We are so fortunate that the little speck of galactic dust we all call home – our Earth – orbits around a somewhat boring little star – our Sun – that does pretty much nothing. Well, nearly nothing but that is another story.
Our galaxy – the Milky Way – and the universe in general is full of unstable stars that vary wildly in their energy output, creating a local environment totally unsuitable for sustaining life as we know it. One such star that has recently being crying out for attention is AG Carinae (AG Car).
The Carina Milky Way is, however, not well placed during November and is seen skirting along the southern horizon during the early evening hours and is not much better placed by the midnight hour. It was towards the end of a recent observing run that I pointed the binoculars towards the Carina Nebula where a number of bright and interesting stars are found.
AG Car was top of the list of stars to check as it had been on the rise in the months prior to it becoming lost in the evening twilight at the end of the 2019/20 observing season.
AG Car is currently listed as a Luminous Blue Variable, a LBV star but this name is a somewhat misleading term and star of this type are better known as the SDOR stars, the S Doradus type stars. The SDOR stars are, as the alternative LBV name suggests, amongst the most luminous stars in their parent galaxies and can be close to 1 million times more luminous than our sun.
They are a diverse group of giant stars, amongst the largest stars in our galaxy, with spectral classification from extremely hot O through to F or even the cooler G, depending upon the current evolutionary state of the individual star. The spectrum of AG Car is presently listed as Ofpe/WN9 – A2Ieq indicating a surface temperature in the order of 26,000K when at maximum. It has a mass some 50 times greater than that of our sun and has been shown to be a highly unstable star with a three magnitude range in apparent brightness. Small amplitude variations are observed on time scales measured in days plus there are large amplitude “outbursts” measured in years.
Deep images such as that recently recorded with the NASA/Hubble Space Telescope show a complex shell of material that has been ejected from AG Car, probably during a series of long ago outbursts. This envelope of dusty material is then pushed outward from the central star by intense stellar winds characteristic of highly luminous stars wherein the rapidly expanding inner material eventually catches up with material from earlier eruptions and which has been slowed as it collides with dark interstellar. This bunching of dusty material produces the ring we see in the HST image, estimated to be some 1 parsec in diameter and contain up to 15 solar masses, surrounding AG Car.
By shedding so much mass during these eruptions, it is possible that AG Car will lose enough of its mass to interstellar space to avoid a core collapse and subsequent supernova explosion. But maybe not….!
Distance estimates to AG Car vary greatly depending upon a number of factors such as the amount of obscuring interstellar material measured (or assumed) to lie between us and AG Car, the actual mass and diameter of the star and the accuracy of the measured parallax obtained by ever more sensitive satellite observatories in earth orbit. Recent estimates place AG Car at an approximate distance of 4,650 parsecs, within the Carina Spiral Arm of our galaxy but somewhat further along our line of sight than the celebrated Eta Carinae that is currently estimated to be some 2,300 parsecs distance.
Earlier estimates have ranged between 2,500 and 6,500 parsecs and the currently accepted value is likely to change in the future as the measurements are further refined.
The light variations of AG Car have been closely followed by amateur astronomers for decades and these observations provide a near continuous record of the recent behaviour, as shown in the accompanying light curve spanning the 25,000 day interval, some 68 years, 1952 through 2020. This clearly shows the irregular and unpredictable behaviour of AG Car, with series of pronounced outbursts that in some cases appear to be semi-periodic in nature.
An interesting feature is the broad deep minimum during 2014 that is followed by a series of outbursts at approximately 2 year intervals (2016, 2018 and 2020) and which has seen each outburst become progressively brighter so that now, at the beginning of the 2020-2021 observing season, AG Car is nudging above magnitude 6 and approaching the historically brightest state yet observed in this star during the 1994 outburst, but which did not experience the semi periodic outbursts in the lead-up to that maximum.
Will AG Car continue to rise during the coming apparition and reach a record peak brightness or has it run out of puff and reduce its light output? Or perhaps it will explode as a supernova – an unlikely event but one that would see it temporarily become the brightest star of the night sky and most probably also be visible to the unaided eye during daylight hours. What the future holds for AG Car is far from certain and it is therefore important that we maintain the long term monitoring.