Archive for the ‘Astronomy’ Category

Recurrent Nova RS Ophiuchi in outburst!

August 10, 2021

The recurrent nova RS Ophiuchi is in outburst for the first time since 2006!

Other known outbursts were observed in 1985, 1967, 1958, 1933, and 1898.

RS Oph outbursts since 1933

The current outburst is shown at upper right of the plot that includes outbursts from 1933 onward.

Around 15 visual observations of the current outburst had been recorded in the AAVSO International Database and the nova had reached around magnitude 4.6 at the time I started writing.

The nova will be visible to the unaided eye from some locations and easily visible in binoculars from suburban locations (at least here in South Australia).

Only a handful of recurrent novae are known and the time between outbursts tends to be measured on a timescale of decades, averaging 20 years for RS Oph but varying +/- 10 or so years.

These systems involve a giant star (in this case, a slow irregular variable with a period of a few hundred days) and a white dwarf star pair in which mass is transferred from the giant to the white dwarf, forming an accretion disk which eventually undergoes a runaway thermonuclear reaction.

Example of a Cataclysmic Variable (RS Oph) from http://apod.nasa.gov/apod/image/0607/rsoph_pparc_big.jpg
Artist’s impression of RS Oph (from APOD)

RS Oph is around 5000 light years distant and so of course, the event we now see happened around 3000 BC.

The magnitude of the RS Oph binary star system increases rapidly from magnitude 10 or 11 to around 4.5 in the space of day, taking around 100 days to return to the baseline in a characteristic decay curve.

Within 2 days it will diminish by around 1 magnitude and by around 2 magnitudes within 5 days.

So, clear skies would be nice! The local forecast does not look great for Adelaide over the next few days.

It was cloudy by the time I read the outburst notification tonight and first started writing this post, but the sky cleared at around 1 am!

I observed the nova low in the west through 7×50 binoculars and estimated its magnitude to be 4.5 (Julian Date 2459436.16237 which is almost 15:54 UT or 01:24 Australian Central Standard Time):

The last month or so of observations. My magnitude 4.5 observation is shown in the cross-hairs.
The last few days of observations. My magnitude 4.5 observation is shown in the cross-hairs.

The 43 and 46 comparison stars (at about 5 and 11 o’clock with respect to the nova) on the following AAVSO finder chart were used for the estimate:

The finder chart needs to be rotated clockwise somewhat more than 90 degrees but less than 180 degrees to match the orientation of the sky shown in the Stellarium image below:

Note the position of the three roughly magnitude 3.5 stars in a straight line above the nova which is below and to the right of the middle of these stars (in the Stellarium image), with Scorpius at far left. These 3 stars occupy much of the finder chart near the nova (which is above and to the right of the middle star on the chart).

While every variable star type has its own interesting qualities, I find novae to be the most fascinating sub-type of cataclysmic variables and probably of all variable types because of the power and unpredictability involved. They are a reminder that we live in a violent, hostile yet endlessly fascinating universe, that we are, as Bernard Lovell said: In the Centre of Immensities.

It will be interesting to see whether the brightness has now peaked. If previous outbursts are anything to go by, it may have. But we’ll see! Novae and their much rarer cousins, recurrent novae, can really keep me up late at night (it’s now 2:55am here)!

Possible Nova in Sagittarius

April 6, 2021

Western Australian amateur astronomer Andrew Pearce has discovered a possible nova (PNV J17581670-2914490) in Sagittarius on April 4. At that time its visual magnitude was around 8.8.

Andrew reported in a message to Variable Stars South today that the object has been classified via spectroscopy as a classical nova.

The nova’s visual magnitude is currently around 8 or a little brighter. As always with these objects, it will be interesting to see how bright it gets.

Here is a finder chart suitable for binoculars (something of a challenge at the current brightness in 7x50s):

Rotating this finder chart a bit more than 90 degrees anti-clockwise will roughly match this image from Stellarium at around midnight AEST.

Zooming out gives more positional context:

Further out still shows even more familiar landmarks in Sagittarius and Scorpius.

I have yet to observe or image the nova but hope to do so over the next few days.

EDIT: After writing this I went out and had an initial look at the area. It was after midnight and the nova was in a good position from my backyard. I was just able to glimpse it in 7×50 binoculars but not well enough to make an estimate. So I took a few quick untracked shots of the region from which it appears that it’s probably around magnitude 7.8 (visual) which is consistent with what I glimpsed through binoculars.

I’ll keep an eye on it over the next few nights and hopefully take some better images.

EDIT: the weather did not improve early enough while the nova was still bright enough for me to observe.

Jupiter and Saturn on Dec 23

December 23, 2020
Canon 1100D with Meade LX-90 8″ scope, ISO 6400, 1/15 second at 21:51 ACDT (click image to enlarge)

The two planets are slowly separating, tonight to 13 minutes of arc, around one fifth of a degree, up from 6.5 minutes of arc on Monday night.

