Monday, March 28, 2011

Tycho's star

In early November of 1572, a new star appeared in the northern constellation of Cassiopeia.  For a time, it rivaled Venus in brilliance and was even visible during the day for about two weeks in November.  Over the next few months it began to fade in brightness until it finally disappeared from view in March of 1574.

One of the people to view this mysterious star was the great Danish astronomer Tycho Brahe (1546-1601).  Tycho (pronounced "tee-koh") noted that this new star did not change position like the planets but stayed fixed in the sky.  This observation was in direct opposition to the classical Aristotelian worldview which held that the heavens were fixed and immutable - the worldview also shared by the church.  Something was terribly amiss in the sky.

Tycho wrote up his and other observations of the new star in De nova et nullius aevi memoria prius visa stella usually known today simply as De nova stella, "the new star".  Today we still call these types of stars "novae" and this one is now known as Tycho's star in his honor.

So, what was this mysterious ephemeral star in Cassiopeia?  Cassiopeia, high in the northwestern sky, looks like either a W or an M depending on your orientation and the supernova was a bit to the right (marked by the circle in the star map).  While astronomers knew its approximate location from sketches left by Brahe, it wasn't rediscovered again until radio telescopes detected it in 1952.

Tycho's star was a Type Ia supernova now known by astronomers as SN1572.  Type Ia supernovae originate in binary star systems.  Imagine such a system with two Sun-like stars.  Stars generate energy by fusing hydrogen into helium in their cores and they eventually run low on hydrogen fuel.  When this happens, they will expand into red giants in an attempt to keep going by fusing helium into carbon.  Eventually, the outer layers of the star are pushed off in an expanding shell of gas (a planetary nebula) and a small compact white dwarf remains behind.

OK, this is all highly simplified and would comprise a couple of hours of lecture in an astronomy class!

So now suppose we have a binary star system where one star is a white dwarf and the other is a red giant.  Gas puffed off by the red giant accretes onto the white dwarf.  When the white dwarf reaches 1.4 solar masses, a series of complex reactions occur resulting in the violent explosion of the star billions of times more brilliant than our Sun - a Type Ia supernova.

Astronomers can now image the remnant of expanding gases from this violent explosion.  Here's a recent image from the Chandra X-Ray observatory - an orbiting telescope which images in the x-ray part of the electromagnetic spectrum.

Pretty damn impressive.  There's some amazing stuff out there we've only been able to "see" in our lifetimes with modern technology.  What wonders await us yet?

Some astronomers have argued that Shakespeare referred to Tycho's star, west of Polaris, the Pole Star, when he wrote Hamlet (Act 1, Scene 1).

     Last night of all,
     When yond same star that's westward from the pole
     Had made his course to illume that part of heaven
     Where now it burns...

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