[meteorite-list] Astronomers Await a Nova

[Gary K. Foote]

Astronomers Await a Nova 

http://www.space.com/spacewatch/061222_ns_nova_star.html

By Joe Rao
SPACE.com Skywatching Columnist
posted: 22 December 2006
06:40 am ET
 

At this time of year it seems almost traditional for stargazers to ponder the age-old 
question of the origin of the Star of Bethlehem.  The Star’s appearance some 2,000 years 
ago is quite possibly one of the best-known celestial events in all of recorded history.  


Many planetariums are currently running sky shows interwoven with a Christmas theme.  
Utilizing special effects projectors, audiences are transported back in time to see the 
night sky as the Magi may have seen it and are then asked if perhaps an unusual gathering 
of planets might have been the fabled star.  Or was that 'sign' a comet, a nova or 
something supernatural?  

The topic has universal fascination, and is why Christmas Star shows still play to packed 
planetarium houses. 

Perhaps the simplest answer that can be offered is that the Star might have been a nova: 
A 'new star' suddenly blazing forth where no star had previously been seen.  While for 
the most part such objects are really dying stars having a final fling of glory before 
descending the long road to ultimate extinction, there are some stars that go through 
such contortions more than once. 

One such star is long overdue to 'pop' and could do so at anytime.  

The star in question is T Pyxidis, in the constellation of Pyxis, the Mariner’s Compass, 
a dim southern constellation that never gets very high for skywatchers in the United 
States.  For instance: as seen from Philadelphia, Indianapolis and Denver, it reaches a 
maximum altitude of just 18-degrees above the southern horizon (your clinched fist, held 
at arm’s length is roughly 10-degrees in width).  Currently, Pyxis is crossing the 
meridian around 2:30 a.m.  By the end of January it will be there just after midnight and 
by early March, just before 10 p.m. 

T Pyxidis is about 6,000 light years away and belongs to a small and seemingly 
"exclusive" group of cataclysmic variable stars called recurrent novae (NR), of which 
there are currently less than ten confirmed members.  T Pyxidis is composed of a dense 
white dwarf and a close companion star. An outburst occurs when the temperature and 
density of the surge of matter dumped from the companion onto the surface of the white 
dwarf reach the nuclear flash point for hydrogen. While material is violently blown off 
in all directions (see comparison photos from both ground-based instruments and the 
Hubble Space Telescope here), the white dwarf itself is not disrupted and soon begins to 
accumulate more matter from its companion, and repeats the cataclysm some years later. 

Astronomers have been patiently waiting for T Pyxidis’s next outburst for more than 20 
years.  

Normally this star shines at magnitude 14-that’s about a thousand times dimmer than the 
faintest star that can be perceived by most human eyes on a dark, clear night. But on 
five occasions, in 1890, 1902, 1920, 1944 and 1967, this star brightened dramatically to 
magnitudes between 6.5  and 7 - a 1,000-fold increase in brightness in the most extreme 
case, making T Pyxidis just bright enough to be glimpsed without any optical aid. These 
eruptions came at an average of just over 19 years apart, and the longest stretch of time 
between them was 24 years.  

But this month marks 40 years since the last outburst. 

It was back on Dec. 7, 1966 that the most recent eruption was first noticed by New 
Zealand amateur astronomer, Albert Jones.  The star had more than doubled in brightness 
to magnitude 12.9.  Just two nights later it was almost four magnitudes brighter and 
after a month it was glowing at magnitude 6.3 before slowly fading back to normal. 

Nobody knows exactly why T Pyxidis has remained quiet for so long, but the general 
consensus is that it may have accumulated an extra-thick coating of nuclear fuel on its 
surface over these past 20 years, which would make it appear extra bright when it finally 
blows its next surge of gaseous debris out into space.  

Who knows?  That night could be tonight!  

Gary Foote
gary at meteorite-dealers.com
http://www.meteorite-dealers.com
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