Type 1a Supernovae - Standard Candle of the Universe?

**Understanding Supernovae Explosions**

How Does Supernovae Explosions Help in Measuring Galactic Distances?

Image by WikiImages - Pixabay

Did you know that astrophysicists often spend more time figuring out a way to measure the distances of distant galaxies and star clusters than they do to actually study them? 

Turns out one of the most intriguing yet tricky processes in the field of astrophysics is to measure different parameters of a distant object - parameters like distance, size, luminosity etc. But that's obvious since there are a few factors that scientists have to take into consideration while finding out different properties of an object or phenomenon - the earth's rotation, parallax, and interstellar dust, just to name a few. Even then, figuring out the distance of an object that is over a hundred thousand light years away still seems complex enough, especially considering the effect of redshift arising due to the expansion of the universe! 

But what if we have a celestial object or a phenomenon that can indicate the distance across millions of light years? For sure, such an object had to be bright enough to be observed by our telescopes here on earth, but what if such a standard candle exists in each and every galaxy in the universe, say...a supernova explosion of a white dwarf?

White Dwarf - Image by WikiImages - Pixabay

What are Type 1a Supernovae bursts?

Supernovae explosions are thought to be one of the brightest phenomena in the universe. They are so bright that they outshine the entire galaxy in which they reside! Supernovae explosions occur when a main sequence star uses up all of its hydrogen and bursts into a gigantic explosion, which is when a star officially reaches the end of its cycle. 

Based on the peak luminosities they give off while the outburst, supernovae can be broadly classified as Type 1 and Type 2 supernovae. Within the Type 1 classification, Type 1a supernovae are considered the brightest among all types of supernovae, with an absolute magnitude of about -19.5 (the absolute magnitude of the sun is 26.74).

A Type 1a supernova occurs when a white dwarf undergoes supernova in a binary star system. White dwarfs are planet-sized dense stars that exhausted their nuclear fuel and got ripped off their outer layers. In most cases, a binary star system consists of a white dwarf and a red giant star that is held together by each other's gravity, causing them to orbit each other. 

However, since the white dwarf is much denser than the red giant, it tends to pull matter from the red giant star, and over time, the white dwarf gains matter and mass. Once the white dwarf reaches a limit of about 1.44 solar masses (also known as the Chandrashekhar Limit), the white dwarf can no longer withstand its high internal pressure and undergoes a tremendously violent supernova explosion, which we call a Type 1a supernova explosion. 

Supernovae of this kind are pretty much happening in all galaxies that we observe, and studies on Type 1a supernovae explosions have shown that occur in all types of galaxies, regardless of their type and region, which makes it an ideal candidate for being a universal standard candle for distance.

Type 1a Supernovae as the Standard Distance Candle

So, what's so special about these Type 1a supernova explosions? In other words, what is the one feature that makes Type 1a supernovae stand out from the rest of the supernovae types? One word - their unique light curve! 

If you were to plot the luminosity (apparent) over time of a Type 1a supernova, you would end up getting a curve with a sharp peak, which then dips further in time. This sharp peak of its light curve is a characteristic feature of all Type 1a supernovae explosions, which is the main reason why it's considered a standard candle for distance in astronomy. Since all white dwarves undergo supernova at 1.44 solar masses, the light curve produced by them will also be comparable. 

Light Curve of a Type 1a Supernova

With that being said, if you were to find a bright object in a galaxy thousands of light years away, you could simply record the light curve of that object and check if it turns out to be the light curve of a Type 1a supernova. If so, you could easily estimate the distance using the apparent magnitude (observed) and the absolute magnitude (known value of -19.5) of the supernova. 

This uniqueness in the light curve of Type 1a supernovae has remarkable implications in cosmology, specifically in indicating the rate of expansion of the universe. By analyzing the amount of redshift undergone by light from such standard candles, we can measure the value of the Hubble Constant (Ho) which expresses the acceleration of the expanding universe. 

White Dwarf - Image by WikiImages - Pixabay

Apart from Type 1a supernovae explosions, there is one more standard candle that we can use to measure distances, namely, the Cepheid Variables. Cepheid Variables are basically luminous pulsating stars with periodic brightening and dimming of their luminosity, which will be discussed in detail in later posts. But for now, let's just end here with those majestic supernovae explosions, one of which could be happening somewhere in the universe right now as you finish reading this sentence!