A nova star is like a vampire that siphons gas from its binary partner. As it does so, the gas is compressed and heated, and eventually it explodes. The remnant gas shell from that explosion expands outward and is lit up by the stars at the center of it all. Most of these novae explode about once every 10 years.
But now astrophysicists have discovered one remnant so large that the star that created it must have been erupting yearly for millions of years.
Apparently not all supernovas work. And when they fail, they leave behind a half-chewed remnant, still burning from leftover heat but otherwise lifeless: a zombie star. Astronomers aren’t sure how many of these should-be-dead creatures lurk in the interstellar depths, but with recent simulations scientists are making a list of their telltale signatures so that future surveys can potentially track them down.
The SPHERE planet-hunting instrument on the European Southern Observatory’s Very Large Telescope captured this image of a white dwarf feeding on its companion star, a type of Red Giant called a Mira variable. Most stars exist in binary systems, and they spend an eternity serenely orbiting their common center of gravity. But something almost sinister is going on between these two.
Astronomers at the ESO have been observing the pair for years and have uncovered what they call a “peculiar story.” The Red Giant is a Mira variable, meaning it’s near the end of its life, and it’s pulsing up to 1,000 times as bright as our Sun. Each time it pulses, its gaseous envelope expands, and the smaller White Dwarf strips material from the Red Giant.
There’s something poignant and haunting about ancient astronomers documenting things in the sky whose nature they could only guess at. It’s true in the case of Père Dom Anthelme, who in 1670 saw a star suddenly burst into view near the head of the constellation Cygnus, the Swan. The object was visible with the naked eye for two years, as it flared in the sky repeatedly. Then it went dark. We call that object CK Vulpeculae.
For the sake of studying the most distant objects in the Universe, astronomers often rely on a technique known as Gravitational Lensing. Based on the principles of Einstein’s Theory of General Relativity, this technique involves relying on a large distribution of matter (such as a galaxy cluster or star) to magnify the light coming from a distant object, thereby making it appear brighter and larger.
The study which details their findings, titled “Predicting gravitational lensing by stellar remnants” appeared in the Monthly Noticed of the Royal Astronomical Society. The study was led by Alexander J. Harding of the CfA and included Rosanne Di Stefano, and Claire Baker (also from the CfA), as well as members from the University of Southampton, Georgia State University, the University of Nigeria, and Cornell University.
To put it simply, determining the mass of an astronomical object is one the greatest challenges for astronomers. Until now, the most successful method relied on binary systems because the orbital parameters of these systems depend on the masses of the two objects. Unfortunately, objects that are at the end states of stellar evolution – like black holes, neutron stars or white dwarfs – are often too faint or isolated to be detectable.
This is unfortunate, since these objects are responsible for a lot of dramatic astronomical events. These include the accretion of material, the emission of energetic radiation, gravitational waves, gamma-ray bursts, or supernovae. Many of these events are still a mystery to astronomers or the study of them is still in its infancy – i.e. gravitational waves. As they state in their study:
“Gravitational lensing provides an alternative approach to mass measurement. It has the advantage of only relying on the light from a background source, and can therefore be employed even for dark lenses. In fact, since light from the lens can interfere with the detection of lensing effects, compact objects are ideal lenses.”
As they go on to state, of the 18,000 lensing events that have been detected to date, roughly 10 to 15% are believed to have been caused by compact objects. However, scientists are unable to tell which of the detected events were due to compact lenses. For the sake of their study then, the team sought to circumvent this problem by identifying local compact objects and predicting when they might produce a lensing event so they could be studied.
“By focusing on pre-selected compact objects in the near vicinity of the Sun, we ensure that the lensing event will be caused by a white dwarf, neutron star, or black hole,” they state. “Furthermore, the distance and proper motion of the lens can be accurately measured prior to the event, or else afterwards. Armed with this information, the lensing light curve allows one to accurately measure the mass of the lens.”
