Blogs / Bad Astronomy

The Local Group of galaxies is our Milky Way’s neighborhood: a few dozen galaxies dominated by our own, as well as Andromeda and a handful of largish ones.

But by far the majority of galaxies in our group — the majority everywhere, really — are dwarfs; small collections of stars and gas with only a few million stars. Lurking just 1.6 million light years away, half the distance to Andromeda, is Barnard’s galaxy, an irregular dwarf with about 10 million stars. Behold!

[As usual, click to embiggen, or get a ginormous 80 Mb TIF.]

This is perhaps the finest near-true color image of Barnard’s galaxy I have ever seen, courtesy of the European Southern Observatory’s 2.2 meter telescope in Chile… a far cry from the small 12.5 cm (5 inch) refractor Barnard himself used to discover it. Given that this diminutive galaxy is in the direction of Sagittarius — toward the center of our galaxy, which is loaded with stars — it’s incredible Barnard found it at all. In fact, most of the stars in this image are inside the Milky Way, between us and the other galaxy.

Barnard’s Galaxy is not precisely irregular, since it appears to have a bar-like structure across it. The red bubbles are regions where stars are being born in large numbers; the UV and fierce winds of subatomic particles from the massive stars being formed carve out cavities in the gas, creating what look like smoke rings. The red glow is characteristic of hot hydrogen, and is a sure-fire way to know that gas is being excited by the stars nearby. The sharp edges to the bubbles are real, due to the gas piling up as it rams gas in interstellar space in a cosmic snowplow effect. There are over 150 separate bubbles like these in the galaxy, some of which have been observed using the Hubble Space Telescope.

There is another feature here that’s not obvious. On the left and right of the galaxy are faint, thick, blue arcs. This is actually gas that’s being blown out by the stars in the center of the galaxy, forming a weak ring surrounding the entire structure. I expect that gas will blow right out of the galaxy entirely; the gravity from the meager number of stars making up Barnard’s Galaxy can’t possibly be enough to restrain it.

Dwarf galaxies are difficult to observe because they are so faint; they fade with distance rapidly, so only nearby ones can be studied in detail. But since they are the most numerous types of galaxy in the Universe, these tiny smudges are well worth studying, and images like this one of Barnard’s Galaxy will help us understand how these galaxies formed, and what they’re made of, and how they behave. We also think that through collisions, these galaxies can grow to become larger, more magnificent ones like our Milky Way, so, as usual, when we study the Universe, looking ever outward, we are actually turning our gaze inwards to learn more about ourselves.

Image credit: ESO.

What happens when two massive spiral galaxies — each with a hundred billion stars — slam into each other head-on at hundreds of kilometers per second?

Beauty.

[Click to embiggen; or go here to get the massive 15Mb TIF image.]

This is the unusual galaxy NGC 2623, seen in this newly-released and breathtaking Hubble Space Telescope image taken in 2007. We’re seeing this vast smash-up caught in the act, a galactic collision already in progress. It appears frozen in time, but that is an illusion of distance: at a distance of a staggering 250 million light years the tremendous velocities of the collision are reduced to a motionless tableau on the human timescale.

But we see a large number of galactic collisions when we catalog the sky, and together with our knowledge of math and physics we have a good understanding of how these encounters play out.

When two massive galaxies approach each other, the gravity of each starts to affect the other. Call them Galaxy A and B. The side of Galaxy B closer to Galaxy A feels more gravity from it, so stars and gas are drawn toward it more strongly than the stars and gas on the far side of Galaxy B. The same is true in the other galaxy. As they get closer, this force strengthens, teasing out long ribbons of material — called tidal tails — that stretch in the direction of the other galaxy.

If the encounter is off-center, then the tails get curved when the galaxies pass, arcing either gently or severely depending on the speed, encounter distance, and mass of each participant. The Hubble image clearly shows the arcing tails from each galaxy in NGC 2623.

Incredibly, even though hundreds of billions of stars are involved, each individual star is far too small to suffer a physical collision. But gas and dust clouds are much bigger than stars (they can be hundreds of trillions of kilometers across, as opposed to stars which are a trifling million or so kilometers in diameter), so collisions between them are common. When clouds collide they collapse and undergo violent bouts of star formation. This too is clear in the image: the blue clumps in the tidal tails are vast regions of clusters of stars being born; over 100 such clusters have been identified in this image in the tail on the right alone.

Collisions like this blast out energy, not just in visible light, but at other wavelengths as well. In infrared alone, NGC 2623 radiates with the power of 400 billion times the Sun’s energy. This makes NGC 2623 a ULIRG: an ultraluminous infrared galaxy. Although relatively rare locally, they are so common at great distance (and therefore earlier on in the age of the Universe) that they comprise as much as half of all the infrared background glow we see in the Universe. The huge amount of infrared comes from the collision itself; star formation produces prodigious amounts of dust which absorb ultraviolet light from newly-born stars and re-radiate it in the infrared. The collision also dumps gas and dust into the central supermassive black holes in the cores of the two colliding galaxies, which piles up in a flat disk outside the black hole, heats up hugely, and again glows brightly.

