Novae (2012 edition)

This season, we're trying something different for our Variable Star of the Season feature. For the January 2012 edition, we asked former AAVSO Councillor Dr. Kristine Larsen to revise and update one of our past Variable Star of the Season articles -- a comprehensive look at galactic novae, past and present by former AAVSO staff members Kerriann Malatesta and Kate Davis. Kristine gave some fresh updates and new insights into the phenomenon of Galactic novae, including three prominent novae that erupted since this article was originally published -- V382 Vel (N Vel 1999), V1280 Sco (N Sco 2007) and KT Eri (N Eri 2009). We hope you enjoy this season's collaborative installment!

A Historical Look at Novae

This gas cloud is illuminated by the flood of light produced by Nova Cygni 1992, resulting in the red emission nebula seen above. The nova itself can be seen above the center of the image as the star engulfed in a red annulus of gas. Credit: P. Garnavich (CfA), 1.2-m Telescope, Whipple Observatory

The word nova is used to describe a star that suddenly increases in brightness, producing one, vivid maximum. Often times there is no bright precursor to the occurrence, hence the Latin-based word meaning "new" seems appropriate to describe such events. Although modern societies normally do not let phenomena in the sky dictate their daily activity, the appearance of such a spectacle was often revered as an omen by ancient cultures. While the people of Europe and the Middle East seemed reluctant to keep records of the events, the Chinese were far less concerned with the implications. The earliest well-documented reports of a new star recorded by many cultures date back to a nova that occurred in 134 BC. Referred to as the star of Hipparchus, this star is said to have led the famous astronomer to the development of his valuable star catalogue (Campbell and Jacchia 1941).

From the earliest nova records until 1900, a total of 161 novae were discovered (Hoffleit 1986). In the past hundred years, however, the count of novae within our own galaxy alone has reached nearly 400, with over 300 of these "new" stars in the AAVSO program. Of these, only about 100 are well-observed, with at least 400 positive observations. The cause for this increase may be attributed to many developments, including the advent of photography, the use of CCD cameras, and most recently the development of automated all-sky surveys. Another 800 novae have been found in the nearby Andromeda Galaxy, suggesting that there are many more novae in our own galaxy that still go undetected. Therefore even though the number of new novae discovered in our galaxy per year has risen to about 8, there may actually be 50% more that reach a maximum visual magnitude of 11 or higher (Bode 2011). With astronomers using space and ground-based telescopes across the electromagnetic spectrum to study novae and their spectra in more detail than ever before, there is an ongoing opportunity for AAVSO visual and CCD observers to take part in coordinated nova observations.

A Word About Novae (and Supernovae)

Artistic impression of an AM Her system (similar to V1500 Cygni).
Image courtesy and copyright Mark A. Garlick / space-art.co.uk

Novae are identified by their spectra and sudden large rise in brightness, typically 8-15 (but sometimes as many as 19) magnitudes. Although many theories to describe the observed outbursts have been postulated over the years (see Hoffleit 1986), it is now thought that the outburst is the result of an explosive event. Novae belong to the class of stars known as the Cataclysmic Variable (CV) stars, along with the dwarf novae, recurrent novae, nova-like, and polar (magnetic) variables. And like all CVs, the physical system is comprised of a very close binary pair, with a white dwarf star as the primary component and a Sun-like, main sequence star as the secondary. Due to evolution and the physics of the system, the Sun-like star loses mass in the direction of the primary companion.

Stars in a binary system exchange mass because one (or both) of them fills their individual so-called Roche lobe. Picture two siblings forced to share a bedroom against their will. The children will usually imagine an invisible line cutting across the room that separates one child’s belongings from the other’s. As long as each child remains on their respective side of the room, they can co-exist peacefully. But over time one of the children might array their toys such that one or more might touch the border between his and his brother’s “space.” As one might expect, the other child gleefully grabs the interloping toy, claiming ownership because it touched his space. In a binary system, each star determines the material that is gravitationally “theirs” by the boundary of its Roche lobe. The two individual lobes meet at a point where material can freely pass from one star to the other. As long as each star is much smaller in size than its Roche lobe, material does not transfer from one star to the other. However, stars tend to expand as they evolve, and when one star fills its Roche lobe, material can flow to the other star. In a binary system, the more massive star evolves faster, fills its Roche lobe to form a red giant, exchanging some of its gas with its companion. When the red giant becomes a white dwarf, it is once again safely inside the boundary of its Roche lobe, and mass exchange ceases. However, the companion star’s evolution has been accelerated due to its force-fed diet of gas, and eventually it fills its Roche lobe, forcing the white dwarf to get a taste of its own medicine.The exchanged matter does not fall directly onto the star, but rather forms a disk about the white dwarf, called an accretion disk. A variety of interaction between the accretion disk and the stars in the system give rise to the types of CVs listed above. Here, our focus is on novae (see also the nova VSOTM GK Per), but readers are encouraged to see past VSOTM for information about dwarf novae (see SS Cyg, Z Cam, U Gem, and SU UMa) and recurrent novae (see RS Oph) types.