In the end, the team determined that lensing events could be predicted from thousands of local objects. These include 250 neutron stars, 5 black holes, and roughly 35,000 white dwarfs. Neutron stars and black holes present a challenge since the known populations are too small and their proper motions and/or distances are not generally known.
But in the case of white dwarfs, the authors anticipate that they will provide for many lensing opportunities in the future. Based on the general motions of the white dwarfs across the sky, they obtained a statistical estimate that about 30-50 lensing events will take place per decade that could be spotted by the Hubble Space Telescope, the ESA’s Gaia mission, or NASA’s James Webb Space Telescope (JWST). As they state in their conclusions:
“We find that the detection of lensing events due to white dwarfs can certainly be observed during the next decade by both Gaia and HST. Photometric events will occur, but to detect them will require observations of the positions of hundreds to thousands of far-flung white dwarfs. As we learn the positions, distances to, and proper motions of larger numbers of white dwarfs through the completion of surveys such as Gaia and through ongoing and new wide-field surveys, the situation will continue to improve.”
The future of astronomy does indeed seem bright. Between improvements in technology, methodology, and the deployment of next-generation space and ground-based telescopes, there is no shortage of opportunities to see and learn more.
Eclipsing binary star systems are relatively common in our Universe. To the casual observer, these systems look like a single star, but are actually composed of two stars orbiting closely together. The study of these systems offers astronomers an opportunity to directly measure the fundamental properties (i.e. the masses and radii) of these systems respective stellar components.
Recently, a team of Brazilian astronomers observed a rare sight in the Milky Way – an eclipsing binary composed of a white dwarf and a low-mass brown dwarf. Even more unusual was the fact that the white dwarf’s life cycle appeared to have been prematurely cut short by its brown dwarf companion, which caused its early death by slowly siphoning off material and “starving” it to death.
For the sake of their study, the team conducted observations of a binary star system between 2005 and 2013 using the Pico dos Dias Observatory in Brazil. This data was then combined with information from the William Herschel Telescope, which is located in the Observatorio del Roque de los Muchachos on the island of La Palma. This system, known as of HS 2231+2441, consists of a white dwarf star and a brown dwarf companion.
White dwarfs, which are the final stage of intermediate or low-mass stars, are essentially what is left after a star has exhausted its hydrogen and helium fuel and blown off its outer layers. A brown dwarf, on the other hand, is a substellar object that has a mass which places it between that of a star and a planet. Finding a binary system consisting of both objects together in the same system is something astronomers don’t see everyday.
As Leonardo Andrade de Almeida explained in a FAPESP press release, “This type of low-mass binary is relatively rare. Only a few dozen have been observed to date.”
This particular binary pair consists of a white dwarf that is between twenty to thirty percent the Sun’s mass – 28,500 K (28,227 °C; 50,840 °F) – while the brown dwarf is roughly 34-36 times that of Jupiter. This makes HS 2231+2441 the least massive eclipsing binary system studied to date.
In the past, the primary (the white dwarf) was a normal star that evolved faster than its companion since it was more massive. Once it exhausted its hydrogen fuel, its formed a helium-burning core. At this point, the star was on its way to becoming a red giant, which is what happens when Sun-like stars exit their main sequence phase. This would have been characterized by a massive expansion, with its diameter exceeding 150 million km (93.2 million mi).
At this point, Almeida and his colleagues concluded that it began interacting gravitationally with its secondary (the brown dwarf). Meanwhile, the brown dwarf began to be attracted and engulfed by the primary’s atmosphere (i.e. its envelop), which caused it it lose orbital angular momentum. Eventually, the powerful force of attraction exceeded the gravitational force keeping the envelop anchored to its star.
Once this happened, the primary star’s outer layers began to be stripped away, exposing its helium core and sending massive amounts of matter to the brown dwarf. Because of this loss of mass, the remnant effectively died, becoming a white dwarf. The brown dwarf then began orbiting its white dwarf primary with a short orbital period of just three hours. As Almeida explained:
“This transfer of mass from the more massive star, the primary object, to its companion, which is the secondary object, was extremely violent and unstable, and it lasted a short time… The secondary object, which is now a brown dwarf, must also have acquired some matter when it shared its envelope with the primary object, but not enough to become a new star.”
