The Mysterious Progenitor of Tycho’s “New Star”

The Mysterious Progenitor of Tycho’s “New Star”
By Judith E Braffman-Miller

The supernova of 1572 is often called Tycho’s Supernova because of the Danish astronomer, Tycho Brahe’s (1536-1601), extensive work on this brilliant stellar blast, De nova et nullius aevi memoria prius visa stella (“Concerning the Star, new and never before seen in the life or memory of anyone”), published in 1573. Reprints of Tycho’s work were overseen by the German astronomer and mathematician, Johannes Kepler (1571-1630), in 1602 and 1610. Tycho’s Supernova exploded in the constellation Cassiopeia, and is particularly noteworthy for being one of only eight supernovae visible to the unaided human eye appearing in historical records. Supernovae are extremely bright explosions that herald the “death” of a star, and they can be so brilliant that for a short time they can even outshine their entire host galaxy. In September 2017, an international team of astronomers announced that their new study of Tycho’s Supernova has shed new light on the true nature of this blast from the past. The research challenges the widely accepted view that this brilliant explosion originated from a type of stellar corpse called a white dwarf, which had been slowly, and catastrophically, sipping up material from its unfortunate companion star–and victim–in a binary system.

Tycho’s Supernova appeared in early November 1572, and it was independently discovered by many individual sky-watchers of that era. Its appearance in our Milky Way Galaxy represents one of the most important observations in the history of astronomy. Indeed, the appearance of Tycho’s “new star” inspired astronomers to revise the ancient models of the heavens. This triggered a revolution in astronomy that started with a realization that improved astrometric star catalogues were needed–and, as a result, there was a need for more precise astronomical observing instruments. This stellar blast also challenged the Aristotelian dogma of the immutability of the stars in the sky.

Type Ia supernovae (SNe 1a) are frequently used as standard candles in modern observational cosmology. In addition, SNe 1a are also thought to play a significant part in galactic chemical evolution. However, the source of these brilliant, gigantic stellar blasts has not been precisely determined, and it remains a topic of considerable debate. Nevertheless, there is almost universal consensus among astronomers that SNe 1a are triggered by the thermonuclear disruption of a white dwarf stellar remnant. The white dwarf that is about to go supernova is composed of carbon and oxygen. It is also reaching the Chandrasekhar mass limit (1.4 times solar-mass)–which means that it has accreted sufficient mass to blow itself up. Any white dwarf with less than 1.4 solar-masses will remain a white dwarf–while a star that exceeds this mass is doomed to end its stellar life in the fiery rage of a violent SNe 1a.

White dwarfs are generally thought to be the lingering core of a small star, similar in mass to our Sun, that has burned its necessary supply of nuclear-fusing fuel in its core, and has gone gentle into that good night by puffing off its outer gaseous layers into the space between stars–leaving its dense core behind in the form of a white dwarf. The newborn white dwarf itself is surrounded by a shimmering, glimmering shell composed of multicolored gases, which are the former progenitor star’s outer gaseous layers. These objects are so beautiful that they are often referred to as the “butterflies of the Cosmos”. Small stars of our Sun’s mass perish in relative peace compared to their more massive stellar kin. Massive stars usually “die” in the fiery fury of a core-collapse Type II supernova explosion that blasts the erstwhile progenitor star to smithereens.

Less massive stars, like our Sun, usually do not go supernova–when they are solitary stars, that is. The fun begins when a white dwarf exists in a binary system with a companion star. When this is the case, some very brilliant fireworks can occur. This is because the vampire-like white dwarf can sip up enough of its companion star’s mass to “go critical”, and it blasts itself to pieces in a supernova conflagration–just like the big guys. This scenario suggests that a sinister, voracious white dwarf is the culprit behind a SNe 1a supernova. However, a second scenario proposes that a SNe 1a can result from an unfortunate binary, composed of a doomed duo of white dwarfs, that are trapped in a fatal gravitational dance. The two waltzing white dwarfs eventually spiral into each other–causing a brilliant SNe 1a. However, these two scenarios differ dramatically in the level of electromagnetic emission predicted to come from the progenitor during the passage of millions of years prior to the catastrophic, final fatal fireworks of a supernova.

Supernova Remnant

Tycho was not the first to detect the brilliant 1572 supernova. However, he was probably the most accurate observer of this strange “new” star. Some almost-as-accurate observations were made by Tycho’s European colleagues, such as Wolfgang Schuler, Thomas Digges, John Dee, Francesco Maurolico, Jeronimo Munoz, Tadeas Hajek, and Bartholomaus Reisacher.

Queen Elizabeth I in England, summoned the mathematician and astrologer Thomas Allen, “to have his advice about the new Star that appeared in the Cassiopeia to which he gave his Judgement very learnedly,” as the historian John Aubrey reported a century later.

