790 ly) from Earth. The shell extends about 150 light-years at this distance. The remnant is estimated to be about 40,000 years old. The remnant’s fragile, tenuous appearance in the optical and near-IR are consistent with other SNRs.
History
Simeis 147 was discovered in 1952 by Grigori Abramovich Shajn and V.T. Hase at the Crimean Observatory on Mount Koshka, Crimea in the former U.S.S.R. (See more about the observatory below.) The discovery was made using photographic plates taken with a 25-inch Schmidt camera.
Sim 147 was catalogued by Stewart Sharpless, an astronomer at the Unites States Naval Observatory in Flagstaff, Arizona. Sharpless compiled his oft-used SNR reference database by examining Palomar Observatory Sky Survey (POSS) plates for HII regions. His second and final catalogue of 313 objects was published in 1959. Simeis 147 appears in his list as Sh2-240.
Simeis 147 was first classified as a possible shell-type supernova remnant (SNR) by Walter Minkowski just one year earlier.
At visual mag 9.0 PSR J0538+2817’s runway O companion star is an easy find for a go-to system.
Why is Simeis 147 so puffy?
Supernova remnants propagate in three successive stages.
• The first phase is the explosive expansion of the star’s envelope mass as a spherical detonation field. The matter moves outward at high velocity, typically ~10,000 km s-1 (from the North Pole to the South Pole in 1.27 seconds). The remnant associated with SN1987A in the LMC is in still this early explosive phase. The ∼300 year old remnant of Cassiopeia A is nearing the end of the explosive phase, billowing outward at 6000 km s 1. Its internal knots, visible in OIII images, are oxygen-rich clumps of matter dredged from the depths of the star’s envelope convection cells for thousands of years before the detonation.
• The second phase of ejection is the Sedov adiabatic blast wave phase. This phase begins when the mass-density of interstellar gas compressed in front of the bubble becomes about equal to the mass-density of the bubble itself. The remnant of Tycho’s Supernova is a Type 1a supernova shock in the blast wave phase. The Crab nebula is a Type IIb core-collapse supernova in the blast wave phase; its expansion velocity is ∼ 900 km s 1 .
• The third phase is the radiative or snowplow phase. When the energy on the inner side of an expanding shock front collides into the billows of gas previously ejected by the star’s stellar wind, it forms a dense cool layer directly behind the shock front. This produces a high-energy rebound propagating back toward the center of the shell. An image shown below of the 1006 Type 1a supernova recorded as a “guest star” by the Chinese shows how the layers of a snowplow shock front hit the density/temperature shell of the initial shock blast and rebound backwards toward the still-expanding gas below. The 40,000 year old Cygnus Loop (Veil Nebula) is in the late snowplow phase; its expansion velocity is 120 km s 1 (roughly Mach 12). The reverse shock compressed the vacated interior where the progenitor exploded, which blasted the gas pockets still there into many small lowdensity gas pockets. Since the pockets were thermally unstable, turbulent cells in the inbound reverse shock transferred energy into the dense clumps of gas in the old core region. This shaped the flocculent puffs we see in both the SN 1006 and Tycho 1572 SNR images.
These are generalized examples. Many variants arise because of the diversity of local conditions. The progenitors of Type II core-collapse SNRs are massive hot stars that expel powerful high-velocity winds. A supernova in such a star occurs in a relatively low-density bubble that has been evacuated by the presupernova star’s intense stellar wind. If the supernova ejecta expands into a low-density bubble it encounters rather little mass ahead of it, hence its free expansion phase is prolonged. The shock shell weakens as it expands. When the shell arrives at the more rarefied gas of a spiral arm or interarm region, it dissolves into champagne flow, a free expansion that feeds into local atomic and molecular clouds; some of it free-flows into local low-density regions.