My main goal tonight was to share the view with Karen, who worked the previous two nights, before Jupiter and Saturn are no longer in the same low power field of view. She enjoyed it.

I didn’t have a lot of time for set up and imaging tonight, but wanted to take an image that emphasised the planets themselves rather than their moons. The focus is not great, and Jupiter is still overexposed, but I like the fact that Saturn’s ring and the planet are distinct here.

The good news is that I have my Meade LX90’s AutoStar back from repair now, and it works well! This will encourage me to start doing tracked, piggy-backed, wide-field photometry again. It’s been awhile.

Jupiter & Saturn in 8″ scope (untracked)

December 22, 2020
Jupiter and Galilean moons plus Saturn with Canon 1100D with LX-90, ISO 1600, 1/5 second at 21:37 ACDT (click image to enlarge)

As mentioned in yesterday’s post, my Meade LX-90 8″ telescope’s AutoStar is being repaired, but tonight I decided to attempt to image the conjunction anyway with manual pointing and no tracking. Fast shutter speed and high gain was important to reduce the effects of rapid movement while obtaining enough detail.

The separation between the two planets was still around one eighth of a degree tonight, well within a low power eyepiece (24.5mm super wide angle) and my Canon 1100D’s sensor frame.

Sky Safari Pro screenshot identifying the four Galilean moons

All four Galilean moons are visible along with Saturn’s rings and the ball of the planet. Io is visible as a “bump” on Jupiter at around 11 o’clock.

Note the reversed telescopic view due to the optics.

The focus is not amazing, but under the circumstances, it turned out reasonably well. The planets were low in the sky as well.

Although I had images containing Saturn’s largest moon Titan, I wasn’t happy with the quality.

I also took a wide-field shot of the pair low on the western horizon, peeping through cloud, not long before the sky became cloud-filled. The exposure and gain make the sky appear abnormally bright.

Jupiter (top) and Saturn with Canon 1100D, 100mm focal length, f2.0, ISO 400, 1 second exposure at 21:53 ACDT

Jupiter-Saturn conjunction view

December 22, 2020

I had a nice view of the Great Conjunction of 2020 last night with Saturn, its largest moon Titan, Jupiter, three Galilean moons (there was a star near Europa that I initially mistook for a moon), all visible in a low power eyepiece.

My Meade LX-90’s AutoStar hand controller is in for repair so unfortunately I had to position the scope manually. With no fine controls or tracking, that was awkward but doable. Imaging, not so doable. There will be plenty from others though.

I expected the two planets to appear a little closer on the sky, but in hindsight, should not have.

I’ll be out again the next couple of nights for another look since, as per my last post, the two planets will still be quite close for the next few nights.

Jupiter & Saturn wide field, Dec 19 2020

December 19, 2020

Two days before the Great Conjunction of 2020 (on Dec 21) in which Jupiter and Saturn will appear at their closest in the sky in nearly four centuries, I took a wide field image of the pair low in the west on Dec 19 at around 9:50pm ACDT (click to enlarge).

Of the two brightest objects near the centre, Jupiter is at left and Saturn at right.

The planets are separated by around 16.5 minutes of arc or 0.275 degrees or a little more than half of the angular size of the full Moon. At the same time on Monday, Jupiter and Saturn will be separated by less than 6.5 minutes of arc or not much more than a tenth of a degree. On Dec 22 and 23 they will still be quite close, at almost 7.5 and 10 minutes of arc respectively.

Jupiter and Galilean moons plus Saturn with Canon 1100D, 100mm focal length, f2.0, ISO 800, 1 second exposure

The following screenshot from Sky Safari pro (iOS) helps with identification. Most of the moons, except for Europa, Ganymede and Callisto are not visible in the wide field image.

Sky Safari Pro screenshot

A cropped portion of the image (click to enlarge) shows the Galilean moons a little more clearly, including Io as a slight bump at lower left of Jupiter. Titan is barely visible at the lower left of Saturn. The resolution is not high enough to see Saturn’s rings or any detail on Jupiter.

Cropped portion of wide field image

I’m hoping that at least one or two nights early next week, the local weather will cooperate for more viewing of the conjunction.

Dad’s old Mac and DSLR photometry

September 29, 2020
Photo by Junior Teixeira on Pexels.com

After upgrading my MacBook to macOS Catalina, WINE stopped working, because 32-bit applications were no longer permitted to run. I had been using WINE to run the Windows IRIS program for image processing and DSLR photometry as part of my minimalist approach.

After my father died in January this year, his older MacBook was passed on to me. It still has Yosemite as the OS, so WINE & IRIS work fine on it!

Given all the times we spent talking about astronomy, the special time we shared watching the 2001 Leonids under a dark country sky, and the “help desk” support I tried to give him over the phone, I know that dad would approve of my use of his old computer in this way.

