White Dwarf Collision-Merger Hypothesis Can Still Be Proven- But Will It?

Artist's depiction of two white dwarf stars in the process of merging.

The issue of merging stars and their effects, including on astrophysical processes, has already been discussed at the end of a previous post for the Laboratory Astrophysics prize talk at the 236th AAS meeting, e.g.

AAS 236: Laboratory Astrophysics Prize Lecture...

This was in the context of one way to account for the r -process, as depicted in one slide:



In this case the collision theorized (bottom of slide to right) is either between two neutron stars (NS) or between a neutron star and black hole.  But what about the possible merger of two white dwarfs?   Recall here that the white dwarf in terms of the Sun's evolution, represents the final stage after it has expended all its hydrogen.  It helps here to review the physical changes in the later stage evolution.

 When only ten percent hydrogen remains (X= 0.10, Y = 0.87 for fractional abundances, say), the Sun is no longer able to generate sufficient energy from its core nuclear reactions to balance the weight of overlying layers. According to a well-known physical principle (the virial theorem), the Sun’s core must contract. The contraction converts gravitational potential energy into thermal (heat) energy that heats the core.[1] By now, hydrogen burning has moved to a peripheral shell around the core, and is ignited by the core heating process. 

The ignition creates radiation pressure that forces the outer shells, layers to expand. This same radiation, however, is now emitted from a much larger surface area. The result of this combination of circumstances is that the Sun becomes a Red Giant.   However,  this stage also cannot be sustained and as the helium nuclear fuel must eventually be exhausted the pressure-gravity balance for the Red Giant, e.g. dP/dr = - G M(r) r / r²


(where P is the outward gas pressure and   r  is the density), can no longer continue.  In this case the force of gravitational contraction wins out with the end phase being a "white dwarf".  This object (the nearest to us is the companion to Sirius, or Sirius B) is mostly constituted of   electron-degenerate matter.   (Isaac Asimov in his book, The Collapsing Universe', one indicated a teaspoonful of its mass would be 15 tons.)

The latest work on merging white dwarfs is from Dr Mark Holland, lead author of a study on the white dwarf, and a physicist from the University of Warwick.   He  postulates that a collision of two such dying stars (see graphic) about 13 billion years ago has produced a star heavier than the Sun.  This evidence, if confirmed, could help astrophysicists better understand the kind of cosmic events that make life on Earth possible. 

The massive star from the merger is currently one of a handful of merged white dwarfs to be identified so far and the first to be discovered from the composition of its gases.  Thus, using the tools and techniques of stellar spectroscopy, the team discovered the star in question (identified as WDJO551-4135) was made up of hydrogen and carbon as opposed to hydrogen and helium.   This is crucial since it discloses the star has already progressed through hydrogen and helium burning and now has passed carbon burning. (Since the nuclear fuel accessed occurs in stages: hydrogen, then helium, then carbon)

But is this not proof  enough? No, what is now needed is to learn more about this merged product's core composition.  Only then can the merger hypothesis be proven.  A key clue may be that while the average white dwarf mass is 0.6 times the mass of the Sun, this merged product is nearly twice the average mass for white dwarfs.  A key aspect of the current finding is that this merged star has a mass of 1.14 the mass of the Sun, or less the the Chandrasekhar limit .  The limit is computed to be 1.44 solar masses for a stellar collapsed core, below which the core cannot give rise to an exploding star or supernova.  

 Another theory related to that of the Univ. of Warwick team is that the star (WDJO551-4135)  may have formed from a merger because of its age and temperature.   In this sense, the star was found to be traveling faster than 99 % of nearby white dwarfs that appear to have the same cooling age. (The faster a star travels, according to Dr. Hollande, the older and colder it is because it has spent more time extracting energy from its neighbors and burning its available fuel.  This suggests the star is older than it appears.)   

What we will eagerly anticipate now is forthcoming results about the core composition, particularly as the merged star may be older than it appears. 