This situation is similar to what astronomers noticed this past summer while studying the binary star system known as WD 1202-024. Here too, a brown dwarf companion was discovered orbiting a white dwarf primary. What’s more, the team responsible for the discovery indicated that the brown dwarf was likely pulled closer to the white dwarf once it entered its Red Giant Branch (RGB) phase.
At this point, the brown dwarf stripped the primary of its atmosphere, exposing the white dwarf remnant core. Similarly, the interaction of the primary with a brown dwarf companion caused premature stellar death. The fact that two such discoveries have happened within a short period of time is quite fortuitous. Considering the age of the Universe (which is roughly 13.8 billion years old), dead objects can only be formed in binary systems.
In the Milky Way alone, about 50% of low-mass stars exist as part of a binary system while high mass stars exist almost exclusively in binary pairs. In these cases, roughly three-quarters will interact in some way with a companion – exchanging mass, accelerating their rotations, and eventually en merging.
As Almeida indicated, the study of this binary system and those like it could seriously help astronomers understand how hot, compact objects like white dwarfs are formed. “Binary systems offer a direct way of measuring the main parameter of a star, which is its mass,” he said. “That’s why binary systems are crucial to our understanding of the life cycle of stars.”
It has only been in recent years that low-mass white dwarf stars were discovered. Finding binary systems where they coexist with brown dwarfs – essentially, failed stars – is another rarity. But with every new discovery, the opportunities to study the range of possibilities in our Universe increases.
Death is simply a part of life, and this is no less the case where stars and other astronomical objects are concerned. Sure, the timelines are much, much greater where these are concerned, but the basic rule is the same. Much like all living organism, stars eventually reach old age and become white dwarfs. And some are not even fortunate enough to be born, instead becoming a class of failed stars known as brown dwarfs.
Despite being familiar with these objects, astronomers were certainly not expecting to find examples of both in a single star system! And yet, according to a new study, that is precisely what an international team of astronomers discovered when looked at WD 1202-024. Using data from the Kepler space telescope, they spotted a binary system consisting of a failed star (a brown dwarf) and the remnant of a star (a white dwarf).
Originally, the white dwarf was identified by the Sloan Digital Sky Survey (SDSS) – designated as WD1202-024 – and was thought to be a solitary star. However, while examining the light-curves of stars that had been surveyed by the K2 mission, Dr. Saul Rappaport (M.I.T.) and Andrew Vanderburg (of the CfA) – the lead author and a co-authors on the study, respectively – noted a curious drop in its brightness.
Whereas the transits of exoplanets are known to cause small dips in brightness, the light curve in this case showed particularly deep and broad eclipses. In addition, between these eclipses, there were changes in brightness which appeared to be due to the cool component (i.e. the brown dwarf) being illuminated by the much hotter white dwarf. This too was unexpected, as it indicated that the transiting object was rather large.
To get to the bottom of this, the team devised a model based on data obtained from K2 mission, the SDSS survey and the Magellan 6.5-m telescope. They also used data from five different ground-based telescopes on three continents, which included amateur-operated 36-cm and 80-cm telescopes in Arizona, the 1-m telescope at the South African Astronomical Observatory, and the 1.6-m telescope at Mont-Megantic Observatory (‘OMM’) in Quebec.
From this combined data, they were able to deduce that their observations were consistent with a hot white dwarf of 0.4 Solar masses being eclipsed by brown dwarf companion of 0.067 Solar masses. They also determined that these two objects, which are seen nearly edge-on, orbited each other with a period of just 71 minutes and 12 seconds – which works out to a speed of about 100 km/s.
But as Lorne Nelson – a professor at Bishop’s University and one of the co-authors on the paper – explained, the team also wanted to address how this system came to be. “We had constructed a robust model but we still had to address the ‘big-picture’ issues such as how this system formed and what would be its ultimate fate,” he said.
