Blaze Star

screenshot of a page in the English Mechanic and Mirror Science, Sept 1869 - Here, the name "Blaze Star, 1866" is chosen as a headline

The variable star T CrB is included in the Yale Bright Star Catalog (HR 5859), here depicted in a star chart in Hoffmann (2017).

The name "Blaze Star" as a nickname for T CrB in eruption is being used since the 19th century. The symbiotic system consists of a red giant and a white dwarf, together normally 10.247 mag bright (V). Occasionally, the system permits nova eruptions (surface eruptions on the white dwarf) which flare it up to mag 3 or 2. Although its usual brightness is below the detection limit of the human eye, the star is included in the Yale Bright Star Catalog (because of its peak magnitude).

Concordance, Etymology, History

On May 12, 1866, at least three observers were the first to observe a star at this location in the sky (PDF).

1. The director of the Athens Observatory, Mr. Schmidt, saw the star with a magnitude of 5 during a sky survey between 8:30 p.m. and 9:45 p.m. (local time).
2. On the same night, John Birmingham in Tuam, Ireland, also reported the star,
3. and W. J. Lynn from the Greenwich Observatory in London saw the object a little later (between 11:30 p.m. and 11:45 p.m.) on his way (not from an observatory) even with the naked eye, because it was already as bright as the main star of the constellation (magnitude 2.2).

The observers agree that in this region of the sky near the galactic pole, where there are few bright stars, it “disturbed”/ “distorted” the shape of the constellation (as they say). The nova was therefore definitely conspicuous and could be identified as a star of magnitude 9.5 in the Bonn survey. This made it clear that this “new star” was an eruption of a known one.

Back then—in 1866—people didn't even know how stars work or where the sun gets its energy from, let alone how stars evolve or why some of them change brightness. They saw it, recorded their observations—and didn't (yet) understand it.

It was not until 1946 that the next bright flare was seen, and in the intervening period, apparently (almost) no one looked at this star – at least, there are no observation data in the data archive (but the light curve also begins in 1866 only after the flare has subsided (at 7 mag).

Note: We have no less than two data points (nova light curves) from the star whose eruption we are now expecting. One of these dates back to before the invention of electrical photometry (Potsdam, 1913), i.e., it is based solely on the estimates of experienced observers: a well-functioning method, but not using the same measurement method we use today. Contrary to everything taught in basic physics courses (or any laboratory course), we have no choice but to compare measurements that school physics teaches us are not comparable (common practice in astronomy).

Expections for next eruption

Three of the other T CrB-class star systems erupt somewhat more frequently: every 20 to 30 years, so that what the press tells us about the impending eruption may not be entirely reliable. We suspect that during the outburst, the star will have approximately the brightness (3 to 2 mag) of the North Star or the main star of a constellation (alf CrB, Alphecca), because this was the case in both documented observations. Many (50%) recurrent novae always erupt with the same brightness – but the peak brightness can also vary (the other 50%). Therefore, it cannot be completely ruled out that the star T CrB will not be quite as bright as last time (e.g., only 4 mag or even only 6 mag) or that it will be even brighter (up to Arcturus brightness) during its upcoming outburst: We will only know for sure when we see it.

Physics of Recurrent Novae

A nova eruption is a surface eruption on a star in a cataclysmic binary star system. These are star systems in which the two stars orbit each other so closely that matter flows from the primary star (donor) to the companion star. The donor can be a red giant star, but it can also be a yellow main sequence star or even a white dwarf. In most cases, the receiving and occasionally erupting “star” is a white dwarf, i.e., the remnant of a sun-like star (i.e., no longer a star itself because it no longer gains energy through nuclear fusion in its interior).

In most cases, we know of at most one eruption as a classic nova from these cataclysmic (or symbiotic) systems. However, there are 30 stars (as of May 5, 2024) that are known or suspected to be “recurrent” novae; twelve of them are outside the Milky Way (galaxy), eight are uncertain.

So we only have ten objects of this type that we can study in more detail in order to make further statistical statements about their behavior – and predictions!

In astronomy, the obvious characteristics of a system are used for classification. In the case of our recurrent novae, these would be the orbital period (period duration) Porb, the amplitude A during eruption, the repetition time τ_r, and the decay time t₃ from the maximum of 3 mag. The period depends on the size of the main star (donor): with a small donor, the orbiting dwarf star must be closer so that matter can flow over, and therefore it has a shorter period due to Kepler's laws.

Characterization of the ten known recurrent novae: In T Pyx stars, the erupting star orbits a dwarf star (blue), in U Sco types it orbits a main sequence star (yellow), and in T CrB types it orbits a red giant (SMH 2024), published in Hoffmann and Vogt (2022)

Characterization of nova behavior: The decay time for novae of the T CrB type is relatively short, i.e., the “new star” is only visible to the naked eye for a few days. The amplitude appears to be at the lower end of the possible range (compared to other systems), but with an outburst of 6 to 9 mag, a star that normally has 10 mag is certainly within the range of naked-eye visibility. The figure on the right shows how much the amplitude of the maximum varies among the individual star systems: it does not depend on the star type and can deviate significantly (by up to 4 mag) from the median mean (i.e., the most common case).

The problem is that the above facts are based on a relatively small sample size. While most of the information about stars, their stages of development, etc., which is derived from Hertzsprung-Russell diagrams, is based on millions/billions of star data, we only have about ten recurrent novae in the Milky Way. The few others recently discovered in the LMC and the Andromeda galaxy may have different properties and are therefore not included in the statistics. So, compared to other astronomical data, these statistics are on rather shaky ground.

Specifically on T CrB

A light curve of T CrB based on the data archive of the AAVSO (2024).

In 2024/ 2025, the press campaigns claimed that the star will erupt soon because the last eruption of T CrB was about 80 years ago, i.e., in 1946. If we think carefully about how many survey programs and systematic sky monitoring observation programs there were at the end of World War II, we can't think of many. At that time, there were no satellite telescopes, and the culture of observational astronomy consisted mainly of individual stargazers who, while observing their favourite objects, might have found an erupting star “by chance.”

It was no different before World War II. So how do we know the recurrence period? It is clearly visible that there is a huge data gap between 1870 and 1930.

Mythology

no mythology

IAU Working Group on Star Names

The name was discussed by the IAU WGSN in 2025 due to the ongoing media hype on its due eruption. No decision yet.

Reference

• References (general)
• Ian Ridpath's website (Star Tales )