I recently carried out photometry of the short period pulsating variable star SX Phoenicis.
This star has a very short period of 79 minutes over which it ranges between around magnitude 6.8 and 7.5! Contrast this with a Cepheid variable whose period is measured in many days or weeks and Miras with pulsation periods of many months.
The light curve above was derived from photometry of more than 400 images, using median stacking to group images close in time in order to reduce the scatter between datapoints. Almost two cycles were captured here. There are not many observations of SX Phoenicis in the AAVSO International Database, so I will very likely contribute more.
SX Phoenicis is the prototype (the first example) of a class of variable stars which has two pulsation modes, one with a period of around 79 minutes (as shown in my light curve above), the other around 62 minutes. The temperature of the star varies between 7,230 and 8,170 degrees Kelvin at minimum and maximum brightness, smallest and largest radius, respectively.
The origin of this kind of star is unclear but one possibility is that it comes from the merger of two stars, creating a single star that is more luminous and more blue than expected of the older galactic halo population in which it resides, a so-called blue straggler.
I recently took an image of the Eagle Nebula (Messier 16) in the constellation Serpens with my Seestar. This object is around 6000 light years distant from us.
M 16: The Eagle Nebula (20 minute exposure, f/5)
The nebula takes its name from imagined outstretched wings. I agree with Karen that a small dark region near the centre of the nebula also bears an eagle-like (possibly even moreso) semblance.
M 16 (region imaged by Hubble and James Webb Space Telescopes)
The Hubble Space Telescope (HST) imaged this region in 1995 with the result dubbed The Pillars of Creation (credit: NASA):
The tips of the finger-like structures glow with stars forming within. The arrowed region in my image is rotated 180 degrees from HST’s.
I’ve had a Seestar S50 since mid-August 2024. The S50 is a small refracting (f/5 apochromatic triplet) telescope with a focal length of 250mm, an aperture of 50mm, and a ZWO-based imaging system that can be controlled with an iOS or Android app.
Images can be taken, enhanced and accessed via the app but the FITS (Flexible Image Transport System, commonly used in astronomy) files can also be downloaded via USB cable for further processing on a computer. This includes photometry, to determine the brightness of targets such as variable stars or asteroids.
So far I’ve taken images of the Triffid nebula, planetary nebulae such as the Dumbbell and Helix nebulae, globular star clusters, the yet-to-erupt recurrent nova T Corona Borealis, a nova in Scorpius (V1725 Sco), a short period (79 minutes) pulsating variable (SX Phe), Luna during the daytime (just because), the Sun, and Pluto from my suburban backyard in South Australia.
I wanted to take two images a few days apart to show Pluto’s movement against the background stars. These images were taken on September 3 and 7:
Note that the images are “decorated” by information and cross hairs because this is a view of the field from the software Tycho Tracker that I use (primarily for variable star photometry).
Here is an animated GIF created from these two images to make the change in location of Pluto on the two dates more obvious. Focus your attention just to the upper right and lower left not far from the centre of the image.
Here are some undecorated images with arrows pointing at Pluto, on Sep 3 and Sep 7 (around 20 minutes of total exposure each, via multiple stacked 10 second exposures):
PlutoPluto
Pluto takes 248 years to orbit around the Sun at an average distance of almost 6 billion km.
How far did Pluto travel along its orbit between September 3 and 7?
At an average speed of 17,096 km/hour, over the 3.96 days between the images I took, Pluto travelled approximately 1,625,000 km. We could arrive at a better result with some trigonometry.
I also measured Pluto’s magnitude on September 7 at 14.64 +/- 0.05, very close to the catalogue value:
The image comparator below provides another way to reveal the location of Pluto on September 3 (upper right of red cross hairs) and September 7 (lower left of cross hairs), by moving the vertical line left or right.
Pluto on September 3 (upper right of cross hairs) and 7 (lower left of cross hairs)
Below are SkySafari Pro screenshots for comparison in case you want to check Pluto’s position for yourself on the dates and times in question.
Being able to see the movement of Pluto is something I’ve wanted to do for myself since Martin George showed me Pluto through the eyepiece of a 14 inch aperture reflecting telescope in Tasmania about 30 years ago.
In September 2024, a nova was independently discovered in Scorpius by Koichi Itagaki (Japan) and Andrew Pearce (Western Australia). Its designation is V1725 Sco.
I like to think of the lovely asterism (arbitrary star grouping) in which the nova appears (arrowed above) as:
The visual band light curve below shows the nova’s (partial) rise to around magnitude 9.5 in early September to around magnitude 13 almost a month later.
