Archive for the ‘Distances’ Category

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3 Ways of Seeing the Cosmos:

April 13, 2017

An Unending – But Necessary – Observing Project

There are some things I don’t blog about because they are too small. Others go unmentioned because they are too large. What I am writing about now is something that started out as an uncomfortable feeling that I was missing out on something, then grew to become an interest and a motivation, and has finally come to be the framework in which I think. Only hints and reflections have made it into my writing so far, whether on this blog or in the pages of Sky & Telescope. The first time I explained it to anyone else was with my friend and fellow observer Steve Sittig about a month ago, and the second time was in the talk I gave at the Three Rivers Foundation star party last month. This is my first attempt to put it into written words – we’ll see how it goes. I’m illustrating this post with slides from my 3RF talk.

This is where we all start out. By necessity – there is nothing else to talk about if we can’t locate things in the sky. But crucially, at this stage to “locate things in the sky” means to figure out how to point at them in the sky as seen from our position on Earth. If we’re talking about a specific object, like M11, the Wild Duck Cluster, then it’s just an address on the surface of an imaginary sphere: RA 18 51, Dec -06 16. That’s important – it’s the basis for an entire science, that of spherical astronomy – but it’s only a start to understanding the structure of the universe and where we are in it.

This stage is what I’ve referred to as “shattering the bowl of the sky” – learning the distances to celestial objects so that we can see space as space, not just a dome with lights over our heads.

An important clue that this is something I should be thinking about came when I first observed M97 and M108. Here are two objects of roughly equal brightness, easily framed in the same field of view in binoculars and telescopes, that are in fact at vastly different scales and distances. We think of M97 as being “out there”, but it’s practically right next door to us in our own galaxy. If the Milky Way is 100,000 light years across – and it is at least that big – then M97 is only 2% of the way across the galaxy from us. It’s right next door. From the perspective of any observers in M108, looking across the intervening 46 million light years, the distance between our solar system and M97 would be unresolvably small.

Here’s an extended example: the major stars and Messier objects of the constellation Lyra. If you’ve been observing for long, you probably know your way around the triangle and parallelogram asterisms, and how to use them to find M57 and M56. It’s natural for us to think of these things as belonging together, because we use them as signposts to guide us when we navigate this part of the sky.

Adding the distances reveals some things. At only 25 light years distant, Vega is in fact closer to us than it is to any of the other stars or DSOs in the field. The other two stars in the triangle are about equally far away, roughly 160 light years, and the other three stars in the parallelogram are ranged between 600 and 1000 light years distant. M57 is a bit further, but M56 is way further out, almost a third of way across the galaxy. Even from M56, our own Sun and all the stars of Lyra would blend into the faint background of field stars that saturate the Orion Spur.

I was really proud of myself for starting to think about my observing targets on this level, and this view of the sky formed the basis for my article “Twelve Steps to Infinity” in the December 2016 S&T.

But it’s not enough. I’ve come to think of this as “army ant observing”. When they are foraging, army ants go out from their bivouacs in straight lines, eating everything they can catch along the way, and come back the same way, just like an observer looking at Lyra. If all we learn about objects in the night sky is how to find them and how far away they are, then we’re still trapped in an Earth-centric view of the universe. We don’t know how the objects relate to each other, any more than a colony of army ants knows that the lizard they devoured on Tuesday hatched out of the same clutch as they one they caught on Friday.

To return to the example of M11, it’s 6200 light years away. That’s pretty darned far for a bright open cluster – the average for Messier and Caldwell open clusters is 3000 light years. We might suspect that to be so bright and so rich at that great distance, the cluster must have many, many stars, and indeed it does, a whopping 2900 of them.

There’s at least one more level of understanding: to hold something of the true 3D structure of cosmos in our minds, and understand how celestial objects relate to each other, without reference to Earth. I illustrated this with a map of our galaxy laser-etched into a cube of crystal. To hold the structure of space in my mind and be able to turn it over and around, view it from all sides, like someone turning that crystal over in their hands – that’s what I aspire to.

