Nova Sagittarii: What a Long, Strange Fade It’s Been

After its initial eruption on March 15th, the brightest nova since 2013 peaked, dimmed, repeaked, and redimmed over and over, with an accelerating downward trend.

Nova Sagittarii 2015 No. 2 on the morning of March 21st, at its first peak brightness.

Nova Sagittarii 2015 No. 2 on the morning of March 21st, when it first peaked in brightness.
Bob King

Update June 19: For three months after its eruption and subsequent rise to magnitude 4.3, Nova Sagittarii 2015 No. 2 fluctuated by at least a magnitude every two weeks or so, on top of an increasing downward trend. As of June 19 it was finally plunging fast, down past magnitude 9, as an old slow nova typically does due to dust obscuration. See the AAVSO's up-to-date preliminary light curve. Follow developments on the AAVSO Forum thread for the nova (the most recent posts are at the bottom).
Update April 12: Will it rebrighten this summer? The nova's long-lingering brightness indicates that this is a "slow nova" and may have bigger things to come. George Gliba remarked in the comments below, "I have seen 20 galactic novae since 1967. That was the year I observed the slow nova HR Delphini. I saw another slow nova in 1995, V723 Cas. Nova Sagittarii 2015 No. 2 looks like it may be a slow nova also. That would mean its peak may not come until July and it may get as bright as 3rd magnitude, if it behaves like HR Del and V723 Cas. It may possibly be a naked-eye object in a dark sky all summer."

[Earlier text, posted March 23rd:]

If you're a slug-a-bed waiting for a comfortable morning to get up before dawn to catch the nova that popped up in Sagittarius on March 15th, your window of opportunity may be closing.

In just the last day, Nova Sagittarii 2015 No. 2 unexpectedly dropped a magnitude to about 5.4 from its peak brightness of about 4.3.

Do you think of Sagittarius just as a summer constellation? In late March it glitters low in the southeast right before the start of dawn for the world's mid-northern latitudes. Farther south it's higher before dawn, but even in Canada's heavily populated latitudes, earlybirds will find it in view from any spot with a view low to the southeast.

And it's getting a little higher every morning. To find when morning astronomical twilight begins at your location, you can use our online almanac. (If you're on daylight time like most of North America, be sure to check the Daylight-Saving Time box.)

Finder chart for Nova in Sagittarius, March 2015

The nova is almost on the midline of the Sagittarius Teapot. The horizon here is drawn for just before the beginning of morning astronomical twilight in early April for a viewer near 40° north latitude. The nova is about 15° above this horizon; by April 10th it will be about 20° up at that time seen from this latitude. Stars are plotted to magnitude 6.5. For more detailed charts with comparison-star magnitudes, see the bottom of this page.
Sky & Telescope diagram

How a Nova Works

This "new" star is definitely one for your logbook. It's the brightest nova in Sagittarius since at least 1898, and the brightest anywhere in the sky since Nova Centauri 2013 peaked in mid-December of that year at magnitude 3.3.

Classical novae are not all alike. Their underlying mechanism is the same: a white-dwarf star collects gas, mostly hydrogen, from a close companion star that's overflowing its gravitational boundary and spilling a stream to the white dwarf, like a trickle of water overflowing a tilted bucket. (In a close binary star, the gravity-and-momentum environment is not intuitive.) As the fresh hydrogen builds up on the white dwarf's surface, the bottom of this layer becomes compressed ever more intensely by the dwarf's powerful gravity. Eventually the layer's bottom becomes dense and hot enough to ignite in a runaway hydrogen-fusion reaction, which quickly spreads around the whole star — the layer becomes an H-bomb in the form of a thin shell.

The hot blast of material expands outward and remains opaque for a while, looking from the outside as if the whole star is swelling enormously. The brightness typically jumps by 10 magnitudes. Yet the shell contains only about 1/10,000 of a solar mass. In the following days or weeks the ever-widening shell cools, thins, and becomes transparent, fading and allowing the original star system in its middle to shine through. The white dwarf eventually settles down to its previous state, the gas stream from its companion resumes, and the cycle begins anew — building toward the next explosion in anything from a few years to tens of thousands of years. The more massive the dwarf and the stronger its gravity, the faster the explosions usually repeat.

In a supernova, by contrast, an entire star explodes completely and for good.