To do this, they used sophisticated computer models to simulate the formation and evolution of WD1202. According to their scenario, the primordial system consisted of a 1.25 Solar mass star and a brown dwarf that were in a 150 day orbit with each other. As the star aged, it began to expand, becoming a red giant that eventually pulled its brown dwarf companion into a much tighter orbit.
They also constructed a 3-D animation to illustrate the effect this had. As Nelson described it:
“It is similar to an egg-beater effect. The brown dwarf spirals in towards the center of the red giant and causes most of the mass of the red giant to be lifted off of the core and to be expelled. The result is a brown dwarf in an extraordinarily tight, short-period orbit with the hot helium core of the giant. That core then cools and becomes the white dwarf that we observe today.”
In addition, their calculations showed that the primordial binary must have formed about 3 billion years ago, and that in less than 250 million years, the white dwarf will begin cannibalizing the brown dwarf. At this point, the brown dwarf is likely to be pulled apart and form a circumstellar disk around the white dwarf, which it will slowly accrete material from.
When this happens, the binary will begin showing the signs of a cataclysmic variable (CV), which include a flickering lightcurve. And in the end, it is likely that the entire system will go out in a fiery cataclysm – aka. a type 1a supernova. It should also be noted that this 250 year period is the shortest pre-cataclysmic variable of any binary system ever discovered, making this find even more of a rarity.
So perhaps the find was not so sad after all. Yes, a failed star is orbiting a star in its death throes, but its important to remember they were not always this way. At one time, WD 1202-024 was a vital star that was orbited by a super-heavy gas giant. Only in approaching death did the two become so tight in their orbits, and the perfect picture of failed stardom and near-stellar-death. And in time, they will come together to produce a cataclysmic explosion. I think we can all agree, its best to go out with a bang!
It’s been over a century since Einstein firs proposed his Theory of General Relativity, his groundbreaking proposal for how gravity worked on large scales throughout the cosmos. And yet, after all that time, experiments are still being conducted that show that Einstein’s field equations were right on the money. And in some cases, old experiments are finding new uses, helping astronomers to unlock other astronomical mysteries.
Case in point: using the Hubble Space Telescope, NASA astronomers have repeated a century-old test of General Relativity to determine the mass of a white dwarf star. In the past, this test was used to determine how it deflects light from a background star. In this case, it was used to provide new insights into theories about the structure and composition of the burned-out remnants of a star.
White dwarfs are what become of a star after it has exited the Main Sequence of its lifespan after exhausting their nuclear fuel. This is followed by the star expelling most of its outer material, usually through a massive explosion (aka. a supernova). What is left behind is a small and extreme dense (second only to a neutron star) which exerts an incredible gravitational force.
This attribute is what makes white dwarfs a good means for testing General Relativity. By measuring how much they deflect the light from a background star, astronomers are able to see the effect gravity has on the curvature of spacetime. This is precisely similar to what British astronomer Sir Arthur Eddington did in 1919, when he led an expedition to determine how much the Sun’s gravity deflected the light of a background star during a solar eclipse.
Known as gravitational microlensing, this same experiment was repeated by the NASA team. Using the Hubble Space Telescope, they observed Stein 2051B – a white dwarf located just 17 light-years from Earth – on seven different occasions during a two-year period. During this period, it passed in front of a background star located about 5000 light-years distant, which produced a visible deviation in the path of the star’s light.
The resulting deviation was incredibly small – only 2 milliarseconds from its actual position – and was only discernible thanks to the optical resolution of Hubble’s Wide Field Camera 3 (WFC3). Such a deviation would have been impossible to detect using instruments that predate Hubble. And more importantly, the results were consistent with what Einstein predicted a century ago.
As Kailash Sahu, an astronomer at the Space Telescope Science Institute (STScI) and the lead researcher on the project, explained in a NASA press release, this method is also an effective way to test a star’s mass. “This microlensing method is a very independent and direct way to determine the mass of a star,” he said. “It’s like placing the star on a scale: the deflection is analogous to the movement of the needle on the scale.”
