Starbirth: It’s Happening!

And Galaxies are Happenin’ Kinds of Places!

There are SO many interesting strands of astronomy research going on these days. I’m reminded of the complexity of it all every time I go to an AAS meeting or open up a week’s worth of press releases to see the latest news.  This past week I spent a couple of days at Jet Propulsion Laboratory, working with a team that’s putting together a set of exhibits for their visitor’s center. One of our discussions was about the topics we could illustrate that show spectacular “things” happening in the universe. Of course, galaxies are a hot research topic, what with their central supermassive black holes (that seem to be playing a bigger role in galaxy evolution than we used to think).  And, we know that galaxies are sites of star formation — which is followed (some millions or billions of years later) by star death.  Star birth and star death are also hot topics in research circles.  And, so you can see that our discussion could get pretty complex — do we show starbirth? Star death? And what about planets?  Lots of those show up in galaxies, too (at least, in the Milky Way they do, and there’s no reason to think that they don’t exist in other galaxies, as well).  Well, we ended up selecting images that show all of those topics in a sort of iconic way.

Star formation in an oddball galaxy. Courtesy Space Telescope Science Institute (click to galacticate).

During the two days I was at JPL, the Hubble Space Telescope’s latest image was released.  It’s a portion of a galaxy (called NGC 2976) that is undergoing bursts of star formation.  Now, normally, you see lots of star formation in spiral galaxies — but if you look closely at this image, you don’t see the typical spiral arms where star-forming regions. this galaxy’s a bit of a strange one because it forms stars but doesn’t really have the look and feel of a spiral where such things are common.

You can spot dusty filaments running through the disk, but those really aren’t spiral arms.  It’s amost like something disrupted what was once a spiral galaxy, roughed it up a bit, caused bursts of star formation, and then things quieted down — leaving the formerly active starbirth regions  (the blue areas) filled with hot, massive young stars.

So, what happened here?  This galaxy had the bad luck to run afoul of some neighboring massive galaxies. The gravitational effect of the interaction stripped away some gas (which is an important ingredient in star formation) and then channeled gas to the galaxy’s inner region.  That compressed gas in the inner area spurred a spate of starbirth that began about  500 million years ago.  The outer regions didn’t have enough gas to form new stars, so you don’t see any regions of starbirth out there.

Now, as it turns out, the inner disk is just almost out of gas. This is because all the star-forming activity has has “eaten” up the available star-forming stuff.  When astrononomers look at this galaxy, they now see a small region of hot new stars and starbirth crêches near the center, and nothing but stars in the rest of the galaxy.

The blue dots in the image are the young blue giant stars residing in the remaining active star-birth regions. They’ll start to die in perhaps tens of millions of years (as opposed to the Sun, which will live about 10 BILLION years), creating gorgeous supernova remnants — which will seed the galaxy with the material for the next generation of stars. For those of you who are stargazers, NGC 2976 is part of the M81 group of galaxies. They lie about 12 million light-years away in the constellation Ursa Major.

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.