Illuminating Reality

Have you heard that the Universe is expanding? Edwin Hubble changed astronomy in the 1920s by discovering this, but did you also know that the expansion of the Universe is actually accelerating?  This was one of the biggest surprises in Cosmology back in 1998, leading to the discovery of the mysterious Dark Energy that makes up the vast majority of the energy density of the Universe.  Now to measure the expansion of the Universe, astronomers needed a way to accurately determine extreme distances.

One of the most effective ways to do this is to look at the apparent brightness of lights of a known magnitude.  Because light energy fades in a very predictable way (inverse square falloff), you can determine the distance accurately as long as you know the absolute brightness.  So try this thought experiment: if you went out on a deserted road at night and placed 100 watt light bulbs along the road every hundred meters, the closest lights would be the brightest and the farthest would be the dimmest - yet in reality they are all of the same absolute brightness.  If you stand next to each of them, they put out 100 watts of energy, but the light traveling from the farther ones loses energy, so it appears dimmer.  So by measuring the brightness of each light you could determine the distance because you know how bright it actually is and how bright it appears; essentially just by measuring the loss of energy over distance.

So how do we do this to measure the distance to stars?  As you can see in the image to the right, stars come in a wide variety of sizes and brightness (click to enlarge).  From the relatively cool, tiny Red Dwarf to the hottest, most massive blue Hypergiants, the brightness of stars varies considerably even amongst the same type.  With so much variation, it would be impossible to accurately use brightness to measure distance.  If only there was a class of bright object scattered throughout the Universe that always had the same brightness?  It turns out there is!

When a star with a mass similar to our Sun runs out of its hydrogen fuel, it can’t generate heat pressure to fight against gravity and the core starts to compress.  This compression generates heat again and causes the core to start fusing heavier and heavier elements – hydrogen into helium, helium into lithium, lithium into berylium, etc.  Heavier elements require increasing amounts of heat and continue to release energy with each pair of atoms fusing.  For a more detailed explanation of this process, check this link out.  For stars like our Sun, the process causes the star to shrink then swell up, shedding material into space until only the ultra-dense core remains.  This is when a star becomes a White Dwarf.

White Dwaves, like the stars they formed from, come in different sizes and brightness, so this isn’t our standard candle.  But something interesting can happen when a White Dwarf orbits another star.  Our star sits in space alone, except for the planets orbiting it, but out there in the cosmos binary star systems where two stars orbit each other are quite common too.  Now if one of those stars becomes a White Dwarf, a situation can occur where the smaller but often equally massive object gravitationally pulls material off it’s neighboring star.  This leaching effect causes matter to collect on the small ultra-dense (and therefore gravitationally powerful) White Dwarf.

How dense is a White Dwarf?  Dense. So dense that one the physical size of the Earth would have a density of 1 x 10⁹ kg/m³.  In case you forgot Scientific Notation, that’s 1 billion kilograms per square meter.  The Earth, on the other hand, has an average density of only 5.4 x 10³ kg/m³ or 5400 kilograms per square meter.

As this material collects onto the surface of the White Dwarf, its mass increases.  But there’s a problem that makes it a ticking time bomb.  As the mass increases, gravity crushes the object down even further, forcing all of the atoms closer and closer.  Eventually there is no room left because the electron density is too great and the object stops shrinking (via the Pauli exclusion principle).  Eventually the mass of the White Dwarf builds until it exceeds the Chandrasekhar limit, triggering carbon fusion which in a sense jump starts the once dead star.  However as soon as this happens, a portion of the matter ignites a runaway reaction which tears the star apart, unleashing a supernova.

And what’s so special about that?  What does this have to do with those 100 watt light bulbs I was talking?  Well, because of the Chandrasekhar Limit, which is roughly 1.38 times the mass of our Sun,  we have an explosion that’s always the same brightness wherever it happens.  In other words, light bulbs with a with a wattage 1.38 times the mass of our Sun blink on across the cosmos.  So astronomer look for these Type 1a Supernova and knowing how bright they really are, determine their distance by measuring the loss of light energy.

UPDATE! So it turns out I missed a paper that throws into question the matter accretion scenario for Type 1a Supernovae.  Rob Knop goes into detail on his blog.