“Oh thick wits. Oh blind watchers of the sky.” This is the tone with which Tycho Brahe, the greatest naked-eye astronomer to ever live, began his landmark work, De Nova Stella, On the New Star*. It was this work that coined the modern term “nova” and later earned the naming of “Tycho’s Supernova”, which I have chosen as the 9th object in my top 10 favorites countdown. Around 9,000 years after the supernova explosion occurred, its light first arrived at Earth in November of 1572. It appeared beside the constellation Cassiopeia, The Queen, and at its peak brightness was visible even during the day.
On seeing the nova for the first time, Tycho wrote: “I was so astonished by this sight that I was not ashamed to doubt the trustworthiness of my own eyes. But when I observed that others, on having the place pointed out to them, could see that there was really a star there, I had no further doubts.” He was so impressed by the sight that he would devote the rest of his life to astronomy, and by excruciatingly careful observations, would be the first to meaningfully undermine the notion of the immutable heavens. For at the time, it was almost universally accepted that the stars and planets were eternally unchanging. To suggest otherwise would not just challenge prevailing opinions, but also the theology of the day.
And so the thinkers of the late 1500’s deemed the new star a “tailless comet”, a mere atmospheric effect. Comets in those days were thought to be combustions in the upper atmosphere, as were any unexpected dynamics in the night sky. Since the heavens were assumed to be changeless, the only possible explanation left was in the atmosphere. But Tycho realized that if this were true, then he should observe a parallax, a slight shift between dusk and dawn in the position of the nova with respect to the background stars, which depends on the distance between object and observer. Such a shift is observed for the Moon, so when Tycho found no parallax, he rightly concluded that the new star must be at least more distant than the Moon.
He also found that the object moved in perfect synchronization with the stars, and eventually decided that this was no atmospheric effect. Rather, it truly was a new star and a major upheaval in our understanding of the cosmos. As you might expect, this idea was not met with open minds, and thus we return to the insult in the preface of his book. A tame response from Tycho, though, who was a man willing to literally die defending the integrity of his ideas. He once lost much of his nose to a sword duel fought over the legitimacy of a mathematical formula. Admirable or foolhardy, you decide.
But history is not the only reason that Tycho’s Supernova, or SN 1572, is a wonderfully interesting astronomical object. It is, after all, a supernova. Or rather it was—all that’s left for us to observe now is the remnant†, the scattered guts of a star that has blown itself apart. This is what’s pictured at the top of the page and here on the left, an x-ray image taken by Chandra and a radio map made using the Very Large Array. A supernova is an explosion that occurs when a star can no longer counterbalance the force of its own gravity. It first implodes, generating a “runaway” nuclear fusion event that creates heavy elements not typically fused in stellar cores. The energy released can outshine entire galaxies, producing more in an instant than the Sun will in its entire lifetime. This is more than enough energy to tear the star apart, liberating all but a dense inner core back into space.
Astronomers have broken supernovae into five categories, and Tycho’s also happens to be of perhaps the most significant variety, Type 1a. These are one of astronomy’s “standard candles”, objects with known luminosities (brightnesses) that can then be used to measure distance given how much fainter they appear from Earth. Type 1as reach over 200 times further than the next best commonly used standard candle, Cepheid variables, and the most distant to be observed is at a staggering 11.8 billion light years. This means that the light observed from Earth left just 2 billion years after the birth of the universe. Wow! And if that wasn’t significant enough for you, the study of Type 1a distances has led to one of the most profound discoveries ever made: not only is the universe expanding, but it’s expanding at an accelerating rate that gravity cannot possibly overtake. This suggests that our universe will end in a Big Freeze, or Heat Death, in which all matter and energy has become so dispersed that it ceases to interact at all. Imagine spreading a single drop of jelly over a piece of toast that grows larger every day.
So how do we know how bright Type 1a supernovae are to begin with? With a major caveat, we know how they form. These explosions happen when a white dwarf, the collapsed remnant of a star similar in size to the Sun, comes too close to the Chandrasekhar limit. Stars are normally held up against gravity by the outward pressure of the nuclear fusion that occurs in their cores, but eventually fuel runs out and the star collapses. Stars like the sun become white dwarfs, which are held up against gravity by the electromagnetic force—positive charges being repelled by negative ones. If enough mass is piled on, even this fails. This is the limit, about 1.38 solar masses, that Indian-American Subrahmanyan Chandrasekhar (namesake of the Chandra space telescope) first worked out. When a white dwarf comes to just a hair below this mass, the runaway fusion described above is initiated and the star explodes. Knowing the mass at the time of the supernova means we can work out the amount of light that should be released and can then decide how far away the object is based on how much less light we receive at Earth.
Here’s the catch: we’re not exactly sure how the white dwarf reaches the Chandrasekhar limit. There are two ways it can happen. The first is the “accretion model”. In this scenario the white dwarf is part of a binary system (very common), in which its companion is so close that material from it is siphoned off onto the white dwarf until BOOM! The mass limit is exceeded and the star goes supernova. The second scenario is the explosive merger of two individual white dwarfs. Both formations very likely occur, but with what frequency? A supernova caused by accretion can occur much more quickly than a merger, so astronomers had long thought that the vast majority of Type 1a events were likely formed in this way.
But recent results from Chandra have called this notion into question. Because of heating within the accretion disk, that scenario should produce much more x-ray emission than a merger. When astronomers surveyed a number of supernovae to look for this, they found 30 – 50 times less x-rays than expected by accretion, suggesting that perhaps mergers occur more frequently than was once thought. If this is true, then our “standard” candle is a bit less standard than we’d like. With accretion, we know exactly how much mass is there when the supernova is triggered. This can’t be precisely pinned down with a merger, adding a new layer of uncertainty to distance measurements made using Type 1a supernovae. Whether this difference is enough to challenge the accelerating expansion of the universe idea, I’m not sure. However, I do know that there is a very fine line between an accelerating and a decelerating universe, and the data is already pretty close to it. I’ve read that the energy releases between the two scenarios shouldn’t be different enough to change our current interpretation, but looking at the data, it seems to me that it wouldn’t take much to fundamentally alter the type of universe we think we’re living in (or to make it prohibitively uncertain using this technique). Either way, this will definitely be one of the most interesting narratives going forward in all of astronomy.
*Click here for a wonderful scanning of an English translation of De Nova Stella published in 1632, 59 years after the original.
†In 2008, astronomers using the Subaru optical telescope in Hawaii were able to observe a “light echo” from Tycho’s Supernova. They detected light from the original supernova explosion that had originally been travelling away from us, but was reflected back in our direction by an interstellar dust cloud. So not only do we get to see the spectacular remnant, but we also get to sample some of the very same light that reached Tycho Brahe’s eyes in 1572! Besides being awesome, this means that a spectrum could be taken to confirm that this was indeed a Type 1a event.