Nova Delphini 2013: A "New Star" In The Summer Night Sky
(The bright new Nova Delphini 2013, as seen from Puerto Rico on August 16th, 03:13UT. Credit: Efrain Morales/Jaicoa Observatory)
A "new" star appeared last week in Northern Hemisphere skies, and is bright enough that with nothing more than a pair of binoculars -- or even just your eyes, if you live in a dark location -- you can see an astronomical wonder for yourself this weekend.
The object is called Nova Delphini 2013. "Nova" (Latin for 'new') is the label for a certain kind of star that quickly flares up in brightness by a factor of up to millions of times, then fades back into obscurity over a period of weeks to months. The label given to this kind of star is an historic relic from an earlier time when such stars only became visible at all in a flare-up, so to earlier astronomers they appeared to be "new" stars that previously did not exist. Only in the 20th century, with the advent of powerful modern telescopes, did astronomers realize that the stars were there all along, and the only "new" thing about them was the flaring phenomenon.
What is a nova? A nova is an event, not just an object. The model for how this works involves two stars: an evolved star like a red giant and a "compact object" such as a white dwarf. The stars must be in close proximity for the mechanism to work, but double stars (pairs of stars that orbit each other) are common in our galaxy. A compact object has a very strong pull of gravity, while a red giant is a big, puffed up star whose gravity is comparatively weak; if these two types of star are close enough in space, the compact object's gravity will strip gas from the outer atmosphere of the red giant. This is shown below in a NASA artist's conception of such a system.
As the gas is transferred from the red giant to the compact object, it spins around in a disk (blues and greens in the illustration) before eventually landing on its surface. The gas that lands on the compact object is mostly hydrogen, and over time a layer of hydrogen accumulates on its surface. The density and temperature of this gas increases steadily over time until a runaway nuclear reaction occurs, and in a matter of seconds the hydrogen is explosively fused into helium. The entire hydrogen surface of the compact object is suddenly blown off into space, briefly forming a "shell" around the compact object. Sometimes this shell remains visible, its atoms excited by the strong ultraviolet radiation given off by the white dwarf as in this example of Nova Persei 1901:
(Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona)
That ejected gas (reddish cloud in the image) becomes part of the interstellar medium -- the stuff between the stars -- while the compact object lives to see another day and begin the cycle all over again. It may be years to millennia between these catastrophic explosions.
"Nova Del", for short, takes its name from the constellation in which it appears, Delphinus (the Dolphin). This compact constellation, consisting mainly of a distinct diamond- or lozenge-shaped group of four stars of about 4th magnitude, appears to be "swimming" just below the faint band of light across the night sky marking the Milky Way. Another nearby constellation, Sagitta (the Arrow), functions just as its name suggests, pointing almost directly at the position of the nova; see the finder chart below.
The nova was discovered last Tuesday by Koichi Itagaki of Yamagata, Japan using a 7-inch (18 cm) reflecting telescope and a CCD camera. It was already at about 6th magnitude and rising quickly in brightness. By midweek, a spectrum of the object was obtained with the FRODOSpec instrument on the 2m robotic Liverpool Telescope on La Palma in the Canary Islands; it showed lines hydrogen, helium, and iron. The hydrogen lines appeared to be broadened by very large velocities in the gas, up to about 2,000 kilometers per second. These indications are all hallmarks of an emerging nova.
In the days since it was first noticed, the nova has increased in brightness by a factor about 100 times. The image below is its light curve, made by plotting estimates of the nova's brightness in the vertical direction against time in the horizontal direction.
The light curve suggests that the peak brightness of about magnitude 4.5 happened on Friday; as of Saturday, the object is beginning to fade, but it remains bright at around 5th magnitude. It will remain bright enough to be easily found from most locations for at least several days to a week or two.
How to find Nova Del: Look east after dark, about halfway from the horizon to the top of the sky, and locate the three bright stars here labeled Vega, Deneb and Altair:
These stars form the "Summer Triangle" and are easy to find under the brightest of city lights. The fainter constellations Delphinus and Sagitta are highlighted to help find the way, and the position of the nova is marked with a yellow "X". There are two approaches from here to find the nova. One is to locate the four main stars of Sagitta, and in particular, find its brightest star Gamma (γ; lowest in the sky of the four stars in Sagitta). Just beyond Gamma in the same direction the arrow points is the star Eta (η) Sagittae, marked with a blue arrow above. Further in the same direction, about as far away as Sagitta is long, is the star 29 Vulpeculae (Vul), also marked with a blue arrow. The nova is almost exactly halfway between these stars. Alternately, find the bright stars Altair and Epsilon (ε) Cygni, marked above. A blue line is drawn connecting them, which passes almost exactly through the position of the nova.
This more detailed finding chart is provided by Sky and Telescope magazine, rotated about 90 degrees so that the direction toward north on the sky is "up" on the image:
Again, find the star Gamma (γ) Sagittae at center-right, then hop from there to Eta (η) Sagittae in the direction of 29 Vulpeculae (upper-left). The nova is almost directly in-between Eta and 29.
What you will see depends on whether you're looking with your eyes alone, or using binoculars or a telescope. With only your eyes, you will perceive a faint, colorless star among many in this part of the sky -- but with the knowledge that you're seeing the outcome of a cataclysmic stellar explosion. Binoculars or a telescope gather more light, bringing some color information to the eyes. Many people will perceive a white or slightly bluish color in the nova, indicative of the high temperatures involved. We're much too far away to see any structure to the explosion itself, so don't expect a stellar mushroom cloud to appear in your telescope eyepiece.
Over the next several weeks, this "new" star will slowly fade back into obscurity. It's too early to know whether we will someday see an expanding shell of gas around the host stars, as in the case of Nova Persei 1901 pictured above. Events like Nova Delphini are uncommon, but they're far from rare. What makes this one special that it is unusually bright; its peak brightness is enough to secure its place in the Top 30 Brightest Novae ever seen. We have not been so well positioned in the Northern Hemisphere to observe a nova since Nova Cygni 1992, more than two decades ago.
Late summer nights are warm; Delphinus swims the edge of the Milky Way and Sagitta flies through the skies. Go outside and let them guide you to the site of a cosmic catastrophe tonight.
