White Dwarf Supernovae: Standard Lampposts on Dark Energy Road

My dear readers,

This special post is part of the Carnival of Cosmology: Dark Energy, hosted by Galileo’s Pendulum, a fellow astronomy blog I discovered less than 12 hours ago, as of the writing of this post.

To begin, I’m going to give a very brief background about dark energy in general, and then I’ll talk about one of the tools we use to measure this enigmatic stuff.  Most of my research has been investigating the nature of dark energy using Type 1a supernovae (pronounced like “Type One-A Super-no-vee,” SNe 1a for short, and also known as white dwarf supernovae.) so it is a topic very dear to my heart as well as my head.

As you may know, the Universe is expanding.  However, what you may not know is that this expansion is not slowing down, as you might think gravity would cause it to do, but it is speeding up.  We can tell this by looking far, far away, deep into space, where the things we see are not quite as bright as we would expect them to be.

One type of candle at many distances – as the candle gets farther away, it appears dimmer. The same goes for white dwarf supernovae and the expanding Universe.
Image courtesy of Lawrence Livermore National Laboratory/Universe Adventure

To quantify this notion, we use “standard candles,” which have a set brightness that we know is the same, no matter where they are in space or when they sent out their light.  A white dwarf supernova is just such a “candle.”**  This particular type of exploding star is very special, in that it doesn’t care what kind of galaxy it calls home (big, small, elliptical, spiral, etc), and it always has the same pattern of brightening and fading.

Strange, to be sure, but this is all because of the specific type of star associated with this explosion: a white dwarf of about 1.44 times the mass of our Sun (or 1.44 Solar masses).  A star similar to our Sun goes through its life cycle (click here for a very plain-English explanation of stellar evolution) and eventually becomes a white dwarf.  Usually a white dwarf will just burn the rest of its energy, cooling down to become a black dwarf.  However, if it is part of a binary system, or has a companion star, and that companion orbits close enough to our white dwarf, the white dwarf will start sucking mass right off of its companion.

White dwarf sucking mass off its companion
Image courtesy of Science Engine – Space

Once it reaches 1.44 Solar masses, the white dwarf becomes too big for its britches and collapses under the force of its own gravity in an explosion so bright that it can rival the brightness of its host galaxy!

So, you can now see how a particular kind of star in a particular setting makes a particular kind of explosion and doesn’t have to be located in a particular type of galaxy.  Even without any constraints on location, this seems like it would be pretty improbable, right?  Well, just remember that there are as many as tens to hundreds of billions of stars in a given galaxy, and with the Hubble Ultra-Deep Field as an indicator, there are billions and billions of galaxies out there, which makes for a lot of potential SNe 1a to be observed.

Type 1a supernovae are super bright!
In 1994, a white dwarf supernova (lower left) occurred in galaxy NGC 4526. The supernova’s brightness rivaled that of its host galaxy – one single star explosion rivaled the brightness of billions of stars combined!
Image courtesy of the High-Z Supernova Search Team/HST/NASA

Now, since there are so many SNe 1a out there and they are so standard, they make an excellent tool for measuring distances, which is exactly how the 2011 Nobel Prize in Physics recipients used them to discover dark energy.  They took a theoretical model of how the brightness should vary with distance and compared that with data from dozens of supernovae that they actually observed (today we have hundreds of observations, and soon there will be many, many more, thanks to the advances in telescope cameras and sensors, as well as the growing number of all-sky surveys being performed).  The disparity between the model and the data grew with distance, thus “dark energy” was discovered.
Head on back over to the Carnival to check out the other great posts!

** I should note that SNe 1a are more like standardizable candles.  If you really want to get technical, check out this article from Lawrence Livermore National Laboratory.


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