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Symbolic Forest

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Blog : Posts tagged with ‘Messier objects’

Under clear skies

Or, some unexpected astronomy

“Is it cloudy or clear?” said The Child Who Likes Animals yesterday evening after finishing his tea.

“I don’t know,” I said, knowing the weather forecast was showing a solid grey sky for the whole of the evening. “Let’s have a look,” knowing there was little risk of us being able to look at the stars.

I opened the back door, and was rather surprised to see clear skies and good visibility. “LET’S GET THE TELESCOPE OUT!” screamed The Child Who Likes Animals Space. I had been planning to head straight to the sofa and the book I’m in the middle of reading,* but agreed that, if we could come up with a sensible plan for what we were actually going to look at, I would set it all up for him.

We fired up Stellarium and I tried to find things that would be interesting to see and straightforward to find. Outside, I had already seen the cross of Cygnus was fairly high in the sky, with Cassiopeia above it, so I make a risky suggestion. “Why don’t we try to see the Elephant’s Trunk Nebula?” I said, knowing that with our inner-city skies nebulas and other deep-sky objects do not put on a very good show. It does, though, have a very child-friendly name.

I set the telescope up on the table outside, and tried to get my eye in. It was immediately obvious that viewing conditions were on average far, far better than they had been on New Years Eve. The Elephant’s Trunk Nebula has the benefit that it’s close to an easy-to-find star, Alderamin (α Cephei). I quickly navigated ten degrees south to the location of the nebula, was confident I was there from the pattern of the stars. And was there anything? Well … maybe. Charitably, I could convince myself that the sky did indeed look slightly brighter than elsewhere in the sky; that I could see hints of structure that were not just inside my own eyeballs and brain. The Children, though, would have none of it.

Luckily, I’d noticed there was something interesting just by the nebula, and confirmed with the computer that I was looking at Herschel’s Garnet (μ Cephei). It’s a red supergiant star, a thousand times broader than our sun, and a distinctive pale peach colour to the eye—the astronomer William Herschel described it as “deep garnet”, hence the name. As its colour is so clear to see, The Children were reasonably impressed.

As it was so clear, we also pointed the telescope at the Pleiades, the view of which wowed The Child Who Likes Fairies (“There’s literally trillions of stars!”) then, the cluster NGC 7686 and the Andromeda Galaxy. The latter was, to my eyes, a slightly clearer fuzzy blob than it has been in the past, but still just a fuzzy blob. The Child Who Likes Animals said he could see it clearly; The Child Who Likes Fairies could not.

Thinking about what we should be able to see in the sky, I did a bit of maths. Say the pupil of one eye when it’s dilated is about 8mm wide—I’m just guessing this part really. We’re using a 150mm telescope: that’s 350 times as large as a pupil, so it gathers 350 times as much light. That’s a difference in astronomical magnitude of just under 6.4. Now, when I was aiming the telescope last night, once my eyes were adapted I could just about make out stars of magnitude 3.8 to 4.2; such as the stars λ Andromedae, κ Andromedae and ι Andromedae, which I used to find NGC 7686 and which I’ll have to write about again some other time.** I suspect that that’s as good as things are ever going to get in our hazy and light-polluted urban sky. But taking the telescope’s light-gathering power into account, if I’ve got the theory correct, I should be able to see things of magnitude 10 to 10.5 or so through it. Next time we have good seeing, I should test this out: look up at a few things and find out what the official magnitude of the stars I can just barely see is.

When I’ve done that, of course, I should probably think about getting some way to record what I can see, rather than just trying to describe it to you. The Child Who Likes Animals’s telescope is a Dobsonian with an altitude-azimuth mount, which means long-exposure photos are probably out, but I’d like to look at our options and see exactly what we could try to do instead.

* The Northmen’s Fury: A History of the Viking World by Philip Parker.

** Because in looking up their names for this post, I’ve discovered they form part of the former constellation named after Friedrich the Great of Prussia, Honores Friderici.

End things as you mean to go on

Looking up at the sky again

Happy New Year!

Last night was the first clear night for a few weeks; and the only clear night at any point in the forthcoming weather forecast. Naturally, as soon as I mentioned the clear skies to The Children, they immediately jumped up and down and shouted “LET’S GET THE TELESCOPE OUT!”

The decking was already slippery with frost, and there was a lot of light and noise from the surrounding houses, but we started out by pointing the telescope to Enif (ε Pegasi), a red supergiant with a very distinctive orange colour. From there we navigated to the nearby globular cluster Messier 15, which I could see in the telescope as a somewhat hazy blob, but which The Children weren’t sure they could actually see.

After they were in bed, I went outside again and looked up at the sky, but it was already filling up with haze and smoke from the fireworks sporadically going off all around the neighbourhood. I tried to find Messier 36, an open cluster in Auriga apparently also called the Pinwheel Cluster, but could not actually make out any of the stars in it. I looked at the Pleiades to calibrate myself, and realised there seemed to be far fewer of its stars visible than normal. As Orion was rising higher in the sky I pointed the telescope at the Orion Nebula, just about visible as a couple of fuzzy points; and the nearby Coal Car Cluster (NGC 1981). The latter’s stars are around magnitude 6.4 to 7.4, which normally would be clearly visible in the telescope even in our inner-city skies, but with last night’s firework haze I could just about make them out.

Ah well, there will be clear nights later in the winter I’m sure. I got up again about 3am to get myself a drink, and the garden was covered in a thick fog of smoke still from all the fireworks of midnight. Fireworks and astronomy do not mix. Hopefully later in the year, too, we’ll be able to take the telescope to somewhere with properly dark skies one night.

One of our galaxies is missing!

