A nova occurs when a star brightens by several magnitudes over a very short time. Like supernovae, they’ve been recorded throughout history. We now know novas are caused by a dance between two stars, where a white dwarf orbits close enough to a companion star that it captures material from its companion until it reaches a critical limit and it’s outer layer explodes. Studying the details of this phenomena is difficult, because a nova is usually too faint to be noticed until it brightens. But thanks to large sky surveys, that’s starting to change. 

Recently a team observed a nova in their data, and knew they had captured that region of sky before. So they went back through their data and were able to document the binary system before, during and after the nova occurred. They found the two stars orbit each other once every five hours, putting them at a distance roughly equal to the diameter of our Sun. Before the explosion, the white dwarf was capturing material at an irregular rate, causing its brightness to “sputter” slightly. After the nova the white dwarf captured material at a more regular rate. This would support the hibernation model, where the white dwarf captures material early on, then the rate of capture dies off. It should be stressed however, that the aftermath of the nova is still young, so we’ll need to collect more data to be sure.

In addition to helping us understand novae, observations like these could also help us understand supernovae. Type Ia supernovae in particular are caused by a similar dance between a white dwarf and companion star, but instead of just the outer layers exploding, the entire white dwarf is ripped apart by a cataclysmic explosion.

Paper: Przemek Mróz, et al. The awakening of a classical nova from hibernation. Nature doi:10.1038/nature19066 (2016)

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In August of last year a star briefly brightened by a factor of 5 in a single day. Known as Gaia 14aae is a cataclysmic variable. It is a binary where the two stars orbit closely enough for the outer layers of one star to be captured by the other. Not only do these stars orbit each other every 50 minutes, their orbits are aligned so that the larger star totally eclipses the smaller one  with each orbit. 

The light curve of the binary system. Credit: Campbell et al.

The smaller star actually has the greater mass. It is a white dwarf with a mass about 80% that of our Sun. The larger star only has a mass 15% of our Sun, but has expanded to 125 times the diameter of our Sun as it approaches the end of its life. One of the striking features about this binary is that it doesn’t contain much hydrogen. This is exactly what you’d expect if the white dwarf has already stripped off much of the outer layer from its companion star. As the white dwarf continues to capture material from the other star, we can expect more flare ups similar to the brightening of last year.

But what makes this system particularly interesting is that it could provide clues as to just how type Ia supernovae might form. These “standard candle” supernovae are used to determine the distances of the earliest galaxies, but we still aren’t entirely sure what causes them. One possibility is the stellar dance of close binaries like Gaia 14aae.

Paper: H. C. Campbell et al. Total eclipse of the heart: the AM CVn Gaia14aae/ASSASN-14cn. MNRAS 452 (1): 1060-1067 (2015)

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If you look at the constellation of Sagittarius (also known as the teapot) in the night sky just before dawn, you might notice an additional star toward the upper middle. It’s currently a 4th magnitude star, which is dim but visible with the naked eye under dark skies. It is the first naked eye nova we’ve had in a while.

A nova is similar to a supernova, but not quite as powerful. It is produced by a white dwarf orbiting a red giant, where material from the red giant is captured by the white dwarf. When a supernova is triggered the entire white dwarf is ripped apart in a catastrophic explosion. With a nova, the hydrogen and helium captured from the red dwarf compresses on the surface of the white dwarf. The heat and pressure from this gravitational compression triggers a fusion reaction of the material. The result is a thermonuclear explosion that is not intense enough to destroy the star, but still produces a massive brightening of the star.

Because the white dwarf is not destroyed by the nova, it is possible for the same star to go nova multiple times. Perhaps the most famous example of such a repeating nova is RS Ophiuchi, which has erupted six times since 1898. There are about 10 observed novae in our galaxy every year, but it is unusual for one to be bright enough to see with the naked eye or small telescope. So if you ever have the chance to look at this new cosmic explosion, it’s worth checking out.

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Last year there was a new nova in the night sky, now officially named Nova Delphini 2013. From Earth, it looked like a fairly dim new star in the constellation Delphinus. Novae such as this one are similar to the more popular supernovae. The popular view of a supernova is that of an exploding star. A large star runs out of hydrogen to fuel, and as a result collapses upon itself. This “core collapse” causes intense nuclear reactions which rip the star apart in a huge explosion, which is why this type of supernova is known as a core-collapse supernova.

There’s another type of supernova known as a thermal runaway supernova. This occurs when a white dwarf is in a close binary system with a red giant star. Material from the red giant is captured by the white dwarf, adding to its mass. But there is a limit to how much mass a white dwarf can have (known as the Chandrasekhar limit). As the this limit is reached, the star begins to collapse. This causes a cascade of rapid fusion within its core, which rips the white dwarf apart. The white dwarf explodes as a supernova. You can read about the details in an earlier post.

