Archive for the ‘Galaxies’ Category

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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.

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My article in the April 2016 Sky & Telescope

March 8, 2016

SnT cover April 2016 - annotated

Getting this posted a bit belatedly, as this issue has been on newsstands for about a week already. When I wrote about my first S&T article last year, I said that my editor, JR, and I had “batted some ideas back and forth and quickly settled on the winter Milky Way”. The other ideas didn’t go away, they just got put off. This binocular tour of the Virgo Messier galaxies is one of those other ideas. Hopefully more will be along in the future – assuming I’m successful in bringing them to fruition, and that the staff – and readers! – of Sky & Telescope continue to be happy with them.

Incidentally, although I aimed the article at binocular users, it should serve as a perfectly cromulent guide for telescopic observations as well.

Have suggestions for how I can improve? The comment field is open.

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Timothy Ferris on galaxies, the universe, and time

November 12, 2014
Abell 2744 from Hubble

Abell 2744, Pandora’s Galaxy Cluster, from the Hubble Frontier Fields. Click through and get lost for a while.

This passage has been lodged in my head since I first read it years ago, and it is still the best short explanation I’ve read for the scale of galaxies and the universe. From Seeing in the Dark, pages 253-254:

Were the Sun a grain of sand, Earth’s orbit would be an inch in radius, the solar system the size of a beach ball, and the nearest star another grain of sand four miles away. Yet even on that absurdly compressed scaled, the Milky Way galaxy would be a hundred thousand miles wide. Galaxies are so big that once you get up to their scale, the universe starts to take on an almost country-cottage intimacy. The larger galaxies in clusters like the Local Group, to which Andromeda and the Milky Way belong, typically lie only a couple of dozen galactic diameters apart from one another – comparable to dinner plates at the ends of a twenty-foot-long dining table. Add in the galaxies’ halos of stars, globular clusters, associated hydrogen clouds, and dark outer disks, and they almost impinge on each other. On the same scale, the Virgo supercluster, of which the Local Group is an outlying member, comprises ten thousand plates scattered across an area not much larger than a football stadium, and the entire observable universe has a radius of only about twenty miles. From a galaxy’s point of view, the universe isn’t all that large.

Andromeda_galaxy_2

The Andromeda Galaxy in ultraviolet light, from NASA’s Galaxy Evolution Explorer. You want to click through for the full image – trust me.

The trouble is that it’s difficult – probably impossible – for a human to make the mental leap to galactic scale. The very concept of space is inadequate for dealing with galaxies; one must invoke time as well. The Andromeda galaxy is steeply inclined to our line of sight, only fifteen degrees from edge-on. Since the visible part of its disk is roughly one hundred thousand light years in diameter, the starlight reaching our eyes from its more distant side is about one hundred thousand years older than the light we simultaneously see coming from the near side. When the starlight from the far side of Andromeda started its journey, Homo habilis, the first true humans, did not yet exist. By the time the near-side light started out, they did. So within that single field of view lies a swath of time that brackets our ancestors’ origins – and that, like the incomplete dates in a biographical sketch of a living person (1944-?), inevitably raises the question of our destiny as a species. When the light leaving Andromeda tonight reaches Earth, 2.25 million years from now, who will be here to observe it? We think of Einstein’s spacetime as an abstraction, but to observe a galaxy is to sense its physical reality.

Andromeda galaxy by Isaac Robers 1899

Andromeda as photographed by Isaac Roberts in 1899 (borrowed from Wikipedia)

…As objects of study, galaxies are bottomless. If we spent eons observing the Andromeda galaxy with ever better equipment, we would, presumably, learn a great deal – indeed, one hopes that this will happen – but there would always be more to learn, if only because so many things keep changing there. To pick a literally glaring example, it is estimated that more than fifty thousand stars have exploded in Andromeda in the past two million years: The light from all those supernovae is already hurtling through space toward our telescopes, part of Andromeda’s past and our future. A galaxy is not so much a thing as it is a grand, glorious exemplification of the scope of cosmic space and time.

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A birthday observing run at the Webb Schools Hefner Observatory

June 16, 2014
Spiral galaxy M81

Spiral galaxy M81

My birthday was June 3. That evening, fellow PVAA member Steve Sittig invited me up to the Hefner Observatory at the Webb Schools in north Claremont. Steve teaches science at the Webb Schools, and he has a particular interest in physics and astronomy. The dome at the Hefner Observatory houses an orange-tube C14 Schmidt-Cassegrain. Observing with us were two other Webb faculty members, Andy Farke (paleontologist, blogger) and science teacher Andrew Hamilton. Andrew Hamilton had brought along his DLSR, a Sony Alpha33—this would turn out to be important.

Starburst galaxy M82

Starburst galaxy M82

We got started a little after 9:00 PM with a look at Jupiter, which was low in the west. We noticed right away that the seeing was pretty darned good. We went on to the waxing crescent moon and then Mars and Saturn. After that we turned to the deep sky. M81 and M82 looked great, so we hooked up Andrew’s DSLR and attempted some photography. We didn’t have a remote shutter or computer control, so we were using only the camera’s native controls, and assessing the results on the LCD screen.

Planetary nebula M57, the Ring Nebula

Planetary nebula M57, the Ring Nebula

After the galaxies, we went on to the Ring Nebula, M57, and then the Great Globular Cluster in Hercules, M13. Even with the 30-second exposures that the camera was natively limited to, we were getting very respectable images. I am including a few here.

