Time to check in on this month's bright nova in Delphinus. After plateauing at magnitude 5.0 last weekend, Nova Del 2013 has been slowly fading ever since. By last night it had slid to 5.8 or just a tad brighter than the naked eye limit. The simple plot or light curve I compiled using my own binocular observations clearly shows its steady decline. Whether the star will continue to fade or bounce back is anyone's guess. Novae can surprise just like comets do. They can also change color as we'll see in a moment.
Now that the moon is rising later, skywatchers with dark, rural skies should be able to spot the nova with the naked eye again very soon. If you've been keeping a log of its brightness, click HERE for instructions on how to build your own light curve. I've included an updated chart for you to use to estimate the star's brightness in the coming nights.
Through the telescope the nova has been a colorful sight. Early on it twinkled pale yellow but now has deepened in hue to yellow-orange. It's still in the fireball phase with the white dwarf star hidden by fiery hydrogen gas and an expanding cloud of debris.
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As novae evolve they'll often turn from white or yellow to red. Emission of what's called hydrogen alpha light gives novae their warm, red color. Hydrogen, the most common element in stars, gets excited through intense radiation or collisions with atoms (heat).
Once energized, hydrogen's electrons "move upstairs", ie. jump from a lower energy level to a higher one. Just as quickly, they can drop back down "downstairs". When they do, each releases a smidge of light in the deep red end of the rainbow spectrum called hydrogen alpha or H-alpha. Nova-red comes from electrons dropping from the "third floor" to the "second floor" inside the hydrogen atom.
Novae take on a pink or red color for several reasons according to Arne Henden, director of the American Association of Variable Star Observers (AAVSO).
"Energy from the explosion gets absorbed by the surrounding material in a nova and re-emitted as H-alpha," said Henden. Not only that but as the explosion expands over time, the same amount of energy is spread over a larger area.
"The temperature drops," said Henden, "causing the fireball to cool and turn redder on its own." As the eruption expands and cools, materials blasted into the surrounding space condense into a shell of soot that absorbs that reddens the nova much the same way dusty air reddens the sun.
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So why does it appear yellow-orange right now?
"That's the underlying continuum (bluish light from the explosion) mixing with the H-alpha from the expanding fireball. Red and blue together make orange."
Finally, I'm often asked how far away the nova is. According to the most recent study (reported 8/23), based on the rate of decline of the nova's brightness, the star is some 13,000 light years from Earth. Very far! That means it must be incredibly brilliant.
A lot's going on right now with Nova Delphini - an expanding fireball, formation of a debris cloud, cooling and reddening. And to think you can sample all this with little more than a pair of binoculars from your front yard. Amazing.
