A Changing Sky

Gazing up at the sky while reading the old texts one would not be amiss in believing that the stars never change. Indeed there are many who insist vehemently that the stars are eternal and unchanging. But the stars do indeed change, often quite visibly, sometimes within the span of a human lifetime.

One such star is L₂ Puppis.

A bright star, one of the few naked eye variable stars that could be seen to fade and reappear without the aid of a telescope much like the far more famous stars Mira and Algol. On star charts the star is found prominently drawn at magnitude 4, buf if you attempt to locate it today you will not find it without the use of a telescope.

I first encountered this star quite recently while starhopping through southern Puppis with an 8″ telescope from the driveway. The chart showed two bright stars close together, L₁ Pup and L₂ Pup, while the view in the finder ‘scope showed only one bright star.

I thought I had taken a wrong turn at Sigma Puppis and retraced my steps… Nope, the chart display in SkySafari, usually extremely accurate was not agreeing with the view in the finder. A star is just missing, a bright 4th magnitude star!

I call up the data on the missing star and sure enough… It is a variable, listed as changing from 2.6 to 6.2 over a 141 day cycle. OK, problem solved… Right?

Intrugued at this point I carefully identify the correct star in the field… This star seems fainter than 6, quite a bit fainter, maybe magnitude 8? That would be 1/6 the brightness, much fainter than this star is purported to get. This is when I call up the AAVSO data on the star to get a current estimate… Magnitude 7.6?!?!

Yes, 7.6 is drastically fainter, recall that magnitude is an exponential scale with each magnitude representing a 2.5 times increase in brightness. This difference indicates the star is about 1/4 as bright as the faintest it is supposed to appear, and 1/27 as bright compared to the charted average of 4th magnitude I had been looking for to start this chase. A bit brighter than my quick estimate at the eyepiece, but still well out of the listed range of variability.

Using the plotting tool on the AAVSO website I see that the star has varied between 6 and 8 for the last few years, never getting anywhere near the fourth magnitude plotted on the chart much less the 2.6 magnitude that it used to occasionally peak at, which would be a prominent star in the sky.

I run the plot back further… In the 1980’s the star behaved as it had historically, varying from around 3rd to 6th magnitude reliably. Then around 1995 its apparent magnitude began to steadily fade for the next decade. Since 2005 the star has ceased to fade and reliably varies from about 6th to 8th magnitude, about 1/10th of its previous average brightness.

An easily naked eye star has essentially vanished from the sky, fading to the point you need optical aid to observe it.

Not much has been written about this star, which is a bit surprising considering it is a case of a bright visual star that has essentially dissapeared. This is probably becuse the star sits among many such stars in the thick galactic starfields of Puppis, a somewhat obscure constellation to the east of Orion and Canis Major. The star is also quite far south at a declination of -44°, not far from Canopus and out of reach for many northern hemisphere observers.

In all my years of watching and reading about the skies I do not recall any mention of this interesting star. The brief but concise Wikipedia entry on L₂ Pup is quite good, but you have to have reason to go looking for it among the thousands of entries concerning stars on the online encyclopedia.

So what has happened here? The short answer is that the star is dying, or by some reckoning is already dead. The star has long since exhausted its hydrogen fuel in the core, it is in the final stages of the giant phase during which it fused heluim and other heavier elements to sustain the fusion reaction that defines a star. It has reached the point when this fusion reaction is either winding down for lack of fuel, or has ceased entirely.

A star that no longer has fusion occuring in its core does not simply go dark, it is still incandescently hot. With gigatons of mass in a vacuum, it will remain hot for billions of years, slowly cooling and slowly fading.

What is left of L₂ Pup is on its way to forming a white dwarf, the exhausted cinder of a star. This is the ultimate fate of any small star, including our own Sun some day billions of years in the future.

At the same time the star has begun expelling a great deal of mass, the ejected material forming what is known as a planetary nebula around the star. Indeed, this gas and dust is likely the current reason for the fading act. As the dust and gas forms the nebula it obscures the star. At the top of this post is an image of the star within its growing cocoon of nebula.

L₂ Pup may never regain it’s brightness. Of course there may be temporary clearings in the nebula, or perhaps outbursts of activity in a now somewhat unstable star as it dies. But these will at best be temporary, the trend leads nothing but downwards.

I will probably keep an eye on this interesting star in the future. It represents an element of clear change over the course of a human lifespan, in a sky where things can seem unchanging to ephemeral creatures such as ourselves.