Stunning stellar explosions occur every second across the cosmos – some observable, most too distant to ever know, all of them fleeting, lasting from seconds to tens of thousands of years. In the age of the universe, these events are brief flashes in the passing of time; for stars it is them gasping for life or their explosive end; in galaxies, local regions can be altered as a consequence. From the perspective of humankind, these transient events contain many undiscovered truths of our universe itself; from its past to present, from all of existence even yet unknown. These events are also known as transient events, affectionately nicknamed “cosmic cataclysms“.
 

What is the difference between Novae and Supernovae?

Among the most famous and spectacular transient events are novae and supernovae. Similar in name, supernovae announce the death and destruction of the most massive stars, while novae are brief periods of life on the surface of already dead sun-like stars.
 

Novae

Latin for new star – a nova occurs when there is a sudden increase in a star’s luminosity. Novae take place when a white dwarf, the remnant of a once sun-like star, reignites nuclear fusion in its atmosphere, causing a visible increase in the star’s apparent brightness.

White dwarfs have already finished fusing hydrogen into helium, but are not massive enough to fuse helium into heavier elements. The inability to continue the fusion process causes the star to lose its outer layers and condense what remains in its core to the size of Earth, with its mass only held together only by the space between electrons.

Dense, dormant, and doomed to quietly fade, white dwarfs have a second chance at life if they orbit another star. These stars have massive gravity, allowing them to attract mass from their partner star, creating a symbiotic binary system. Stolen hydrogen collects in the atmosphere of the white dwarf until temperatures rise to the point nuclear fusion begins anew on the surface of the star. The energy released from this process emits light across the cosmos, causing the intense increase in brightness we know as a nova.

Novae can remain visible for days up to months, and fade after the collected mass is exhausted. All of the newly generated energy escapes into space, this interaction leaving no lasting impact on either star. Given enough time, there is the possibility for this process to begin again, and very few white dwarfs have been observed to go nova repeatedly at predictable intervals; a recurrent novae. One such example is T CrB, which flickers approximately every 80 years and has been the subject of a recent Unistellar Network campaign.

Novae may be mostly harmless, but some white dwarfs meet a different fate after stealing too much mass in another kind of cosmic transient event: a supernova.
 

Supernovae

Supernovae are massive stellar explosions announcing the end of a star’s life. All natural heavy elements in the universe, including precious metals, are created in the aftermath of these cosmic cataclysms. There are two main classifications of supernovae: Type Ia, and Core Collapse Supernovae.
 

Type Ia

Type Ia supernovae occur when the reignition of a white dwarf’s nuclear fusion destroys the star in its entirety, leaving nothing behind in the aftermath except a cloud of stellar dust to slowly disperse into the cosmos.

In symbiotic binaries,  the white dwarf can accumulate too much mass to remain a stable star. Upon exceeding 1.44 solar masses, the white dwarf befalls a different fate, where increased core temperatures trigger a rapid, uncontrollable sequence of internal nuclear fusion. This is a thermonuclear runaway effect, which generates massive amounts of energy inside the white dwarf. As the dormant stellar mass cannot handle the increasing temperature and pressure, it is torn apart in the blink of an eye, obliterated in a type Ia supernova.

The explosion lasts seconds, and brightness can increase up to hours, and an afterglow can linger for months. As the energy from this event dissipates, a diffuse nebula is left wake of this star’s destruction; a sparse cloud of potential star-forming matter drifting apart in the stellar wind.

As the limit for a white dwarf’s mass before succumbing to supernovae is universal, Type Ia supernovae are uniform in luminosity. Knowing the inherent brightness of these events and comparing it to our perceived brightness allows us to calculate the distance to the supernova. These standardized luminous events help us map our universe and are sometimes called standard candles.
 

Type II (Core Collapse)

The glorious deaths of the most massive of stars, Type II Supernovae – or Core Collapse Supernovae – occur when a star’s core mass loses against its own gravity and collapses inwards on itself, violently expelling its outer layers in a magnificent display.

The most massive stars in the universe boast the greatest size and luminosity, but are also burdened with the shortest lifespan; the largest stars have lifespans 3,000 times shorter than our Sun. These stars fuse through their elements quickly to fight against their own gravity, until an iron core is left and a supernova becomes imminent. Energy cannot be generated from iron fusion, and in the absence of generating energy, the star cannot support its own mass against its gravity. The star’s core collapses inwards on itself within a fraction of a second, resulting in a core collapse supernova.

The outer layers of the supergiant star are blown away, dispersing into a cloud of stellar matter shrouding the implosion. However brief the supernova, residual energy from the event leaves an afterglow lasting months, and the dispersing supernova remnant can linger for hundreds of thousands of years. Because the progenitor of a Type II supernova is an actively fusing star (as opposed to a dead stellar core), astronomers detect hydrogen in the spectra of these explosions — those outer layers sent into space will contain hydrogen. In contrast, a Type Ia supernova contains no hydrogen, as the culprit white dwarf would have already shed its hydrogen layers eons beforehand.

The resulting body of a core-collapse is determined by the mass of the stellar core at the moment of supernova. Massive cores exceeding the mass of 3 times of our sun collapse into black holes, while cores between 1.4 to 3 solar masses become young neutron stars – the densest known stars in the universe, held together by the space between neutrons, containing the mass of multiple suns within the space of a city.

Type II supernovae herald the formation of the densest objects in the known universe; a byproduct of these powerful cataclysmic events are Gamma Ray Bursts.
 

Gamma Ray Bursts

Gamma Ray Bursts (GRBs) are the most powerful and luminous events since the Big Bang; short, powerful bursts of the highest energy radiation resulting from supernovae, the merging of neutron stars, or the birth of black holes. The burst itself is a narrow beam of electromagnetic radiation travelling at light speed, as luminous as a few trillion suns, and powerful enough to vaporize anything within hundreds of light years.

These powerful events are brief, most lasting only seconds. The duration of these events can determine the cause; bursts longer than 2 seconds are the byproduct of Type II supernovae, shorter bursts are the result of the merging of neutron stars. However, there are still some mysteries; for instance, scientists are still debating the cause of the longest observed GRB in history, a burst in 2021 lasting almost a minute. Regardless of the burst duration,  a GRB can leave in its wake a temporary optical afterglow.These lingering signatures are sometimes bright enough to be observed with a Unistellar telescope, allowing scientists to measure or constrain the glow’s brightness.

Although common across the universe, with up to one detected per day, GRBs no longer occur in our galaxy. Instead, they are more often found in intense star forming regions, where excited matter is in abundance, forming clusters of massive stars to live out short, spectacular lives.
 

Join the Night Watchers

Across the universe, stars will always inevitably lose their fight to time. Some meet devastating ends, countless others quietly fade, and few get a chance to be reborn, however momentarily on a cosmic scale. From a quiet neighborhood of a peaceful galaxy, Unistellar and citizen astronomers can study these transient events, allowing us a small glimpse of far too distant truths with every detection. From novae, to supernovae, to Gamma Ray Bursts, these fleeting events may contain the story of the cosmos’ past and hints at the future of the universe; it is up to vigilant, enthusiastic scientists and astronomers around to capture such timely celestial events and bring to earth this cosmic data. Last year, over 200 citizen astronomers made contributions to our Cosmic Cataclysms program through observations with their Unistellar telescopes, and anyone can also make their own discoveries by joining our Unistellar network!
 

To become a Night Watcher, all you need is your Unistellar telescope and excitement about these explosive events. You learn how to observe with our Cosmic Cataclysms program here, and check out the newest targets here. 

This article was originally published by Bonnie Chi at science.unistellar.com.