The Whirlpool Galaxy is an astrophotography classic, all swirling arms and bright stars. A new composite image of optical and X-ray wavelengths layers in a map of neutron stars and black holes within the galaxy, bright X-ray sources highlighting an ongoing galactic collision.

The Whirlpool Galaxy is Messier Object 51 (M51), one of the original fuzzy blobs categorized by early astronomers squinting through their 'scopes. While the optical-light image from the Hubble Space Telescope is probably familiar, this composite image adds a new layer of nearly 1 million seconds of X-ray observations from Chandra.

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Each of the bright purple points is an X-ray source, most commonly an X-ray binary where a neutron star or black hole is orbiting a more mundane companion star. The exotic compact star steals gas from its companion. The gas accelerates as it spirals within the intense gravitational field, heating to millions of degrees where it emits X-rays.

The Whirlpool Galaxy is a massive spiral galaxy like our own, although we can't feasibly get such a beautiful vantage point to observe the Milky Way. While we're set to collide with the Andromeda Galaxy in the distant future, the Whirlpool Galaxy is already mid-merge. While X-ray sources are scattered through both the Whirlpool Galaxy and the smaller galaxy merging into it, they're most dense in the region surrounding the collision where the surging galactic dynamics are triggering waves of star formation. The most massive of those stars race through their fusible materials in just a few million years before exploding and collapsing into compact objects. The remains of those explosions, neutron stars or black holes, are X-ray sources detectable by Chandra.

The Whirlpool Galaxy in optical wavelengths [left, credit: NASA/STScI] and X-ray wavelengths [right, credit: NASA/CXC/Wesleyan Univ./R.Kilgard, et al]. Top image is a rotated composite of these two images.

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Most of the X-ray binaries contain neutron stars, but at least ten are bright enough to contain a black hole, and eight of those are munching on a companion star more massive than our sun. In ten years of observations, Chandra has found that the X-ray sources stay consistently bright over time, indicating a steady supply of super-heated gas.

Last time Chandra surveyed the galaxy, it found roughly 100 X-ray sources. The new, longer observations reveal closer to 500 X-ray sources, of which 400 are likely within the galaxy, and the remainder are either in the foreground or background. Aside from providing insight into galaxy dynamics and stellar evolution, carefully mapping known X-ray sources helps us recognize glitches when observing gamma ray bursts.

The Whirlpool Galaxy is about 30 million light years away. The scale is as mind-boggling as it always is in astronomy: the field of view in the image is 6 by 10 arcmin, or roughly 52,000 by 87,000 light years across at the depth of the galaxy.

Read more on the Chandra website.