Reading Time: mins
In the blog “Are all Supernovae the same?” We discovered that all Supernovae (SN) are explosions of stars. However, there are two main types; Core Collapse SN which are when large stars explode at the end of their lives, and Type Ia SN which are when an old white dwarf star becomes too large by steeling material from a companion and explodes.
But how do we actually discover new Supernovae explosions? And is this something new or have astronomers been discovering them for hundreds of years?
Have you ever looked up at the sky in wonder? Was one of your questions does it change? If we think about times scales, we are around for ~100 years (most people might think it’s a little less but astronomers deal in such big numbers we often say it’s the same order of magnitude so that’s near enough for us)! Compare this to 13.8 billion years ago when the Universe formed – too long to even understand, then our Galaxy (the Milky Way) formed within a billion years of this and our own Solar System about 4.6 billion years ago. So the answer for most of the Universe is, no there’s not really a change in our life times – unless we look at SN and suddenly, the sky is flickering. Looking at the first animation here of the night sky from 1985 to 2013, each point on here appearing is a SN we’ve detected exploding and at the top you can see the timeline. On average we think each galaxy in the universe experiences a SN explosion every 500 years. And there are millions to billions of galaxies. Thus, when we start looking with dedicated telescopes, we see hundreds of them.
The stripe of lots of dots (SN) appearing after about one minute is called Stripe 82 – this is part of the Sloan Digital Sky Survey (SDSS). This part of the sky survey was scanning this stripe of the sky every few days looking for SN and this is dataset formed basis for my PhD analysis.
People have been asking questions for thousands of years about what’s new with our skies – ancient Chinese astronomers noticed a bright ‘guest’ star in Taurus the bull in our own galaxy on the 4th July 1054. Although the historical records are not precise, the bright new star likely outshone Venus, and for a while was the third-brightest object in the sky, after the Sun and Moon. It shone in the daylight sky for several weeks, and was visible at night for nearly two years before fading from view. Sadly, all the SN we discover nowadays need telescopes to view them as they are more distant in other galaxies.
It is likely that sky watchers of the Anasazi People in the American Southwest also viewed the bright new star in 1054. Historic research shows that a crescent moon was visible in the sky very near the new star on the morning of July 5, the day following the observations by the Chinese.
From June or July 1056, the object was not seen again until 1731, when an observation of the now quite faint nebulosity was recorded by an English amateur astronomer John Bevis. However, the object was rediscovered by French comet-hunter Charles Messier in 1758, and it soon became the first object in his catalogue of objects not to be confused with comets, now known as the Messier Catalogue, so the object is often known as M1. In 1844, astronomer William Parsons, better known as the Third Earl of Rosse, observed M1 through his large telescope in Ireland. He described it as having a shape resembling a crab, and since then M1 has been more commonly called the Crab Nebula. However, it was not until the 20th century that the association with Chinese records of the 1054 “guest” star was discovered.
In reality, it’s a vast, outwardly rushing cloud of gas and debris: the scattered fragments of a supernova, or exploding star. The estimated distance to what’s left of this star – the Crab Nebula – is about 6,500 light years. So the progenitor star must have blown up some 7,500 years ago. The glowing relic has been expanding since the star exploded, and it is now approximately 11 light-years in width.
When Tycho Brahe was on his way home on November 11, 1572, his attention was attracted by a star in Cassiopeia which was shining at about the brightness of Jupiter and which had not been seen in this place before. Other European observers claimed to have noticed it as early as the preceding August, but Tycho’s precise measurements showed that it was not some relatively nearby phenomenon, such as a comet, but at the distance of the stars, and that therefore real changes could occur among them. Over the next two years, the supernova dimmed until it could no longer be seen with the naked eye.
It wasn’t until the 1950s that the remnants of the supernova could be seen again with the help of telescopes. This image from NASA’s Wide-field Infrared Survey Explorer (WISE). The red circle visible in the upper left part of the image is SN 1572 remnant, often called “Tycho’s Supernova”.
When the star exploded, it sent out a blast wave into the surrounding material, scooping up interstellar dust and gas as it went, like a snowplough. An expanding shock wave travelled into the surroundings and a reverse shock was driven back in toward the remnants of the star. Previous observations by NASA’s Spitzer Space Telescope indicate that the nature of the light that WISE sees from the supernova remnant is emission from dust heated by the shock wave.
I just want us to look a little deeper into Tycho Brahe himself, he made the most accurate celestial observations of his time and challenged the prevailing belief in how the universe was organized. While most people may think of scientists as stodgy academic types, Brahe’s flamboyant lifestyle and untimely death would have made some of today’s wild celebrities look like choirboys. He was a wealthy member of the nobility. In 1566, 20-year-old Brahe fought a fellow student in a duel over who was the better mathematician. As a result, he lost a large chunk of his nose. For the rest of his life, he donned a metal prosthetic nose to cover the disfigurement!
