Dispelling a Common Misconception About Novae

Something that has bothered me over the years in my many discussions with other amateur astronomers and people, in general, is a common misconception they have about novae. The misconception to which I refer is that novae are scaled down versions of supernovae and are the explosive death of stars like our sun. This misconception is somewhat understandable, given the similarity of terms (i.e., “supernova” and “nova”), but that’s where the similarity ends.

To set the record straight, when stars like our sun have stopped burning thermonuclear fuel in their cores they become very unstable and start ejecting successive shells of ionized gas until their cores have been reduced in mass to about 0.6 solar masses. The shells of ionized gas slowly expand into a planetary nebula, which in time, disperses into and becomes a part of the interstellar medium. The carbon-oxygen core, which has been unveiled during this process, becomes an Earth-sized object known as a white dwarf. Thermonuclear reactions do not occur in white dwarfs and the source of their energy output is purely thermal in origin. In a word, stars like our sun go out with a whimper rather than a bang.

So then, what is a nova if not a dying dwarf star like our sun? The answer to this question is that a nova is a class of cataclysmic variable. More specifically, a nova is a close binary pair in which one member, the primary (a white dwarf), draws ionized, hydrogen-rich gas, off the outer layers of it’s companion star, the secondary (either a main sequence or red giant star). If the secondary is a red giant then it is known as a recurrent nova. If the secondary is a dwarf star of the spectral type G or K–our sun is of the spectral type G2 V–then it is known as a dwarf nova. If a nova has a main sequence star as it’s secondary that is not of the spectral type G, K or M then it is known as a classical nova.

Roche Lobes and the Mechanics of Novae

When talking about close binary systems, such as novae, and the gravitational interaction between the secondary and it’s primary, it is necessary to talk about a concept known as equipotential surfaces. Put simply, equipotential surfaces are imaginary surfaces surrounding either a celestial object or–in the case of novae–a close binary system, where the gravitational field is constant. In close binary systems the equipotential surface forms a figure-eight or an hour-glass configuration, like the one shown in the illustration below. The figure-eight shape delineates what is referred to as the Roche lobes–named after Edouard Roche who was the first to discover what is known as the Roche limit. The Roche limit is defined as the minimum distance (i.e., the Roche radius) from the center of a body at which a satellite in orbit around it will remain in gravitational equilibrium. (Reference for illustration below: Illingworth, Valerie. The Facts on File Dictionary of Astronomy, Third Edition, 1994, pg. 146).

Within the Roche radius a satellite (e.g., such as the components that make up the rings of Saturn) will eventually spiral down to the surface of it’s primary, i.e., the planet Saturn. Outside the Roche radius an orbiting body (e.g., such as our moon) will slowly move away from it’s primary, i.e., the Earth.

In a close binary system the point where the Roche lobes intersect, the first Lagrangian point (L1, in the illustration), is where the gravity of the secondary and that of it’s primary cancel each other out (i.e., a gravity-free zone). When a secondary expands to fill up it’s Roche lobe, the binary system of which it is a part, becomes what is known as a semi-detached binary. And when this occurs, the only place where the gas can flow is out through the Lagrangian point and toward the primary. It is in this manner that hydrogen-rich gas is transferred from the secondary to the accretion disc spiraling inward toward the surface of the primary. This is the basic mechanism behind the gas transfer of all novae.

The Designation and Nomenclature of Novae

The designation of novae consist of the genitive case of the name of the constellation and the year in which they were first observed (e.g., Nova Cygni 1975). Novae designations also include a variable star designation (e.g., V1500 Cyg), but this practice has only been in existence since 1925. Before 1925 novae were assigned numbers in the order that they were discovered (e.g., GK Per 2)–Novi Persei discovered in 1901. So, novae discovered before 1925 would have a designation which would appear like this:

Nova Persei 1901
(GK Per 2)

The designation for novae after 1925 would look like this:

Nova Cygni 1975
(V1500 Cyg)

or

Nova Herculis 1934
(DQ Her)

Admittedly, the difference in nomenclature between the two examples (above) of novae discovered after 1925 needs some explaining. Actually, the system of designating variables, in general–not just novae–was first devised in 1862. The letters used to designate a newly observed variable in a particular constellation were R through Z. Once the number of variables in a particular constellation exceeded the number of letters from R–Z, then the sequence was started all over again, using double letters from R–Z, (i.e., RR–RZ, SS–SZ, TT–TZ all the way up to ZZ). If more variables are discovered in the constellation after they have reach the designation ZZ, then they start over with AA–AZ, BB–BZ (excluding the letter J) and so on, up to QZ. Of course, this only allows for 334 variables in a given constellation before you completely run out of letters. So, what do you do when you discover the 335th variable in a particular constellation? Well, the answer to this question is the former example, above. You use a “V” for “variable” followed by the number which reflects the variable’s rank in the order of discovery–starting, of course, with the number 335. In the example above, Nova Cygni 1975 was the 1500th variable discovered in the constellation Cygnus. That’s pretty much how novae and other variables are designated. (Source on stellar nomenclature: Illingworth, Valerie. The Facts on File Dictionary of Astronomy, Third Edition, 1994, pp. 306, 439).

