Images From Science: Cassiopeia A

Thursday, December 4, 2008

 

Supernova remnant Cassiopeia A. Click to enlarge.

On November 11, 1572, the Danish astronomer Tycho Brahe saw what he thought was a new star. Outshining the planet Venus, it was visible for several months until it faded. What Brahe actually saw was a supernova - the titanic explosion of a dying massive star. Its designation "Cassiopeia A" is derived from the fact that it lies in the direction of the constellation Cassiopeia, about 11,000 light-years from Earth.

Interestingly, astronomers have been able to do spectral analysis on the supernova not just by using the light that reaches us today, but by using the light that originated not long after the event itself.  How? They measure it's light echo. A ‘light echo’ is light from the original supernova event that bounces off dust particles in surrounding and background interstellar clouds and reaches Earth many years after the direct light passes by. The diagram below explains the concept:

Credit: Subaru Telescope

So, using the diagram above, let's imagine that the dust cloud behind the supernova is 10,000 light years behind the supernova. The supernova radiation that bounces off it takes an appropriately longer 10,000 years to reach Earth than does the radiation that travels directly toward us. In effect, we get a "snapshot" of the radiation that is 10,000 years older than the light that reaches us in a straight-line fashion. Using this technique, astronomers can study the supernova event at a much earlier point in the event's history than by seeing the light that reaches us today. They can compare the two sets of radiation measurements (reflected and direct) and deduce the evolution of the stellar blast over the millenia. It's a nifty way of using spacetime as a time-traveling technique.

Using this technique, astronomers were able to obtain information about the nature of the original blast, and determine its origin and exact type, and relate that information to what we see from its remnant today.

They found Cassiopeia A's star was suffered what is called a Type Ia supernova. In comparing this supernova with other Type Ia supernovae outside our galaxy, they were able to show that Tycho's supernova is typical of this type of supernova, and indeed is the first confirmed and precisely classified supernova in our galaxy. This finding is significant because Type Ia supernovae are the primary source of heavy elements in the Universe, and play an important role as cosmological distance indicators, serving as standard candles because the level of the luminosity is always the same for this type of supernova. That means when astronomers detect a Type Ia supernova, they can measure its relative brightness (i.e. not how bright it is, but how bright it appears because of distance) and make an extremely accurate calculation of how far away it is. That's very useful when observing a supernova event in a distant galaxy - they'll know exactly how far away the galaxy is.

This kind of 'standard candle' distance measurement creates a huge problem for Young Earth Creationists (YECs, appropriately), who maintain that the universe can be no more than 6,000 years old based on genealogical lineages outlined in the Old Testament. Any type Ia supernova over 6,000 light-years away tends to blow a big fat hole in their young universe "hypothesis", and it's always fun to watch "creation scientists" try to explain it away. Yes, the very existence of the universe itself is a testament to fundamentalist foolishness!

Source: Universe Today