The interstellar medium within a spiral arm is clumpy, thready, and turbulent. In the interarm regions the gas medium is relatively smooth. Local inhomogeneities fragment the propagation of blast waves and snowplow shells. The parts of a shock front passing through an irregularly clumped volume of gas can enter density pockets that support the formation of a slower snowplow shell. If other parts of the front encounter a locally rarefied region, they remain in the blastwave phase. The result is the bulbous look of Sim 147. Massive stars are still in their natal clusters when they explode. In a large cluster the most massive O and B stars will supernova within the first 5 to 20 million years. They will not have had time to move very far from the association’s central region. The expanding shells of multiple supernovae will eventually make a common superbubble that can grow to hundreds of parsecs in diameter. Simeis 147 was caused by just one supernova. Imagine a dozen or more from the same cluster as they merge into a single superbubble. The LMC’s Tarantula Nebula and M33’s NGC 604 are examples of the tumult of multi-supernova bubbles blasting hot ionized gas into a galaxy. See these SILCC sims (1, 2, 3) of what this does to a galaxy over several million years.
Where do high-velocity electrons get their energy?
The mechanism that converts the kinetic energy of the expanding shock into energetic electrons goes by the cumbersome but exact term magnetohydrodynamic instability. Each time a free ion bounces back and forth across a shock front, it gains energy. The result is ultra-energetic cosmic rays that can pack the mass of a baseball traveling near the speed of light.
Why is Sim 147 so transparent?
Mentally freeze-frame a boiling kettle and you have a pretty good image of the internal motions of “empty” space. Consider the non-boiling water as empty space and the rising steam bubbles as an expanding supernova explosion. Whether the hot gas is a bubble on the boil or a gas/dust bubble in a galaxy, it expands as it heats up. We often think of interstellar space as a consommé of cold atoms and hot photons. In reality the space between stars — e.g., the vast volume between us and Alpha Centauri — is more like a super-thin vegetable soup. It is made of long, slender, noodle-like dust filaments, wraith-like atomic clouds hundreds of times the diameter of the Solar System, egg-yolk sacs of molecular hydrogen clouds surrounded by an albumin of atomic hydrogen. Not to mention the 411 to 413 photons spanning radio to gamma energies zipping through any given cubic centimeter every second. (The numbers differ because so do opinions.) Of these, 406 are from the Big Bang’s primal fireball, the Cosmic Background Radiation.
The mechanism of Type 1a SN explosions is far from settled science. See here, here, and here.
Space dust is unimaginably sparse — The Local Bubble averages one dust particle per cubic meter. The motions of dust particles do not necessarily relate to the motions of gas particles. Gas and dust co-mingle but don’t respond to space’s multitude of forces in the same way. Dust particles fly through the interstellar medium under the influence of magnetic fields and gravity. Magnetism doesn’t affect gas until it is ionized. Some dust is siliceous (sand, basically) because it was seeded with silicon from core collapse supernovae and late-stage AGB stars. Over half (57%) of space dust is carbonaceous. It formed from carbon molecules in the atmospheres of low surface-temperature AGB red giants and Cepheid variables. Carbon dust is blobby and amorphous. Siliceous dust is needleshaped and responds to magnetic fields. Carbon dust is soot that plays a vital catalytic role in the formation of the molecular hydrogen which collapse into stars. (Atomic hydrogen can’t make stars; only molecular hydrogen can do that).
Particle densities in galactic space vary enormously. A handy number in the case of the galactic plane is one particle per cm3. The cores of molecular clouds rise to a million particles per cm3 (though supernovae don’t occur there). Densities outside the galactic plane can go as low as one particle per 10,000 cm3. The term “particles” can include free electrons, ions, atoms, molecules, and dust. As a reference point, the average particle density of the universe is 5.9 protons per cubic meter. (Protons rather than particles, because the WMAP Wilson Microwave Anisotropy Probe built to detect the density was configured to probe proton signal.)
