Why do massive stars survive while white dwarfs explode?

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Edward Herrick-Gleason

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We know that when a white dwarf accreting matter from a larger companion star exceeds 1.4 solar masses it explodes as a type Ia supernova. However it occurred to me that there are of course millions of stars with greater masses than that. Why do they all not do the same, or what is it about white dwarfs in particular that causes them to do this when other more massive stars do not?

There are indeed innumerable stars far more massive than 1.4 solar masses. In fact, many of the stars that one can see with the unaided eye are more massive than that. One of the most massive known stars, R136a1, located in the Tarantula Nebula, could be up to 230 times more massive than the Sun, and it has not collapsed – at least not yet. How can such a highly massive star remain intact while a white dwarf will collapse if it becomes a mere 1.4 times as massive as the Sun?

The answer lies in the counterbalance. The spherical shapes of active stars such as the Sun are sustained by the delicate balance of two antagonistic forces: gravitational contraction pulling inward and energy pressure pushing outward. This ‘hydrostatic equilibrium’ prevents the star from collapsing under gravity or dispersing under the influence of internal pressure, which originates in the stellar core and is transported outward through the surrounding layers, where thermonuclear fusion reactions create heavier elements from lighterones and, in the process, convert some material directly into energy.

A white dwarf is a stellar remnant in which all fusion reactions have ceased. This planet-sized object does not collapse immediately because its structure is supported by electron degeneracy pressure. This is a quantum-mechanical effect related to Pauli’s exclusion principle. Essentially, since no two electrons can occupy the same energy state, the presence of so many electrons in such close confines exerts a pressure that resists further compression.

However, this pressure cannot increase indefinitely. If a white dwarf in a binary system accretes enough material from a companion star so that its mass exceeds approximately 1.4 solar masses, it becomes unstable, triggering a runaway thermonuclear explosion as a Type Ia supernova.

This value, known as the Chandrasekhar limit after Subrahmanyan Chandrasekhar (1910–1995), the Indian-American astrophysicist who calculated it, marks the upper mass limit for white dwarfs. Fortunately, the upper limit for stars supported by fusion is much greater.

So, with all the myriad worries that relentlessly assail us each day, we can at least be assured that our Sun will remain ceaselessly active and beautifully spherical for quite some time to come.

Edward Herrick-Gleason served as Director of the Southworth Planetarium in Portland, Maine, USA for a quarter of a century. He now lives in St. Johns, Newfoundland.