Spitzer: A Pinwheel Galaxy Rainbow (M101)

[bystander]

A Pinwheel Galaxy Rainbow (M101)
NASA JPL-Caltech | Spitzer | 2012 May 23

---

This image of the Pinwheel Galaxy, or M101, combines data in the infrared, visible, ultraviolet and x-rays from four of NASAs space telescopes. This multi-spectral view shows that both young and old stars are evenly distributed along M101s tightly-wound spiral arms. Such composite images allow astronomers to see how features in one part of the spectrum match up with those seen in other parts. It is like seeing with a regular camera, an ultraviolet camera, night-vision goggles and X-Ray vision, all at once!

The Pinwheel Galaxy is in the constellation of Ursa Major (also known as the Big Dipper). It is about 70% larger than our own Milky Way Galaxy, with a diameter of about 170,000 light years, and sits at a distance of 21 million light years from Earth. This means that the light were seeing in this image left the Pinwheel Galaxy about 21 million years ago - many millions of years before humans ever walked the Earth.

The red colors in the image show infrared light, as seen by the Spitzer Space Telescope. These areas show the heat emitted by dusty lanes in the galaxy, where stars are forming.

The yellow component is visible light, observed by the Hubble Space Telescope. Most of this light comes from stars, and they trace the same spiral structure as the dust lanes seen in the infrared.

The blue areas are ultraviolet light, given out by hot, young stars that formed about 1 million years ago. The Galaxy Evolution Explorer (GALEX) captured this component of the image.

Finally, the hottest areas are shown in purple, where the Chandra X-ray observatory observed the X-ray emission from exploded stars, million-degree gas, and material colliding around black holes.

Credit: X-ray: NASA/CXC/SAO; IR & UV: NASA/JPL-Caltech; Optical: NASA/STScI

M101: A Pinwheel in Many Colors
NASA SAO | Chandra X-ray Observatory | 2012 May 24

[Ann]

This multi-spectral view shows that both young and old stars are evenly distributed along M101s tightly-wound spiral arms.

Very interesting, but I don't think I entirely agree.

This composite image does not, in fact, trace the old yellow stars in M101. The infrared filter at 8.0 µm doesn't see the old yellow or red stars very well, since this wavelength is in the far infrared, and cool red stars emit copious amounts of red and near infrared light at about 700 nanometers (or 7.0 µm), but not a lot of far infrared light. The far infrared filter is much better at tracing dust, but dust is more strongly associated with young stars and star formation than with old red stars.

What about the Hubble image that traces optical light? In this case, the filter used for the Hubble image was centered at 435 nm, which is far into the blue part of the spectrum. Cool old stars will barely show up at all through such a filter. They will be even more invisible when seen through an ultraviolet filter, particularly a fairly far ultraviolet filter centered at 150 nm. Finally, there is no reason to believe that old red stars will show up particularly strongly through an X-ray detector.

So in my opinion, while the picture of M101 through four different filters is interesting and worthwhile, it doesn't say much about the distribution of old stars in M101.

Ann