Amateur Astronomy


#1341

Not pure astronomy but its complicated…


#1342

My close encounters of the 1st kind recently eventually morphed as I slowly realised I was looking at a train of about 20+ of the starlink satellites a few nights ago… that was interesting.

What would a plague be without a Comet, though it’s usually in the reverse order in terms of the play out of malady catastrophic.

@ps200306 Is this or will this be visible to us mere mortals anytime soon?


#1343

Comets are notoriously fickle. They can show lots of promise but eventually fizzle. This one can already be seen with binoculars and may yet become a naked eye object. Or it may not. We can’t predict the volatility of a comet’s surface and thus the size of coma and tail it will develop. That’s especially true of long period comets like this one, which hasn’t approached the Sun since Stonehenge was built and folks figured out how to smelt bronze.

But hope springs eternal. Comet ATLAS already has a visible coma, half a million miles wide. It’s greenish – caused by ultraviolet light from the Sun exciting cyanogen (a carbon-nitrogen compound) which boils at around -20°C. If it keeps on stewing, we’ll see something in the next six weeks. It’ll be low to the horizon in the north west, moving back closer and closer to sunset as it heads for rendezvous with our star. They say it could be brighter than Venus, but that’s it’s integrated magnitude – a fuzzy ball will never appear as bright as a point-like star or planet of the same magnitude. But we may as well rave about it now before it fails to live up to expectations.


#1344

Well whadya know. That didn’t take long. Before it reached naked eye visibility it’s already started to fade again. Astronomers think it may be breaking up.


#1345

I saw this and then thought are these closures having any impact on tracking Comet Atlas?


#1346

No. The world’s largest telescopes don’t look at comets. They are multi-million dollar pieces of equipment with observing time booked years in advance. They don’t generally have time to look at novel objects of interest. A few specialised sites (such as PanSTARRS) conduct transient object surveys, but the largest telescopes are not suitable for that as you need wide fields of view. PanSTARRS is a pair of 1.8 metre scopes with a 3 degree field of view. Longitudinal studies of individual comets are – and probably will remain – the domain of amateurs.

Even though it’s fading, ATLAS is still easily visible to very modest telescopes. There’s even still binocular observations in the last 24 hours, albeit with a pair of 20 x 100’s from Sweden. It’s an easy spot with an 8-inch scope, and dozens if not hundreds of observations are reported every night from around the world. Amateur observations are of genuine scientific value. They measure integrated magnitude and estimate coma and tail angular size. You can see a few here: https://cobs.si/

As for tracking, gravity is a curious thing. We know exactly what Comet ATLAS (or its remnants, should it not survive its solar encounter) will do for the next century at least, but have much less idea over thousands of years. You can see some of that here: https://theskylive.com/c2019y4-info

Think of it like shooting an arrow from a bow almost vertically into the air. Initially it races upward, mostly unaffected by air currents and breezes. At the top of its trajectory it hangs suspended, where it wavers and flutters. Its fletching is buffeted by even the gentlest breezes so that we can’t predict where it will hit the ground.

A long period comet is a bit like that. Our Sun is the archer who gathers up the falling arrows and shoots them back out to space. The comet is the arrow which spends just a few years in the inner solar system (“near the ground”) and although it can be strongly gravitationally affected by planetary encounters we know how to calculate that with great precision. Thousands of years from now ATLAS will reach the “top” of its trajectory, a hundred billion kilometres from the Sun. There it will spend century after century, moving at mere centimetres per second. What it might encounter in those frozen depths of space we have no idea – nothing very large, we can say with certainty, but like the floating arrow it doesn’t take much to perturb it.

The next return to the inner solar system will see it on an altered trajectory so that we don’t know in advance how it will interact with the planets. On its current orbit we know it has been slingshotted so that it will take 400 years longer to come back next time. The very farthest comets can be substantially affected by the gravity of passing stars over millions of years near aphelion. That is why there will always be an enormous reservoir of hitherto unseen comets waiting to be flung in our direction. They are long lost companions that formed at the same time as our own planet more than four billion years ago, but were banished to the icy wastes by close encounters with the nascent giant planets.


#1347

#1348

Thanks for that! The Pillars of Creation are one of my favourite astronomical objects.