King Fredrick II of Denmark was so impressed by Tycho Brahe that he gifted him a small island named Hven and a large amount of money to set up a personal observatory there! I wish this still happened for astronomers today!! Tycho made the people of the island build his a grandiose casted called Uraniborg in honour of Urania, the muse of astronomy. In the castle Brahe hosted huge lavish parties. As well as, collecting various animals including elk and moose who reportedly used to drink alcohol with Brache and the guests. He died in 1602 at the age of 54, and the rumour was that he died of an exploding bladder! The custom of the time wouldn’t allow people to get up from the banquet to go to the toilet, he drank a huge amount and collapsed, dying 11 days later in much pain due to a burst bladder. However, in 1901 scientists opened his grave and examined the body. They claimed to find mercury, which started new rumours that he was instead poisoned. People at the time believed another astronomer Johannes Kepler was responsible. However, in 2010 the body was again examined and the levels of mercury was thought to be too low to have caused his death.
So let’s get back to talking about Supernovae!
We observe the same patch of sky many times – and look for a change – a sudden new ‘star’ which wasn’t there the last time we looked. So you can have a go – here’s the constellation of Orion the hunter. Can you spot the new ‘star’ or fake supernovae we have added here for you to find?
Now the next images show what it’s really like when we search for SN nowadays, as they are nearly all in other galaxies. A galaxy with a new point of light which shines as bright as all the billions of stars in the galaxy.
Therefore, to find SN we need to search huge areas of the sky comparing new images to old ones to look for new points of light which appear.
Large survey telescopes such as the Sloan Digital Sky Survey and the Dark Energy Survey (along with many others) have repeatedly scanned the same regions of the sky every few days to discover hundreds of new points of light. We compared the old and new images by eye a minute ago, but with these volumes of SN to discover astronomers use image subtraction, where the old image is taken away from the new image to see if there is a new point of light to be discovered. Once the software tells us there is a new point of light, we can look at it for a few days or weeks to discover if it is a SN by building up a light curve. The light curve is the brightness of the SN over time. It shows how the SN gets brighter quickly over a few days and then fades gradually over a few months. Different types of Supernovae explode differently so their LC looks different.
With these large survey telescopes it’s quite easy to discover new SN. However, it takes a long time to identify which sort of explosion they are. We have to use larger telescopes and take a spectrum – where you break the light up into its constituent parts so you can see what elements are inside. We are looking for silicon. This is a sign that it was an old dense star that exploded – so a Type Ia. Each spectrum takes approximately an hour on a large telescope (>4m diameter mirror), and as we look for SN further away in the Universe we will need to use even large telescopes for longer.
These are a few photos from when I’ve been to telescopes in Chile in 2015, while I was a Post Doctoral researcher at Cambridge University, to take spectra of new SN and identify if they are Type Ia or core collapse SN. This is the NTT (new technology telescope) which was built in the 80s so not so new now. It has panels on the back to adjust the mirror to keep it in really accurate focus – it was the test for the Very Large Telescope in Chile some of you might have heard of.
Well the future is here! We are now finding 1000s of SN – but we can’t separate them into subclasses the way we have been doing in the past – using spectra. There’s not enough telescope time; it would take thousands of hours on the world’s largest telescopes, and apparently not all astronomers only what to look at supernovae! So we are finding too many SN – we have to come up with a new way of classifying them which doesn’t need so much telescope time. And we found it – to use the light curve itself. As we just saw Type Ia and Type II SN have different shape light curves, so we can use the light curve to determine which SN we are looking at. It is not quite as accurate as using the spectra of the SN, however if we have large samples then a small amount of contamination from the wrong classification of SN is acceptable.
This is what I spent my PhD doing – working on methods to use SN to classify Type Ia SN and assessing the amount of contamination from the other types of SN that would not mess up the measurements of dark energy.
Astronomers have been searching for SN for hundreds of years. We look for new points of light in the sky which weren’t there before. Nowadays large survey telescopes carry out image subtraction to discover new SN. We then look at the light curve to see if it is a SN. Then, take a spectrum to discover which type of SN. However, we are entering the era when there are too many SNs to take spectra of them all and are now using the light curve itself to classify the different types of SN.
This post was written by Dr Heather Campbell for Mission Astro.
If you have any questions about this post or our services, please email us on [email protected].
Take me to the FREE taster session for Mission Astro!
Copyright © 2025 Just Good Science Ltd
Website and course built and managed by Web X Design Studio