Classical novae may all work alike, but their light curves often behave differently. Clearly, other things influence the course of events. Sudden brightness drops like the one happening now are unusual. It could halt or even reverse at any time.

About 10 novae in the Milky Way are discovered each year, out of the 40 that are estimated to take place throughout the galaxy.

Here is the AAVSO's list of recent observations.

The nova's yellow color in the eyepiece seems to be deepening. Here's a color image of its spectrum taken March 17th, by Jerome Jooste in South Africa using a Star Analyser spectrograph on an 8-inch reflector. Note the wide, bright emission lines. They're flanked on their short-wavelength ends by blueshifted dark absorption lines: the classic P Cygni profile of an object with a thick, fast-expanding cooler shell or wind.

Below are comparison-star charts from the AAVSO, for estimating the nova's brightness. Stars' visual magnitudes are given to the nearest tenth with the decimal points omitted. The nova is at declination –28° 55′ 40″, right ascension 18h 36m 56.8s (2000.0),

Check back for more updates.

Comparison-star chart for Nova Sagittarii 2015 No. 2

The cross at center is Nova Sagittarii 2015 No. 2. Magnitudes of comparison stars are given to the nearest tenth with the decimal points omitted. The frame is 15° wide, two or three times the width of a typical binocular's field of view. Courtesy AAVSO.

Nova Sgr 2015 No. 2 deeper comparison-star chart

And here's a deeper, magnified comparison-star chart, 3° wide, for when the nova fades further.


For more to see with your binoculars, check out Gary Seronik's Binocular Highlights book!

9 thoughts on “Nova Sagittarii: What a Long, Strange Fade It’s Been

  1. Graham-Wolf

    Past it’s peak, and fast fading out. Let’s keep watching and recording it’s denoument (as devoted astronomers do). A rather nice distraction from C/2014 Q2 Lovejoy. Thanks to Alan and the S&T team for keeping this exploding star very firmly in the public eye!
    Adios Nova Sgr 2015 No 2..

    Graham W. Wolf:- Barber Grove Observatory
    Lower Hutt, New Zealand

  2. Anthony BarreiroAnthony Barreiro

    We had another clear morning in San Francisco today, 25 March. Around 0545 PDT I was able to see the nova again with 8×42 binoculars. Based on the comparison chart I guessed it was between fifth and sixth magnitude. It’s a pleasure to see something new in the heavens.

  3. RussRusty

    After finishing photography of the recent lunar eclipse (Saturday morning, April 4th), I was able to spot the nova with the naked eye before twilight got too bright. This was verified by viewing with some 7 X 50 binoculars. I was observing from Bone Mountain in the Oregon Coast Range at 3665 ft. (1117 meters) elevation, with snow on the ground. It’s nice to have a bright nova for this spring. It should be interesting to follow in the coming months.

  4. Graham-Wolf

    Well done:- Rusty!
    I managed to get a quick peek with 10 x 50 binocs at Nova Sgr during Lunar Eclipse Totality when the sky was very dark. Located Low in the East and below tail of Scorpius. Eclipsed Moon was high in the near NW, locally. Excellent views of BOTH from the Hutt Valley in NZ in clear skies, with a 20kt Northerly.
    Graham Wolf:- Lower Hutt.

  5. George Gliba

    I have seen 20 galactic novae since 1967. That was the year that I observed the slow nova,
    HR Del, I also saw another slow nova in 1995, V723 Cas. N. Sgr 2015 No. 2 looks like it may
    be a slow nova also. That would mean that its peak may not come until July and it may get as
    bright as 3rd nagnitude if it behaves like HR Del and V723 Cas did. That also means that it may
    possibly be a naked-eye object in a dark sky all summer. Let’s hope I am correct.

  6. George Gliba

    If Nova Sgr 2015 No. 2 isn’t a slow nova of the HR Del type as V723 Vul was,
    then it may still be a slow nova of the DQ Her type, as NQ Vul was in 1976,
    and may fluctuate for a few weeks before it drops rather fast out of naked-eye
    range. I recently found this out checking some other slow novae light curves.
    Although I didn’t follow NQ Vul visually much, I remember it distinctly as it was
    a 7th magnitude star among the stars of the Coat Hanger asterism. Interestingly
    it was the great English nova, comet, and meteor observer G.E.D. Alcock who
    discovered HR Del and NQ Vul, and almost found DQ Her. I say almost because
    he was observing meteors the night it was discovered by Prentice and quit early.

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