The other day I posted about the record-breaking sighting of the youngest crescent Moon — the Moon wasn’t even new yet!

There was some confusion over the image I posted along with that article. That image was taken many hours before the actual record was made, when the Moon was still 19 degrees from the Sun.

Martin Elsässer, the man who broke the record, has posted a new image, showing what the Moon looked like just 10 minutes shy of its conjunction (closest passage) with the Sun:

That is an amazing picture. You can barely see the crescent at all; I highlighted it with red lines so you can see it. The closer the Moon gets to the Sun, the less of the lit day side we see, so this picture tells you just how close it was. At this point, it was less than 5 degrees from the Sun. Incredible.

Thanks to the folks at Fark for picking up on this story, and for the link to Herr Elsässer’s page.

Update (5/7/08): The image I had posted originally was distorted due to the wrong picture being made available to the press (like me!). I got a nice email from Joerg Dietrich, one of the astronomers who took the data, with a link to the correct image. I have updated both the image and the link. Sorry, and enjoy!

We’ve known for a long time that most of the Universe is invisible. 72.1% of it is dark energy, about which we know very little. 23.3% of it is dark matter, which was only recently tagged for real and for sure; we still don’t know what particles make it up, but we’re on the verge of finding out.

Normal matter — us — makes up just 4.6% of the Universe’s energy and mass budget. But here we are! At least, here we mostly are: actually, we only see roughly half of the normal matter in the Universe. Stars, galaxies, and warm-to-middling gas aren’t too hard to spot in general, but they only make up about half of what we expect to see of normal matter.

Where’s the other half?

Let’s turn the wayback machine to about 13.6 billion years or so ago. The Big Bang is old news at this point, but the first stars have yet to be born. Matter and energy are mixed everywhere, but some of it is different. What we now call dark matter is starting to clump together through gravity, forming long sheets and filaments far bigger than any galaxy we see today. This forms a grid, a framework, upon which normal matter starts to fall. Eventually, galaxies and clusters of galaxies and clusters of clusters of galaxies will form along these cosmic skeletons.

Fast forward to today. Bang! We see galaxies everywhere… well, not exactly everywhere. We see them lying in those long sheets and filaments, showing us where the dark matter structures are, like dew drops on a spider’s web.

But that’s just the stars and galaxies, remember? It’s only half. Where’s the other normal matter?

The hypothesis is was that it would be in the form of very hot gas strung out along those filaments as well. Hunting for it would be hard: it would be very diffuse, making it dim, and very hot, meaning it would only emit at short wavelengths, like extreme ultraviolet or X-rays.

Hey, we have telescopes that can see those!

And now we have (and more pictures can be found here). Astronomers upped the odds of finding the gas by looking around galaxy clusters, where it would be denser, and also doing something clever: looking near clusters that are near each other in the sky due to perspective. One would actually be farther away than the other, but peering very nearly along the angle separating them they would look like they’re right next to each other. Since we’d be looking along a long thin cylinder of gas, that would make it appear brighter than if we saw it through its side.

The picture above shows the galaxy clusters Abell 222 and 223, both about 2.5 billion light years away. The visible light image just shows them as clumps of points, but remember: each dot is a massive galaxy like our own! The technicolor bit is from the XMM-Newton orbiting X-ray observatory, and shows the hot gas. Since these are separate clusters, they should be detached from each other. But instead, they’re connected by a gas bridge of ten-million-degree plasma. That’s the missing stuff! That’s made up of baryons; particles like protons and neutrons, atomic nuclei and the like. Look around you: everything you see is made of baryons (and leptons, which include electrons), so this gas is your kin.

It’s a bit more rarified, though: there are only about 30 baryons per cubic meter in this bridge. Good thing it’s big (about 4 million light years wide) and we’re looking down its length! But then, that’s why so much of this stuff is missing. It’s really hard to detect.

According to the models, there is enough stuff in this bridge to extrapolate the existence of the rest of the missing normal matter. Of course, we only have a data set of one, which is a bit rocky, but I suspect more of these will be found now that we know they’re out there.

And may I add, phew! It’s always nice when half the stuff you can’t find finally turns up.

During the live video chat on Sunday, I was asked a good question: why don’t nebulae, gas clouds in space, dissipate? What holds them together?

Here’s my answer:

The basic answer is: gravity. The clouds aren’t like clouds on Earth, or balloons filled with air; nebulae are immense objects with vast amounts of mass. Their own gravity holds them together, and can even cause them to collapse and form stars. And wouldn’t you know it, this goes against claims made by creationists that stars can’t form from gas clouds, so I included that as well in the video.

