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SYFY WIRE Bad Astronomy

A Face-On Blast From the Past

By Phil Plait
The central region of the spiral galaxy M61 seen by Hubble. Credit: ESA/Hubble & NASA. Acknowledgements: G. Chapdelaine, L. Limatola, and R. Gendler.

Every week, the folks at Hubble release a lovely image taken using their fine observatory. It’s always fun to see what they’ll show next, but this week’s gave me a bit of a surprise and a wonderful wave of nostalgia.

The image is M61, a spectacular face-on spiral galaxy:

The central region of the spiral galaxy M61 seen by Hubble. Credit: ESA/Hubble & NASA. Acknowledgements: G. Chapdelaine, L. Limatola, and R. Gendler.

The shot is a bit unusual in that it’s composed of only two colors: an image taken in blue light (shown as blue in the picture) and another in the near-infrared (colored yellow in the picture). I rather like the way they combine. The spiral arms in a galaxy are where most stars are born, and the brightest ones are young, hot, and blue. That makes the spiral pattern pop out in blue. Those stars die young, so older populations tend to look redder, like in the center of the galaxy. Light-absorbing dust blocks the glow of both, and you can trace the whirlpool right down to the center of the galaxy.

The surprise, though, was the shape of the image itself. The step pattern (sometimes called a batwing or Stealth bomber shape) is a giveaway that this image was taken using Hubble’s Wide Field/Planetary Camera 2, installed on the observatory in late 1993. The WFPC2 had four detectors on it: three that had a wide field of view and one that was higher resolution, able to zoom in a bit more on smaller objects (like planets). They were arranged in that odd manner that became an iconic look for Hubble for many years. In 2002 the Advanced Camera for Surveys was installed on Hubble and took over the main imaging functions, and in 2009 astronauts removed WFPC2 and replaced it with the Wide Field Camera 3. That was a built-in part of the Hubble mission: the ability to take out old tech and replace it with more advanced, modern instruments.

I used the original WFPC a bit for some work shortly after I got my Ph.D. and played around some with WFPC2 data as well. I haven’t really thought about it in some years, so seeing this image of M61 was a nice little blast from the past. It was taken as part of the Hubble’s Hidden Treasures program, to highlight older or less well-known images from the venerable observatory. It was a great choice.

And a pretty one, too. M61 is part of the Virgo Cluster, a nearby collection of more than 1,000 galaxies, about 60 million light-years away. M61 is not all that different from our own Milky Way; it’s about 100,000 light-years across, has well-defined spiral arms, a bar in the center (the rectangular feature you can see in the Hubble picture; the Milky Way’s bar is much larger, though), and, like all big galaxies, has a ginormous black hole in its very core. Although it has roughly the same mass as the one in the center of the Milky Way, unlike ours, the M61 central black hole is actively gobbling down matter, which heats up as it falls in, getting intensely bright. That makes M61 a so-called “active galaxy,” though somewhat on the low end of the scale (unlike, say, Herc A).

And hmmmm. Right now the constellation of Virgo is high in the sky once it gets dark out and is bright enough to spot pretty easily in a small telescope. I may have to give it a try with mine. It won’t look quite as magnificent as it does via Hubble, but that’s OK. There’s just something about seeing that light with your own eyes, knowing it’s traveled for tens of millions of years across the vast reaches of space to finally reach us. It’s one of the reasons so many amateur astronomers do what they do. Me included.

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