Quick Thoughts - SNIa and the Universal Distance Ladder
How far away are space objects?
It might not be initially clear why we should care about this, seeing as all astrophysical objects are so far away from us that their distances are incomprehensible to humans. However, knowing the distances to objects actually tells us a fair deal about their intrinsic properties, and some quite embarassing mishaps can occur when distances fail to be measured correctly.
Consider the case of Stephenson 2-18[1], a red supergiant commonly claimed to be the largest star in the universe. Initial estimates found the star to be some 2,100 times wider than the Sun, smashing upper limits of around ~1,500 Solar radii[2] predicted by mathematical models. This conundrum was eventually resolved when it was discovered that estimates for the star's distance were in fact grossly in error. It turns out that Stephenson 2-18 is some 20,000 light-years from Earth[3], a fifth of the original estimate of ~100,000 light-years. Upon this discovery, Stephenson 2-18 went from a distant, impossibly luminous record-breaker to a nearby, relatively ordinary (if still quite bright) red supergiant. Though undoubtedly a disappointment for fans of the erstwhile largest star ever, this development certainly proved fortunate for astrophysicists' understanding of stellar physics.
Stars are not the only objects for which reliable distance estimates are critical. The left galaxy, PGC 45917, was long thought to be a close companion of the one on the right, PGC 45918. As it turns out, the latter is some 12 times as far away as the former!
(Image: ESA/Hubble)
A simple diagram of stellar parallax.
(Image: By PdeQuant - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=91597298)
Given the importance of accurate distance measurements in astrophysics, it is unsurprising that we have a wide variety of measurement methods ranging from the planetary to the universal scale. Together, these are collectively known as the cosmic distance ladder.
For nearby objects like planets and some stars, we can determine their distances directly through parallax. As Earth moves through its orbit, the resulting change in our line of sight causes their positions in the sky to shift relative to the background. This is a simplistic method which can be very accurate with sufficiently precise measurements, but it is only good for objects within a few thousand light-years of Earth, as further objects' parallaxes become undetectably small.
For objects beyond the limit where parallax is usable, we have to rely on indirect methods for determining their distances. All of these methods rely on finding 'standard candles'. Standard candles are objects whose intrinsic luminosities can be determined easily, since if we know an object's intrinsic luminosity we can then compare that to its apparent brightness to extrapolate distance. The most famous standard candles are the Cepheid variable stars[4], which powerful observatories like Hubble can see out to some 50-60 million light-years from Earth[5]. Unfortunately, most galaxies in the universe are much more than 60 million light-years away and so cannot be searched for Cepheids, forcing us to rely on other methods.
This is where the subject of this post comes in. A Type Ia supernova is an incredibly bright nuclear explosion that occurs inside a collapsing
A symbiotic binary.
(Image: NASA/Hubble)
For a long time, it was believed that most Type Ia supernovae occurred in symbiotic binaries[8], where a white dwarf slowly 'eats' a close-orbiting stellar companion. In a supernova-producing symbiotic binary, the white dwarf slowly grows until reaching the Chandrasekhar limit, collapsing, and exploding. Critically, the collapse will always happen immediately upon reaching the limit, so the explosion should always have precisely 1.4 solar masses of nuclear fusion fuel to 'burn'. Therefore, this model predicts that all Type Ia supernovae are fundamentally the same, precisely the characteristic of a standard candle!
Because all Type Ia supernovae are supposed to be exactly the same[9] and they are visible from across the universe, we have used them extensively[10] to investigate the vast majority of galaxies too dim and distant for other means of distance determination. It is safe to say that a great part, if not all, of our understanding of the distant universe is thanks on them.
Now imagine the suprise when we found out that Type Ia supernovae are actually not all identical after all!
The central star of this nebula, Henize 2-428, is actually a pair of white dwarfs whose collective mass exceeds the Chandrasekhar limit. It is becoming increasingly clear that these types of systems are a significant source of Type Ia supernovae.
(Image: ESO/VLT)
It turns out that symbiotic binaries are responsible for no more than 20% of Type Ia supernovae[11]. Instead, the majority of Type Ias arise from a different kind of binary star - one where two close-orbiting white dwarfs with a combined mass of over 1.4 solar masses merge, collapse, and explode. Since white dwarfs can have any mass under 1.4 times the Sun's, a merger-driven Type Ia supernova could theoretically have as much as 2.8 solar masses' worth of fuel - twice as much as the symbiotic Type Ia! If we can no longer assume that all Type Ia supernovae have the same amount of fuel, we also cannot assume they all have the same luminosity - logically, more fuel should result in greater brightness[12]. If we cannot safely assume that all Type Ia supernovae have the same brightness, then they are useless as a standard candle. Indeed, the 'Champagne Supernova' SN 2003fg[13] was obviously overluminous, suggesting abnormal Type Ia supernovae are not uncommon.
Consider that practically everything we know about the distant universe is rooted in our studies of Type Ia supernovae. If they truly are not standard candles like we once believed, what becomes of the grand theories that they spawned, like dark energy? Who’s to say that we aren’t making a Stephenson 2-18-sized error?
Footnotes
[1] aka. Stephenson 2 DFK-1, RSGC2-01 or St2-18
[2] As per Levesque et al. (2005)
[3] As per Davies et al. (2007)
[4] Cepheids are a type of aging star a few times the mass of the Sun that swell and shrink cyclically as a result of particular instabilities in their cores. The speed of these pulsations is mathematically related to their intrinsic luminosities, allowing us to determine their innate brightness.
[5] As per Freedman et al. (1994)
[6] Only common carbon-oxygen white dwarfs can explode as Type Ia supernovae, since these elements have fusion ignition temperatures that can be achieved in a super-Chandrasekhar white dwarf's collapse. The much rarer oxygen-neon-magnesium white dwarfs produced by intermediate-mass stars instead transform into
[7] Electrons obey a rule known as the Pauli Exclusion Principle, which prevents multiple electrons from occupying the same energy level. The matter of a white dwarf is tightly compressed by gravity, so most of its densly packed electrons must occupy very high (energetic) energy levels since the lower levels are quickly filled to capacity. This energy counteracts gravity, supporting the star with a force known as electron degeneracy pressure.
[8] As per Lieb and Yau (1987)
[9] There is technically a distinct subclass of Type Ia supernovae, the Type Iax, where the white dwarf is not completely consumed and the explosion is not as bright. They are quite rare and we're not entirely sure why they are different from normal SNIa.
[10] Such as Guy et al. (2010) and Möller et al. (2022)
[11] As per Gonzáles-Hernández et al. (2012)
[12] To be completely fair, an overmassive progenitor actually decreases the brightness of a supernova, since the explosion uses more energy climbing out of the progenitor's gravitational well. However, this effect is not enough to force overmassive SNIa to be the standard brightness.
[13] Described by Branch (2006)