When a Type 1a supernova detonates, its shock wave propagates a searingly hot high-velocity expansion sphere. In a typical spiral arm medium of one particle cm3, a supernova shock expands outward, cools, and slows down at a rate inversely proportional to the local gas density. It plows into gas/dust overdensities and compresses them into hot lenses with the same curvature as the shock front. Dust filaments respond to an advancing hypersonic shock wave by compressing into thin sheets tens of centimeters thick that are pushed to the front surface of the shock wave. Long-slit spectra of shock fronts acquired perpendicular to the curve of Sim 147’s bubble shells show the signatures of alpha and transferric elements instead of H1, Hα, and H2. This tells us how the heavier atoms from oxygen to rubidium made in a star core are injected into the galaxy (and more pertinently, into us.)
When Sim 147’s carbon-oxygen core deflagrated into 56Ni, 56Co, and 56Fe, the blast wave hurled vast amounts of carbon and oxygen into space at speeds of millions of miles per hour. This generated a forward shock wave that sped ahead of the ejecta. The high pressure gas behind the forward shock expanded into the ejecta, prompting a reverse shock whose viscous turbulent shocks heated the ejecta. Eventually the reverse shock wave traversed the ejecta all the way back to the core. The end result was that the interior of the supernova shell came to the flocculent thermal stability that we observe in the images of SNR 1006 and Tycho 1572 on the next two pages.
An observer surfing along with the front edge of the ejecta would see the reverse shock moving inward and the forward shock moving outward. We distant observers see both.
Case study: SNR 1006
It's the year 1006 and look who blew in the door.
In the year 1006 a "new star" appeared near what is for us Beta Lupii. In just a few days it became brighter than the planet Venus. We now know that the event heralded not the appearance of a new star, but the cataclysmic death of an old one. It was likely a white dwarf star that had been pulling matter off an orbiting companion star. When the white dwarf mass exceeded the Chandrasekhar stability limit, it detonated.* Material ejected in the supernova produced tremendous shock waves that heated gas to millions of degrees and accelerated electrons to extremely high energies.
In Wiki’s telling, SN 1006 was a supernova that was likely the brightest observed stellar event in recorded history, reaching an estimated 7.5 visual magnitude, exceeding by sixteen times the brightness of Venus. Appearing between April 30 and May 1, 1006 ACE, this "guest star" was described by observers in China, Japan, Iraq, Egypt, and Europe, and possibly recorded in a North American Hopi Native American petroglyph. Some reports state it was clearly visible in the daytime. Modern astronomers now consider its distance from us to be about 7,200 light-years.
The 1006 supernova produced a shell-type remnant. In these, the initial detonation hurls hypersonic star ejecta in a roughly spherical ball. At first the ball expands at up to Mach 100, or >10,000 km s-1. This is the free expansion phase and it can lasts hundreds of years. Eventually the ejecta cools to about 200 K and slows to 20 km s-1. The shock front continues to expand, but the internal heat is no longer driven by the pressure of fiercely hot gas trying to come to thermal equilibrium with the space around it. For the next 10,000–20,000 years. the gas expansion is driven by adiabatic cooling, expanding at a pace which keeps the interior temperature nearly constant.
Eventually the shock front slows to 10 km s-1, which is the average speed of sound (Mach 1) in the interstellar medium (ISM) in a spiral galaxy arm. When a shock front goes subsonic it slowly diffuses into the general gas medium of the galaxy, fittingly labeled the Disappearance Phase. Overall, the entire process can take a million years or more.
* A detonation accelerates its component material to supersonic velocities. Deflagration means the ejecta expands at subsonic speeds.
SN 1006 was likely a mass-transfer binary in which a white dwarf star pulled matter off a larger lower-mass companion star. When it detonated, the bright outer shock front did not spherically encircle the remnant because the orientation of the poloidal magnetic field was perpendicular to the visible shock front. Immediately behind the shock wave a counter-shock or reflection wave pushed backwards towards the center of the remnant. The forward shock created temperatures in excess of 10,000 K as it expanded. The reverse shock slammed into the ejecta still flowing outward, producing temperatures into the millions of degrees K. The flocculent (fluffy) red features seen throughout the interior are dense gas pockets heated by the reverse shock.
