Quick Thoughts - SNIa and the Universal Distance Ladder
Space is big. Really big.
Space is really, really far away. For example, Alpha Centauri, our closest stellar neighbor is 4.37 light-years away, which translates to about 41 trillion kilometers. To put that in perspective, that’s like driving from New York to Los Angeles… a billion times!
But just how far away is that, exactly?
Quantifying the Unquantifiable?
You might be thinking, “How can it possibly matter if a star is 30 trillion or 40 trillion kilometers away? We can't exactly visit them anyway, so who cares whether they're this incomprehensible distance or that other incomprehensible distance?" Fair point. But here’s the kicker: getting those distances right can save us from some pretty hilarious cosmic-sized blunders. Enter , the that briefly held the title of “Biggest Star in the Universe”—and then had to hand it back in shame.
The Rise and Fall of a Stellar Celebrity
The once-recordholder in all its glory.
From 2MASS survey (CC BY-SA 3.0)
Picture this: Stephenson 2-18 struts onto the scene, clocking in at a whopping 2,100 times wider than the Sun1. That’s a problem, because according to the rulebook of stellar physics, stars aren’t supposed to get bigger than about 1,500 solar radii2. Scientists were scratching their heads, wondering if they’d have to rewrite the laws of the universe.
Nope. Turns out, it was just another case of “someone didn’t check their work.” Early estimates pegged Stephenson 2-18 at 98,000 light-years away1. But after some awkward recalculations, we realized it was actually only 20,000 light-years away3. That’s still a trek, but it meant the star wasn’t the oversized diva it seemed—it was just closer, pulling off the ultimate optical illusion. Like when your cat looks huge in a photo because it’s sitting two inches from the camera.
Once the distance was corrected, Stephenson 2-18’s record-breaking size evaporated faster than a puddle in the desert. Fans of the star were crushed, but stellar physicists everywhere breathed a sigh of relief. No need to toss out the textbooks—just the original measurements.
The Takeaway: Measure Twice, Claim Once
So we've seen how inaccurate distances can lead to physics-defying results. But the case of Stephenson 2-18 raises an important question: What other supergiant-sized distance errors could be messing up our cosmic wayfinding?
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
[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)
References
1 Stephenson, C. B. (1990). A Possible New and Very Remote Galactic Cluster. The Astronomical Journal, 99(6), 1867-1868.
2 Levesque, E. M., Massey, P., Olsen, K. A. G., Plez, B., Josselin, E., Maeder, A., & Meynet, G. (2005). The effective temperature scale of Galactic red supergiants: Cool, but not as cool as we thought. The Astrophysical Journal, 628(2), 973–985. https://doi.org/10.1086/430901
3 Stephenson, C. B., MacKenty, J., Najarro, F., & Herrero, A. (2007). A massive cluster of red supergiants at the base of the Scutum‑Crux Arm. The Astrophysical Journal, 671(1), 781–801. https://doi.org/10.1086/522224
Freedman, W. L., Madore, B. F., Mould, J. R., Hill, R., Ferrarese, L., Kennicutt, R. C., Saha, A., Stetson, P. B., Graham, J. A., Ford, H., Hoessel, J. G., Huchra, J., Hughes, S. M., & Illingworth, G. D. (1994). Distance to the Virgo cluster galaxy M100 from Hubble Space Telescope observations of Cepheids. Nature, 371, 757–762. https://doi.org/10.1038/371757a0
Lieb, E. H., & Yau, H.-T. (1987). A rigorous examination of the Chandrasekhar theory of stellar collapse. The Astrophysical Journal, 323, 140–144.
Guy, J., Sullivan, M., Conley, A., Regnault, N., Astier, P., Balland, C., Basa, S., Carlberg, R. G., Fouchez, D., Hardin, D., Hook, I. M., Howell, D. A., Pain, R., Palanque-Delabrouille, N., Perrett, K. M., Pritchet, C. J., Rich, J., Ruhlmann-Kleider, V., Balam, D., ... Walker, E. S. (2010). The Supernova Legacy Survey 3-year sample: Type Ia supernovae photometric distances and cosmological constraints. Astronomy & Astrophysics, 523, A7. https://doi.org/10.1051/0004-6361/200913657
Möller, A., Smith, M., Sako, M., Sullivan, M., Vincenzi, M., Wiseman, P., Armstrong, P., Asorey, J., Brout, D., Carollo, D., Davis, T. M., Frohmaier, C., Galbany, L., Glazebrook, K., Kelsey, L., Kessler, R., Lewis, G. F., Lidman, C., Malik, U., ... Varga, T. N. (2022). The dark energy survey 5-yr photometrically identified type Ia supernovae. Monthly Notices of the Royal Astronomical Society, 514, 5159–5177. https://doi.org/10.1093/mnras/stac1691
González Hernández, J. I., Ruiz‑Lapuente, P., Tabernero, H. M., Montes, D., Canal, R., Méndez, J., & Bedin, L. R. (2012). No surviving evolved companions of the progenitor of SN 1006. Nature, 489(7417), 533–536. https://doi.org/10.1038/nature11447
Branch, D. (2006). Astronomy: Champagne supernova. Nature, 443(7109), 283–284. https://doi.org/10.1038/443283a