The large amplitude outbursts in novae are generally associated with the ejection of a shell of matter from the surface of the white dwarf by thermonuclear processes. A white dwarf is a stellar corpse, meaning that the main source of power (fusion of lighter elements into heavier elements in its core) has been exhausted. Most of its outer hydrogen-rich layers have already been released into space (by stellar winds or the formation of a planetary nebula), leaving the surface of the white dwarf strongly enriched with He, C, N, O, Ne, and other elements. Hydrogen flows from the secondary via the accretion disk and eventually forms a layer on the surface of the primary. The extremely dense white dwarf star possesses a high gravitational force, and with continuing transfer of matter the base layer becomes compressed and heated until the critical temperature for thermonuclear reactions is achieved; runaway processes then give rise to an explosion. This explosion results in the expulsion of a shell of gas from the surface of the white dwarf, hence the large amplitude outburst (typically equivalent to more than 10,000 times the luminosity of the sun). Note that because only the outer layers of the white dwarf are affected, this process can repeat over time. For most systems, outbursts are estimated to reoccur every 1000-100,000 years; those systems that have recurring outbursts on much shorter scales (so that more than one outburst has been seen) are recurrent novae.

From observed peculiarities in comparison with other stars, T Aurigae (Nova Aur 1892) was the first nova to be recognized as a result of explosive processes (Starrfield and Shore 1998). If the white dwarf is force-fed too much gas too fast, it cannot shed the excess mass quickly enough through a nova outburst, and it reaches the Chandrasekhar limit (1.4 times the mass of our sun). At this point the white dwarf becomes unstable and explodes, creating a Type Ia supernova. Other types of supernovae are related to the collapse of the core of massive stars or white dwarfs. For more information on supernovae, see Supernova 1987a. T Aur is also an important player in the history of variable stars, because the detailed study of its outburst in 1892, compared to that of supernova S And (1885) helped lead to the recognition in the 1920s that supernovae are a different sort of celestial animal than their dimmer cousins. This discovery had important implications for our understanding of the size and scope of the Milky Way in particular, and the universe in general, because both novae and supernovae are used as “standard candles” to estimate astronomical distances (Bode 2011).

The shell of Nova FH Ser ejected in 1970 and imaged in 1997 with the Hubble Space Telescope. Image courtesy of Tim O'Brien, Gill & O'Brien (2000)

By looking at the visual light curves of novae, we see that they can be further classified based on their behavior according to the duration of rise to maximum and the fall to minimum light. The GCVS recognizes the following types of novae:

Na: Fast novae. Fast novae have an extremely rapid rise to maximum. Maximum brightness is maintained for just a few days at most, followed by an initially steep decline which later slows and may become reasonably smooth. The fading may be marked by a prolonged series of pronounced fluctuations. The brightness reaches three magnitudes below maximum within 110 days.

Nb: Slow novae. Slow novae generally have a gradual rise to maximum and may remain there for several weeks or months before declining. They tend to fade slowly at first, after which the rate of fading quickens. These novae often have a well-known sudden “dip” and then second rise. The overall three-magnitude decrease of these novae (which ignores the temporary dip) may take 150 days or more.

Nc: Very slow novae. A small group of ultra-slow novae that have light curves that exhibit similarities to the preceding varieties, however, the maximum extends over years (in some cases more than a decade). Decline also proceeds with extreme slowness. The progenitors of these novae sometimes show 1-2 magnitude long period fluctuations before the actual nova outburst.

For a diagrammed view of the process that gives rise to a nova outburst and for a look at the different types of novae, including sample light curves, see the VSOTM GK Per. These differences are related to the mass and/or chemistry of the white dwarf, which in turn are related to the location of the white dwarf within our galaxy (for example, in the disk versus the central bulge). Therefore novae are sometimes also classified by the elements seen in their spectra, in particular iron or neon (Della Valle et al. 2002). For example, V1974 Cyg (Nova Cyg 1992) was a “neon” nova.