Novae (Classical Novae)

It is worth noting that the word “nova” comes from the Latin adjective: novus -a -um, meaning “new, young, fresh, unexpected or unusual”. The nomenclature is quite appropriate considering that when novae were first observed, in centuries past, they were regarded as new stars that had previously not been there. It was not that long ago that astronomers realized that novae were actually close binary systems that periodically increased in brightness. It is especially easy to understand why it has taken so long to understand the origins of classical novae since they are far from short-lived events. When one considers that in the case of a classical nova it takes somewhere in the order of 10,000–100,000 years before enough material has been accumulated by the primary in its accretion disc to trigger a thermonuclear explosion, it doesn’t take a tremendous stretch of the imagination to see how, in comparison to the span of a human lifetime, such an event would seem like a one-time deal. A probable explanation for this long period between explosions is the rate of hydrogen gas transfer from the secondary to the accretion disc of the primary–approximately 10^-9 solar masses per year . . . not exactly a very speedy delivery system.

It has been said by those who have observed and studied classical novae that in the pre-nova state the accretion disc is the brightest part of the system and that it appears as a blue shimmering light. I’ll have to take their word for it since I, myself, have never actually seen one.

Recurrent Novae

Recurrent novae do not take quite as long to occur as do classical novae. In fact, the transfer rate of hydrogen from the red giant to the accretion disc around the primary is about a thousand times faster than it is in the systems that produce classical nova. As a result of this faster rate of transfer, a particular recurrent nova can be observed over decades, as opposed to the millennia it takes for a classical nova event to occur.

A good way for astronomers to discern a recurrent nova is through it’s spectrum. Because a spectrograph will show the spectrum of both the red giant (which is cooler than the accretion disc around the primary) and the spectrum of the hotter accretion disc around the primary, the system will appear as a single star with two temperatures–a physical impossibility. This is a tell-tale sign that what is being observed is not a single star but rather a close binary system–specifically, a recurrent nova.

The change in brightness in a recurrent nova is not as great as it is with a classical nova and they fade much faster after brightening than the latter.

Dwarf Novae

Dwarf novae are divided into three sub-groups: U Geminorum stars, Z Camelopardalis stars and SU Ursae Majoris stars. Most dwarf novae fall into the first subgroup, i.e., the U Geminorum stars. Typically, these novae brighten within a few weeks to a few months–remaining at their brightest for only a few days. After maximum brightness, they gradually dim and repeat the cycle weeks or months later.

Z Camelopardalis stars are more unpredictable and complex in their behavior than U Geminorum stars. Z Camelopardalis stars sometimes plateau-out for a period of time while dimming from maximum brightness. These plateaus of intermediate brightness may last for several days or for as long as many months. There’s really no way to predict how long these plateaus of brightness will last. I call the stars of this particular subgroup, “odd-ball novae”.

The last subgroup, the SU Ursae Majoris stars, aren’t quite as peculiar as Z Camelopardalis stars, but they too sometimes have an irregularity with respect to there brightening. Specifically, SU Ursae Majoris stars sometimes become extraordinarily brighter than normal. To the best of my knowledge, no one really knows why this is so; that has yet to be determined.

There are two competing models that have been proposed that attempt to explain the mechanics of dwarf novae. The first model–which is the more widely accepted of the two–is called the disc instability model. It proposes that the brightening of the accretion disc is caused by thermal fluctuations that occur periodically in the disc. Without any explosion occurring at this time, there is an increase in temperature hence a brightening in the disc. Sometimes though, enough mass does build up in the accretion disc around the primary so that a nova explosion does occur.

The second model, which is not as popular as the first, is called the mass-transfer instability model. It proposes that the secondary in the system–a dwarf star of either the spectral class K or G, as mentioned in the beginning of this article–periodically expands and fills its Roche lobe and transfers hydrogen-rich gas from its outer layers, via the Lagrangian point, to the accretion disc of the primary. When enough mass has built up in the accretion disc a nova explosion occurs.

Concluding Thought

I hope that this article has shed some light on the true nature of novae and has helped to dispel any confusion regarding their origins. Keep looking up and maybe one day you’ll become one of the initiated who can say: “I have seen and have done so with ‘new eyes’.”

Copyright © 2003 Eric F. Diaz

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