Or, more astronomy for beginners

Yesterday afternoon, sitting at my desk as dusk was falling, the skies were clear and I could clearly see the moon and Mars rising in the sky. As soon as I logged off from work, I scampered downstairs and went outside, and saw Jupiter and Saturn just visible above the rooftops at the back of the house. “Let’s get the telescope!” I said to The Child Who Likes Animals Space. “Before they set!”

“Before what set?” said The Child, but I was already rushing off to get his telescope and set it up in the back garden.

By the time I’d hoicked it out of its box, Jupiter had already gone down below the roofline, but Saturn was still there. Sadly, I could only see it when I was stood up. The telescope, sitting on a camping table, was too low down to spot it. I briefly considered setting the smaller camping table on top of the larger camping table and making some sort of rickety makeshift telescope-tower, but it would probably have ended in some sort of injury to one or both of us. So, like we’d done before, we looked at the moon, we looked at Mars, and we looked at random bright stars. “Point it at that blue thing!” he shouted. “That’s a very hot star!” This time, at least, he was a lot calmer and could stand still looking through the eyepiece without having to break off every couple of seconds to run back and forth with excitement. I experimented with holding my phone camera in front of the eyepiece. Worst. Astrophotography. Ever.

Worst. Astrophotography. Ever.

After The Children decided they’d rather go inside and watch TV I left the telescope set up; and after they’d gone to bed, the skies were still clear. Time for some telescope practice for me, I decided. Using an app on my phone to show me roughly where things were, I tried focusing on key visible stars then swinging the telescope sideways to find a nearby Messier object. The results? Not very successful, other than a possible sighting of M29, the Cooling Tower Cluster.

Getting a rough fix and vaguely hoping to spot the thing clearly wasn’t working. So, I fetched my laptop, and fired up Stellarium in Night Mode. I picked a target—the Triangulum Galaxy—and went looking for it.

Although the skies were clear, the seeing wasn’t great. Even a newbie like me could tell that the seeing wasn’t good. Vega normally stands out to me like a sore thumb, but last night it didn’t really appear any brighter or more significant than the stars of the Northern Cross to its south, which normally are noticably fainter. Nevertheless, I could see Hamal and Sheratan, the brightest stars in Aries, and could spot the telescope nicely onto Hamal and swing it between the two. Zooming in on Hamal in Stellarium, and flicking my head back and forth from the telescope eyepiece to the dim red screen of the computer, I could slowly navigate my way upwards from star to star until I reached Mothallah, which I hadn’t managed to see with the naked eye.

Road map of the stars

From there, I could similarly hop south towards the spot where the Triangulum Galaxy should be, navigating from star to star and matching the scene in the sky to the screen of the computer. But when I reached it: nothing. Just a blank patch of sky. I found Mothallah again, then worked my way across by a different route. Still nothing. The Triangulum Galaxy has been stolen!

Let’s try the Andromeda Galaxy instead, I thought, given that it’s one of the brightest galaxies in the sky. I found Mirach by eye, spotted the telescope on to it, and walked over to where the galaxy should be. Another blank patch of sky, with a faint hazy blob in the middle of it. Hurrah! A faint hazy blob!

I’m almost glad I hadn’t found it the other day when The Child Who Likes Animals had asked me to find it, because I suspect if I had he’d have been awfully disappointed. I was a bit puzzled, though, because in theory the Andromeda Galaxy is of naked-eye magnitude. I should have seen much more than a fuzzy blob, surely?

A quick note about how astronomical “apparent magnitude”, or brightness to you and me, works. It’s a relative scale based loosely on the subjective scales used by ancient astronomers, and as it’s relative it’s written down as a number without units. The higher the number, the fainter the thing is; and a difference of five in the apparent magnitude number means “a hundred times brighter”. If you have a calculator to hand you can work out that a drop in magnitude of 1 therefore means “2.5119 times brighter”.* The star Vega, mentioned earlier, has magnitude 0, so a few things in the sky have negative magnitudes: Sirius is -1.47, Jupiter varies from -1.66 to -2.94, and Venus from -2.98 to -4.92, almost 100 times as bright as Vega.

On a good night, with a clear dark sky, the human eye should in theory be able to see things as faint as magnitude 6 or so. Last night was clearly nowhere near that: I could see Sheratan at 2.655 and Albeiro (in the Northern Cross of Cygnus) at 3, but couldn’t see Mothallah at 3.42. Through the telescope, though, I was happily stepping my way across the sky using stars of 8 to 8.4, roughly speaking, stars around 140 times less bright than the faintest I could see without it.

The Triangulum galaxy, though, is officially** of magnitude 5.72. Andromeda is considerably brighter still, around the same magnitude as Mothallah. So what was going on?

A galaxy isn’t a point of light, like nearly all stars are. Nearly every star in the sky, other than a handful of stars like Betelgeuse when seen through very high magnifications, appears to be just a single point of light to the viewer here on Earth. A galaxy, by comparison, covers a broad chunk of the sky. That small fuzzy blob I could make out in Andromeda was really just the very brightest core of the galaxy; and the rest of that 3.4 magnitude of light is spread out over an area wide enough to fill my entire eyepiece. Through the telescope, it just becomes a vaguely paler area of sky.

Hopefully at some point we will get the telescope out on a night with rather better viewing conditions, and be able to see all of these things properly. Until then, it seems strangely unintuitive to be able to see hundreds of dimmer stars but not a theoretically many-times-brighter galaxy. That, though, is just how the physics works.

* Apparently, the formal definition “five means a factor of 100” was set down by a Victorian astronomer with what I think is a great name, N R Pogson. The relationship 1:2.5119 is therefore known as Pogson’s Ratio.

** I say “officially”. I mean “according to what I’ve just read on Wikipedia”, of course.