The images in the video, if you’re curious, are of the Orion Nebula (seen here too), very young stars forming in the Orion Nebula, and an artist’s drawing of a young planetary system still forming.

Paranal is a 2600 meter-high mountain in Chile, and the location of the very large Very Large Telescope telescope^*, an 8-meter monster that is one of the largest in the world. It creates amazing pictures of the heavens, as you might expect.

But you don’t always need a telescope to see jaw-dropping beauty. The folks at the European Southern Observatory just released some really nice shots taken outside the dome. Here’s one of the setting Sun:

The blue flash is the even more rare cousin of the unusual green flash. Basically, the light from the setting Sun is bent by the Earth’s air. But the Earth is curved, so the closer an object is to the horizon, the more air it must pass through. Also, different colors of light are bent differently by air; the shorter wavelengths (violet, blue, green) are bent more then longer (yellow, orange, red), and these effects add up to generate a transient but very pretty flash of color. The circumstances needed for a flash are particular, so they don’t happen terribly often.

The ESO released other images, including a green flash and spectacular shots of the gegenschein and zodiacal light: sunlight reflected back to us from dust particles in space. These are phenomenally difficult to see or photograph, so the pictures are particularly noteworthy and very, very pretty.

Someday I’d like to get to a site so dark I could see that for my own self. Wow.

Pictures like this are so cool: they remind us that there are things out there you’ve probably never even heard of, yet are incredibly beautiful and just waiting to be seen. All you have to do is want to know about them.

Globular clusters are one of those types of astronomical objects that make everyone happy: they are incredibly beautiful to observe, jaw-dropping even in small telescopes; and they are also tailor-made laboratories for studying stellar evolution, an environment where studying how stars age and interact is almost too easy.

But that last bit has run into a problem of late. A wrinkle has turned up that makes examining globulars a bit more complicated than previously thought.

Globular clusters (or just GCs) are roughly spherical collections of hundreds of thousands or millions of stars held together by their own gravity. They look a bit like beehives, and in fact the individual stars orbit the center on mostly randomly distributed paths, so a time-lapse movie (lasting millions of years) of a GC would strongly remind you of bees around a hive.

Early on, astronomers noticed that GCs appeared to lack massive stars, and in fact when examined closely it was seen that all stars above a cutoff mass were gone. This implied that a GC forms all at once from a cloud of gas, with all the stars switching on simultaneously, or near enough. A star’s lifetime is dependent on its mass, and more massive stars live shorter lives. Some high-mass stars explode after 10 million years, some after 100 million. A GC older than that will therefore not have those kinds of stars in it. They’ll all have died.

The Sun will turn into a red giant when it’s about 12 billion years old. So if you don’t see any Sun-like stars in a GC, you know it must be older than that age. By observing the kind of stars in a GC, we can get an idea of its age! In fact, this caused a problem some years ago: the oldest GCs looked to be older than the Universe itself! It turns out this was due to astronomers not knowing the age of the Universe very well, and as time went on we figured out that the Universe was older than first thought (it’s 13.73 billion years old now) and the paradox was resolved.

Anyway, over time, the stars inside a GC orbit around, and because they are so tightly packed together, encounters between two stars are common. They pass close enough to gravitationally affect each other, changing their orbits. In general, if two stars of different mass pass each other, the lower mass star will gain energy, boosting it to a larger orbit, and the higher mass star will lose energy, dropping it to the center of the cluster. Over time, you get "mass segregation", with the hefty stars all in the center and the lighter-weight ones relegated to the cluster’s suburbs.

Not only that, but the stars near the center can actually interact and become bound to each other, forming binary stars. That takes time, though, billions of years. First the stars have to fall to the center, and then they need time to interact. So another way to get the age of a GC is by looking at the binaries in the core. This is called the dynamical age of the GC — how long stars have been interacting with one another — as opposed to the actual ages of the stars in it.

Binaries in the core reveal themselves through X-rays. High mass stars explode and leave behind neutron stars or black holes. If one of these is orbiting a normal star, then it can siphon off gas from the star and gobble it down, which produces a lot of X-rays (see here for details). So detecting these binaries is not terribly hard: point your X-ray telescope at a GC and count up the sources of X-rays in the middle.