I’ll never get there, really. It’s an impossible project. There’s just too much stuff out there. But that’s okay. As a paleontologist, I’m familiar with the problem of envisioning other worlds based on incomplete information. And everything I do learn, every step up the long road to the stars, deepens the experience of observing for me.

Back to M11. It’s a rich cluster, and it’s situated in the constellation Scutum (as seen from Earth…), not far from the center of the galaxy in Sagittarius. But in fact it has nothing to do with the central bulge of the Milky Way galaxy. The galactic center is 27,000 light years away from us, and M11 is only a little over 6000, a bit over one-fifth of the way. Instead of being part of the galactic center, M11 is one of the clusters that marks the Sagittarius arm of the Milky Way, which is the next arm inward, between us and the galactic center. M11 is still out in the burbs, with us, not downtown. In contrast, the globular clusters in Sagittarius, Scorpio, and Ophiuchus actually are related to the galactic center – they are swinging by it, like comets sweeping past the Sun, on incredibly long, elliptical orbits that carry them tens or hundreds of thousands of light years out into the galactic halo.

Next Steps

So here’s my ambition. We have loads and loads of observing guides, like NightWatch and Turn Left at Orion, based on Level 1. Again, that’s not a bad thing, and it’s of necessity. We all have to stand and walk before we can run, and finding things in the night sky is the first step. But as a community, we have people on this already, not only in the vast majority of existing observing guides, but also in the observing features – including most of my own! – published in astronomy magazines, blogs, and online fora.

There are also resources that address Level 2. Stephen James O’Meara’s books live there – they address not just where to find things in the sky, and what they look like, but also what they are, how far away they are (and how their appearance relates to their distance from us), and even, in some cases, how they relate to other nearby objects.

Things get better – a bit – when we get beyond our own galaxy and observe others. The Astronomical League has an observing program for the Local Group and galactic neighborhood, for example. And lots of observing guides and articles on the Virgo Cluster include some basic astrophysical data, including the fact that M87 is a monster elliptical and the central galaxy of the cluster.

A diagram I drew to help get my head around the shape of the galaxy and our place in it. Based on images and data from NASA.

What we don’t have many of, and what those of us who aspire to Level 3 desperately need, are observing guides that address the relationships of celestial objects within the Milky Way. I want an observing guide to the clusters and nebulae of the Sagittarius arm, for example, something that will tell me that M11 is not just 6200 light years away, but that it’s related to M16 and M17 because all three objects are in the same spiral arm of the galaxy. I want a guide to the Perseus arm, and another to the Orion Spur and Gould’s Belt. One of the reasons I’m so excited by Allan Dystrup’s “Classic Rich Field” thread on Cloudy Nights is that Allan has provided a wealth of information on easily-observable OB associations in the nearby reaches of the galaxy.

There is one book that does address the internal structure of the Milky Way, albeit more from an astrophysical than observing perspective. It’s The Guide to the Galaxy, published in 1994 by Nigel Henbest and Heather Couper. A couple of decades on, I assume that at least some of the information in the book has been superseded by new discoveries. But it’s still an interesting and useful resource, and with used copies going for just over two bucks on Amazon, a low risk for anyone who wants to investigate (I have a copy already).

UPDATE April 17: This is what I get for posting in the middle of the night. Several commenters reminded me of resources that do address Level 3 that I forgot to mention. Among them are Craig Crossen’s books Binocular Astronomy and Sky Vistas and Bill Tschumy’s resources at ThinkAstronomy.com, including his program “Where is M13?” and his essays “Milky Way Rising” and “Escape From Plato’s Cave“. The oversight was particularly dumb since I have Crossen’s Binocular Astronomy on the bookshelf next to me, I’ve corresponded with Bill Tschumy before and he’s been very generous with his thoughts, and in “Escape From Plato’s Cave” he lays out basically the same manifesto as I have in this post. Chalk this up to tiredness – I certainly meant no slight to the other observers and authors who have trod this path before me.

