Category Archives: galaxy classification

Galaxies Are Like People

They Come in All Shapes and Sizes

Artist's concept of the Milky Way Galaxy. Credit: NASA JPL

You grew up in this galaxy.  It’s what astronomers call a “barred spiral” and if you were an alien living on a planet a galaxy with a “top down” view of the Milky Way, this is what the view would be.  The central area, called the “bulge” is filled with stars and, of course, our own supermassive black hole.  The bar of light extending out from the center is a sort of “transport” mechanism for gas and other materials toward the core. The spiral arms are where a lot of the latest star-forming (and star death) action is taking place.  Our planet is about 2/3 of the way out from the center, between a pair of spiral arms. We don’t live in the center of this stellar city, but more like in the outskirts where the action is a bit quieter. That’s good for us, since being too close to the center might not be good for our solar system’s health.

Astronomers are well aware that not all galaxies look like this. In fact, the Milky Way didn’t always look as it does now. It has evolved, just as all other galaxies have throughout the cosmos. To study the changes that galaxies go through, astronomers have categorized them by shape (their “morphology”) and their sizes, as well as other characteristics like the ages of their stars and the metal content they have.

A new, colorful collection of galaxy specimens has been released by NASA's Wide-field Infrared Survey Explorer, or WISE, mission. Image credit: NASA/JPL-Caltech/UCLA

NASA’s WISE mission (the Wide-field Infrared Survey Explorer), is studying distant galaxies and today has released images of an assort mix of colorful and shapely galaxies. Just as people come in all sizes and variations on the two-arms, two-legs, height and weight arrangements of our bodies, galaxies also exhibit a wide array of variations on the standard theme.

The new collection of nine galaxies shows off this diversity, with members of different sizes, colors and shapes. Infrared light from the galaxies, which we can’t see with our eyes, has been translated into visible-light colors that we can see. Blue colors show older populations of stars, while yellow indicates dusty areas where stars are forming.

This collage of WISE images shows everything from “grand design spirals,” with their elegant swirling arms, to so-called “flocculent” galaxies, which look more patchy and nebulous.  All these galaxies are close enough to us that WISE can see details of their structures. Some show sinuous arms and central bulges filled with packed-together stellar populations and possibly even central supermassive black holes.

Some of the galaxies are oriented toward us nearly face-on, such as Messier 83, and others are partly angled away from us, for example Messier 81. One galaxy, NGC 5907, is oriented completely edge-on, so that all we can see is its profile. The edge of its main galaxy disk appears pencil-thin, and its halo of surrounding stars is barely visible as a green glow above and below the disk.

The arms of the galaxies come in different shapes too. Messier 51 has arms that look like a spiral lollipop, while the arms of the flocculent galaxy NGC 2403 look choppy, perhaps more like layered frosting. Astronomers think that gravitational interactions with companion galaxies may lead to more well-defined spiral arms. One such companion can be seen near Messier 51 in blue. Some of the galaxies also have spokes, or spurs, that join the arms together, such as those in IC 342.

As astronomers scan the universe, they’ll be able to dig more deeply into the different galaxy shapes they see. Just as images of different people at different ages tell us about how humans are born, age, and die, images such as these give important clues about a galaxy’s evolutionary history and the stars it contains. Not only will this work help us understand the life stories of all galaxies, it contributes to a greater appreciation of our own Milky Way and the changes it went through that led to the creation of our own Sun and planets.

A Cosmos of Galactic Content

We’re In Your Universe, Classifying Your Galaxies

As every astronomy enthusiast knows — and as many people are now learning by looking at great images from Hubble Space Telescope and other observatories — there are countless galaxies out there. They seem to exist as far as we can detect — as far back as the earliest epochs of cosmic history after the Big Bang. Last week, I posted about an image that HST took that showed galaxies as they looked at least 13 billion years ago. The big job for astronomers now is to understand the long line of evolution that begins with the first shreds of galactic matter (stars, gas, dust) that clumped together early in a galaxy’s existence — and continues through galaxy mergers and acquisitions to make the galactic objects we see today. At the same time, astronomers need to classify each galaxy they see by shape, mass, distance, color, and size.

The image shows collections of galaxies as generated by a computer model. The yellow objects are most distant and therefore appear as they were 13 billion years ago. The closer galaxies appear as they look in more recent times. Courtesy A. Benson (University of Durham), NASA / STScI. Click to galaxify.

