Observations

A new star in Sagittarius: an excerpt from a meeting of the Sydney City SkyWatchers

Alan Plummer of Linden Observatory in the Blue Mountains sends the following fascinating article about an object that he is observing:

Come on a trip: Set out almost exactly for the center of the Galaxy, to be found in the direction of the constellation Sagittarius, and keep going for some thousands of light years. Remember that the Galaxy is a spiral, and that we’re traveling right through the crowded plane of the disc, toward the hub. Continue on out of our Local Arm of the Milky Way and across the more spacious inter-arm void, into Sagittarius-Carina Arm. And maybe further still.

Somewhere there, a couple of months ago (our time) was a binary star system that probably looked a bit like that pictured below. The system was small and dim, in the Galactic scheme of things, and certainly no ‘superstar’. Yet for the previous few tens of thousands of years a white dwarf star with a disc structure around it had slowly gained mass from a close companion. Finally, the dwarf formed a shell held tight around itself by gravity with enough mass to explode in thermonuclear fury. The system then increased in luminosity tens of millions of times; blowing away the disc, and is today heaving megatons of mass into space in an expanding and brightening shell. (The seat of the explosion was the bottom of the shell, on the dwarf stars’ surface.) Both stars will probably come to resemble the picture again, in a century or three.

Space-art_Mark A. Garlick

In some binary systems the newly gained mass falls on to the white dwarf star through an accretion disc. Image by Mark A. Garlick

Yukio Sakurai of Japan announced in mid April the discovery of the new object at magnitude 10.3, and neither the USNO catalogue nor the Digitized Sky Survey indicates any precursor at that position; it was to far away, and to dim. Sakurai is a dedicated amateur astronomer who surveys the sky with digital CCD equipment searching for novas, and he has many to his credit.

This discovery is indeed confirmed to be a nova, and is designated Nova Sgr 2007, also V5558 Sgr. More specifically, it’s a classical nova as opposed to the very different dwarf and supernova explosions. The above generalizations tell some of our current understanding of classical nova explosions; it will take time for this system to be described more uniquely.

V5558 Sgr is a classified as a slow nova, as is evident by the light curve reproduced below from the American Association of Variable Star Observers. A fast nova could have come and gone completely during the fifty day span of this figure, but V5558 Sgr has not yet peaked. (Or maybe just peaked; wait and see.) The curve commences from two days after discovery until today, the 6th of June. The purple boxes are the author’s estimates, the grey dots are those of other visual observers, and the green dots are photometric measurements.

Light curve for V5558 Sgr_AAVSO

Light curve for V5558 Sgr reproduced from the American Association of Variable Star Observers

As this is written, the nova is visible in binoculars as an eighth magnitude star, and will be easily found with a small telescope from suburban skies for a while yet. I encourage you to try: the nova can be found by plotting the coordinates R.A. = 18h 10m 18.27s Decl. = -18° 46′ 52.1″ (2000.0) on a chart, or use the chart below.

Finding chart for V558 Sgr_Alan Plummer

Finding chart for the nova V5558 Sgr, courtesy Alan Plummer

Alan Plummer alan@alplummer.com.au

3 responses to “A new star in Sagittarius: an excerpt from a meeting of the Sydney City SkyWatchers

  • > I was pleased to see you using my CCD measurements of the nova.
    > I am observer NMR and most of the AAVSO data is down to me.

    Wow, really? We better rename the star after you then, change the first A in AAVSO to N

  • Hi Martin,

    Always a pleasure to ‘meet’ another observer. I will watch out for your observations in future so I can credit them. I see you’ve observed the nova recently – my weather has been too bad, but hope for tonight?

    You will have seen the SOHO observations when in the nova was in conjunction with the Sun? Wonderful ‘alternate’ use of a satellite.

    Thanks for the comment.

    Regards, Alan

  • I was pleased to see you using my CCD measurements of the nova. I am observer NMR and most of the AAVSO data is down to me.

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