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[1] According to the virial theorem: 2K + W = 0 for any spherical system in equilibrium, where K is the gas kinetic energy (K = 3/2(y-1)U) and W is the gravitational potential energy. From this one can obtain the binding (or total) energy of a star as: E(S)= K + W. Combining the two equations, E(S) = W/ 2 = -K. Thus, the total energy of the star is negative and equal to half the gravitational potential energy, or the negative of the gas kinetic. Hence, if E(S) decreases, K increases, but W decreases, i.e. contraction.

See also:

• White dwarf stars often exploded at lower masses in the distant past

Selected Questions - Answers From All Experts Astronomy Forum (Origin of Planetary Nebula)

Question:  I am interested in knowing about the origin of the planetary nebula, such as the Ring nebula in the constellation Lyra. Basically, how did the term originate and also what is the physical natures of these nebulas?  - Amateur astronomer, Dallas TX


Answer:  The Ring Nebula you referenced is shown in the accompany image from Mt. Palomar Observatory.  But, of course even in a good telescope you will not see it like that but rather more like the image shown below, taken with my Schmidt -Cassegrain Celestron scope in August,  1980..


The reason for the radical difference  is that no color enhancement has been applied to the second image.  Apart from some extra magnification it "is what it is".   This is pretty well how it appears in the telescope eyepiece, except much dimmer and fuzzier!

As to the origin of the name "planetary" this can be traced to the early observers being misled by the condensed, roundish appearance by surmising the objects were planetary like hence planetary nebula.   But when observed in a small telescope this is easily dispelled. After all,  what planet- like object has a hole in its center?  It was only after the invention of the spectroscope that the real nature of the object was established.

It was found in particular that the atoms of the ring  -like gas cloud (such as for the object shown) emitted a great deal of radiant energy.  This was compatible with temperatures in the thousands of degrees Celsius.  Again, this would confirm the objects were in no way "planet like".  The detection of the direction of expansion of the gaseous material also substantiated this.

What exactly is a planetary nebula and how does it come about? We are still not 100 % certain of the physical details but there is mounting evidence these objects are associated with the later stages of a star's life, especially  for moderate to relatively low mass stars like the Sun. Basically, it is found that the central star typically belongs to the white dwarf class. A star which has already had its heyday as far as nuclear fuel consumption in its core is concerned.

The best way to think of a white dwarf is as the partially collapsed core of such a star, e.g. like the Sun. Most plausibly, the white dwarf stage is triggered after a star like Sun becomes a Red Giant.  This would coincide with the time  when all the nuclear fuel (H, He)  is exhausted.  In the explosive instability process itself, two things happen:   1) The ejected gaseous surface- the ring if you will -   becomes the expanding shell of gas around the remnant core, and 2) the remnant itself collapses since with the outer, radiative layers blown away the star can no longer support itself against gravity.  

Note here that the time between when the star ceases to fuse Helium and the ejection of its outer gas layers -i.e.  to become a planetary nebula- is inconceivably long,  on the order of 8 billion years. 

The outer expanding gas shell is what we are actually calling the planetary nebula. The central  remnant core meanwhile can attain temperatures up to 100,000 K.   As the last of the fuel is exhausted the hot core  becomes a white dwarf.  In his excellent book, 'The Collapsing Universe', Isaac Asimov describes in detail the collapse process peculiar to the white dwarf phase and might be worth a read.  From his parameters he estimates that just one teaspoonful of such an object could tilt the scales at 15 tons.

Ultimately - as Asimov and many others have pointed out- the white dwarf's further contraction (from which it extracts its energy) is prevented by degenerate gas pressure. The white dwarf can now only radiate what limited stored energy it has left.   How long would it take to radiate all the remaining stored energy away and then become a black dwarf?  We can't say for sure, but some,   like Asimov, believe it could be up to several trillion years, maybe more.

Parsing the origin of the white dwarf definitely gives one a cosmic perspective.

A Planet In The Throes Of Destruction: A Glimpse Into the Future of Earth

For the first time in astronomical history earthlings are getting a glimpse of what it will be like when their own home planet is destroyed by its star, the Sun, some five billion years in the future. Thanks to the planet- hunting Kepler  Space Telescope we now can put together the process of a Sun-like star's end point - accompanied by the destruction of a planet orbiting it at a distance of 570 light years - in the constellation Virgo.

