Tue, 21. May 2013
Astro-Speaker Watch - May 2013
An American astronomer, based in Chile, visiting Australia.
That's what we were treated to this month at the Macartur Astronomy Forum!
Dr. Mark Phillips is currently Associate Director for Magellan & Associate Director at Las Campanas Observatory, Chile. His Wikipedia entry tells us:
"Mark M. Phillips is internationally recognized astronomer in the observational studies of all classes of supernovae. He is well known for his work on SN1986G, SN1987A, the Calán/Tololo Supernova Survey, the High-Z Supernova Search Team, and especially for the Phillips relationship. This relationship has allowed the use of Type Ia supernovae as standard candles, leading to the precise measurements of the Hubble constant H0 and the deceleration parameter the latter implying the existence of dark energy or a cosmological constant in the Universe."
His talk was actually a behind the scenes look at how Saul Perlmutter, Brian P. Schmidt & Adam G. Riess were awarded the Nobel Prize in Physics for 2011 and it seemed to me that Dr Phillips was a little modest about his own role in this amazing discovery, as I will describe below.
Dr. Mark Phillips answers a question from ex-president John Rombi.
The story began with a little history about a guest star which was observed by Tycho Brahe in 1572 - which can be observed to this day as a supernova remnant in Cassiopeia - and its light echo which reached us in 2008. It continued with the discovery by Slipher and Hubble that the Universe is expanding - but the question remained: was the Universe open, closed or flat? (is the Universe expansion speeding up or slowing down?).
How do supernova occur?
Stars evolve along predictable paths, depending mainly on their mass.
Stars like the Sun, will evolve from Main Sequence to Red Giant and then to White Dwarf without ever exploding as a supernova - but when a giant star with an iron core finally runs out of fuel, the core collapses and the outer layers implode and rebound in a massive, catastrophic explosion.
Supernovae occur in two situations: (i) the core collapse of a giant star; and (ii) a thermonuclear supernova in a white dwarf star which acretes mass from a companion star and reaches a critical density. They will leave behind either a neutron star or a black hole.
In the observable Universe, there is on average one supernova occuring every second, which is rather astonishing when you consider it is over four hundred years since one occurred in our own galaxy!
A typical giant star will pass through several stages as it converts its lighter elements (Hydrogen, Helium) into heavier ones (Carbon, Neon, Oxygen, Silicon) before going supernova (and producing all the other 'natural' elements).
Then it begins to get even more interesting.
Type Ia supernovae occur when a white dwarf star accumulates mass from a companion star; reaches a critical mass of 1.38 solar masses; and becomes unstable, leading to a supernova. They follow predictably similar light characteristics but more work was needed to develop their data to a standard candle to measure distances. This is the point where Mark somewhat glossed over his personal role in tying down Type Ia supernovae to work out the fate of the Universe. My ears pricked up when he said that he was embarrassed to have this effect named after him. Wikipedia again:
"The 'Phillips Relationship' is the relationship between the peak luminosity of a Type Ia supernova and the speed of luminosity evolution after maximum light. During the course of the Calán/Tololo Supernova Survey, Mark M. Phillips discovered that the faster the supernova faded from maximum light, the fainter its peak magnitude was. It has been recast to include the evolution in multiple photometric bandpasses and as a stretch in the time axis relative to a standard template. The relation is typically used to bring any Type Ia supernova peak magnitude to a standard candle value."
Type Ia supernova characteristics can thus be reduced to a linear graph, from which distance can be accurately inferred from the maximum brightness of a Type Ia supernova!
From this, measurements showed that the Cosmological Constant was different in the distant past:
So we can infer the Universe will expand forever - infinite Universal dilution!
As the Universe expands, the galaxies beyond our gravitationally bound Local Group will disappear from view as the expansion of space becomes faster than the speed of light. Our Local Group of galaxies, by that time, will have consolidated into a single super galaxy and any astronomers evolving at such a time will be unable to determine the expansion of the Universe because there will be no galaxies to measure and the Cosmic Microwave Background will become so stretched that it will be undetectable.
It's nice to know we live in a cosmic epoch in which the ability to study the origin and fate of the Universe is still in our hands.
One ugly potential scenario is worse than just mere infinite expansion:
Big Rip doesn't sound too nice.........
Our guest, Dr. Mark Phillips, provided us with a very clear picture of what supernovae are; how we measure them, and why they are so important in evaluating the very nature of the Universe.
He modestly downplayed his own part in determining the accelerating expansion of the Universe - and I hope the above has set the record straight.