I made (and submitted to AAVSO) 6 observations of the nova with my Seestar.
The differential photometry aperture rings (red bullseye) are centred on the nova and the green highlighted stars are reference stars. Note the deliberate defocus, so that the light of the stars is spread across multiple sensor pixels, as is common for “one-shot colour” sensors such as the S50, DSLRs and others. My tests so far suggest this may be less necessary than for the DSLR photometry I’ve done in the past, possibly because of the differences between sensor sizes and the number of arc seconds per pixel.
One of the things I love about doing variable star photometry is the endless variation in the star fields being imaged, and the endlessly varying asterisms I see and can imagine about.
Here’s a Sky Safari screenshot of the region around the not-yet-visible-to-the-unaided-eye T CrB (aka The Blaze Star, bottom right):
at 10:34pm Adelaide (ACST) time on Sunday June 16. To disambiguate, Alphecca is the bright star (magnitude 2.2) to the lower left of that name, as is the case for Arcturus and Izar (which each appear in a less crowded part of the field).
Here’s a cropped iPhone picture of that region at the same time, above the roof of my house, showing Arcturus, Izar, Alphecca and the location where the Blaze Star (T CrB) will appear, as bright as Alphecca:
I showed an AAVSO finder chart in a previous post. This is the one I’m using for comparison stars down to magnitude 8, with T CrB in the cross-hairs at centre, and getting very familiar with through 7×50 binoculars from my backyard:
Here, Alphecca is the star marked “22” (magnitude 2.2) to the left of the cross-hairs, with Arcturus and Izar out of the field to the left. The 37 and 28 comparison stars are also visible in my iPhone image at right near the centre line.
I was having a conversation recently in which someone made the claim that “star hopping” (visually hopping between stars, with a star chart as your guide, to find a target object) is dead in the age of computerised telescopes. With visual variable star observing using binoculars, this is not the case. You have to get to know the field for every new variable star you want to estimate the brightness of. With DSLR photometry, before I added plate solving to my partially manual processing method, that remained true. Even now, especially when using a simple tripod, I still need to locate the right field.
In my suburban sky, at the low altitude of CrB, with 7×50 binoculars, I can see down to magnitude 7.1, so the reference (comparison) star just to the upper right of the cross-hairs.
In any case, if you want to be prepared for the T CrB eruption, get to know the field and the reference stars you can use to assist in an estimation.
One of the things I showed (reproducing a result from Brad Schaefer’s 2023 paper) was the similarity between the light curves around the eruptions of 1866 and 1946. The VStar plot below shows visual band data for the two eruptions in which the difference between the two eruption peaks has been added to the times of the 1866 observations.
I also showed the similarity between observations leading up to the 1946 eruption (the rise and dip) and recent observations:
and the two overlaid:
How much time remains before the next eruption is uncertain, but the signs are that it’s only weeks or in the worst case, a few months away.
T Corona Borealis — T CrB for short — is one of ten known recurrent novae. At a distance of around 3000 light years, it was first discovered as a nova eruption in 1866 by John Birmingham. Outbursts occur approximately once every 80 years. It may have been observed in 1217 and in 1787 as well.
T CrB is expected to explode again this year reaching naked eye visibility, around magnitude 2 or 3. The last outburst was in 1946. As always when we talk about astronomical objects, these events happened long ago, 3000 years ago in this case, with the evidence only expected to reach us this year due to the speed of light.
This is likely to be the brightest nova of a generation, certainly of the known recurrent novae. It will quickly rise from a visual band magnitude of 9 or 10 within a day or two to become visible to the naked for a few days, remaining a binocular object for a week or thereabouts, then returning to its pre-eruption magnitude within a month or so.
Exactly when it will brighten is uncertain and is discussed by nova expert Brad Schaefer and others in this AAVSO article, but the prediction is 2024.4 +/- 0.3, so May or June, but it could be earlier or later.
There was a pre-eruption dip in the light curve, 1.1 years before the 1946 outburst. A similar dip happened in March 2023 as shown in the last two years of T CrB observations in V and B bands, more prominent in the B band.
T CrB is located low in the north-eastern sky from Adelaide starting in the late evening. This Stellarium screenshot shows the circumstance for Apr 8 at midnight when T CrB is around 15 degrees above the horizon. Waiting an hour or two will help make observing easier with T CrB culminating at around 29 degrees above the horizon, but the region is viewable from around 11:30pm with a clear NE horizon.