The star, having reached the end of its evolution and life is now in the process of annihilating its own solar system, with one of the Earth-like planets being vaporized by the star's searing heat and ripped apart by its gravity (as depicted in the artist's conception shown above).  According to the lead author of the study, Andrew Vanderburg, of the Harvard-Smithsonian Center for Astrophysics:

"Every second it's losing up to 10 million kilograms or 22 million pounds of material."

Vanderburg and his colleagues - according to an HSCA release -   also made additional observations using a number of ground-based facilities, including : the 1.2-meter and MINERVA telescopes at Whipple Observatory, the MMT, MEarth-South, and Keck telescopes. Combining all the data, they found signs of several additional chunks of material, all in orbits between 4.5 and 5 hours - generating a comet-like debris field. The main transit was particularly prominent, dimming the star by 40 percent. This disclosed the remnant of the Earth like planet.

The dimming transit signal combined with the comet-like pattern suggests the presence of an extended cloud of dust surrounding  the fragment. The total amount of material is estimated to be about the mass of Ceres, the largest main-belt asteroid in our solar system. It is conceivable that this is the reduced mass that's been left after the white dwarf  rent most of the original mass asunder.  (See below.)

According to Carole Mundell, head of astrophysics at the University of Bath, England:

"It's a glimpse into the future of Earth. It reinforces the idea that we are in a much more hostile environment than we sometimes imagine."

The stable lifetime of the Sun depends on how long before it consumes ninety percent of the hydrogen in its core. Theoretical investigations using data from nuclear reaction rates and cross sections suggest the Sun’s Main Sequence lifetime at 8-10 billion years. Since it already has spent 4.5 billion of those years, there are anywhere from 3.5 to 5.5 billion years remaining. Most astrophysicists put the balance at from 4.8 to 5 billion years.

When only ten percent hydrogen remains, the Sun is no longer able to generate sufficient energy from its core nuclear reactions to balance the weight of overlying layers. According to a well-known physical principle (the virial theorem), the Sun’s core must contract. The contraction converts gravitational potential energy into thermal (heat) energy that heats the core.[1] By now, hydrogen burning has moved to a peripheral shell around the core, and is ignited by the core heating process. The ignition creates radiation pressure that forces the outer shells, layers to expand. This same radiation, however, is now emitted from a much larger surface area. The result of this combination of circumstances is that the Sun becomes a Red Giant. This translates into the Sun increasing its diameter to up to 200 times its present size.

Afterwards, it collapses to a much smaller, but vastly denser object known as a white dwarf. Even if the Earth were to survive the red giant phase (humans, of course, will be long since dead - baked to a crisp possibly a billion years earlier) the powerful gravimetric forces of the white dwarf will claim it and other planets.  This is possible because the gravitational attraction of a typical white dwarf is 350,000 times that of Earth's surface gravity. Thus, if we take Earth's g -value at 10 N/kg (or m/s/s) the typical white dwarf's g-value would be 3,500, 000 N/kg. No ordinary mass would be able to withstand such a gravitational pull.

Thus, the extreme gravity of the dwarf will likely cause the nearer planets to fall toward the white dwarf and disintegrate under enormous gravitational tidal forces. This seems to be the scenario now unfolding in the constellation Virgo and which the artist's conception portrays.

We should count ourselves lucky that we are able to merely observe the death throes of a planet like ours from a great distance. But we should know one day the same cataclysm will befall our own home, Earth.

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[1] According to the virial theorem: 2K + W = 0 for any spherical system in equilibrium, where K is the gas kinetic energy (K = 3/2(y-1)U) and W is the gravitational potential energy. From this one can obtain the binding (or total) energy of a star as: E(S)= K + W. Combining the two equations, E(S) = W/ 2 = -K. Thus, the total energy of the star is negative and equal to half the gravitational potential energy, or the negative of the gas kinetic. Hence, if E(S) decreases, K increases, but W decreases, i.e. contraction.

See also:

http://www.space.com/23756-white-dwarf-stars.html