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