The deflection measurement yielded highly-accurate results concerning the mass of the white dwarf star – roughly 68 percent of the Sun’s mass (aka. 0.68 Solar masses) – which was also consistent with theoretical predictions. This is highly significant, in that it opens the door to a new and interesting method for determining the mass of distant stars that do not have companions.
In the past, astronomers have typically determined the mass of stars by observing binary pairs and calculating their orbital motions. Much in the same way that radial velocity measurements are used by astronomers to determine if a planet has a system of exoplanets, measuring the influence two stars have on each other is used to determine how much mass each possesses.
This was how astronomers determined the mass of the Sirius star system, which is located about 8.6 light years from Earth. This binary star system consists of a white supergiant (Sirius A) and a white dwarf companion (Sirius B) which orbit each other with a radial velocity of 5.5 km/s. These measurements helped astronomers determine that Sirius A has a mass of about 2.02 Solar masses while Sirius B weighs in at 0.978 Solar masses.
And while Stein 2051B has a companion (a bright red dwarf), astronomers cannot accurately measure its mass because the stars are too far apart – at least 8 billion km (5 billion mi). Hence, this method could be used in the future wherever companion stars are unavailable or too distant. The Hubble observations also helped the team to independently verify the theory that a white dwarf’s radius can be determined by its mass.
This theory was first proposed by Subrahmanyan Chandrasekhar in 1935, the Indian-American astronomer whose theoretical work on the evolution of stars (and black holes) earned him the Nobel Prize for Physics in 1983. They could also help astronomers to learn more about the internal composition of white dwarfs. But even with an instrument as sophisticated as the WFC3, obtaining these measurements was not without its share of difficulties.
As Jay Anderson, an astronomer with the STScI who led the analysis to precisely measure the positions of stars in the Hubble images, explained:
“Stein 2051B appears 400 times brighter than the distant background star. So measuring the extremely small deflection is like trying to see a firefly move next to a light bulb. The movement of the insect is very small, and the glow of the light bulb makes it difficult to see the insect moving.”
Dr. Sahu presented his team’s findings yesterday (June 7th) at the American Astronomical Society meeting in Austin, Texas. The team’s result will also appear in the journal Science on June 9th. And in the future, the researchers plan to use Hubble to conduct a similar microlensing study on Proxima Centauri, our solar system’s closest stellar neighbor and home to the closest exoplanet to Earth (Proxima b).
It is important to note that this is by no means the only modern experiment that has validated Einstein’s theories. In recent years, General Relativity has been confirmed through observations of rapidly spinning pulsars, 3D simulations of cosmic evolution, and (most importantly) the discovery of gravitational waves. Even in death, Einstein is still making valued contributions to astrophysics!
When we do finally learn the full truth about our place in the galaxy, and we’re invited to join the Galactic Federation of Planets, I’m sure we’ll always be seen as a quaint backwater world orbiting a boring single star.
The terrifying tentacle monsters from the nightmare tentacle world will gurgle horrifying, but clearly condescending comments about how we’ve only got a single star in the Solar System.
The beings of pure energy will remark how only truly enlightened civilizations can come from systems with at least 6 stars, insulting not only humanity, but also the horrifying tentacle monsters, leading to another galaxy spanning conflict.
Yes, we’ll always be making up for our stellar deficit in the eyes of aliens, or whatever those creepy blobs use for eyes.
What we lack in sophistication, however, we make up in volume. In our Milky Way, fully 2/3rds of star systems only have a single star. The last 1/3rd is made up of multiple star systems.
We’re taking binary stars, triple star systems, even exotic 7 star systems. When you mix and match different types of stars in various Odd Couple stellar apartments, the results get interesting.
Consider our own Solar System, where the Sun and planets formed together out a cloud of gas and dust. Gravity collected material into the center of the Solar System, becoming the Sun, while the rest of the disk spun up faster and faster. Eventually our star ignited its fusion furnace, blasting out the rest of the stellar nebula.