Astronomers are making a comprehensive study of such ULIRGs using a fleet of telescopes including Hubble, Spitzer, Chandra, GALEX, 2MASS, VLA, and even the venerable IRAS satellite which surveyed the sky in infrared in the 1980s, and in fact first discovered the ULIRGs.

Why study them? Because galaxies as large as our Milky Way almost certainly started off small and grew to their present size by colliding and merging with other galaxies. Studying ULIRGs is a way of examining how our galaxy came to be… and it’s a glimpse of our future as well. In a billion years or more, we will suffer a massive collision with the Andromeda Galaxy. Our own clouds of gas and dust may smash into those in Andromeda, creating huge waves of star formation and blasting out light at all wavelengths. What will our fate be then? The Earth may survive — the Sun will still be around for this event — and the gravitational repercussions may toss us out of the new galaxy, or drop us down to the core.

It may seem academic, but astronomers thirst for understanding of these events. We want to know how we came to be, and where we are headed. That knowledge may have little or no practical use for our own survival (or at least not for a few million millennia), but for now, for today, we learn more about galaxies in general, more about the physics of cosmic collisions, and more about the interaction of gas and dust on a truly mind-numbing scale.

And of course, we get to gaze on lovely images, illusions of placidity and gentleness to be sure, but lovely nonetheless.

Image credit: NASA, ESA and A. Evans (Stony Brook University, New York & National Radio Astronomy Observatory, Charlottesville, USA)

NASA’s Swift satellite is a modern success story: designed to peer at the Universe in ultraviolet, X-rays, and gamma rays, it is on constant lookout for gamma-ray bursts, explosions so vast they are second only to the Big Bang itself.

Swift scans the skies, constantly observing, always on its toes for that fleeting blast of high-energy light. But it also does other science as well; an orbiting camera like that has many uses. For three months in 2008, astronomers used Swift to target the nearest major spiral galaxy like our own: M31, the Andromeda Galaxy. And what they got was this gorgeous picture:

Wow. You absolutely want to click that to embiggen it most cromulently — you’ll get a whopping 4400 x 200 pixel version.

This image is incredible, both scientifically and logistically. It is the combination of 330 images, totaling 24 hours of solid observations, and amounted to a hefty 85 gigabytes of data. It covers three UV wavelengths: 192.8, 224.6, and 260 nanometers, which are just outside the range the human eye can see.

The image is huge; the full Moon would just fit over the apparent size of the central bulge of the galaxy. Over 20,000 individual sources of ultraviolet light can be found. Some science can be seen just with just a glance: for example, the light coming from the spiral arms is clumpy, and from the bulge it’s smooth. The arms are where you find patches of giant gas clouds forming newly born stars; the most massive of these blast out UV light and fierce winds which make the clouds themselves glow in UV.

But the bulge at the core is smooth, because stars there are old; star formation long ago ceased in the galactic center. The UV glow is mostly from tightly packed stars, not from gas. There are so many stars that the individual sources blend together into what looks like a continuous glow (not unlike a digital image itself, where individual pixels blend together to make what looks like a smooth picture).

This image is the most detailed ever taken of our big neighbor in the ultraviolet, and I have no doubt it will be used as an atlas for higher-resolution cameras aboard Hubble and future spacecraft. Pictures like this are scientifically incredibly useful; they are roadmaps we can use to plan out our travels ahead.

And they are also just very, very cool.

Image credit: NASA/Swift/Stefan Immler (GSFC) and Erin Grand (UMCP)

The hits from space keep on coming! Take a peek at this new image from Europe’s Herschel space telescope, which peers at the Universe’s far-infrared light:

[As usual, click it to embiggen.]

Very pretty! This image is a composite of five separate images taken with two cameras (PACS and SPIRE), which together cover a wavelength range of light of 70 out to 500 microns — and, bearing in mind the reddest wavelength the human eye can see is about 0.8 microns, you can see that this is way, way out in the infrared.

What you’re seeing here are cold dust clouds in the constellation of Crux, the Southern Cross. It was thought these regions would be fairly smooth on these scales, but Herschel is revealing that in fact they’re pretty turbulent. You can see ribbons and filaments of material here, caused by stars forming deep in these dense clouds. You can the odd proplyd or two; small (well, much bigger than our solar system but small-looking here) disks of matter, very dense clouds with stars forming in their cores. Proplyd is short for protoplanetary disks, because these structures are in the process of forming solar systems much like ours. And you can also see long fingers of material; towers of matter where newly-born stars are eroding and blowing away the dust with their stellar winds. In a sense, these are like cosmic sandbars, material being sculpted by fluids flowing past them.

Star formation can take place in such thickly-choked regions, but visible light cannot penetrate them; even to Hubble this would be dark material and we could only see the very surface of these clouds. But Herschel sees the kind of infrared light that passes right through the dust, so astronomers can look into the hearts of these areas and learn about star formation. We know quite a bit already, but there are still gaps in our knowledge because these clouds are so thick and difficult to study. With Herschel now on the prowl, we can expect to find out a lot about how stars are born… and also to see more pretty images like this one.