Parting Shot: Bricks and Boards

It may seem like I’m being dismissive of observing guides – or observers! – that prefer to work at Level 1 or Level 2. That’s not my intention. As far as observing goes, I like Uncle Rod’s dictum that there’s no wrong way to do amateur astronomy. As long as you’re out soaking up photons, or letting your equipment soak them up for you, good on ya. May a thousand gardens grow. I started as a purely recreational observer, and stargazing for no more noble purpose than personal aesthetic enjoyment still occupies a lot of time out under the stars. It also, eventually, fired the curiosity and the hunger for a more informed and encompassing view of the universe. I see these approaches as complementary rather than conflicting. I would never have desired to learn the secrets of celestial objects – their relationships, the processes that shape them, their origins and fates – if I hadn’t fallen in love with them in the first place.

As far as observing guides go, here’s my thinking. The God’s Eye View of the cosmos is an edifice of the imagination. Each of us that wants to understand the structure of the cosmos on this level will have to build our own mental model to play with and learn from. And we need raw material. Those observing guides, books, and articles that never get beyond Level 1 or Level 2 are still good and useful things: they’re bricks and boards that we can build with. We may have to dig into online databases and astrophysical literature to find the connections we’ll need to join them, but we’ll get farther if we don’t have to invent everything from scratch. Everything is potential grist for the mill.

I have recommended the monthly Evening Sky Map to countless people as a way for them to learn the sky. I wonder how much progress I could make if I learned the distances to all of the targets on each month’s lists, and looked for connections among them?

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Observing report: Saturday night stargazing on Mount Baldy

April 2, 2017

Waxing crescent moon, afocal shot by Eric Scott with Samsung Galaxy S6, shooting through Orion XT10 reflecting telescope.

London and I went up Mount Baldy last night with our friends Thierra Nalley and Eric Scott. Marco Irigoyen and Leandra Estrada joined us up on the mountain. We went up to look for comet 41P, but that didn’t pan out.

Since we went ostensibly to look for the comet, I brought the XT10 for firepower, and lots of binoculars. We got set up at Cow Canyon Saddle at about 8:30. Our first target was Orion, before it could sink into the light dome over LA. Second targets were the Pleiades and the Double Cluster. The Double Cluster in particular looked magnificent. I’ve been on a small-scope kick for a while so the XT10 hasn’t been out much, and I tend to forget what a potent instrument it is, especially under dark or semi-dark skies.

The skies on Mount Baldy last night were definitely semi-dark. Even three days shy of first quarter, the waxing crescent moon was bright enough to throw shadows and rather seriously degrade the darkness of the sky.

I tried for the comet but just couldn’t see it. I had the chart, knew where to look, and swept the area repeatedly with binoculars of all sizes and with the XT10, and I got bupkiss. This was after catching the comet easily in 7×50 binos every time I looked for it in Texas last weekend – but I wasn’t fighting any moon then. I think the comet is so big and diffuse that the surface brightness is low, and therefore it is easily swamped by moonlight. It certainly was not evident last night.

While we were in the neighborhood of the Big Dipper, we had a look at Mizar and Alcor, the famous double star in the dipper’s handle. Then for comparison we checked on Sigma Orionis, and then Marco wanted a look at Jupiter. After Jupiter we went on an extended tour of the deep sky, in which we observed:

  • M81, M82 (interacting galaxy pair)
  • M97, M108 (planetary nebula and galaxy in same field)
  • M3 (globular star cluster)
  • M37 (open star cluster)
  • M35 (open star cluster)
  • M104 (Sombrero galaxy)

In addition, we also saw three more open star clusters with our naked eyes and/or binoculars: the Hyades, M44, and the Coma Berenices star cluster.

