Monthly Archives: August 2011

M101 Supernova and the Cosmic Distance Ladder

Last week, on August 24, there was a very fortuitous and major discovery by UC Berkeley and Lawrence Berkeley National Lab astronomers of a nearby Type 1a supernova, named PTF 11kly, in the nearby Pinwheel Galaxy. This galaxy in Ursa Major is also known as M101 (the 101st member of the Messier catalog). Type 1a supernovae are key to measuring the cosmological distance scale since they act as “standard candles”, that is, they all have more or less the same absolute brightness. Dark energy was first discovered through Type 1a supernovae measurements. These supernovae are due to certain white dwarf runaway thermonuclear explosions.

Supernova in M101

Supernova in M101 (Credit: Lawrence Berkeley National Laboratory, Palomar Transient Factory team)

Three photos on 3 successive nights, with the supernova not detectable on 22 August (left image), detectable (pointed to by green arrow) on 23 August (middle image) and brighter on 24 August (right image).

A white dwarf is the evolutionary end state for most stars, including our Sun eventually, after it exhausts the hydrogen and helium in its core via thermonuclear fusion. Some of the star’s outer envelope is ejected during a nova phase but the remaining portion of the star collapses dramatically, until it is only about the size of the Earth. This is due to the lack of pressure support that previously was generated by nuclear fusion at high temperatures. This is not a supernova event; it is the prior phase that forms the white dwarf. The white dwarf core is usually composed primarily of carbon and oxygen. The collapse of the core is halted by electron degeneracy pressure. The electron degenerate matter, of which a white dwarf is composed, has its pressure determined by quantum rules that require that no two electrons can occupy the same state of position and momentum.

A Type 1a supernova is formed when a white dwarf of a particular mass undergoes a supernova explosion. It was shown in the 1930s by Chandrasekhar that the maximum mass supportable in the white dwarf state is 1.38 solar masses (1.38 times our Sun’s mass). In essence, at this limit, the electrons are pushed as close together as possible. If the white dwarf is near this limit and sufficient mass is added to the white dwarf, it will ignite thermonuclear burning of carbon and oxygen nuclei during a very rapid interval of a few seconds and explode as a supernova. The explosion is catastrophic, with all or nearly all of the star’s matter being thrown out into space. The supernova at maximum is very bright, for a while perhaps as bright as an entire galaxy. The additional mass that triggers the supernova is typically supplied by tidal accretion from a companion star found in a binary system with the white dwarf.

Because they all explode with the same mass, Type 1a supernovae all have more or less the same absolute brightness. This is key to their usefulness as standard candles.

In 1998 two teams of astronomers used these Type 1a supernovae to make the most significant observational discovery in cosmology since the detection of the cosmic microwave background radiation over 30 years earlier. They searched for these standard candle supernovae in very distant galaxies in order to measure the evolution and topology of the universe. Both teams determined the need for a non-zero cosmological constant, or dark energy term, in the equations of general relativity and with their initial results and others gathered later, its strength is seen to be nearly 3/4 of the total mass-energy density of the universe. These results have been confirmed by other techniques, including via detailed studies of the cosmic microwave background.

One needs two measurements for each galaxy to perform this test: a measurement of the redshift distance and a measure of the luminosity distance. The redshift distance is determined by the amount of shift toward the red portion of major identifier lines in the host galaxy’s spectrum, due to the expansion of the universe (the host galaxy is the galaxy in which a given supernova is contained.) The apparent brightness of the supernova relative to its absolute brightness provides the luminosity distance. Basically the two teams found that the distant galaxies were further away than expected, implying a greater rate of continuing expansion – indeed an acceleration – of the universe during the past several billion years compared to what would occur without dark energy.

What is exciting about the M101 supernova discovery last week is that it is so nearby, so easy to measure, and was caught very soon after the initial explosion. By studying how bright it is each day as the supernova explosion progresses, first brightening and then fading (this is known as the light curve), it can help us tie down more tightly the determination of the distance. This in turn helps to provide further precision and confidence around the measurement of the strength of dark energy.

References:  Perlmutter et al. 1999 “Measurements of Omega and Lambda from 42 High-Redshift Supernovae” Astrophys.J.517:565-586  Reiss et al. 1998 “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant” Astron.J.116:1009-1038