Daniel Pope

Cassiopeia (KAS-ee-oh-PEE-ah) is the Queen of our night sky. I associate her with the beginning of a new school year. Every year at this time Cassiopeia is high and bright in the northeast and I wonder – what new things will I learn in this new year? 

An event in the stars of Cassiopeia on a night in early November 1572 changed forever how the study of astronomy was viewed and pursued. Up until then, no one had successively argued against Aristotle’s (384-322 BC) position that nothing in the sky beyond the planets ever changed. On Nov. 11, 1572 Tycho Brahe (1546-1601) saw a very bright new star appear in Cassiopeia. It was so bright that it could be seen in the daytime but it faded from view by March 1574. Its approximate location is near the top star in Cassiopeia in the graphic. Tycho studied this new star extensively and wrote a book about it that became popular and was widely read.  This greatly increased his stature as an important astronomer of his day. He presented this new star as evidence that Aristotle’s philosophy was flawed. Astronomers would no longer use the stars just as a calendar in the sky but they would study the stars with the hope that one-day they would come to understand how the Cosmos works. By the early 1600s the telescope had been invented and astronomers used them to search for this new but faded star. They were not successful.  The word ‘supernova’ (a star that explodes at the end of its life cycle) came into our terminology in the 1930s.  It was not until the 1950s that a remnant of a supernova was found at this spot in the sky – 7,500 light years from us. In recent years astronomers have determined that not all supernovae have the same origin or cause.  Supernovae are classified as of type Ia, Ib, Ic, or type II.  The 1572 supernova appears to be of type Ia.  All supernovae of type Ia have the same mass and from that astronomers have concluded they have the same luminosity or brightness. Type Ia supernovae are so bright that they can briefly out shine all the stars in a galaxy. For this reason the Ia supernovae are used as the ‘standard candle’ for measuring distances that are so extreme that the rate of expansion of the universe has to be taken into account. The term ‘redshift’ is used to describe the distances rather than the ‘light year’ term.

Two weeks ago we described the Delta star in Cepheus, the King, as being the ‘key’ that unlocked the size of our Universe.  That distance measuring process depended on Cepheid variable type stars and the mathematical technique of parallax. This method works well for our Local Group and nearby galaxies but cannot be used to measure the distances to galaxies that are so far away that the rate of expansion of the Universe has to be taken into account.

Since a type Ia supernova has been discovered in the direction of Cassiopeia, you could say the ‘standard torch (candle)’ has been passed from King Cepheus, to his Queen, Cassiopeia. 

When “radio” astronomy came into existence in the 1940s the strongest radio source in the sky outside of our Solar System was detected just a few degrees above the top star in Cassiopeia in the graphic. It was named Cassiopeia A (abbreviated Cas A) and is 11,000 light years away from us. It has been determined that this is a result of a type II supernova. From calculations, it appears that the explosion took place about 1680. No one has found documentation that anyone observed the explosion – perhaps it was not bright enough to be noticed in 1680. Astronomers are learning a great deal from these two relatively close by but different types of supernovae. 

But wait – there’s not one, not two but three supernovae remnants in Cassiopeia.  SN1181 is just below the “W’s” lowest star. It was bright enough to be observed and documented in 1181 for six months by astronomers in China and Japan. It was located in the 1960s by radio telescopes but its type and distance is yet to be determined.

In 1998 two separate studies using type Ia supernovae led to the discovery that the rate of expansion of our Universe is increasing rather than decreasing as had been assumed since the 1920s.

 The Cassiopeia part of our sky is getting a great deal of attention from our present day astronomers. By studying supernovae of different types, astronomers are gaining a better understanding and giving better explanations of the life cycles of stars of different sizes.

Next week we will use mighty Jupiter to help us locate the constellation Capricorn.

 Clear skies.