We finished up on the moon, and then Jupiter again. We spent quite a bit of time getting pictures of both with Thierra’s and Eric’s phones. By coincidence, they both have the Samsung Galaxy S6, which has a very full-featured slate of camera options. Leandra is a pretty talented photographer and she was able to coach us on what settings to use. I think the results are pretty astounding, for handheld shots using phones. Here are the two best images of Jupiter, captured by me using Thierra’s phone and Leandra’s advice:

Here’s a composite of Jupiter and the Galilean moons – the planet was overexposed in the original to get the moons to show up, so I replaced it with the better of the two shots above.

And here’s a comparison screenshot from Sky Safari Pro 5 identifying the moons – from left to right in the above image they are Callisto, Europa, Io, and Ganymede.

As usual, the view at the eyepiece was about an order of magnitude more detailed than what the photos captured. One thing that I had never seen before with one of my own scopes was a band of ruffled white clouds within the north and south equatorial belts (the prominent orange-brown stripes on either side of the equator). The barest hint of this survives in the photos. It was a pretty mesmerizing view. For eyepieces we used a 32mm Plossl (37.5x), 28mm RKE (43x), 24mm ES68 (50x), 14mm ES82 (86x), 8.8mm ES82 (136x), and 5mm Meade MWA (240x). The most used were the 28mm RKE, 14mm ES82, and 5mm MWA. If you’re wondering why we used both a 32mm Plossl and a 24mm ES68 – since they give the same true field of view – we used the Plossl during the afocal photography because it gives a wider exit pupil, which is easier to keep the camera’s aperture centered inside.

Even though we missed the comet, I was pretty happy with what we did see – at least one of every major class of deep-sky object, including all of the stages of the life cycle of stars. In the disk of the Milky Way, new stars are born from vast nebulae of gas and dust, like Orion. In time, heat and light from the newborn stars push away the remnants of their birth clouds, leaving behind only the stars themselves, as open star clusters (‘open’ as opposed to globular). Over time, the stars in open clusters drift apart to become ‘field stars’ like the Sun, no longer gravitationally bound to their siblings. When the run out of fuel, stars blow themselves apart in supernovae if they are 8 times the mass of the Sun or larger, whereas smaller stars blow off their outer layers of gas to form planetary nebulae like M97. Whether stars die suddenly in supernovae or slowly as planetary nebulae, the matter blown out by dying stars enriches the galactic gas and dust clouds, and in time it will be incorporated into new generations of stars and planets. We are products of this process – all of the elements in our bodies other than hydrogen were born by fusion in the hearts of stars, and seeded into the galaxy’s spiral arms when those stars died.

Farther out, globular clusters like M3 orbit the core of the galaxy on long elliptical orbits that are not flat, but come looping in from all directions. The stars in globular clusters are typically very old, 12 billion years or more. We know very little about how and why globular clusters formed, and how they came to have such weird orbits. Probably they are some kind of developmental leftover from the formation of the earliest galaxies in the first billion years after the Big Bang – astrophysical fossils, if only we knew how to interpret them.

All of these processes are going on in other galaxies as well, especially spiral galaxies like M81, M104, and M108.

To put all of that into context, here are all of the objects we observed again, this time ranked from closest to farthest:

In our solar system:

  • moon – 240,000 miles or 1.3 light seconds
  • Jupiter – 370 million miles or 33 light minutes (currently – Jupiter is about 5 AU out from the sun, but right now we’re on the same side of the sun so it’s only 4 AU from us)

In our spiral arm of the Milky Way galaxy (the Orion spur):

  • Mizar and Alcor (double star) – 83 light years
  • Hyades (open star cluster) – 151 light years
  • Coma Berenices cluster (open star cluster) – 280 light years
  • M45 (Pleiades; open star cluster) – 440 light years
  • M44 (Beehive; open star cluster) – 577 light years
  • Sigma Orionis (multiple star) – 1255 light years
  • M42, M43 (Orion nebula; star-forming region) – 1344 light years
  • M97 (planetary nebula in same field as M108) – 2030 light years
  • M35 (open star cluster) – 2800 light years

