Archive for the ‘Astronomy’ Category

Waiting for U Sco: nearby gamma ray transient detected

September 29, 2021
The Fermi Gamma-ray Space Telescope (source: NASA)

Two days ago I wrote Waiting for U Sco. Today, here are a few excerpts from ATel #14941 (The Astronomer’s Telegram):

The Large Area Telescope (LAT), one of two instruments on the Fermi Gamma-ray Space Telescope, has detected a new transient gamma-ray source, Fermi J1623-1752.

A possible counterpart to the LAT transient is the recurrent nova U Scorpii with position offset by 0.18 deg from the LAT position, thus just outside the 95% confidence error circle, but within the 99% confidence error circle. U Sco has a known recurrence interval of 10.3 years (Schaefer 2010, ApJS, 187, 275) prior to its outburst in January 2010, thus a new outburst from U Sco could be anticipated. The last observation of U Sco in the AAVSO lightcurve database was on 2021 Sep 19.

Because Fermi normally operates in an all-sky scanning mode, regular gamma-ray monitoring of this source will continue. In consideration of the ongoing activity of this source, we encourage multi-wavelength observations, particularly optical follow-up of U Sco.

ATel #14941

I read about this in a post by Alexandre Amorim (Brazil) in the AAVSO novae forum. Robert Fidrich (Hungary) soon followed up with an iTelescope observation to say that there was no eruption visible.

Given that U Sco almost certainly had an outburst in its “seasonal gap” (unobservable due to position on the sky), probably in 2017, what this current gamma ray event means seems pretty uncertain.

I’ll continue imaging the U Sco region until it’s too low tonight (in between the clouds) and, consistent with my last post, whenever I can thereafter.

The Crab Nebula

September 25, 2021

We live only a few conscious decades, and we fret ourselves enough for several lifetimes.

(Christopher Hitchens)

I recently gave a 5 minute talk to a non-technical group about the Crab Nebula (M1, the first object in Charles Messier’s catalog) that related to a sense of wonder in Science. I thought the brief content was worth sharing here.

Chinese astronomers in 1054 witnessed a “guest star”, an explosion that gave rise to a remnant several light years wide called the Crab Nebula.

The object is several thousand light-years distant from earth, yet visible in a small telescope, although with nothing like the detail shown in the Hubble Space Telescope image of Figure 1!

Figure 1: The Crab Nebula, a Hubble Space Telescope image mosaic (source: NASA)

At the heart of the nebula is an object composed only of neutrons, a spoonful of which would weigh a billion tonnes, spinning 30 times per second, energising the nebula, wave-like rings expanding outward from the centre like ripples in a pond (Figure 2).

The original star ran out of the elements that had sustained its nuclear fusion for millions of years, its core collapsing then rebounding in a fraction of a second, ending as a supernova explosion.

Figure 2: The central region of the Crab Nebula (source: NASA)

The event synthesised elements heavier than iron that spread out into the space between the stars, to be incorporated into new stars and their planetary systems, some perhaps going on to develop life.

The iron in our blood, along with trace elements like zinc and iodine, come from such stars.

So, not only are we star-stuff, as Carl Sagan liked to say, but in fact we owe our existence to massive stars that burned bright and lived short lives (millions vs the billions of years our sun will last) then died, so that we could live for just a few decades.

RS Ophiuchi Decline Continues

August 16, 2021

The expected decline of the recurrent nova RS Oph continues, almost 3 magnitudes in 7 days. I’ve managed only 5 observations with a couple of cloudy nights between the last two. Luna at 60% illumination in nearby Scorpius tonight made estimating its magnitude (7.2) in 7×50 binoculars a little trickier tonight.

Recurrent Nova RS Ophiuchi Update

August 12, 2021

Since my Aug 9 observation of RS Oph, the AAVSO alert notice was published by Sara Beck: Alert Notice 752: Rare Outburst of Recurrent Nova RS Ophiuchi. My 4.5 visual estimate was listed there along with others from Australia, Brazil, the UK, the US, Belgium and Russia.

The next night was cloudy and wet, but there were enough clear periods tonight for another estimate, 5.7 visual, as shown below:

The last 2 years of RS Oph visual data (including photometry from CCDs, DSLRs)

The next two plots show more detail:

The last week of RS Oph visual data
The last few days of RS Oph visual data

I suggested in my last post that based upon previous outbursts, RS Oph will have reduced by 1 magnitude in 2 days, which it has done. A little more actually, from 4.4 on Aug 9 to 5.7 on Aug 11.

These objects are a reminder that we live in a violent, ever changing universe.

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
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.