Category Archives: galaxies

galaxies, hubble space telescope

It’s Classified

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Galaxies in the Young Universe

For a long time, well before the invention, creation, and deployment of the Hubble Space Telescope, classifying galaxies was easy for the closest ones and quite a bit tougher for the more distant ones. When I say “classify” I mean determining a galaxy’s size and shape and structure — its morphology.  There are several main shapes: spiral, elliptical, lenticular, irregular — and within those classifications you get subclasses like barred spirals and so forth.  There was also a class of galaxies called “peculiar” because they looked — well, peculiar.

Making out the shape of a distant, dim blob of light is difficult unless you have a really good, strong telescope able to look out through the guck. So, until we had such observatories as Hubble and Gemini and Spitzer and European Southern Observatory and others, it was easier to determine the morphology of galaxies that were easier to see. That also meant astronomers classified galaxies that were closer to us. Edwin Hubble, for whom the HST is named, developed a classification scheme that astronomers came to call the “tuning fork” diagram because it … looks like a tuning fork. Nearby galaxies were observed and fit into this scheme and that helped astronomers talk definitively about the structure and characteristics of, oh say, a barred spiral galaxy and compare it to the characteristics of an elliptical galaxy.

Data from the Hubble Space Telescope and the Sloan Digital Sky Survey show that galaxies at a time when the universe was about half its present age, looked different than galaxies today. Courtesy HST and Sloan Digital Sky Survey. Click to galacticate.

But, the burning question always has remained — what are more distant galaxies like? Do they follow the same morphological classification scheme? And if so, what does that tell us about the time in which they formed and in which we observe them? Remember: when we look at distant galaxies, we are seeing them as they appeared very long ago. Looking at a galaxy that is six billion light-years away only tells us what it looked like roughly six billion years ago. Did galaxies conform to the same kinds of shapes they do in the closer and more recent universe? That was a question that modern telescopes were designed to answer.  Scientists used data from both Hubble Space Telescope and the ground-based Sloan Digital Sky Survey to create a tuning fork for the more distant universe.  The image above shows the tuning fork for the local galaxies in the top section. The bottom section is a classification of more distant galaxies that lie some six billion light-years away.

Even just a quick look at the image will show that the nearby galaxies — also known as the “local universe” is mostly (about 72 percent) spiral galaxies.  Another 15 percent are lenticular (labeled S0), and only three percent are ellipticals (indicated by the letter E).

Now, look at the bottom half of the image and you see that among the galaxies as they existed half the age of the universe ago, there are way more peculiar galaxies (more than half, actually). Ellipticals are just about four percent, 31 percent are spirals, and 13 percent are lenticular.

Astronomers think that many of the pecular galaxies will evolve to become spirals through galaxy mergers.  That process of interaction and merger is how the Andromeda Galaxy was formed, and indeed, our own galaxy is in process of gobbling up smaller, dwarf companions.

So, what’s the take-away message from this work?  There are a couple: Classification is more than just make-work.   Classification helps us trace the history of galaxy formation. Ultimately, it brings us back to the present-day universe and helps us understand why it looks the way it does.  If you want to read more about the observations behind this result, check out the Hubble Europe web page story.

astronomy news, astrophysics, galaxies, hubble space telescope, starbirth

Starbirth: It’s Happening!

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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.