But different stellar nebulae can lead to the formation of multiple stars instead. What you get depends on the mass of the cloud, and how fast it’s rotating.
Check out this amazing photograph of a multiple star system forming right now.
In this image, you can see three stars forming together, two at the center, about 60 astronomical units away from each other (60 times the distance from the Earth to the Sun), and then a third orbiting 183 AU away.
It’s estimated these stars are only 10,000 to 20,000 years old. This is one of the most amazing astronomy pictures I ever seen.
When you have two stars, that’s a binary system. If the stars are similar in mass to each other, then they orbit a common point of mass, known as the barycenter. If the stars are different masses, then it can appear that one star is orbiting the other, like a planet going around a star.
When you look up in the sky, many of the single stars you see are actually binary stars, and can be resolved with a pair of binoculars or a small telescope. For example, in a good telescope, Alpha Centauri can be resolved into two equally bright stars, with the much dimmer Proxima Centauri hanging out nearby.
You have to be careful, though, sometimes stars just happen to be beside each other in the sky, but they’re not actually orbiting one another – this is known as an optical binary. It’s a trap.
Astronomers find that you can then get binary stars with a third companion orbiting around them. As long as the third star is far enough away, the whole system can be stable. This is a triple star system.
You can get two sets of binary stars orbiting each other, for a quadruple star system.
In fact, you can build up these combinations of stars up. For example, the star system Nu Scorpii has 7 stars in a single system. All happily orbiting one another for eons.
If stars remained unchanging forever, then this would be the end of our story. However, as we’ve discussed in other articles, stars change over time, bloating up as red giants, detonating as supernovae and turning into bizarre objects, like white dwarfs, neutron stars and even black holes. And when these occur in multiple star systems, well, watch the sparks fly.
There are a nearly infinite combinations you can have here: main sequence, red giant, white dwarf, neutron star, and even black holes. I don’t have time to go through all the combinations, but here are some highlights.
For starters, binary stars can get so close they actually touch each other. This is known as a contact binary, where the two stars actually share material back and forth. But it gets even stranger.
When a main sequence star like our Sun runs out of hydrogen fuel in its core, it expands as a red giant, before cooling and becoming a white dwarf.
When a red giant is in a binary system, the distance and evolution of its stellar companion makes all the difference.
If the two stars are close enough, the red giant can pass material over to the other star. And if the red giant is large enough, it can actually engulf its companion. Imagine our Sun, orbiting within the atmosphere of a red giant star. Needless to say, that’s not healthy for any planets.
An even stranger contact binary happens when a red giant consumes a binary neutron star. This is known as a Thorne-Zytkow object. The neutron star spirals inward through the atmosphere of the red giant. When it reaches the core, it either becomes a black hole, gobbling up the red giant from within, or an even more massive neutron star. This is exceedingly rare, and only one candidate object has ever been observed.
When a binary pair is a white dwarf, the dead remnant of a star like our Sun, then material can transfer to the surface of the white dwarf, causing novae explosions. And if enough material is transferred, the white dwarf explodes as a Type 1A supernova.
If you’re a star that was unlucky enough to be born beside a very massive star, you can actually kicked off into space when it explodes as a supernova. In fact, there are rogue stars which such a kick, they’re on an escape trajectory from the entire galaxy, never to return.
If you have two neutron stars in a binary pair, they release energy in the form of gravitational waves, which causes them to lose momentum and spiral inward. Eventually they collide, becoming a black hole, and detonating with so much energy we can see the explosions billions of light-years away – a short-period gamma ray burst.
The combinations are endless.
It’s amazing to think what the night sky would look like if we were born into a multiple star system. Sometimes there would be several stars in the sky, other times just one. And rarely, there would be an actual night.
How would life be different in a multiple star system? Let me know your thoughts in the comments.
In our next episode, we try to untangle this bizarre paradox. If the Universe is infinite, how did it start out as a singularity? That doesn’t make any sense.