In the next spiral arm out from the galactic center (Perseus arm):

  • M37 (open star cluster) – 4500 light years
  • NGC 869/884 (Double Cluster; open star clusters) – 7500 light years

In the galactic halo of the Milky Way:

  • M3 (globular star cluster) – 34,000 light years

External galaxies:

  • M81, M82 (interacting galaxy pair) – 11 million light years
  • M104 (Sombrero galaxy) – 31 million light years
  • M108 (galaxy in same field as M97) – 46 million light years

That is very satisfying to me, to take in such a menagerie of celestial objects, at so many scales and distances, in the space of a couple of hours armed only with a comparatively inexpensive telescope and an idea of what’s out there to be seen. I can’t wait for next time.

Saturday night astro crew. Left to right: Marco Irigoyen, Leandra Estrada, London Wedel, Matt Wedel, Thierra Nalley, Eric Scott. Photo courtesy of Eric Scott.

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From km/s to parsecs per million years

February 15, 2017

I’ve been following a fascinating thread on Cloudy Nights called “Classic Rich Field“, in which Danish observer Allan Dystrup is posting his observations and sketches of stellar associations, especially OB associations of big, hot, young stars. It’s a fascinating observing program, not least because he’s doing it all with a 55mm telescope. I’ll no doubt be talking more about OB associations in the future, as my interest in this area – both intellectual and aesthetic – has been steadily growing over the last few years.

Canadian astronomer Glenn LeDrew has made some very substantial contributions to the thread as well. His two posts on runaway stars alone are worth copying, pasting, and saving (1, 2). Especially for this arresting fact that he related:

1 km/s is almost exactly 1 parsec/million years [1.023 pc/myr, in fact, according to this page – ed.]

What a stunning way to put the scale of a parsec into common terms – there are about as many kilometers in a parsec as there are seconds in a million years. Or, if you prefer the more familiar light years, about as many kilometers in a light year as there are seconds in 313,000 years. Suddenly the universe feels ungraspably, inhumanly big. Which of course it always has been – it’s just easy to forget that.

Who thinks on these scales, besides astronomers? Paleontologists. And the question that popped into my head immediately upon reading that near-equivalence was, “If an object had been traveling at 1 km/s since the Late Jurassic, how far would it have traveled?” By Late Jurassic I was thinking about 145 million years ago, when Apatosaurus, Stegosaurus, and Allosaurus roamed the American West. When I go dig each summer, it’s in sediments laid down during that time, preserving the bones of those animals. And if you travel at 1.023 pc/myr, after 145 million years you will have traveled 1.023 x 145 = 148.3 parsecs, or 485 light years.

milky-way-sketch-10-galaxy-diameter-and-thickness-with-earth-distance

A diagram I made for my RTMC talk last year, showing the size of the galaxy and our place in it.

That’s nothing. That’s barely farther than the distance to the Pleiades, one of the closest open star clusters to earth. It’s about a third of the way to the Orion Nebula. (I have these distances loaded in RAM for a reason.) It’s a little less than half of the 1,100-light-year thickness of the ‘thin disk‘ of the Milky Way, which holds about 85% of the stars in the disk of the Milky Way, including our sun.

That’s amazing. If you started out right on the centerline of the plane of the galaxy, halfway between the bottom and the top of the galactic disk, and you started flying directly up or down (galactic north or south) at 1 km/s (fast as a speeding bullet), you’d have to fly for more than 170 million years to reach the edge of thin disk. To get to the edge of the galaxy’s halo at 30,000 parsecs would take 29.3 billion years, or just over twice the age of the universe (and 3.5 times the age of the Milky Way itself).

Space is big, y’all.