Brightest supernova since 1954 peaks this week

[ewmayer]

Brightest supernova for 57 years hits peak tonight | Mail Online

There will be a once-in-a-lifetime event in the night sky over the next few days – as a star exploding 21 million light years away becomes so bright it will be visible through binoculars across Britain. [EWM: And more generally across the entire northern hemisphere.]

The supernova is predicted to reach its brightest between September 9 and 12, and will be the brightest since 1954, visible all over Britain, weather permitting. A team of scientists at Oxford University are tracking it using the Hubble Space Telescope.

The explosion is so bright because the star is very close to Earth, cosmically speaking, in the Big Dipper constellation, Ursa Major. Most supernovae are more than 1 billion light years away.

t will appear, blueish-white, just above and to the left of the last two stars in the Big Dipper. Watchers are advised to stay away from street lights for maximum visibility.

The supernova explosion was first detected on August 24, by University of California at Berkeley scientist, Peter Nugent, using the wide angle 1.2-meter Samuel Oschin Telescope in California. It has grown brighter by the minute. Located in the Pinwheel Galaxy, it was 20 times brighter in one day.

It has been among the earliest detections of a supernova in history, and has caused huge excitement among astronomers who are tracking it using all available equipment with a view of the galaxy.

After a few days it will fade away and be visible only with a telescope until mid-October. The best way to spot it is in the hours after nightfall, by looking east of the ‘handle’ of The Plough constellation.

Supernovae of this type, classified as a 'Type 1a' event, occur when a super-dense white dwarf star, about the size of Earth but with more mass than the sun, explode. The blast hurls matter in all directions at nearly one-tenth the speed of light.

The matter from the explosion will eventually form new stars and planets. Such events, which include one in five supernovae, provide scientists with essential information on how the universe expands.

Get out your star atlas and look up M101, the "pinwheel galaxy". Since full moon is tomorrow evening (12 Sep), a few hours after sunset starting around Wednesday may be best for viewing ... the SN may be a bit past peak brightness by then, but won't be competing with the full moon.

[ewmayer]

[I thought the topic of supernovae deserved its own thread, especially for the kind of more-involved physics discussion I hope to 'ignite' here.]

A few followups to the above post on the M101 supernova. This is SN of Type 1a. These are thought to occur when a white dwarf star slightly below the Chandrasekhar limit of ~1.4 solar masses accretes enough mass from its surroundings (typically from a close binary companion of more ordinary stellar type) to reach or approach the limit, at which point the core of the white dwarf begins to collapse due to electron degeneracy pressure no longer being able to counteract the star`s self-gravity, and the resulting rapid pressurization and heating leading to a runaway thermonuclear reaction - a kind of whole-star version of a hydrogen bomb, to use a crude analogy.

Because the physics of such events is thought to be reasonably well-understood and the Chandrasekhar limit is thought to be identical for all stars irrespective of their evolutionary history and interior chemical composition, type 1a supernovae have come to be used as "standard candles" for gauging distance, a kind of cosmological-distance analog of the Cepheid variable.

But if one reads the summary of the current consensus (such as it is) regarding type 1a supernovae, one should immediately question the "standard candle" claim:

The current view among astronomers who model Type Ia supernova explosions is that this limit is never actually attained, however, so that collapse is never initiated. Instead, the increase in pressure and density due to the increasing weight raises the temperature of the core,[3] and as the white dwarf approaches to within about 1% of the limit,[11] a period of convection ensues, lasting approximately 1,000 years.[12] At some point in this simmering phase, a deflagration flame front is born, powered by carbon fusion. The details of the ignition are still unknown, including the location and number of points where the flame begins.[13] Oxygen fusion is initiated shortly thereafter, but this fuel is not consumed as completely as carbon.[14]

Once fusion has begun, the temperature of the white dwarf starts to rise. A main sequence star supported by thermal pressure would expand and cool in order to counter-balance an increase in thermal energy. However, degeneracy pressure is independent of temperature; the white dwarf is unable to regulate the burning process in the manner of normal stars, and is vulnerable to a runaway fusion reaction. The flame accelerates dramatically, in part due to the Rayleigh–Taylor instability and interactions with turbulence. It is still a matter of considerable debate whether this flame transforms into a supersonic detonation from a subsonic deflagration.[12][15]