We glossed over it in this episode, but one of the most interesting effects of multiple star systems are novae, explosions of stolen material on the surface of a white dwarf star. Learn more about it in this video.
There are times when I really wish astronomers could take their advanced modern knowledge of the cosmos and then go back and rewrite all the terminology so that they make more sense. For example, dark matter and dark energy seem like they’re linked, and maybe they are, but really, they’re just mysteries.
Is dark matter actually matter, or just a different way that gravity works over long distances? Is dark energy really energy, or is it part of the expansion of space itself. Black holes are neither black, nor holes, but that doesn’t stop people from imagining them as dark tunnels to another Universe. Or the Big Bang, which makes you think of an explosion.
Another category that could really use a re-organizing is the term nova, and all the related objects that share that term: nova, supernova, hypernova, meganova, ultranova. Okay, I made those last couple up.
I guess if you go back to the basics, a nova is a star that momentarily brightens up. And a supernova is a star that momentarily brightens up… to death. But the underlying scenario is totally different.
As we’ve mentioned in many articles already, a supernova commonly occurs when a massive star runs out of fuel in its core, implodes, and then detonates with an enormous explosion. There’s another kind of supernova, but we’ll get to that later.
A plain old regular nova, on the other hand, happens when a white dwarf – the dead remnant of a Sun-like star – absorbs a little too much material from a binary companion. This borrowed hydrogen undergoes fusion, which causes it to brighten up significantly, pumping up to 100,000 times more energy off into space.
Imagine a situation where you’ve got two main sequence stars like our Sun orbiting one another in a tight binary system. Over the course of billions of years, one of the stars runs out of fuel in its core, expands as a red giant, and then contracts back down into a white dwarf. It’s dead.
Some time later, the second star dies, and it expands as a red giant. So now you’ve got a red dwarf and a white dwarf in this binary system, orbiting around and around each other, and material is streaming off the red giant and onto the smaller white dwarf.
This material piles up on the surface of the white dwarf forming a cosy blanket of stolen hydrogen. When the surface temperature reaches 20 million kelvin, the hydrogen begins to fuse, as if it was the core of a star. Metaphorically speaking, its skin catches fire. No, wait, even better. Its skin catches fire and then blasts off into space.
Over the course of a few months, the star brightens significantly in the sky. Sometimes a star that required a telescope before suddenly becomes visible with the unaided eye. And then it slowly fades again, back to its original brightness.
Some stars do this on a regular basis, brightening a few times a century. Others must clearly be on a longer cycle, we’ve only seen them do it once.
Astronomers think there are about 40 novae a year across the Milky Way, and we often see them in other galaxies.
The term “nova” was first coined by the Danish astronomer Tycho Brahe in 1572, when he observed a supernova with his telescope. He called it the “nova stella”, or new star, and the name stuck. Other astronomers used the term to describe any star that brightened up in the sky, before they even really understood the causes.
During a nova event, only about 5% of the material gathered on the white dwarf is actually consumed in the flash of fusion. Some is blasted off into space, and some of the byproducts of fusion pile up on its surface.
Over millions of years, the white dwarf can collect enough material that carbon fusion can occur. At 1.4 times the mass of the Sun, a runaway fusion reaction overtakes the entire white dwarf star, releasing enough energy to detonate it in a matter of seconds.
If a regular nova is a quick flare-up of fusion on the surface of a white dwarf star, then this event is a super nova, where the entire star explodes from a runaway fusion reaction.
You might have guessed, this is known as a Type 1a supernova, and astronomers use these explosions as a way to measure distance in the Universe, because they always explode with the same amount of energy.
Hmm, I guess the terminology isn’t so bad after all: nova is a flare up, and a supernova is a catastrophic flare up to death… that works.
Now you know. A nova occurs when a dead star steals material from a binary companion, and undergoes a momentary return to the good old days of fusion. A Type Ia supernova is that final explosion when a white dwarf has gathered its last meal.