Astronomers did this recently using the Chandra X-Ray Observatory. And there’s the problem: in several GCs, they found too many X-ray binaries.

Chandra image of two GCs: NGC 6397 (left) and NGC 6121 (right); 6397 is old, but it has far more binaries than expected, making it look younger.

When the GC is young, you don’t expect to see too many binaries in the core. When it’s middle aged, you see quite a few as the stars in the center interact, and then when it’s old the number tapers off again (as the normal stars die off and the source of X-rays shuts off). What the astronomers found is that in some clusters that were presumed to be really old (due to the age determined by looking at the stars in them), there were still more binaries than expected, as if they were younger.

Why? Well, all this also depends on how dense the cores of the GCs are. A less dense core should have fewer encounters between stars, and therefore fewer binaries. But one older GC which was expected to only have a few binaries had quite a few more than predicted. In other words, the stars themselves in that GC are old, but the core appears to be somewhat more immature.

What this means is that age is not the only thing that drives the number of binaries in the core, and that they are not the simple laboratories that has always been assumed. Most likely, this doesn’t affect things too much; they can still be used to study how stars age and interact, but you have to be more careful when poking around in the details. As usual, the Universe is a wee bit more complex than we usually assume. But the beauty of it, too, is that this complexity can be revealed, and we can revise our ideas to accommodate it.

So obviously, you have to be careful when dating heavenly bodies. They might look older on the outside, but be younger and less mature on the inside.

If there’s a life lesson in there, you’re welcome to determine it your own self.

What happens when a smallish galaxy plows right through the center of a bigger one?

This:

Holy Haleakala. That’s Arp 148, and it’s magnificent. That elongated galaxy was probably not quite so stretchy before it hit, but the gravity of the other galaxy drew it out. In turn, its own gravity drew in stars and gas from the bigger galaxy, which then expanded as a ring as the smaller galaxy plunged on. It almost looks like a freeze-frame image of a bullet shattering a drum head.

If you like that, you’ll love this: Hubble has released 59 such images of galaxy collisions today (the US version of the release is here), celebrating Hubble’s 18th anniversary in space. It launched on April 24, 1990.

The galaxy pictures are stunning. The one on the left is Arp 256, two spiral galaxies interacting as they pass each other for the first time. Long tendrils are being drawn from both galaxies, and the blue regions indicate epic bursts of star formation (young, massive stars are blue and extremely luminous). Someday the Milky Way and Andromeda galaxies will look very much like this… a billion years from now, when they pass each other. I’d write more here, but golly, I have a book coming out in October with lots more details.

This one is even weirder: it’s a red elliptical galaxy and a bluish spiral interacting. The blue galaxy looks to be small to my eye; it’s getting totally disrupted by the elliptical. There’s some indications of dust getting blown every which-way in the elliptical too. This will be a very interesting system in about another 500 million years or so.

I’ll leave you with one more: NGC 6050.

Two magnificent spiral galaxies, each about the same size, slide toward one another and are just now beginning their slow dance. I can almost imagine them spinning like buzz saws into each other, tearing both to shreds (in fact, they look a whole lot like the animation of colliding galaxies used in my short astronomy video on Hulu — and yes, we’ll be posting that on internationally-accessible servers soon). The two spirals will no doubt merge completely into an elliptical… unless they’re moving too quickly. They are both part of the Hercules Cluster of galaxies, 650 million light years away (the press release says 450 million, but the 2MASS catalog says 650). Hercules has over 100 galaxies in it and is therefore pretty massive; all that mass means a lot of gravity, and that in turn means the component galaxies are screaming along at high velocity. It’s possible these two beauties will continue on their way, passing through each other, distorted, beaten, but surviving.

I wonder how that story will end; lover’s embrace or ships passing (literally) in the night? With images like these, astronomers will learn a lot more about how galaxies behave when they collide, and that will point the way to better, more detailed observations. Eventually we’ll know how the story goes, from start to finish.

Our own future is wrapped up in these images, writ large across the sky. As usual in astronomy, and in science as a whole, by looking outwards we learn more about ourselves.

Happy anniversary, Hubble.

I just got word from the folks at Bigelow Aerospace — whose goal is to have an orbiting hotel for people who want a really spectacular view out their window — have tweaked a camera onboard their Genesis II satellite, and it’s now returning higher-res images. These are really stunning views of the planet, and well worth taking a look!

That’s Baja California in the background, with Genesis II suspended in front. And you should really check out their fisheye view, too! Very cool stuff.