Case study: Tycho 1572
We’ve come a long way since Tycho Brahe recorded an unusually bright star in 1572. Today we know such “guest stars” are mostly supernovae or bright novae. SN ejection clouds like this and the others shown above are abundant sources of newly-formed elements, termed “metals” by astronomer consensus. Cosmic metallicity is why we can breathe air made of oxygen and nitrogen, and use carbon atoms in many different ways (some of them not good for us). SNRs are also the primary source of cosmic rays which hurtle around space, packing outsize amount of mass into their tiny protons.
The Chandra X-ray telescope discovered these odd-looking stripes in the Tycho supernova remnant. Image analysis and computer modeling suggested that these are the probable source of the most intensely energetic cosmic rays known to science. Electrons with energies of a trillion electron volts (1012 eV) produced the X-ray emission seen by Chandra. The blue edge of the circle represents the outer shell of the blast wave of the supernova remnant. The lighter features are the stripes. The upper square box to the right is a close-up of a region far away from the stripes; the black lines show twisted but not tangled magnetic field lines. The red squiggle shows an electron spiraling around one of these lines. The middle panel shows magnetic fields are much more tangled and particle motions are much more turbulent. In the bright stripe the tangling of the magnetic fields and the turbulence is even stronger. Strongly energetic tangled fields are believed to be the origin of the most energetic teravolt cosmic rays in our galaxy.
Dust in space is just as messy as it is in your optical system.
2D simulation of aerodynamic grains in supersonic turbulence. In this simulation by Hopkins & Lee 2015, the grain sizes are 0.001 to 1 μm and Mach number is M 5. At the instant this “snapshot” in the simulation was frozen, Mach speed and magnetic energy were in equilibrium. The colors show local gas densities relative to the mean gas density of the parcel (blue). The thin black steaks show how the compression and rarefaction of a multitude of small shock fronts (turbulence) tends to flatten a rather chubby filamentary network of dust particles into very thin sheets. Notice the fractal-like coherence in filament patterns. These show that turbulent shocks affect dust pretty much the same on large scales and small. Nearly all the dust lies along razor-thin filaments. In this simulation the dust filaments are not associated with the gas clumps.
Where does space dust come from?
Cepheid variables are an often overlooked source.
RS Puppis is a beauty, and it tells a great tale. (1, 2, 3, 4.)
Dust is one of the least-told stories in popular accounts of why things in the sky look the way they do. Dust is most often described as carbon nanoparticles smaller than soot or cigarette smoke (technically 0.001 to 0.1 µm) that originate in the cool atmospheres of AGB red giants. Latestage AGB stars dredge up huge amounts of nuclear material from their core-envelope interface. The convective envelopes become abundant with carbon, nitrogen, oxygen, and other α-process elements assembled from helium nuclei.
That doesn't explain the presence of siliceous dust, which is mainly silicon, calcium, and sulfur. Some of it comes from late-AGB stars, some originates in Type II supernovae, and some is gas expelled by winds from the surfaces of Cepheid variables as they expand and contract.
This Hubble image of the Cepheid variable RS Puppis is a vivid study in mass loss through wind ejection. Classic Cepheids are 10 times more massive than the sun. RS Puppis’s surface gravity is only ~0.016 of Earth’s (and about half that of the Sun’s). Its higher-velocity ejecta escapes ballistically into the Galactic medium, forming a huge cocoon of dust. Some particles will escape forever; slightly lowervelocity ejecta will arc high above the star’s disc, slow, and fall back into the maelstrom.
The atoms that escape merge into the Galactic medium. Over great periods of time the atoms absorb enough incoming electrons and photons that feed chemical reactions which end in complex molecules. The energy consumed in these processes exerts a cooling effect on the Galactic medium. By virtue of their absorption they remove the energy from incoming photons and electrons, cooling their surroundings.
