A Candle Above the Wind

I could not resist the title which, quite obviously, is a distortion of the famous Elton John song about Marilyn Monroe. But how do I justify its use with the subject of this blog? Well, this blog is about candles, in fact the biggest ones. Like Monroe, who was (and still is) the standard of beauty for all of humankind, the candles I’m going to talk about are also heralded as ‘standards’. Let’s not stretch this analogy any further and dive straight into the subject for this bitesize blog: How do we measure distances of faraway galaxies in space?

Well, definitely not with rulers. But let’s take a step back and ask why would we need to measure those distances at all? For all practical purposes, they are infinitely far away. There’s no way we are going there anytime soon, we aren’t even in Mars yet! The answer of course is ‘because we can and we want to’. We want to know how far we can see with our telescopes and lenses; we want to know the stretch of our quite probably lonely existence and of course we want to claim it for us. After all: finders keepers. But beyond all these adventurous motives, there’s a scientific query as well. Einstein’s theory tells us that universe is not a static arena in which all the drama is taking place. Rather, it’s very much an active participant to all of this. So the collective behaviour of faraway galaxies speak little of their individual properties but speaks volume about the behaviour of our universe itself at large distances. Thus knowing the distance becomes very important for cosmologists who make models about these behaviours. Through these models they try to answer the most fundamental questions we ask: How did all this begin? How will our universe end? Is it eternal? Etc…

So we’ve established its importance and now we’re back to rulers. What can we possibly use to measure these humongous distances? The answer lies in one of the most violent events in all of universe, the death of a star, Supernovae.

The life cycle of a star is a complex phenomenon and its ultimate fate depends largely on what its mass is. Stars are basically nuclear reactors for fusion. In their early stage, stars are nothing but large collection of hydrogen bound by gravity. In fact, the gravitational pull is so large that the hydrogen atoms in stars get enough energy to fuse together and make helium. This fusion gives out insane amount of energy that acts as a fuel for the star and they continue to shine for millions of years. Our Sun is still in this stage. Once most of the hydrogen is converted to helium, the latter starts to fuse and create heavier elements inside the core of the star. But all this fuel comes to an end after a stage. It is at this point, stars start taking different routes on the process of dying.

The bigger stars, i.e. bigger than 8 times the mass of our Sun, swells up and becomes what is known as red giant. Although swollen up, its core continues to shrink under gravity. The temperature rises to billions of degrees (yes! Billions with a B). Ultimately the core uncontrollably shrinks extremely fast and then recoils. This produces an extremely powerful shock wave that expels all the matter of the star violently. This is how a Supernova occurs. The energy produced in this burst is so huge that supernovas shine brighter than whole galaxies.

Figure 1: After and before the supernova explosion in 1987. Brighter than whole galaxies

There’s one more way a supernova can be created. This happens in a binary star system, where two stars revolve around each other. If one of the stars of the binary is a White dwarf (which is a politically incorrect way of saying it’s an extremely dense star made of carbon and oxygen), then sometimes it starts absorbing the other star of the binary. If it absorbs too much, its density becomes so large that the carbon and oxygen at its core fuses uncontrollably, ultimately detonating the star. This is called Type IA Supernova. The previous mechanism is called Type II.

 This type IA supernovae are the candles we require. It turns out that all these supernovae have the same luminosity when they explode. Luminosity is the measure of how bright the event is when measured from a certain distance. Now obviously as you look from further and further away, the luminosity decreases. Thus measuring the luminosity of a particular supernova in a distant galaxy from earth can automatically tell us about how far those galaxies actually are. But wait! This seems a bit circular, isn’t it? How do we know the luminosity of these events in the first place? Don’t we have to measure that? Won’t we need to know the distance to measure the actual luminosity? Yes. Fortunately, we achieve these by measuring the nearby supernovae in neighbourhood galaxies. In this scale, there are other methods for calculating distances, for example by using stellar parallax (It’s another simple but cool method. But I won’t be talking about it).

There are a lot of problems though with this method. Firstly, it’s only conjectured that the luminosity of all type IA supernovae are same. If they aren’t, the distances calculated will be erroneous. Secondly, supernovae are rare, type IA are rarer. Thus astronomers have to patiently wait for their occurrence and keep scanning the sky looking for them. I’d use the phrase needle in a haystack but in this case, they haystack is the size of the observable universe!

Figure 2: Remnants of past supernovae are visible in sky

Supernovae are historically significant to us as well. The oldest known sighting of supernovae were actually recorded in ancient Indian texts. They’ve also helped us formulate early cosmological models. Euclid had a theory that beyond earth, sun and our planets the rest of the universe is just a static frame. That claim was refuted when Greek scholars suddenly spotted a supernova explosion, visible to the naked eye. Of course almost all of interstellar oxygen and iron comes from supernova explosions, so without these violently beautiful events, life wouldn’t exist at all.

Thus we see the death of a star apart from being an extremely beautiful and aesthetic phenomenon, is also extremely important for scientists, philosophers and even to all living creatures ignorant to it. Maybe Elton John can be quoted again to pay our homage to this wonderful phenomenon:

Your candle burned out long before

Your legend ever did

Elton John

By Arindam Bhattacharjee (IISER Pune)

References:

  1. How to blow up a star https://www.nature.com/articles/d41586-018-04601-7
  2. Supernovae by H. Bethe https://physicstoday.scitation.org/doi/pdf/10.1063/1.881256
  3. Standard candle in astronomy https://universe-review.ca/R02-07-candle.htm.
  4.  Your Bones Are Made Out of Exploded Stars, Scientists Say            https://futurism.com/bones-exploded-stars
  5. https://www.nasa.gov/mission_pages/chandra/news/kepler_remnant.html

Images:

Cover: Image of a recently discovered supernova SN2020jfo superimposed with Monroe to set the theme of the blog.

Figure 1: from NASA.

Figure 2: by Callum Potter for BBC’s Sky at Night magazine.


Author’s Bio: I’m an Int. PhD student from IISER Pune whose interest lies in anything remotely connected to Theoretical Physics especially Gravity. When I’m not doing physics I have various indulgences ranging from playing football, reading poetry and cooking what I believe to be food. I believe there’s nothing better than a wonderful debate over tea and most of the time I overdo both of these. I’d love to hear comments/criticism about this piece from the readers (Twitter: @quarantinedmind).

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