Quick Thoughts - SNIa and the Universal Distance Ladder
How far away are the stars?
It might be initially unclear why we should care about this, seeing as they are all at incomprehensibly large distances from us. What does it matter if Alpha Centauri is 30 trillion or 40 trillion kilometres away from us?
Though it may be unintuitive, knowing the distances to space objects actually tells us a fair deal about them. Some quite embarassing mishaps can occur when distances fail to be measured correctly.
Consider the case of Stephenson 2-18[1], a red supergiant once claimed to be the largest star in the universe. Initial estimates found it to be some 2,100 times wider than the Sun, smashing previous theoretical limits of around ~1,500 Solar radii[2]. This conundrum was eventually resolved not by a new theory of stellar dynamics but by the fact that measurements of the star's distance were 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. Once this error was rectified, the theory-breaking record-breaker vanished. Though a disappointment for fans of the star, this development certainly proved fortunate for 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 methods ranging from the planetary to the universal scale. Together, these are collectively known as the cosmic distance ladder.
For nearby objects, we can determine their distances directly through parallax. Basically, the optical position of planets and nearby stars shifts with respect to the background as our viewpoint changes through Earth’s orbit. This is a simple method which can be very accurate out to a few thousand light-years of Earth, where the optical shifts become undetectable.
For objects beyond a few thousand light-years, we have to rely on indirect methods, which rely on 'standard candles'. These are objects whose intrinsic luminosities can be determined somehow. We can then compare luminosity with visual brightness to determine the distance to the standard candle and other objects (we assume are) close to it. The most famous standard candles are Cepheid variable stars[4], which powerful observatories like Hubble can find in galaxies as far as 50-60 million light-years away[5]. Unfortunately, most galaxies in the universe are much more than 60 million light-years away and so cannot be searched for Cepheids.
For distances ranging from 60 million light-years to the edge of the observable universe, Type Ia supernovae are our standard candle of choice. These are incredibly bright thermonuclear explosions which happen on collapsing
A symbiotic binary.
(Image: NASA/Hubble)
It was long believed that most Type Ia supernovae occurred in symbiotic binaries[8], where a white dwarf slowly 'eats' a close companion star. As it steals mass from its companion, the white dwarf grows until reaching the Chandrasekhar limit and immediately collapsing. Since the explosion happens immediately when the white dwarf reaches the limit, it should always have 1.44 solar masses of fuel available. Barring small variations in fuel efficiency, therefore, all Type Ia supernovae should produce the same amount of energy and thus they will all have the same intrinsic luminosity, making them a good standard candle.
As Type Ia supernovae are the brightest and most detectable standard candle we have[9], we have used them extensively[10] to investigate the vast population of distant galaxies. It is safe to say that a great part, if not all, of our understanding of the distant universe rests on their use as standard candles.
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 create no more than 20% of all Type Ia supernovae[11]. The rest of them arise from a different kind of binary star - binary white dwarfs. A pair of low-mass stars can both die and become white dwarfs while remaining in orbit then fall together and merge as their orbit decays through the emission of gravitational radiation. Since white dwarfs can have any mass under 1.44 times the Sun's, a merger-driven Type Ia supernova could theoretically have as much as 2.9 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 results in greater brightness[12]. If we cannot safely assume that all Type Ia supernovae have the same brightness, then they cannot be a standard candle. Indeed, the 'Champagne Supernova' SN 2003fg[13] was obviously abnormally bright, suggesting that overluminous 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 we cannot rely on them as a standard candle like we once believed, what becomes of the astrophysical theories that rest on our understanding of the distant universe? 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 what is known as electron degeneracy pressure.
[8] As per Lieb and Yau (1987)
[9] There is 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. These are quite rare and distinguishable from other SNIa so they are not problematic for standard candle measurements.
[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 does also decrease the brightness of a supernova, since the explosion uses more energy climbing out of the progenitor's gravitational well. However, this does not compensate for the increase in brightness provided by the extra fuel.
[13] Described by Branch (2006)