If you thought the Lagoon yesterday was pretty, then reset your awe-meter. Check. This. Out.

D’ya like that? Huh? Do ya? Had enough? No? Then check THIS out!

Jeebus. Click either to brobdingnangate. In fact, you can get massively huge versions here and here. We’re talking 30 and 40 Mb each, so be ye fairly warned, says I.

Those magnificent images are of the galaxies NGC 4402 and NGC 4522, respectively, as seen by the Hubble Space Telescope’s Advanced Camera for Surveys (from before the recent repair mission). They’re both spiral galaxies in the Virgo Cluster, the nearest large collection of galaxies to us, roughly 60 million light years from Earth.

If they look funny to you, then good! The Virgo Cluster is massive, and has a lot of gravity. The galaxies bound to it are moving like bees surrounding a hive, each in its own orbit going every which way. These galaxies are screaming through the cluster at speeds of 10 million kilometers per hour, a truly terrifying velocity.

There is an ethereal gas distributed between the galaxies called the intercluster medium. It’s incredibly thin, but over the size of a galaxy — especially when said galaxy is barreling through it at such tremendous speed — the gas can exert significant pressure, called ram pressure. The pressure is actually blowing the galaxies’ internal gas clouds out into the cluster itself, making them look a little bit like pickup trucks driving down a highway with dirt copiously pouring out the beds^*. This is especially obvious in NGC 4522 (the lower one), where you can see bright blue splotches, which are regions of intense star formation, along with dark lanes of dust actually above the galactic plane.

In NGC 4022, you can see how the ram pressure is roiling up the dust in the galaxy, and also blowing it back, though apparently not as briskly as in the other galaxy.

These pictures are incredible. Poke around them; you can see amazing detail in the galaxies themselves, as well as hundreds, maybe thousands of background galaxies.

It’s been a while since we’ve seen deep, glorious pictures of spiral galaxies from Hubble. Now that ACS is working again, and it’s being joined by the equally powerful Wide Field Camera 3, we’ll be seeing lots more of these. Get used to it.

Image credits: NASA and ESA.

I have been behind in my cool-astronomy-posting, and haven’t mentioned the GigaGalaxy Zoom project, an ambitious and totally awesome three-part image series by the European Southern Observatory. The first part was to create a magnificent all-sky view of the heavens; the second was a zoom in on the Milky Way showing a region choked with stars and dust.

The third is of the Lagoon Nebula, a star forming region a quadrillion miles across and 400 light years in toward the center of the galaxy. And the image? Well, if you want the full-res version, you’d better have some room on your drive: it has 370 million pixels and will eat up a whopping 700 Mb of disk space.

And what does it look like? Heh heh heh. Like this:

[You know the drill, click to embiggen. Do it! Now!]

Oh. Wow. And that’s low-res! Here’s the monster one if you want it.

The depth and detail are simply and truly jaw-dropping. You can zoom in and see young stars, massive stars, dark clouds, ribbons and sheets of gas sculpted by vast winds of subatomic particles blown off of supergiant stars.

The Lagoon was always a favorite target of mine in the summer months when the center of the Milky Way in Scorpius and Sagittarius would just clear my neighbor’s trees. A nearby streetlight always made observing in that direction a pain, but even from a distance of 40 quadrillion kilometers away the nebulous glow of gas and newly-born stars still shone through. I wouldn’t have been able to imagine back then that I’d be able to zoom in on the Lagoon using a 2.2 meter telescope equipped with a 67 megapixel camera!

There is science in this picture, to be sure. We can study it to look at the shape of the nebula, how it interacts with the stars and other nearby nebulae, and much more. But you know what? At this exact moment, I don’t care.

Because my oh my, the Universe is a beautiful place. And sometimes, for just a little while, that’s enough.

The spacecraft MESSENGER is just three days away from its third encounter with Mercury. The past two have been nothing short of frakkin’ amazing, so I’m really looking forward to this final pass. Even though it was 1.3 million kilometers away from the tiny planet on the 25th, it snapped this serene shot:

I love moody astronomical pictures. Here, a crescent Mercury sits in the inky black as MESSENGER screams down on it at 3.3 km/sec (2 miles per second). ON this final pass, the spacecraft will slow enough that it will be able to enter orbit around Mercury in March 2011. The nominal lifespan of the mission will be for one solid (Earth) year of observations, and will map the planet with 18 meter resolution. That’s the size of a house.

That is so going to rock. But that’s 1.5 years away, and we’re still waiting on this third pass. I hope to be able to post pix when they come in, but I’m traveling to the UK for TAM London not long after, so we’ll see. But you can always keep an eye on the MESSENGER website, as well as Emily Lakdawalla’s blog. They’ll be up-to-date… especially, if I know her, Emily, who will be spending the flyby looming over her keyboard and hitting "refresh" every 30 seconds or so.