Category Archives: galaxies

galaxies, galaxy classification, galaxy shapes

Galaxies Are Like People

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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.

astronomy, astronomy news, galaxies, galaxy interactions, galaxy shapes, hst news, Hubble Space Telecope

It Was 21 Years Ago…

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That Hubble Went out to Play

To celebrate the 21st anniversary of the Hubble Space Telescope's deployment into space, astronomers at the Space Telescope Science Institute in Baltimore, Md., pointed Hubble's eye at an especially photogenic pair of interacting galaxies called Arp 273. This image is a composite of Hubble Wide Field Camera 3 data taken on December 17, 2010, with three separate filters that allow a broad range of wavelengths covering the ultraviolet, blue, and red portions of the spectrum. Hubble was launched April 24, 1990, aboard Discovery's STS-31 mission. Click to enlarge (and you WANT to see this one bigger).

And what a time it’s been!  As you can see by this image, the most famous of the Great Observatories is still crankin’ out some stunning visions of the cosmos.  Take this image, for example. It’s a pair of interacting galaxies, slightly farther along in their gravitational dance than the two I wrote about in my last entry. They are an interesting looking grouping called Arp 273.

The larger of the spiral galaxies in the group, known as UGC 1810, has a disk that is distorted into a rose-like shape by the gravitational tidal pull of the companion galaxy below it, known as UGC 1813. Not only are these two cosmic behemoths changing each other’s shapes, but in the process, they’re spurring huge swaths of star-forming factories in the process. Those are the blublogs at the top of UGC 1810, and the bluish clouds of light at the tip of the lower galaxy.  The image (embiggenate to see it better) shows a tenuous tidal bridge of material between the two galaxies that are separated by tens of thousands of light-years from each other.

Even more unusual are the off-center spiral patterns of each galaxy.  Even if you didn’t know anything else about these galaxies, just one look at the off-kilter spirals would tell you that something has happened. In this case, one galaxy has dived through the center of the other. The smaller one probably sliced right through its larger companion above it in this image.

Notice how the spiral arms of UGC 1810 (the upper one) are warped off-kilter with respect to each other.  The inner set is offset out of the plane of the galaxy.  This must have been a titanic interaction!

As if this wasn’t weird enough, there’s also a possible mini-spiral in the upper right arms of UGC 1810.

Astronomers have seen many interacting galaxies — enough to be able to understand something of how and why they form. In this case, the larger galaxy of the pair is about five times more massive than its smaller companion.   In unequal pairs such as this, the relatively rapid passage of a companion galaxy produces the lopsided or asymmetric structure in the main spiral. Also in such encounters, the starburst activity typically begins in the minor galaxy earlier than it does in the major galaxie. These effects could be due to the fact that the smaller galaxies have consumed less of the gas present in their nucleus — and that gas is what you need for stars to form. The gravitational shock waves spur “bursts” of star formation as the gas is compressed and heated during the interaction.

Arp 273 lies in the constellation Andromeda and is roughly 300 million light-years away from Earth. This image is just one of a stream of cosmic visions sent back by Hubble during its 21 years on orbit.  Currently, the telescope is in great shape and should continue its work for some time to come.