Things That Go Bang in the Night

White dwarfs are hot glowing remnants of stars that were at least somewhat like the Sun. Over billions of years of time isolated white dwarfs gradually cool down (possibly crystalizing into Earth-sized diamonds!) and fade away. But white dwarfs that are located in binary star systems can have a much more spectacular time than just gradually dimming from view. 

Artist's illustration of a white dwarf (right) pulling gas from its companion star. Credit: NASA/CXC/M.Weiss

 

In some of these binary systems hydrogen gas is pulled off of the white dwarf's companion star which piles up on the surface of the white dwarf. Eventually there's enough gas there that it detonates producing a nuclear explosion on the surface of the white dwarf. This is what astronomers call a nova. 

The nova explosion does not destroy either star and can eventually repeat. There are 10 of these recurring novae that have been observed to repeat in our Milky Way galaxy. 

One of these recurring novae is known as U Scorpii (U Sco for short). Normally U Sco is too faint to observe with my Unistellar eVscope, but it is an easy target when a nova outburst occurs. On June 6th I got word that U Sco had an eruption underway so I decided to take a look. 

U Scorpii (bright star at center)

One characteristic of this nova is that it fades in brightness very rapidly. To record its fade I have been looking at it periodically since June 6th. Sure enough, it will soon be back to its normal too-faint-for-me-to-see status. The image below shows U Sco dropping in brightness by a factor of 100 in just 10 days. 

The nova is still dropping in brightness and will soon fade from my view entirely. 

A second type of explosion involving white dwarfs is known as a Type Ia supernova. This is a much bigger deal. Again we are talking about white dwarfs in binary star systems, only instead of an explosion on the surface of the star, the entire thing blows apart, most likely from the merger of two white dwarfs (although, maybe not - astronomers are working on that). 

These explosions are big enough for my little telescope to be able to see them from distances of tens of  millions of light years (whereas U Sco is just ~60,000 light years away). 

Last year I was able to observe one of these explosions, supernova 2021hiz, in an obscure galaxy known as IC 3322A. I followed up my observation a year later to produce this comparison showing the galaxy with and without the supernova:

The galaxy is just a faint smudge of a line (it's a spiral galaxy seen from the side) and the supernova is the bright dot on the upper edge that blinks on and off. Notice that the light of the supernova is actually brighter than the rest of the entire galaxy. 

Think about that. One supernova explosion can outshine all the light produced by hundreds of millions of stars. That's amazing.

Another bright Type Ia supernova (sn 2022hrs) took place this spring in a galaxy known as NGC 4647. NGC 4647 is conveniently in the sky next to one of the galaxies in the Messier Catalog, M60. Last year I photographed all of the objects in the Messier Catalog, which means that I had a before image to go with the new image of the supernova. 

Here's the before and during comparison:

This supernova exploded some 63 million years ago and light from it arrived on Earth in mid April 2022. I am happy to have had the opportunity to catch it. 

By the way, the last supernova observed in our Milky Way galaxy was also a Type Ia supernova. It was recorded by Johannes Kepler back in the year 1604.