Beauty of star in death throes
WEST of the Southern Cross, lost among the vast number of stars making up the glow of the Milky Way, is a star that is in its death throes and has been a great subject for the Hubble Space Telescope.
The “Hubble” has shown it to be quite a spectacular object and its most recent photograph has quickly become one of my favourites.
The star is called AG Carinae, the designation indicating that it varies in brightness. It is seen within the constellation of Carina, the keel of the great ship Argo, which was divided up into smaller constellations because of its large size.
AG Carinae is a type of star called a Luminous Blue Variable, or LBV for short. It is an extremely massive star – at least about 55 times as massive
as our sun – and is very unstable. It is also a star that could explode – any time – and there is a possibility it has already done so!
The variability in its brightness has long been known. For much of the time, it is rather below the level of visibility with the unaided eye but can be seen through binoculars.
Occasionally, however, it becomes just visible without any optical aid, reaching about magnitude six on the brightness scale used by astronomers.
In 1950, astronomer Andrew Thackeray discovered there was a shell of material around the star. The star is
fighting to survive: This shell was ejected from the star about 10,000 years ago and comprises at least 10 per cent of the mass the star had at the time.
The latest image by Hubble is not the first but it is very rich in detail. It makes for great computer wallpaper but of course its real purpose is for research into these fascinating objects.
Another famous LBV, although in a special classification of its own – is the star Eta Carinae, also in the constellation of Carina. It is in almost the same direction as AG Carinae and is another star that is a candidate for becoming a supernova.
Because of their far southern location, AG and Eta Carinae are well suited to observation from our part of the world. Indeed, Tasmania and southern parts of mainland Australia are great locations for gazing at stellar sights such as the Southern Cross, the “Pointers”, and the Milky Way.
From much of the northern hemisphere, these stars never even rise above the horizon, just as we never see the Big Dipper from Tasmania.
The Hubble Space Telescope has produced many spectacular images. Recently, I was watching episodes of Star Trek: Voyager, in which Hubble images are seen displayed
inside the starship. The fact the series is set late in the 24th century means the images would by then be almost 400 years old. It is a testament to the quality of the imagery!
I mentioned earlier that an explosion of AG Carinae may already have taken place. This is because of the great distance to the star – about 20,000 light years – meaning it has taken 20,000 years for its light to reach us.
If a disaster had befallen AG Carinae within that time, we would not yet know about it. However, when it happens, the star will temporarily appear about as bright as the
planet Venus. In my talks and planetarium presentations, I have often been asked about the methods by which the stars are named. In this case, the letters “AG” are not actually initials. They arise because the Roman lettering of normal stars within each constellation ended with Q for the faintest ones visible to the unaided eye in the most “populated” constellations, so the letters R to Z were assigned to variable stars. After the sequence reached Z, it continued with AA, AB, and so on.