They’re just a small part of the much larger Eagle Nebula. You can see them just below and left of centre in this image:

They’re an example of a general phenomenon colourfully called “elephant’s trunks”. The canonical example is actually called the Elephant’s Trunk nebula, also part of a larger nebular region called (rather less colourfully) IC 1396:

It’s not by accident that elephant’s trunks are associated with larger clouds. Stars form when giant molecular gas clouds collapse under gravity. Anyone who studied second level science will remember the gas laws. As a gas is squeezed it heats up and the pressure rises, resisting further compression. In order to squash all the way down to a star the cloud must start off incredibly large and incredibly cold, just 20 degrees above absolute zero. This is the so-called Jeans criterion, which I wrote about fairly awkwardly five years back.

Cold molecular clouds are opaque and able to shield their internal bulk from radiation coming from the general surroundings, which would otherwise heat them. Generally a large cloud fragments into a whole lot of overdense knots from which individual stars will form. As soon as they do so the new stars start to produce their own radiation and stellar winds. This quickly starts to blow away the remaining cloud. But some of the denser knots in the cloud are more resistant to dispersal. While they remain intact, they shield a tube of molecular gas behind them from the corrosive radiation. This is how an elephant’s trunk forms.

The radiation pushing on the end of the elephant’s trunk can cause it to further densify and trigger star formation within it. Infrared telescopes which can see through gas and dust at long wavelengths can see protostars forming inside, as in propertyspire’s Hubble image. Some of these are mere hundreds of thousands of years old. In the grand scheme of things elephant’s trunks are quite transient, destined to thin out and reveal the new stars that have been born inside.


#1349

The moon looks very clear atm.

Worth going out the back with a pair of binoculars to take a look.


#1350

3 posts were split to a new topic: Starlink Gazing


#1351

https://www.quantamagazine.org/black-hole-paradoxes-reveal-a-fundamental-link-between-energy-and-order-20200528/


#1352

Most “comet of the century” announcements turn out to be duds as they have a habit of fizzling without reaching their potential. But this one has already survived perihelion on July 3rd and is visible to the naked eye. Comet NEOWISE (less glamorously known as C2020 F3) can be seen from Ireland this month. It’s reportedly at magnitude 1.6 which makes it easily naked eye visible, and allegedly you can spot a tail through binoculars. It’s the brightest northern hemisphere comet since Hale-Bopp in 1997.

It’s close to the northern horizon about an hour after sunset and gets higher as it moves northeastward throughout the night. Look for the bright star Capella in Auriga – you can’t miss it, it’s the only bright star in that rather sparse northern sector of the sky. NEOWISE will be to the left and a little below Capella.

image

Why not familiarise yourself with Perseus while you’re at it, in preparation for the Perseid meteor shower in mid-August.


#1353

Anyone spotted NEOWISE yet? I’ll be keeping an eye out from Sunday night onward, as the current weather front moves away to the south east and high pressure tries to build. The cloud might break up enough early next week for some views.

On the plus side, NEOWISE rises higher above the northern horizon as the month goes on. It is past its first magnitude peak, but seems to be holding steady at mag 2. The important thing is that it’s easier to spot as it moves away from the Sun and the glare of sunrise and sunset. It reaches its closest to Earth – around 100 million kilometres – on July 23rd.

NEOWISE has also developed distinct dust and ion tails, as can be seen in the great photos being taken around the world. Ion tails are caused by the solar wind lighting up outgassed material from the comet. While the gases are always blown directly outward from the Sun, the effect on dust particles depends on mass. So we see them trailing after the comet in its curved orbit, a bit like a car skidding round a bend. This image from the Parker Solar Probe which is orbiting the Sun captures NEOWISE on July 5th, just a couple of days after perihelion. That’s when the comet’s orbit is undergoing the greatest change in angle, so the separation between the tails is more of a handbrake turn than a skid:

Both tails point away from the Sun so that on this outbound leg from the solar system the tail of the comet is actually streaming out in front of it. The comet is only doing a laggardly 8 km per second or so, while the solar wind zips past at 400 km/s. The tail will gradually dissipate as the distance from the Sun increases. Three thousand years from now the comet will be at the apex of its orbit, far beyond where the solar wind has ceased to blow. Instead it will feel the drifting winds of interstellar space (though that hardly conveys the extraordinary emptiness at just one atom per cubic centimetre). The Sun from out there will be a pointlike but intensely bright star, about 100 times brighter than the brightest planet Venus seen from Earth. But the sunlight will be ten times dimmer than a full moon, possibly enough to see your hand in front of your face but no more.