Regardless of the exact details of nuclear burning, it is generally accepted that a substantial fraction of the carbon and oxygen in the white dwarf is burned into heavier elements within a period of only a few seconds,[14] raising the internal temperature to billions of degrees. This energy release from thermonuclear burning (1–2×1044 J[4]) is more than enough to unbind the star; that is, the individual particles making up the white dwarf gain enough kinetic energy that they are all able to fly apart from each other. The star explodes violently and releases a shock wave in which matter is typically ejected at speeds on the order of 5000–20,000 km/s, or roughly up to 6% of the speed of light. The energy released in the explosion also causes an extreme increase in luminosity. The typical visual absolute magnitude of Type Ia supernovae is Mv = -19.3 (about 5 billion times brighter than the Sun), with little variation.[12] Whether or not the supernova remnant remains bound to its companion depends on the amount of mass ejected.

My Comment: If one is talking about simple balances involving gravitational self-attraction and electron-degeneracy repulsion, then the idea of a "chemistry-independent result" becomes at least plausible. But any time you read stuff like "deflagration flame front", it tells you chemistry is likely to matter, possibly quite a lot.

This is important because a few years ago there was a headline-making study of very distant type 1a SN which - if one believed them to be valid standard candles - would imply that the expansion of the universe is in fact accelerating at large redshifts, which would imply that Einstein`s famous "cosmological constant" is nonzero. To its credit, the wikipage on standard candles acknowledges the problem:

A significant issue with standard candles is the recurring question of how standard they are. For example, all observations seem to indicate that type Ia supernovae that are of known distance have the same brightness (corrected by the shape of the light curve). The basis for this closeness in brightness is discussed below; however, the possibility that the distant type Ia supernovae have different properties than nearby type Ia supernovae exists. The use of Supernovae type Ia is crucial in determining the correct cosmological model. If indeed the properties of Supernovae type Ia are different at large distances, i.e. if the extrapolation of their calibration to arbitrary distances is not valid, ignoring this variation can dangerously bias the reconstruction of the cosmological parameters, in particular the reconstruction of the matter density parameter.[7]

That this is not merely a philosophical issue can be seen from the history of distance measurements using Cepheid variables. In the 1950s, Walter Baade discovered that the nearby Cepheid variables used to calibrate the standard candle were of a different type than the ones used to measure distances to nearby galaxies. The nearby Cepheid variables were population I stars with much higher metal content than the distant population II stars. As a result, the population II stars were actually much brighter than believed, and this had the effect of doubling the distances to the globular clusters, the nearby galaxies, and the diameter of the Milky Way.

The author of the above ("headline-making study" link) Astrophysics Spectator article expresses skepticism (which I share) about the rapidity with which the nonzero-cosmological constant interpretation was embraced by the astrophysics community:

A peculiarity of this scientific episode is the reaction of the scientific community; it rapidly settled on the non-zero cosmological constant as the explanation for the results. Perhaps this is a wish, because discovering new physics is more interesting than either uncovering systematic effects or refining conventional physics. My own experience in these matters, however, suggest that the more mundane possibilities are much more probable than new physics.

We've seen his type of thing before - the common thread seems to be the involvement of those lovers of theoretical exotica, particle physicists and cosmologists. Remember bubble fusion? When I first heard about this then-new discovery in 2002 I immediately thought (admittedly, being a fluid dynamics PhD may have biased my thinking here) "spherically symmetric shock wave collapse", the same kind of thing used to produce ultra-high pressures in implosion-type nuclear weapons. But nooooooo, that possibility was much too mundane for the particle physicists - we soon had luminaries like Steven Weinberg trotting out exotic hypotheses about virtual particles, zero-point energies and the Casimir effect. (Note that the existence of bubble fusion is still not established due to repeatability problems and further clouded by academic-misconduct issues, but the main point for the present discussion is that all current theoretical models involve "boring" old shock-wave physics, not zero-point-energy "as featured in the popular SciFi TV series Stargate:Atlantis!").