The way to do that is take data about galaxies that we already know about, from surveys and observations by such telescopes as HST and the Two Micron All-sky Survey (2MASS, a ground-based observatory) and dump that data into computer models that slice and dice the data. The output?  An image that looks remarkable like actual survey images of galaxy fields.  That’s what two astronomers — Dr. Andrew Benson of the California Institute of Technology (Caltech) and Dr. Nick Devereux from Embry-Riddle University — did. Their work was just announced and is written up in the Monthly Notices of the Royal Astronomical Society.

Their results explain the diversity of galaxy shapes we see.   Their image (to the left) contains images of data from HST and 2MASS, and looks very much like that Hubble Deep Field image we saw at the AAS meeting last week.

Now, galaxy shapes are an interesting substory in astronomy history because, up until less than a hundred years ago, astronomers didn’t know what galaxies were, much less how to classify them. We didn’t even know WE lived in a galaxy until the early years of the 20th century.  Some of the most flashy arguments of that time were about just what the “spiral nebulae” were that astronomers were observing. There was, in fact, a well-known public debate in 1920 called the Shapley-Curtis Debate, and it really was about the scale of the universe and these spiral nebulae that seemed to be scattered around in nearby space.

It really took the advent of larger telescopes, reliable photographic instruments, and the science of astrophysics before astronomers fully understood the nature of galaxies. And, once they began looking out at the cosmos with improved instruments, they saw galaxies everywhere!  As far as we can see, we detect galaxies. And, they come in an amazing array of shapes. Classifying galaxies became a growth industry.

Astronomers began with the basic shapes:  elliptical, spiral, and irregular. But, as astronomers examined more galaxies, the simple three classes began to fragment into subclasses. Edwin Hubble (for whom the HST is named) came up with a classification scheme that is named after him, and is the basis for the more extensive classifications astronomers use today.

The Hubble Galaxy Classification scheme, modified by more recent studies of galaxlies. On the left are elliptical galaxies, with their shapes ranging from spherical (E0) to elongated (E7). Type S0 is intermediate between elliptical and spiral galaxies. The upper right line of objects stretch from Sa (tightly wound spiral) to Sc (loosely wound spiral). The lower right line shows the barred spirals that range from the tightly wound SBa to loosely wound SBc types. Credit: Ville Koistinen

If you look at this scheme and then look at the image above — or at any HST image of galaxies — you’ll see these basic shapes. Astronomers see these shapes back almost to the earliest epochs of galaxy formation — although the very earliest galaxies often looked more like shreds of material, as opposed to the more fully formed objects we see today.

So, what does galaxy classification do for us? Certainly it helps astronomers understand how many different galactic shapes there are out there. And, how many of certain types of galaxies lie at huge distances from us (at presumably earlier times) versus how many are relatively nearby and more recent.

But, look at this another way — at some level and for many galaxies, we’re seeing an evolutionary history as we look at the shapes. For example — astronomers can see places where spiral galaxies are colliding. In the fullness of time, those collisions will produce NOT new spirals, but elliptical galaxies with old stars with very little starbirth activity, but sporting supermassive black holes (and possibly high-speed jets).  In the process, the colliding galaxies will tear out huge tidal streams of gas and dust, where starbirth activity will go off like firecrackers.

Spirals, on the other hand, are hotbeds of star formation. How do they form?  Like other galaxies, they are created as smaller galaxies merge and coalesce and the complex gravitational interactions shape the spiral arms.  Our Milky Way galaxy is a barred spiral, and its shape implies that it has experienced a pretty complex history, with only a few minor collisions and at least one episode where the inner disk collapsed to form the large central bar.

And, there’s a lot of evidence that black holes play a part in galaxy formation and evolution as well. And, just to make things interesting, galaxies are very likely all embedded in haloes of dark matter — invisible “stuff” that seems to affect the evolution of galaxies as they ride along the expansion of the universe and traverse the cosmos.

As it turns out,  some irregular galaxies are also the sites of intense star formation.  In at least one case, which we saw at the American Astronomical Society meeting last week, the Small Magellanic Cloud (which is an irregular galaxy in the Milky Way’s neighborhood), is glowing with star-forming activity.

Galaxies are fascinating objects — just when we think we understand them, we find more of them to study– and classify. There’s more than enough work to do, and the observatories of the future (like James Webb Space Telescope and others) will give us deeper looks at more distant galaxies and their structures.