NEOWISE over the north western US, © Kenneth LeRose:

A clear view of the ion and dust tails, © Sebastian Voltmer:

Closer to home, this is from Spookwoman over on boards.ie, snapped from Waterford:

Impressed that she managed to get the double tail. This is with a Canon 750d and a 70-300mm zoom, no telescope or tracking mount. 75 x 4 second exposures with image stacking software. Here’s another one from her, over Waterford City:

More great pictures on NASA’s “astronomy picture of the day” Facebook page. You’re not going to see anything like the above if you’re lucky enough to spot NEOWISE with the naked eye, but I’m told the tail is clearly visible in binoculars and up to five degrees long. That’s ten moon diameters. In astronomical photos it is revealed to be 15 degrees long. It is certainly millions of kilometres in length, though previous comet tails have been up to a billion kilometres long! Here’s a spotter’s guide for the next couple of weeks, courtesy of astronomynow.com. You’ll be able to use the stars of the Big Dipper as pointers:


#1354

Finally bagged the comet tonight. Not an easy spot, given I was looking north across Dublin bay toward the lights. It didn’t get dark enough to see it until about 11.30pm. I reckon its worse than 3rd magnitude at this stage, possibly closer to 4th, and definitely not naked eye visible from my location. Needed a bit of star hopping with binoculars before finally seeing it. Once spotted, the tail was easily visible fanning out above it. But the whole thing is a faint fuzzy smudge, possibly a bit underwhelming to anyone expecting a streaking fireball.

I found it easiest to use two of the Big Dipper stars to point down to the northern horizon, then scan back up with binoculars to Talitha and Alkaphrah (ι and κ U. Majoris) which are the two stars on a slant to the right of the comet above the tree in the picture. They were also not naked eye visible but a fairly easy spot in the binocs, occupying about half the field of view. From there you can almost get the comet in the same field of view to the left. Bear in mind the comet will be moving slightly higher and westward over the coming nights.

Apart from the comet, managed to entertain companions with two successive bright passes of the ISS (visible all week this week), a couple of other satellites, a bright meteor, and Jupiter and Saturn very bright near opposition.


#1355

I looked briefly on Saturday night but no joy. Spotted the ISS a few weeks back. Definitely surprised by the brightness of the pass. Does the low earth orbit subject them to more or less radiation that higher orbits?


#1356

No, the height makes no difference to the intensity of sunlight on the satellite. Even geosynchronous orbit – which is roughly a hundred times higher than the ISS – is only a couple of hundredths of a percent different in distance to the Sun. The height does make a difference to the flux of reflected light received on the ground. Like any object emitting or reflecting light, the flux follows an inverse square law due to the angular width changing with distance. It depends on other details too, like the reflectivity and orientation of the surfaces. The height also makes a difference to how long the satellite stays in sunlight on a flyover. The ISS is as bright as it is compared to other LEO satellites mainly because of its size, being the length of a football pitch. It subtends an angle roughly equivalent to the planet Venus, so it’s not surprising that it shines about as brightly as Venus, since they’re both fairly reflective objects.


#1357

Light refraction near the horizon can be quite deflected. When you ‘see’ the lower limb (edge) of the sun touching the horizon, the upper limb has in reality just passed below it!! Anything less than 10 degrees above the horizon there are (very) annoying additional corrections for light refraction (alterations in air pressure, temperature etc.) as the light passes so obliquely through the atmosphere. So perhaps brightness maybe affected, but 10 degrees is very low.


#1358

Tonight


#1359

I went out for a look at 1.30 am to see if it was worth going further afield. I’ve been watching the Perseids from Brittas Bay the last couple of years, as it’s easily accessible, decently dark, and low-lying (i.e. good for weather compared to mountains). Although the sky had cleared, it was still fairly hazy and I reckoned the last quarter moon was going to wash out a lot of meteors. So I didn’t bother. Might look again tonight when the moon is less of an issue, although I usually don’t bother except on the night of absolute maximum. The Perseids were quite rewarding the last couple of years, though.


#1360

It was pretty good last night until the fog rolled in. Space X junk is getting a bit irritating though.