Quick Thoughts - Type Ia Supernovae and the Cosmic Distance Ladder
Introduction
Space is vast. But just how vast?
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The Vastness of the Universe
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 once-recordholder in all its glory.
From 2MASS survey (CC BY-SA 3.0)
The Rise and Fall of a Stellar Celebrity
Stephenson 2-18 strutted onto the scene back in the ancient year of 2010. Scientists clocked in the red supergiant at a whopping 2,100 times wider than the Sun1, which is a problem because the rulebook of stellar physics says that stars aren’t supposed to get bigger than about 1,500 solar radii2. We were scratching our collective heads, wondering if we’d have to rewrite the laws of the universe.
Nope. Turns out, it was just another case of “someone got sloppy and didn’t check their work.” Early estimates pegged Stephenson 2-18 at 98,000 light-years away1. But after some hurried recalculations and some new-and-improved data, we realized it was actually only 20,000 light-years away3. That’s still quite a trek, but it meant the star wasn’t the oversized diva it seemed - it was just closer. 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 a couple bad 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?
The Cosmic Distance Ladder
A simple diagram of stellar parallax.
PdeQuant (CC BY-SA 4.0)
Parallax: Real, Reliable Distances
Imagine you're looking at a nearby tree with a distant mountain in the background; if you move your head, the tree seems to shift more than the mountain does. This is parallax, and astronomers can use it to determine precise and unbiased distances.
To measure a distance with parallax, we observe a nearby star or planet from opposite sides of Earth’s orbit around the Sun, six months apart, to see how it shifts against the background. The amount of this shift, called the parallax angle, is measured carefully. With some basic geometry, this tiny angle helps calculate how far the star is from Earth. It’s like using your eyes to gauge distance, but with telescopes and the Earth’s orbit doing the heavy lifting!
Parallax distance determination is direct, so the accuracy of your result only depends on the accuracy of your instruments. But for the overwhelming majority of things in the universe, we can’t use it. We are instead forced to use indirect methods to determine their distance, the collection of which we call the cosmic distance ladder.
Cosmic Yardsticks: Measuring the Universe with Stellar Fireworks
When parallax becomes undetectable beyond a few thousand light-years, you can’t just whip out a cosmic tape measure. Instead, astronomers use “standard candles,” which are space objects that shine with a predictable brightness. By comparing how bright they look from Earth to how bright they actually are, we can guesstimate their distance (and the distance of anything nearby). The rock stars of standard candles are , which blink like cosmic lighthouses and can be spotted by big-shot telescopes like Hubble in galaxies up to 50-60 million light-years away4. But here’s the bummer: most galaxies are way farther than that, so Cepheids just don’t cut it for the deep, deep cosmos.
For those super-distant galaxies, we turn to the ultimate showstoppers: Type Ia supernovae. These are that go out with a bang, exploding in a thermonuclear tantrum so bright they can outshine entire galaxies. White dwarfs are held up by some fancy quantum physics that keeps them from collapsing until they hit a cosmic weight limit of 1.44 times the Sun’s mass, called the Chandrasekhar limit. If they pork up past that, it’s game over: they collapse, heat up to a gazillion degrees, and trigger a runaway nuclear explosion that lights up the universe like a galactic firework show, visible from billions of light-years away!
A symbiotic binary.
NASA, ESA, Leah Hustak (STScI)
Type Ia Supernovae: Not Quite Twins
For ages, we thought most Type Ia supernovae were born out of cosmic sibling rivalries: a white dwarf star greedily siphoning material from a still-living companion in a “symbiotic binary” star system5. Once the white dwarf gobbles up enough to hit the Chandrasekhar limit, it’s showtime! The star collapses in a spectacular explosion, always with the same amount of fuel, so it should, in theory, always blast out the same amount of light. That predictable brightness made Type Ia supernovae the go-to cosmic yardstick6, perfect for measuring distances to far-flung galaxies7.
These stellar explosions are the brightest, flashiest standard candles in our toolbox, so we’ve leaned on them hard to study the universe’s most distant corners. Pretty much everything we know about those far-off galaxies? Yep, we owe it to these galactic pyrotechnics. So, imagine our shock when we learned that Type Ia supernovae aren’t all identical twins after all!
The Problem of Type Ia Supernovae
Henize 2-428, a pair of young white dwarfs lighting up a small nebula. In a few billion years they will collide to produce a not-so-standard Type Ia supernova.
ESO/VLT (CC BY 4.0)
The Cosmic Fireworks That Keep Throwing Curveballs
Turns out, our old pal the symbiotic binary only accounts for about 20% of Type Ia supernovae8. The rest? They come from a cosmic rom-com gone wrong: two long-passed white dwarfs locked in a decaying orbital dance, spiraling closer as they bleed until—smash!—they merge. Since each white dwarf can weigh anything below 1.44 solar masses, their merger could pack nearly double that, with up to 2.9 solar masses of fuel. More fuel, more boom, which means these explosions can shine brighter than the “standard” Type Ia we thought we knew, like a firecracker sneaking into a fireworks show.
Type Ia supernovae have been our trusty cosmic rulers, lighting up distant galaxies so we can measure the universe. But if they’re not all shining with the same brightness, they’re not the reliable standard candles we thought they were. Take SN 2003fg, nicknamed the “Champagne Supernova,”9 which was so dazzlingly bright it basically screamed, “I’m not like the others!” Turns out, these overluminous oddballs might not be that rare. Since our entire understanding of the far-off universe hinges on these supernovae, this plot twist is like finding out the map you’ve been using has some seriously wonky scales.
Are we staring down another Stephenson 2-18-style oopsie, where our cosmic calculations go up in smoke? Stay tuned, because the universe loves keeping us on our toes!
References
1 Stephenson, C. B. (1990). A Possible New and Very Remote Galactic Cluster. The Astronomical Journal, 99(6), 1867-1868. https://doi.org/10.1086/115464
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 Davies, B., Figer, D. F., Kudritzky, R.-P., 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
4 🔒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
5 Lieb, E. H., & Yau, H.-T. (1987). A rigorous examination of the Chandrasekhar theory of stellar collapse. The Astrophysical Journal, 323, 140–144. https://doi.org/10.1086/165813
6 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
7 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
8 🔒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
9 🔒Branch, D. (2006). Astronomy: Champagne supernova. Nature, 443(7109), 283–284. https://doi.org/10.1038/443283a
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