A nova is similar to a thermal runaway supernova. It too is produced by a white dwarf orbiting a red giant. When a supernova is triggered the entire white dwarf is ripped apart in a catastrophic explosion. With a nova, the hydrogen and helium captured from the red dwarf compresses on the surface of the white dwarf. The heat and pressure from this gravitational compression triggers a fusion reaction of the material. The result is a thermonuclear explosion that is not intense enough to destroy the star, but still produces a massive brightening of the star. You can see an artistic take on such an event in the image above.

Because the white dwarf is not destroyed by the nova, it is possible for the same star to go nova multiple times. Perhaps the most famous example of such a repeating nova is RS Ophiuchi, which has erupted six times since 1898. If RS ophiuchi continues to accrete mass, it will eventually reach the Chandrasekhar limit and become a supernova.

There are about 10 observed novae in our galaxy every year, but it is unusual for one to be bright enough to see with the naked eye or small telescope. So if you ever have the chance to look at such a new cosmic explosion, it’s worth checking out.

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A recurrent nova is a stellar nova that occurs from the same star more than once.  You’re probably more familiar with a supernova, where a star is ripped apart in a cataclysmic explosion.   Novas occur when a white dwarf orbits with another star and captures some of the star’s outer material. This material forms an accretion disk around the white dwarf, which gradually falls to its surface.  When material accumulates on the surface of the white dwarf, it can trigger a nuclear explosion that causes it to brighten similar to a supernova, but not nearly as intense.  Since the explosion doesn’t destroy the star, it is possible for a nova to occur again after more material has accumulated.

The most famous recurrent nova is RS Ophiuchi, which becomes a nova about every 20 years.  Other recurrent nova occur at different rates.  Recurrent nova caused by more massive white dwarfs tend to have shorter repeat times than those with less mass.  This seems to be due to the fact that stars with more gravity can accumulate matter from the companion star more quickly.  Some recurrent nova have irregular repeat times, or have novae with highly varying brightnesses.  This seems to be due to disruptions in the accretion disk when the star explodes.

It is thought that recurrent novas could be a precursor to the white dwarf becoming a supernova.  If the material cast off by the nova is less than the material accumulated each time, then the white dwarf will gradually increase in mass.  Eventually this bring its mass to the Chandrasekhar limit, which is the upper limit for the mass of a white dwarf.  Beyond that point the star will collapse, which can trigger a supernova explosion.

Background: The star as a Nova.
Inset: Hubble image of the star when inactive.

Recently astronomers have discovered a fast recurrent nova in the Andromeda galaxy.  A paper on the nova was recently published in Astronomy and Astrophysics, and presents some of the initial results.  The star, known as M31N 2008-12a has a nova outburst about once a year.  This is extremely rapid, since the next highest frequency rate is about once a decade.  The star also brightens quickly and dims quickly, on the order of a few days.  When observed in x-rays, it was found the star emits x-rays during the nova period for about 10 days.  Since x-rays are generated by nuclear interactions, this indicates that the nuclear interactions on the surface only last about that long.

All of this points to M31N 2008-12a being a very massive white dwarf star.  Since the novae occur so rapidly, it is clear that material from its companion star continues to accrete at a regular basis.  It would seem that this star is on its way to becoming a supernova.  Just how soon that might be is unknown, but it’s worth keeping an eye on.  It could explode at any time.

Of course on a cosmic scale “any time” could be anywhere from tomorrow to thousands of years.

Paper: M. Henze, et al. A remarkable recurrent nova in M 31: The X-ray observations. Astronomy & Astrophysics, 563, L8 (2014)

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In November of 1572, Wolfgang Schuler observed a new star in the constellation of Cassiopeia.  A few days later, it was observed by Tycho Brahe, who began taking careful observations of this visiting star, which came to be known as Tycho’s supernova.  By the time Tycho observed the star, it was about as bright as Jupiter.  Eventually it reached a brightness rivaling Venus before gradually fading from view over the next year and a half.

The appearance of this new star, or stella nova as it was known in Latin, was a major discovery.  The prevailing view at the time was that the stars were fixed and unchanging.  Planets wander through the sky and comets appear from time to time, but the stars were eternal.  Tycho’s observations demonstrated this was not the case, and it spurred a new interest in making accurate measurements of the heavens.

The remnant of this supernova wasn’t discovered until the middle of the last century, first by radio telescopes in the 1950s, then optically in the 1960s.  Since then it has been studied in great detail, and you can see a modern composite image below.  We now know it was a type Ia supernova about 8000 to 9000 light years away.

About 30 years later, in 1604, Kepler observed a supernova in the constellation of Ophiuchus.  Since then, there hasn’t been an observed supernova in our galaxy, which is unfortunate and a little perplexing.  Studies indicate that a galaxy of our size and type should have a supernova about once every 50 years.  That we haven’t seen another in 400 years seems to indicate that there’s a crucial aspect of supernova physics we don’t understand, or astronomers are having a 400 year long streak of bad luck.

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