Archive for the ‘Stars’ Category

The Northern Crown

Posted: June 9, 2012 in Astrobites, Observing, Stars

The short nights of late spring don’t give much time to do astronomy.  Despite this there are some interesting constellations to be explored!  One of my favourites is Corona Borealis – the Northern Crown.  At this time of the year it is high in the southern sky when it gets dark.  If you’ve not seen this little constellation before, well, it’s easy to find by star hopping from The Plough to the brilliant orange star Arcturus and then onto Corona Borealis.  Like this:

It an easily recognisable constellation – just a small semi-circle of stars.  And one of the few that actually resembles the object it is meant to represent:

The brightest star in the constellation is called Alphekka (or Gemma).  It’s a second magnitude star about 75 light-years away.  Alphekka is an eclipsing binary star and it falls  *very*  slightly in brightness every 17 days as its unseen companion star eclipses it.

The Northern Crown is well known to astronomers for a couple of its odd variable stars!  The first is known as T Corona Borealis, or more informally, The Blaze Star.  It is normally invisible to the unaided eye but it erupted dramatically in 1866 and 1946, becoming the brightest star in the constellation.  The Blaze Star is a recurrent nova; a white dwarf star being fed gaseous material by a red giant companion.

A critical point is aperiodically reached and the material ignites in a thermonuclear explosion visible from Earth.  After a few months the star has returned to its usual dim self.

The second oddball star is called R Corona Borealis (it doesn’t have a more exciting name).  Binoculars or a telescope are needed to see the star because it usually hovers on the border of naked eye visibility at magnitude 6.  At irregular intervals of months or years this star dramatically fades by a factor of 30  – requiring a very big telescope to see.  The best model to explain the unusual behaviour of the star suggests that carbon dust (soot!) builds up in the star’s atmosphere preventing light getting out and causing it to fade.  On the inside of the star the radiation cannot escape and the pressure rises until the carbon dust is blown out again causing the star to return, albeit temporarily, to normal brightness.

I downloaded a more detailed finder chart from the AAVSO showing the location of R CrB:

The numbers next to the stars are actually brightness labels (so 46 means magnitude +4.6).  Naked eye limit from a dark sky are those labelled with numbers less than about 65.  Here’s a light curve showing how the light from the star has brightened and dimmed over the last 20 years:

R CrB tumbled in brightness in 2007 and has been hovering near minimum brightness since.  The star will eventually recover dramatically – will it be in the next few weeks or months?  Time will tell so watch out!

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Aldebaran Spectrum

Posted: November 2, 2011 in Spectroscopy, Stars

I’m determined to master the spectroscopy thing by the Spring.  Here’s a heavily processed image of the spectrum of the star Aldebaran in Taurus that I got at the observatory last night.

It’s very low resolution – I couldn’t get the distance from the Star Analyser to the Meade DSI II camera big enough to really bring out the absorption lines clearly.  I think I’ve figured out how to do it next time.  Nevertheless, you can see some dark bands in the spectrum.

Here’s a profile of the brightness along the middle of the spectrum.

The scale shown for wavelength is extremely rough and ready!  The troughs of the graph correspond to places where particular wavelengths of the light have been absorbed by the atmosphere of Aldebaran or by our atmosphere.

Aldebaran is an orange giant star; the star is off the main sequence because hydrogen fusion in the core has stopped.  It has swelled into a giant star with a cooler orange “surface”.

The next step for me is to consistently be able to set up with a higher resolution and then calibrate the resolution it using a well known spectra (i.e. Vega).

All good fun!

In Northumberland on late on autumn evenings there’s a lonely star near the southern horizon.  It often goes unnoticed – perhaps hidden by nearby (or even distant) buildings or trees.  The name of the star is Fomalhaut (pronounced “fum-al-hort”).

Looking south during mid-October 2011 at 10.30pm.

Fomalhaut is a tough star to see from these northerly latitudes.  You can find it using the popular “Square of Pegasus” stars shown above.  Follow a line from the two right-hand stars down to the horizon.  It’s typically only visible for just a few hours each night and it climbs just 6 degrees above the southern horizon at most.  The further north you are, the harder it gets!  Way up past 61 degrees north it doesn’t rise at all.

Fomalhaut is a 1st magnitude star and compared with the stars of the Summer Triangle, it is brighter than Deneb but doesn’t quite rival Altair.  But Fomalhaut at its best is shining through 10 times as much air as a star overhead.  It’s light is scattered and the star is dimmed significantly.  If the air is unsteady it can twinkle like few other stars in the sky can!

Here’s a picture Fomalhaut at visible and near infrared wavelengths:

Fomalhaut - the brightest star in Piscis Austrinus.

I had great fun assembling this image from individual infrared, red and blue images lifted from the STScI Digitized Sky Survey website!

Fomalhaut is a white, main sequence star just 25 light years away.  It’s the brightest star in the constellation Piscis Austrinus (the Southern Fish).  Astronomers have found that Fomalhaut is a shade over twice the Sun’s mass and it puts out 18 times more light than the Sun does.  Interestingly the motion of Fomalhaut through space suggests that it shares a birth place with several other bright stars in the sky – the so called Castor Moving Group.  So Fomalhaut formed from the same nebula as Castor (in Gemini), Vega (in Lyra) and Zubenelgenubi (in Libra) along with others.  These stars are widely scattered across the sky today but their common motion puts them at the same location in space sometime in the last 300 million years!

Fomalhaut is well known to astronomers because it is surrounded by a warm, dusty disk of material resembling the Kuiper Belt in our solar system.  In recent years the HST has actually directly imaged a planet orbiting within the disk – the planet is named Fomalhaut b.

Fomalhaut b is trillions of times fainter than its parent star – it’s only visible in the HST picture because the star is mostly blocked out and some delicate image processing has been applied after that (the white circle shows the location of the star).  The mass of the planet is not know with certainty but may be half as massive, to twice as massive as Jupiter.  It’s at a great distance from Fomalhaut – about 115AUs, which is nearly four times the distance of Neptune from the Sun.  It will take Fomalhaut b about 870 years to go around its star.

So Fomalhaut is not actually as lonely as it appears in our skies!