Of course on the flip side or the argument, if one believes type 1a interior chemistry to be non-negligible, one must explain how one could have explosive-yield-affecting chemistry which is not obviously apparent in the resulting SN spectroscopy and light curve. Perhaps this newest nearby supernova - which promises to be the most-studied one ever - will help answer some of these questions.

And lastly, using the above "about 5 billion times brighter than the Sun" number we can derive a quick peak apparent visual magnitude estimate of the M101 supernova:

1. Sun is ~1/50,000 ly from earth, visual magnitude -27;
2. Supernova is ~21,000,000 ly from earth;

So, ignoring light extinction via interstellar dust and such, the supernova should reach an apparent brightness of roughly

5000000000/(50000*21000000)^2 = 1/220,500,000,000,000 = 1/2.2e14 relative to the sun.

Using that the logarithmic astronomical stellar magnitude is constructed so each factor 100 in relative brightness corresponds to a difference of 5 in magnitude, the supernova should reach a peak of 5*log(2.2)/log(100) = +36 magnitude versus the sun, meaning an apparent magnitude +9, which should be barely visible in a pair of 35-50mm binoculars, and nicely visible in a telescope of 4" (100mm) diameter and up.

Every 2.5x attenuation of the supernova brightness will add one to the visual magnitude estimate, so a 2.5-fold dimming due to intervening dust would make the peak magnitude 10.

[fivemack]

I think that the non-zero cosmological constant also emerges from analyses of the cosmic microwave background, though it looks as if the statistics are a bit fiddly - they work better if you use a prior distribution on H_0, but there are a few independent ways of getting that (Cepheids and the Sunyaev-Zeldovich effect)

http://arxiv.org/pdf/astro-ph/0403509v1 suggests that the carbon-to-oxygen ratio in the progenitor (which is the obvious chemistry that might be different in the earlier universe) doesn't make much difference to the amount of nickel produced in the explosion, and it's the nickel which produces the light peak.

[ewmayer]

Originally Posted by fivemack

http://arxiv.org/pdf/astro-ph/0403509v1 suggests that the carbon-to-oxygen ratio in the progenitor (which is the obvious chemistry that might be different in the earlier universe) doesn't make much difference to the amount of nickel produced in the explosion, and it's the nickel which produces the light peak.

Thanks for the link - I am underwhelmed by the quality of the numerical simulations, but at the very least it makes for a good survey article of recent work in this area.

As to specifics: They do indeed find that total energy output of the SN is significantly variable with C/O ratio of the WD progenitor ... but as you note, their simulations indicate that the resulting peak brightness resulting mainly from decay of radioactive ⁵⁶Ni is relatively insensitive to the explosion-energy variation. That elicits a "hmmm....." ... but their numerical simulations are of sufficiently poor resolution that it raises a big red flag. 2 obvious points:

1. Any modern high-end numerical simulation of such complex 3-D physics involving a huge range of spatiotemporal scales (any time you have turbulence and flame and/or shock fronts you get this issue) cries out for use of locally adaptive meshing (dynamically adding more grid points/cells in regions of steepening local gradients in order to maintain decent numerical resolution.) This has become more-or-less standard over the past 2 decades in high-resolution computational fluid dynamics - the Michigan AeroE department where I dd my PhD had a very active research group in this area.

2. Even if your code does not support [1], you *must* provide data showing how sensitive your results are to the grid resolution! (I.e. your numerical results must be shown to be "mesh independent".) The authors say they ran on a 256^3 spatial grid ... fine, now do at least one run on a finer (or coarser) mesh and compare the results to see how much the major conclusions change. If the authors here did such a convergence test they fail to mention it. That is so basic when numerical simulation is concerned that it leaves me very dubious about inferring physics from the numerical results.

Still a good find as far as sparking discussion, though.