New observations of exploding stars reveal pauses, flickers and flares not reliably seen before

Dec 03, 2010

Astronomers have traced the waxing and waning light of exploding stars more closely than ever before and seen patterns that aren't yet accounted for in our current understanding of how these eruptions occur.

Using data from a sensitive instrument aboard a satellite that images the entire sky every 102 minutes, they studied four of these stars, or novae, that exploded so violently their light would have been visible without a and measured their brightness over the course of the outburst.

Three of the novae stalled before reaching a peak, and all flickered or flared as the explosions ran their course, they report in The .

The instrument they used – the Solar Mass Ejection Imager – was developed by a team led by astrophysicist Bernard Jackson at the Center for Astrophysics and Space Sciences at the University of California, San Diego, to study the sun. Rebekah Hounsell, a graduate student at Liverpool John Moores University in Britain made the measurements while visiting UC San Diego.

Because starlight is a distraction for Jackson's team, noise they must subtract from their data so that they can focus on the sun's outer corona and the heliosphere, they make detailed maps of stellar light, including its brightness.

In those maps Hounsell identified the four novae by finding points of light that rapidly brightened and dimmed over the course of days.

Wavering Light

Other astronomers had observed a pause in the brightening of novae, or "pre-maximum halt" before, but some thought it an anomaly. The precise time-scale and repeated observations of the current study confirms it, they authors say.

"The reality of this halt as found in all three of the fast-declining novae observed is a challenge to detailed models of the nova outburst," said one of the authors, astrophysicist Mike Bode, of Liverpool John Moores University.

Two independent teams of theorists have already begun to refine their models of how novae explode in response.

Astronomers typically characterize novae's changing light with curves smoothly fit to more sporadic observations, but the rapid cadence of the solar imager captured glimmers that hadn't been observed before. All flickered as their light dimmed and one nova, the slowest of the four to dim, flared brightly twice after reaching its peak luminosity.

These novae are white dwarf stars that steal matter, in the form of hydrogen, from a companion star, often an aging, expanding red giant. As hydrogen accumulates the white dwarf's gravity pulls it in and condenses it until it ignites, setting off a runaway nuclear fusion reaction.

The team speculates that the post-peak flares may correspond to changes in the dynamics of that reaction that still need to be explained.

Catching Missing Stars

"Before Hounsell looked through these data, most novae were observed only after their peak luminance. The instrument's very even cadences and uniformly exposed images allow us to trace the entire evolution of these explosions as they brighten and dim," UC San Diego's Jackson said.

Data from the imager, which has been in operation aboard the Coriolis since January 2003, allows astronomers to measure novae that they initially missed.

"Even today novae are mainly discovered by amateur astronomers around the world who then alert their professional counterparts to conduct observations," Hounsell said.

As many as five novae bright enough to be detected by SMEI explode in our galaxy each year, Allen Shafter, astronomy professor at San Diego State University and one of the co-authors of the report have previously estimated, but more than half have gone undetected.

"The instrument assures that the brightest and most rapidly evolving novae – ones that brighten and then fade within a few days – are not overlooked," Shafter said. "The high time resolution of these observations has opened up a new window into the study of novae in our galaxy."

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User comments : 10

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Tuxford
1.9 / 5 (9) Dec 03, 2010
So novae are not necessarily runaway fusion explosions. Oh what a surprise... Explode... no... wait.... now explode again. These models do not include a genic energy heat imbalance (photon-blue shifting in stellar cores) as a contributing source, as they must not upset current nuclear physics models. Astrophysics is constrained by nuclear physics (where is all that dark matter anyway?) and nuclear physics is conveniently ignoring astrophysics observations when they don't fit the standard model. Each science being hamstrung by the other.

What if the First Law of Thermodynamics is only a very accurate approximation, whose violation is so small that in the Earth's gravitation environment it is simply undetectable in the laboratory? And what if such violation is more extreme in other gravitational environments, such as the core of star. Read LaViolette's 'SubQuantum Kinectics," and think different.
Nik_2213
3.7 / 5 (3) Dec 03, 2010
"novae are not necessarily runaway fusion explosions"
Huh ??
Sorry, it didn't imply that, only that the process is more complicated than current models.

Which you would expect, given the vagaries of novae accretion disks etc...
Skepticus
not rated yet Dec 03, 2010
All of this just proves that stellar explosions are not what you've come to see and take heart from Star Trek and the like.
yyz
3.8 / 5 (5) Dec 03, 2010
These observations are looking at flickering phenomenon that can occur during the sharp, fast rise of a nova from quiescence to maximum light and shortly thereafter. Oftentimes this variability is missed due to the extremely fast rise times of these events. Flickering is commonly observed in dwarf novae, cataclysmic variables, and symbiotic stars in outburst both in the rise to maximum light as well as the gradual (or rapid) fading post-maximum. Flickering in novae was first observed visually by astronomers in the late 19th century, often on timescales of minutes to hours.
jamesrm
1 / 5 (6) Dec 04, 2010
Whats the bet they will invoke
"D-A-R-K M-A-T-T-E-R" to fix their models, or maybe they will take my idea of "Serendipitous matter"© more seriously :)

invoke "to call on (a deity, Muse, etc.), as in prayer or supplication"

rgds
jms
axemaster
not rated yet Dec 04, 2010
It seems likely to me that these "halts" (which just means that the brightness gets stuck at a certain level for a while) would be due to the formation of very strong magnetic fields. Energy gets absorbed into the fields, preventing the brightness from increasing, and then the field lines "snap", causing sudden flickerings.
BadMan
not rated yet Dec 04, 2010
Is it possible that this will question thier use as standard candles?
eachus
5 / 5 (2) Dec 04, 2010
All of you take a deep breath, and repeat after me, "This side of the house is white."

It takes a lot of effort to see what you didn't see, and in this case account for the fact that we only see one side of the star. Well, actually white dwarf stars spin pretty fast, so while we see only one side of the star at a time, we can see the whole surface over a period of a few hours. Oh, and don't forget the donor star that is providing the hydrogen.

The net result is that the light intensity early in the nova will depend on where on the surface of the star the ignition point is relative to the observer. If the ignition point is visible, there will come a point where the explosion wavefronts are all on the other side of the white dwarf.

Finally as the nova proceeds, most of the light will be coming from the gases blown off the white dwarf. All the hydrogen remaining on the star will have been converted to Helium, but the gasses blown off will still be burning.
omatumr
1 / 5 (2) Dec 04, 2010
To understand large scale events like this in the cosmos, we must first understand basic interactions of the smallest particles in the nucleus.

That is the central message of this new video:
http://www.youtub...yLYSiPO0
eachus
3 / 5 (2) Dec 04, 2010
All of you take a deep breath, and repeat after me, "This side of the house is white."

It takes a lot of effort to see what you don't see. In this case the fact that we only see one side of the star. Well, actually white dwarf stars spin pretty fast, so while we see only one side of the star at a time, we can see the whole surface over a period of a few hours. Oh, and don't forget the donor star that is providing the hydrogen. It rotates and moves too.

The net result is that the light intensity early in the nova will depend on where on the surface of the star the ignition point is relative to the observer. If the ignition point is visible, there will come a point where the explosion wavefronts are all on the other side of the white dwarf.

Finally as the nova proceeds, most of the light will be coming from the gases blown off the white dwarf. All the hydrogen remaining on the star will have been converted to Helium, but the gasses blown off will still be burning.

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