This unprocessed, untracked image was taken with my DSLR (Canon 1100D, 100mm lens, f2.0, ISO 100, 10 secs) on Apr 7 at 2am, so it’s a little further rotated anti-clockwise than the Stellarium view above. The red arrow points to where T CrB will become visible and the green arrow points to alpha CrB (Alphecca). This shows the bright stars of the constellation of Corona Borealis.
Here is an AAVSO finder chart, with stars only down to magnitude 5. You’ll need to rotate it slightly clockwise to match the views above.
The comparison star marked 22 is the magnitude 2.2 star Alpheccca (alpha CrB). Izar and Arcturus (epsilon and alpha Bootis) do not appear in this finder chart, and are at upper left of the constellation Corona Borealis in this orientation. T CrB may approach Alpheccca in brightness.
I’ll be looking out for T CrB whenever I can stay up late enough or get up early enough until the outburst happens, using just the unaided eye in the first instance. Once visible in outburst, I’ll make estimates with 7×50 binoculars and time-permitting, DSLR images for subsequent photometry, submitting both to the AAVSO International Database.
Messages will also be posted on the AAVSO nova forum when the T CrB outburst happens.
I took wide field DSLR images of PNV J17261813-3809354, now Nova Sco 2024 or V1723 Sco, on February 11, 12, 15, and 17 at around 5:30am Adelaide time (ACDT) with subsequent calibration and photometry yielding observations submitted to the AAVSO International Database (AID).
The nova is marked on this image (click to expand):
Trailing is becoming apparent on this 10 second exposure, visual band image (calibrated and median stacked from a subset of images), as is the deliberate defocussing to spread the light over multiple elements of the DSLR’s Bayer array.
Here is the light curve as of February 17:
After around 9 days, there are only 116 visual band observations from observers around the world, 49 from DSLRs, 27 from some other imager, and 40 from visual observing (binoculars, telescope).
The nova seems to have peaked at around magnitude 6.8 or 6.9 and as of the time of writing (February 17) is dimmer than magnitude 8 and is steadily declining.
My 4 visual band DSLR observations are shown in purple, with the one under the cross hairs at magnitude 8.1 in close agreement re: time and magnitude with a visual observation made by Andrew Pearce (the discoverer). I have also submitted blue and red band observations, not shown above.
My imaging gear is fairly minimal, as shown below:
Canon 1100D DSLR, 100mm f2 lens on 25+ year old Manfrotto tripod (USB connection to my dad’s old Mac)
Custom built light box and DSLR to obtain flat frames for calibration (Mac in dark at right)
You can’t choose the time or sky conditions. Here are hand-held iPhone 13 images of the some of the pre-dawn skies, before and after observation on Feb 15 and Feb 11:
Feb 15, 05:12am ACDT, eastern sky Scorpius to upper left of tree at rightFeb 15, 05:49am ACDT (Venus rising)Feb 11, 05:17am ACDTFeb 11, 05:53am ACDT (Venus rising)
The Central Bureau for Astronomical Telegrams reports that Andrew Pearce in Western Australia (Nedlands) discovered a transient in Scorpius on 2024-02-08, at a visual magnitude of 7.8, and 7.4 on 2024-02-09.
A CCD observation by Andrew a little over half a day later gave a visual (Johnson V) magnitude of 7.2.
It was also independently discovered by Y. Sakurai in Japan (Mito), estimated at magnitude 7.1 on 2024-02-09, and found on images by R. H. McNaught, New South Wales (Coonabarabran).
The transient is designated PNV J17261813-3809354 and it’s position on the sky corresponds to Gaia DR3 5974053153713533184, a 19.4 visual magnitude star.
From nothing observed in the field on 2024-02-07 (Andrew’s DSLR), it rose by more than 10 magnitudes within a day.
The Astronomer’s Telegram has classified the object as a nova near visible peak, however the behaviour of these objects can sometimes be surprising.
The following Stellarium fields show the transient’s approximate position (click to enlarge) at about 3:30am Australian Eastern Daylight Time.
The chart will need to be rotated by 90 degrees anti-clockwise to match the sky shown.
I observed the field with 10×50 binoculars this morning from around 5am to 6am Adelaide time (ACDT).
There was a significant amount of cloud around the area that made observing the nova difficult. I proceeded to take DSLR images before sunrise and there is some apparently useful data there. I will do the photometry later today and post an update.
Some updates are also starting to flow into AAVSO and Variable Stars South forums.
