Cosmic Vision: Witnessing Fireworks

Professor Katherine Blundell OBE

Accounts of occasional celestial spectacular events in past centuries have provided crucial information for modern-day astrophysicists. One such example is the so-called Great Eruption of Eta Carinae which was for a time in the mid-19th century the third brightest object in the night sky.

Interpretation of the spectacular structure of this so-called supernova imposter would be hard without information from eye-witnesses of days gone by.


The Homunculus Nebula, surrounding Eta Carinae, imaged by WFPC2 at red and near-ultraviolet wavelengths. Credit: Jon Morse (University of Colorado) & NASA Hubble Space Telescope

Eta Carinae (η Carinae, abbreviated to η Car), formerly known as Eta Argus, is a stellar system containing at least two stars with a combined luminosity greater than five million times that of the Sun, located around 7,500 light-years (2,300 parsecs) distant in the constellation Carina. Previously a 4th-magnitude star, it brightened in 1837 to become brighter than Rigel, marking the start of its so-called “Great Eruption”. It became the second-brightest star in the sky between 11 and 14 March 1843 before fading well below naked eye visibility after 1856. In a smaller eruption, it reached 6th magnitude in 1892 before fading again. It has brightened consistently since about 1940, becoming brighter than magnitude 4.5 by 2014.

At declination −59° 41′ 04.26″, Eta Carinae is circumpolar from locations on Earth south of latitude 30°S, (re Johannesburg: lat. 26°12′S); and is not visible north of about latitude 30°N, (re Cairo: lat. 30°2′N).

The two main stars of the Eta Carinae system have an eccentric orbit with a period of 5.54 years. The primary is a peculiar star, similar to a luminous blue variable (LBV), that was initially 150–250 M☉ of which it has lost at least 30 M☉ already, and is expected to explode as a supernova in the astronomically near future. This is the only star known to produce ultraviolet laser emission. The secondary star is hot and also highly luminous, probably of spectral class O, around 30–80 times as massive as the Sun. The system is heavily obscured by the Homunculus Nebula, material ejected from the primary during the Great Eruption. It is a member of the Trumpler 16 open cluster within the much larger Carina Nebula.

Although unrelated to the star and nebula, the weak Eta Carinids meteor shower has a radiant very close to Eta Carinae.

In 1827 William Burchell specifically noted Eta Carinae’s unusual brightness at 1st magnitude, and was the first to suspect that it varied in brightness. John Herschel, who was in South Africa at the time, made a detailed series of accurate measurements in the 1830s showing that Eta Carinae consistently shone around magnitude 1.4 until November 1837. On the evening of December 16, 1837, Herschel was astonished to see that it had brightened to slightly outshine Rigel. This event marked the beginning of a roughly 18-year period known as the Great Eruption.

Eta Carinae was brighter still on January 2, 1838, equivalent to Alpha Centauri, before fading slightly over the following three months. Herschel did not observe the star after this, but received correspondence from the Reverend W.S. Mackay in Calcutta, who wrote in 1843, “To my great surprise I observed this March last (1843), that the star Eta Argus had become a star of the first magnitude fully as bright as Canopus, and in colour and size very like Arcturus.” Observations at the Cape of Good Hope indicated it peaked in brightness, surpassing Canopus, over March 11 to 14, 1843 before beginning to fade, then brightened to between the brightness of Alpha Centauri and Canopus between March 24 and 28 before fading once again. For much of 1844 the brightness was midway between Alpha Centauri and Beta Centauri, around magnitude +0.2, before brightening again at the end of the year. At its brightest in 1843 it likely reached an apparent magnitude of −0.8, then −1.0 in 1845. The peaks in 1827, 1838 and 1843 are likely to have occurred at the periastron passage—the point the two stars are closest together—of the binary orbit. From 1845 to 1856, the brightness decreased by around 0.1 magnitudes per year, but with possible rapid and large fluctuations.

From 1857 the brightness decreased rapidly until it faded below naked eye visibility by 1886. This has been calculated to be due to the condensation of dust in the ejected material surrounding the star rather than an intrinsic change in luminosity Below left

William John Burchell (23 July 1781 – 23 March 1863) was an English explorer, naturalist, traveller, artist, and author. His thousands of plant specimens, as well as field journals from his South African expedition, are held by Kew Gardens, and his insect collection by the Oxford University Museum.

image Above right

Sir John Frederick William Herschel, 1st Baronet KH FRS (7 March 1792 – 11 May 1871) was an English polymath, mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint, and did botanical work.


SN 1994D (bright spot on the lower left), a type Ia supernova within its host galaxy, NGC 4526

A supernova is a powerful and luminous stellar explosion. This transient astronomical event occurs during the last evolutionary stages of a massive star or when a white dwarf is triggered into runaway nuclear fusion. The original object, called the progenitor, either collapses to a neutron star or black hole, or is completely destroyed. The peak optical luminosity of a supernova can be comparable to that of an entire galaxy before fading over several weeks or months.

Cosmic Vision (

Professor Katherine Blundell OBE


Katherine was appointed Gresham Professor of Astronomy in 2019. She is a Professor of Astrophysics at Oxford University and a Research Fellow at St John’s College. Before this she was one of the Royal Society’s University Research Fellows, a Research Fellow of the 1851 Royal Commission, and a Junior Research Fellow at Balliol College, Oxford.

Her research interests include the evolution of active galaxies and their life cycles, the accretion of material near black holes and the launch and propagation of relativistic jets (jets of plasma emitted by some black holes). In her research she uses electromagnetic imaging and spectroscopy, as well as computational techniques.

She is also a renowned science communicator and set up a worldwide network of five schools-based Global Jet Watch observatories, which collect data on evolving black hole systems and nova explosions in our Galaxy, helping to inspire the next generation of scientists in South Africa, Chile, Australia and India.

Her awards include a Philip Leverhulme Prize in Astrophysics, the Royal Society’s Rosalind Franklin Medal in 2010, the Institute of Physics Bragg Medal in 2012, the Royal Astronomical Society’s Darwin Lectureship in 2015 and an OBE for services to astronomy and the education of young people in 2017.

Blundell’s first lecture series for Gresham College is called Cosmic Concepts, starting 2 October 2019, and she will be looking at how concepts developed in physics and cosmology have led to some of our most surprising discoveries about the Universe.

Professor Blundell’s lecture series are as follows:

2020/21 Cosmic Vision

2019/20 Cosmic Concepts

All lectures by the Gresham Professors of Astronomy can be accessed here.

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