Far away, in Ming dynasty China, the strange, brilliant, and mysterious object became an issue between Zhang Juzheng and the youthful Wanli Emperor. In keeping with the cosmological tradition of that time, the emperor was warned to halt his own misbehavior because the new star was considered to be an evil omen.

Less magical, and more reliable, reports note that the new star itself appeared sometime between November 2 and 6, in 1572, when it became bright enough to rival Venus in Earth’s sky. The supernova remained visible to the naked eye all they way into 1574, before it started to dim gradually, finally vanishing from view.

The distance to the remnant of Tycho’s Supernova had earlier been calculated to be between 6,500 and 16,300 light-years. However, more recent estimates suggest a distance of between 8,000 and 9,800 light-years.

The hunt for a remnant of Tycho’s Supernova came up empty-handed until 1952. That year, Hanbury Brown and Cyril Hazard reported a radio detection at the Jodrell Bank Observatory. This detection was later confirmed, and its position more precisely measured, by Baldwin and Edge using the Cambridge Radio Telescope in 1957.

The supernova remnant was discovered in the 1960s by astronomers using a Palomar Mountain telescope in California. This first optical detection of Tycho’s Supernova spotted it as a very dim nebula, and the remnant was later imaged by a telescope on the international ROSAT spacecraft. The supernova has currently been confirmed as a Type Ia–in which the culprit behind the stellar explosion is a white dwarf that has accreted enough matter to “go critical”, blasting itself to pieces. This type of explosion does not normally produce the spectacular nebula that is more characteristic of core-collapse Type II supernovae, that herald the “death” of a massive star that has reached the end of that long stellar road, after having burned its necessary supply of nuclear-fusing fuel. For example, a Type II event produced the supernova of 1054 which created the Crab Nebula. A shell of gas is still in the process of spreading from its center at approximately 9,000 kilometers per second. A more recent study suggests a rate of expansion below 5,000 kilometers per second, however.

In October 2004, a paper appearing in the journal Nature reported on the discovery of a star, similar in type to our own Sun. The recently discovered star is dubbed Tycho G, and it is thought to be the unfortunate companion star that contributed its mass to the voracious and vampire-like white dwarf that triggered Tycho’s Supernova. A later study, published in 2005, provided more detail about this star: Tycho G was likely a main-sequence (hydrogen-burning) star or subgiant before the horrific blast, but some its mass was torn away and its outer layers were shock-heated by the supernova explosion. The current velocity of Tycho G is, probably, the most important evidence that it was once the tragic companion star to the white dwarf. Tycho G is zippng along at a rate of 136 kilometers per second, which is more than four times faster than the mean velocity of other stars in its stellar backyard.

However, this discovery has been challenged. Tycho G is far away from the center of the remnant, and it does not reveal the rotation expected from a companion star.

The Mysterious Progenitor Of Tycho’s “New Star”

A sinister white dwarf, in the process of sipping up the material from its companion star, becomes a source of large amounts of X-ray and extreme UV photons. The currently most widely accepted accretion scenario suggests a luminous, searing-hot progenitor that would ionize all of the ambient gas within a radius of approximately 300 light-years (Stromgren sphere). After the white dwarf has paid for its crime, in the fiery raging explosion of the supernova that destroys it, the source of ionizing emission vanishes. But it takes a very long time for the interstellar gas to recombine and become neutral again. An ionized nebula will hang around the supernova for about 100,000 years after the blast. Therefore, the detection of even small quantities of neutral gas in the neighborhood of a supernova can help astronomers place tight constraints on the luminosity and temperature of the progenitor.

Astronomers now know, based on optical observations, that Tycho’s Supernova remnant currently is expanding into primarily neutral gas. As a result, the remnant itself can be used as a way to probe its environment. This means that astronomers can then go on to exclude hot luminous progenitors that would have manufactured a Stromgren sphere larger than the radius of the currently existing remnant. This rules out steadily nuclear-burning white dwarfs (supersoft X-ray sources), as well as disk emission emanating from a Chandrasekhar-mass white dwarf that is in the process of devouring more than one solar-mass in about 100 million years (recurrent novae).

The lack of a surrounding Stromgren sphere is consistent with the merger of a duo of binary white dwarfs, rather than the greedy and destructive eating habits of a sinister white dwarf sipping up the matter of its companion star–and going out with a bang, as a result. However, other more exotic scenarios may still prove to be viable explanations.

It was more than four centuries ago that Tycho Brahe observed his “new star” in the night sky above our Earth. The “new star” was brighter than the planet Venus when it first made its appearance, but it dimmed over the course of the following year. Astronomers now know that Tycho had observed the thermonuclear blast that destroyed a white dwarf–the result of a Type Ia supernova conflagration. Because of its relative proximity to our own planet, as well as its noteworthy history, Tycho’s Supernova is certainly one of the most well-documented examples of a Type Ia blast.


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