M13, the Great Globular Cluster in Hercules

M13, the Great Globular Cluster in Hercules

Our results were pretty primitive compared to what people can do with dedicated astro cameras and post-processing, but we still had a grand time, and the process was sufficiently rewarding that we stayed out until almost two in the morning. All in all, a pretty darned good birthday present. Hopefully we’ll be able to reconvene and shoot some more this summer. I’ll keep you posted.

Many thanks to Andrew Hamilton for permission to post these photos.

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Mission 13: Pegasus to Andromeda

December 7, 2009

Mission Objectives: Constellation, Galaxy

Equipment: Sky map, Naked eye, Binoculars, Telescope

Required Time: 5 minutes

Related Missions: Cassiopeia and the Double Cluster

Instructions: Go outside after dark, look up high in the east, and find a big square of stars surrounding a whole lot of nuthin’. That’s the Great Square of Pegasus. If it doesn’t jump out at you, punch it up in Stellarium, print out  a free sky map, or follow the other middle leg of the Cassiopeia W, the one that doesn’t point to the Double Cluster.

The Great Square of Pegasus isn’t all in Pegasus; the star at the northeast corner actually belongs to the neighboring constellation of Andromeda. But the square is such a handy signpost that most people ignore the official constellation boundaries as set out by the International Astronomical Union.

That northeast corner star is the anchor for two almost identical chains of stars, one of which looks like a fainter copy of the other. Go from the second star in the brighter chain to the second star in the dimmer one, and then on in the same direction for an equal distance, and you’ll come to M31,  the Great Nebula in Andromeda.

M31 was named Back In The Day when the term “nebula” was used for any hazy patch in the sky. These days “nebula” means an interstellar cloud of gas and dust, any one of the many that litter the arms of spiral galaxies. They come in lots of flavors, which I won’t cover here; the important thing is that nebulae are comparatively tiny parts of galaxies.

M31, or the Andromeda Galaxy, is not just a galaxy; for stargazers in the northern hemisphere, it’s THE galaxy. From a dark site you can see it with the naked eye, and in fact at two million light years away, it is the most distant object that can be easily observed without optical aid (I qualified that with “easily” because there are a handful of more distant galaxies that can also be seen with the Mark 1 eyeball; pick up the current issue of Astronomy magazine and check out Stephen O’Meara’s column to learn more).

In binoculars, the Andromeda Galaxy looks like a pretty oval haze with a bright core. As you go from binoculars to small telescope to big telescopes, the amount of visible detail increases but the field of view usually decreases, and it can be hard or impossible to fit the whole thing into the field of view of a long focal-length telescope. Think of that, a galaxy so big and so close you can’t see it all with most scopes! It’s so close that people with monster Dobs regularly amuse themselves by picking out its globular clusters, whereas small-scope folks like me find the globs in our own galaxy to be plenty challenging.

The very, very small version of Rob Gendlers' very, very large M31 mosaic.

If you want to see M31 in all its glory, you must get over to Rob Gendler’s site and check out the stupendously huge mosaics on his galaxies page. One of his images has a resolution of 21,904 x 14,454 pixels and at least as of 2009 was the highest resolution image ever made of a spiral galaxy, period.

You may also know that the Andromeda Galaxy is destined to collide with our own Milky Way in a few billion years, setting off massive bouts of star formation as the two repeatedly pass through each other and eventually merge into something bigger and stranger, probably an elliptical but possibly a super-spiral or even a ring galaxy. Should be a pretty good show for whoever is around to see it.

Don’t wait up though. M31 is high overhead in the early evening and pretty good viewing until the wee hours. Go check it out.

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Friday pretty picture two-fer

December 4, 2009

First up, check out this awesome polar ring galaxy from APOD:

Ring galaxies are weird beasts to begin with, with a giant ring of stars, gas, and dust around a central core instead of the usual spiral arms. Polar ring galaxies are even weirder, in that the ring is offset from the axis of the central disk. Think about what it would be like to live on a planet in such a galaxy: depending on where the planet was located and the season, there might be two “milky ways” of light arching over the night sky.

And speaking of the Milky Way, check out these awesome posters celebrating the view of the night sky from the US National Parks. The posters were created by Dr. Tyler Nordgren, who toured the national parks a couple of years ago to document the night sky and educate people about light pollution. I got to see him speak at an SBVAA meeting last year, and I’m looking forward to his forthcoming astrophotography book, which will chronicle his experiences on his national park tour.

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Astronomy Quote #2

November 10, 2009
M33colormosaic5LL_gendler_full

M33, the Triangulum galaxy, with pink star-forming regions and bluish clusters of newly-formed stars.

Once the sky was fully dark I had a look at the Triangulum galaxy, which at a distance of less than three million light years from Earth is a local object by intergalactic standards. Its rangy spiral arms, tangled with glowing clouds of gas, spilled out beyond the field of view. As often happens, I was struck by the fact that all of these things, unimaginably big or small or hot or cold as they may be, really are out there. Like giant squid or loaves of French bread–and unlike, say, postmodernism or public opinion polls–they confront us with the regality of the materially real.

– Timothy Ferris, Seeing in the Dark, p 64

 

Photo from APOD.