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Saturday, March 21, 2015

Nova in Sagittarius Brighter Than Ever – Catch it with the Naked Eye!

Nova in Sagittarius Brighter Than Ever – Catch it with the Naked Eye!:

Nova Sagittarii 2015 No. 2 photographed this morning when it was easily visible to the naked eye at magnitude +4.4. The nova has been on the upswing since its discovered less than a week ago. Credit: Bob King

Nova Sagittarii 2015 No. 2 in the Sagittarius “Teapot”was easily visible with the naked eye at magnitude +4.4 when this photo was taken today March 21. The nova has been steadily brightening since its discovery less than a week ago. Credit: Bob King

Great news about that new nova in Sagittarius. It’s still climbing in brightness and now ranks as the brightest nova seen from mid-northern latitudes in nearly two years. Even from the northern states, where Sagittarius hangs low in the sky before dawn, the “new star” was easy to spy this morning at magnitude +4.4.

While not as rare as hen’s teeth, novae aren’t common and those visible without optical aid even less so. The last naked eye nova seen from outside the tropics was V339 Del (Nova Delphini), which peaked at +4.3 in August 2013. The new kid on the block could soon outshine it if this happy trend continues.

This view shows the sky facing south-southeast just before the start of dawn in mid-March from the central U.S. The nova's located squarely in the Teapot constellation. Source: Stellarium

This view shows the sky facing south-southeast shortly before the start of dawn in late March from the central U.S. The nova is centrally located within the Teapot. Source: Stellarium

Now bearing the official title of Nova Sagittarii 2015 No. 2, the nova was discovered on March 15 by amateur astronomer and nova hunter John Seach of Chatsworth Island, NSW, Australia. At the time it glowed at the naked eye limit of magnitude +6. Until this morning I wasn’t able to see it with the naked eye, but from a dark sky site, it’s there for the picking. So long as you know exactly where to look.

The chart and photo above will help guide you there. At the moment, the star’s about 15° high at dawn’s start, but it rises a little higher and becomes easier to see with each passing day. Find your sunrise time HERE and then subtract an hour and 45 minutes. That will bring you to the beginning of astronomical twilight, an ideal time to catch the nova at its highest in a dark sky.

Use this AAVSO chart to pinpoint the nova's location and also to help you estimate its brightness. Numbers shown are star magnitudes with the decimal points omitted. Credit: AAVSO

Use this AAVSO chart to pinpoint the nova’s location and also to help you estimate its brightness. Numbers shown are star magnitudes with the decimal points omitted. Credit: AAVSO

To see it with the naked eye, identify the star with binoculars first and then aim your gaze there. I hope you’ll be as pleasantly surprised as I was to see it. To check on the nova’s ups and downs, drop by the American Association Variable Star Observers (AAVSO) list of recent observations.

Seeing the nova without optical aid took me back to the time before the telescope when a “new star” in the sky would have been met with great concern. Changes in the heavens in that pre-telescopic era were generally considered bad omens. They were also thought to occur either in Earth’s atmosphere or within the Solar System. The universe has grown by countless light years since then. Nowadays we sweat the small stuff – unseen asteroids – which were unknown in that time.

AAVSO light curve showing the nova's brightening trend since discovery. Dates are at bottom, magnitudes at left. Credit: AAVSO

AAVSO light curve showing the nova’s brightening since discovery. Dates are along the bottom, magnitudes at left. If the trend continues, Nova Sgr #2 could outshine the 2013 nova in Delphinus very soon. Credit: AAVSO

Novae occur in binary star systems where a tiny but gravitationally powerful white dwarf star pulls gases from a close companion star. The material piles up in a thin layer on the dwarf’s hot surface, fuses and burns explosively to create the explosion we dub a nova. Spectra of the expanding debris envelope reveal the imprint of hydrogen gas and as well as ionized iron.

Nova illustration with an expanding cloud of debris surrounding central fireball emitting red hydrogen-alpha light.

Artist’s view of a nova with an expanding cloud of debris surrounding the central fireball emitting red hydrogen-alpha light.

Shortly after discovery, the nova’s debris shell was expanding at the rate of ~1,740 miles per second (2,800 km/sec) or more than 6.2 million mph (10 million mph). It’s since slowed to about half that rate. Through a telescope the star glows pale yellow but watch for its color to deepen to yellow orange and even red. Right now, it’s still in the fireball phase, with the dwarf star hidden by an envelope of fiery hydrogen gas.

As novae evolve, they’ll often turn from white or yellow to red. Emission of deep red light from hydrogen atoms – called hydrogen alpha – gives them their warm, red color. Hydrogen, the most common element in stars, gets excited through intense radiation or collisions with atoms (heat) and re-emits a ruby red light when it returns to its rest state. Astronomers see the light as bright red emission line in the star’s spectrum. Spectra of the nova show additional emission lines of hydrogen beta or H-beta (blue light emitted by hydrogen) and iron.

There are actually several reasons why novae rouge up over time, according to former AAVSO director Arne Henden:

“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.

Nova Sagittarii’s current pale yellow color results from seeing a mix of light – blue from the explosion itself plus red from the expanding fireball. As for its distance from Earth, I haven’t heard, but given that the progenitor star was 15th magnitude or possibly fainter, we’re probably talking in the thousands of light years.

Wide view of the Sagittarius-Scorpius region with some of the brighter star clusters and nebulae labeled for binocular browsing. Credit: Bob King

Wide view of the Sagittarius-Scorpius region with some of the brighter star clusters and nebulae labeled for binocular browsing. Credit: Bob King

In an earlier article on the nova’s discovery I mentioned taking a look at Saturn as long as you made the effort the get up early. Here’s a photo of the Sagittarius region you can use to help you further your dawn binocular explorations. The entire region is rich with star clusters and nebula, many of which were cataloged long ago by French astronomer Charles Messier, hence the “M” numbers.

About Bob King

I'm a long-time amateur astronomer and member of the American Association of Variable Star Observers (AAVSO). My observing passions include everything from auroras to Z Cam stars. Every day the universe offers up something both beautiful and thought-provoking. I also write a daily astronomy blog called Astro Bob.

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