Star formation requires intensely cold conditions in the depths of molecular clouds. Those clouds are abundant with dust, which does two things: (a) acts as a coolant that slows particle velocities down to gravitational star-forming energies, and (b) eventually deposits their complex mix of elements into new stars, which end up with a higher metallicity than the generation that came before them.
Watch how an AGB giant seeds the Galaxy with dust here.
Strolling the Galleria Galactica: Supernovae Remnants
RCW 86 Circinis, the oldest known SNR. Tycho Brahe’s 1522 SNR in Cassiopeia.
Vela SNR Hα OIII 11-12000 yro. SNR B0519-69.0 T 1a SNR in LMC 600 G34.7-0.4
Events in space take place on such a large and long time scale that we do not notice the violence of their beauty.
Readings
Cosmic Challenge: Simeis 147, Phil Harrington, Cloudy Nights, January 2021
Simeis 147, Michael E. Bakich , Astronomy Magazine, January 8, 2024
Simeis 147--Another supernova remnant in Taurus, Stewart L. Moore, Journal of the British Astronomical Association (Vol. 120, Issue 6), Dec. 2010
Sh2-240 - Simeis 147 - The Spaghetti Nebula, Gregg Williams, Astrobin February 28, 2022
Further Research
arXiv:2401.17312: Study of X-ray emission from the Sim 147 nebula with SRG/eROSITA: X-ray imaging, spectral characterization, and a multi-wavelength picture. Abstract: Simeis 147 (Sim 147, G180.0-01.7, "Spaghetti nebula") is a supernova remnant (SNR) extensively studied across the entire electromagnetic spectrum, from radio to giga-electron volt γ-rays, except in X-rays. Here, we report the first detection of significant X-ray emission from the entire SNR using data of the extended ROentgen Survey Imaging Telescope Array (eROSITA) onboard the Russian-German Spektrum Roentgen Gamma (SRG). The object is located at the Galactic anti-center, and its 3 deg size classifies it among the largest SNRs ever detected in Xrays. A&A May 17. 2024.
arXiv:2401.17261: Study of X-ray emission from the Sim 147 nebula by SRG/eROSITA: supernova-in-the-cavity scenario. Abstract: The Simeis 147 nebula (Sim 147), particularly well known for a spectacular net of Hα emitting filaments, is often considered one of the largest and oldest known supernova remnants in the Milky Way. Here, and in a companion paper, we present studies of X-ray emission from the Sim 147 nebula using the data of SRG/ eROSITA All-Sky Survey observations. Many inferred properties of the X-ray emitting gas are broadly consistent with a scenario of the supernova explosion in a low-density cavity, e.g., a wind-blown-bubble. Submitted to A&A January 2024.
arXiv:2309.13670: Discovery of a one-sided radio filament of PSR J0538+2817 in Sim 147: escape of relativistic PWN leptons into surrounding supernova remnant? Abstract: The pulsar is plausibly associated with the supernova that gave rise to the Spaghetti Nebula (Simeis 147). The structure is one-sided and appears to be almost aligned (within 17 degrees) with the direction of the pulsar's proper motion, but in contrast to the known cases of pulsar radio tails, it is located ahead of the pulsar. At the same time, this direction is also approximately (within 5 degrees) perpendicular to the axis of the extended non-thermal X-ray emission around the pulsar. Accepted to MNRAS
arXiv:2107.12897: High resolution H-alpha imaging of the Northern Galactic Plane, and the IGAPS images database. Abstract: …Herbig-Haro objects are outlined and, as part of a discussion of mosaicking technique, we present a very large background-subtracted narrowband mosaic of the supernova remnant, Simeis 147. We lay out a method that exploits the database via an automated selection of bright ionized diffuse interstellar emission. Announced July 2021.
Eleven older references on ADS Astrophysical Data Service.