Certainly better than having it sit idle.

Thanks dad.

Photo by David Besh on Pexels.com

Nova Ret 2020 Update #2

July 24, 2020

As of July 24 2020, there are more than 100 Nova Ret 2020 observations, most visual, with a handful of CCD submissions. Mine (visual) are shown in purple.

Most observers are from Brazil (10), then Bolivia and Australia (5 each), with one from Italy and South Africa.

The southerly declination of the target and comparison stars in Reticulum (almost -55o degrees) makes northern hemisphere observations difficult.

A linear fit shows an overall decline of around 0.17 magnitudes per day, but there’s a lot of spread in the data and novae are unpredictable.

Nova in Reticulum!

July 17, 2020

Rob McNaught at Siding Spring, Coonabarabran, NSW found a magnitude 5.3 object in the constellation Reticulum on July 15 2020.

MGAB-V207’s pre-brightening magnitude was 15.8 (Johnson V). Just before I wrote this, it was designated N Ret 2020 in VSX and the AAVSO International Database.

I observed the nova at around 5:30am this morning (July 17) in 7×50 binoculars. My estimate, based upon 4.95 and 5.45 visual magnitude comparison stars, was 5.2.

It may have been brighter but that was my best estimate, given the seeing quality at the time, and a lack of coffee. Having said that, I’m confident that it was reasonable.

At the time of my submission to AAVSO this morning, there were 6 observations, including mine. Andrew Pearce in WA submitted his second observation (brighter) soon after mine.

This is the first bright nova since 2018! So I’m a little bit excited.

To get familiar with the field, I started with Stellarium:

and this AAVSO finder chart, which needs to be rotated by about 45 degrees anti-clockwise to match the Stellarium view:

In general, the sky was lovely this morning. After estimating the nova’s brightness, when the dawn became evident, I took this quick shot of a conjunction of the crescent Moon, Venus, and Aldebaran (alpha Tauri) low in the NE sky (1/5 sec, ISO 200, f 2.0, unprocessed) with our recently pruned walnut tree (thanks Karen!) visible at upper right:

Between the nova, satellites passing through my binocular field while observing it and the stars of Reticulum, a stray meteor, the Luna-Venus-Aldebaran conjunction, Orion rising in the east, and the general beauty of the sky, it was an uplifting start to the day.

I was again reminded that there is a hidden sky, waiting for all to see.

Variable stars and novae in particular, always reinforce to me the dynamic, constantly changing universe of which we are a small part. In what often feels like a dystopian world, especially in 2020, I find it oddly comforting that the Universe just keeps doing its thing, irrespective of us. Astronomy is a great way to get some perspective.

Current Betelgeuse Dimming Event

December 29, 2019

The semi-regular late-type variable supergiant star alpha Orionis, better known as Betelgeuse, pulsating with a period of around 420 days at a distance of 640 light years, one of the largest stars visible to the unaided eye and normally in the top 10 list of brightest stars in the sky, is currently undergoing a rapid dimming event.

betelgeuse-size-comparison-with-the-solar-system

source: ESO

If Betelgeuse was located at our sun’s position, it would engulf all the inner planets and extend out to the vicinity of Jupiter’s orbit.

When this star ends its life in a supernova explosion it will be visible in daytime and cast shadows at night.

There have been recent questions about the possibility of Betelguese going supernova (or having done so already) and whether the current dimming may be a sign of it.

ATel 13365 has this to say about the dimming event:

The current faintness of Betelgeuse appears to arise from the coincidence of the star being near the minimum light of the ~5.9-yr light-cycle as well as near, the deeper than usual, minimum of the ~425-d period.

It’s interesting to look at past observations of the star.

Here are the last 2 years of visual and Johnson V observations:

alfOri2yrs

The last 5 years of observations:

alfOri5yrs

The last 10 years of observations:

alfOri10yrs

The last 25 years of observations:

alfOri25yrs

In this and the next plot you can see the transition to image-based photometry about 50 years ago in the form of Johnson V data points.

The last 50 years of observations:

alfOri50yrs

The last 100 years of observations:

alfOri100yrs

It’s apparent that there have been similar dimming events over the last 100 years, e.g. in 1947, 1985 and other years.

alfOri100yrsWithFilterGT1.3.png

It’s not obvious that the current dimming event is significantly more rapid than others that came before it, at least by looking at the slope of a linear fit of dimming events.

Alpha Orionis is a bright star, varying between around magnitude 0 and 1.3. It’s easily observable in the north-eastern late evening sky at the moment, along with comparison stars such as Aldebaran and Pollux.

You can create a finder chart via the AAVSO website. An easier way to get started is to see pages 2 and 3 and the chart on the last page of the AAVSO Citizen Sky southern “10 star” tutorial.

The submission of observations to the AAVSO is encouraged and there’s no need for binoculars or a telescope.