Getting to Know a Few Novae

Click on one of the extraordinary novae below to learn more about it.

Observing Novae

Once a nova has been discovered, observers should plan to observe the object every clear night. The early data are crucial in helping astronomers understand the evolution of the nova as it progresses. The AAVSO generally provides finder charts with a comparison star sequence for brighter novae. If an AAVSO chart is not available, be sure to use a standardized sequence so that your observations will be compatible with those from other observers. You may then submit your observations to the AAVSO for inclusion in the AAVSO International Database.

It is common for brightness estimates of a nova to become less frequent as the nova dims in light. We must stress, though, that it is of extreme importance to continue monitoring a nova as it fades. A weekly, or even monthly report of a nova will help to keep an eye on the progress of the event. For those novae whose light has faded from the view of the telescope-aided eye, we encourage CCD observers to add them to their observing programs so that we may monitor their behavior as they fade to very faint magnitudes. Novae have been known to unexpectedly brighten, so no nova should be considered “boring,” even after it has appeared to reach a minimum. In addition, novae can recur; therefore any nova should be followed to the limits of one’s technology, just in case it acts up again.

T Pyxidis, a recurrent nova, unexpectedly went into outburst in early 2011. It was assumed that T Pyx had become dormant after its last scheduled eruption never materialized. The system is still under study, but amateur observers have made an enormous number of time-series observations of this nova, making it one of the best-observed nova events in AAVSO history.

Visual observations aid astronomers in piecing together the events that take place throughout the life of a nova. It is important to observe novae as early in the outburst and in as many wavelengths as possible, for doing so will give us more clues to the processes that occur as the event evolves. For instance, early observations at all wavelengths can provide information about the expanding photosphere, and other parameters such as elemental abundances. Looking at the nova early in the decline in ultraviolet, optical, and infrared wavelengths yield information about the energetics and rate of mass loss. X-ray observations later on are helpful in the construction of nebular models, while a look through the infrared is crucial in providing information about how the dust forms and evolves (Starrfield 1988). As stated above, AAVSO observers can aid in completing our understanding of individual novae by engaging in coordinated observing programs, such as those announced on the Alert and Special Notices page.

Is That a Nova I See?

So what should you do if you think that you have seen a nova (or supernova) event? Here are some steps to take that will expedite the verification of the suspect. Following these simple actions will ensure that the astronomical community is alerted as swiftly as possible:

Check to make sure that the object you are looking at is not a planet. You can find the planetary positions by any number of means, including Sky & Telescope's interactive tools, or a freeware software program such as Stellarium.
Check to see if the suspect is a minor planet. The Central Bureau for Astronomical Telegrams (CBAT) maintains an interactive, user-friendly web page will allow you to check for known minor planets in the field.
Check to make sure the object is not a field star omitted from whatever source you are using as a reference. Compare the field with other catalogs you may have or contact a friend who has other references. You should also check VSX to make sure you haven't inadvertently "discovered" a known variable such as a Mira or dwarf nova!
If you still suspect that you have found a nova, contact CBAT and follow the instructions listed on their website.

We suggest that you inform CBAT of your potential finding, but if you choose to report to the AAVSO, please let us know if you've contacted CBAT as well. Also, be sure to include the following information in your report:

the location (position) of the suspect
the date and time of observation
the brightness estimate
the color of the suspect
equipment used to make the observation (size of telescope, binoculars, etc.)
whether it is a visual estimate, photographic estimate (including film specifications), or made with a CCD camera (including filter, if any)
information about the chart(s) and comparison star(s) used
any weather or other conditions that may be of significance
what checks have already been made (e.g. checked for planet, asteroid, missing field star)

This report may then be sent to the AAVSO at aavso@aavso.org. If you have an image of the field, please attach the image file to the e-mail and make sure that the suspect is indicated in the field. Again, please be sure to alert CBAT of any suspected activity.

[Note, please also read our article on How to report new variable star discoveries.]

Getting the Word Out

Once the object has been verified as a nova (or supernova), CBAT will issue an International Astronomical Union Telegram Circular (IAUC) which will contain pertinent information about the finding so that others may study the object. (The IAU Circulars also contain information about other astronomical events as well.) Upon receiving the IAUC, the AAVSO will issue an Alert Notice for all novae (and brighter supernovae). The Alert Notice includes much of the information contained in the IAUC, but may also make note of a comparison star sequence which should be used to make estimates of the nova, and may contain other useful observing information as well.

For More Information

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