We’re on a Collision Course with a Gas Cloud

Smith Cloud

A giant cloud of mostly hydrogen gas with enough material to make over a million suns is heading towards our Milky Way at a speed of 45 miles per second. Called the Smith Cloud (after Gail Bieger-Smith who discovered it in 1963), this 9,800 × 3,300 ly high velocity cloud (HVC) is about 40,000 ly distant and is expected to slam into our Milky Way galaxy in about 27 million years, causing the birth of many new stars a quarter-way round the galaxy from us.

Smith Cloud is located in the constellation Aquila, the Eagle

The Smith Cloud is located in the constellation Aquila, and has an apparent diameter around 11° across its long axis. It is only visible using radio telescopes (spin-flip transition of neutral atomic hydrogen), or by detecting hydrogen absorption lines Doppler shifted and superimposed upon the spectra of more distant stars that are shining through the cloud.

The origin of the Smith Cloud is unknown. It may have originated within the Milky Way galaxy itself, or it may be extragalactic. The upcoming collision may not be the first time the Smith Cloud has encountered the disc of the Milky Way. It may be embedded in a large halo of dark matter which would have kept the cloud from being completely disrupted during any past encounters.

The Smith Cloud is a great example of an object that would never have been discovered were it not for radio astronomy. Felix J. Lockman, who has published extensively on the Smith Cloud, has created Radio Astronomy: Observing the Invisible Universe for The Great Courses. Dr. Lockman’s engaging lecture style, his clear explanations, and thorough knowledge of the subject matter makes this the perfect introduction to the subject. Highly recommended!

Incidentally, Jay Lockman discovered a region in Ursa Major that is relatively free of neutral hydrogen gas and dust, thus affording a clearer view into the distant universe. It is named, appropriately, the Lockman Hole.


Alig, C. et al. “Simulating the Impact of the Smith Cloud.” The Astrophysical Journal 869 (2018): 1-6.
arXiv:1901.01639 [astro-ph.GA]

Hu, Y. et al. “Magnetic field morphology in interstellar clouds with the velocity gradient technique.” Nature Astronomy (2019): 1-7.
arXiv:2002.09948 [astro-ph.GA]

Lockman, F.. “Accretion Onto the Milky Way: The Smith Cloud.” Proceedings of the International Astronomical Union 11 (2015): 9 – 12.
arXiv:1511.05423 [astro-ph.GA]

M81 and M82 from HST

The galaxy pair M81 and M82 in Ursa Major must rank near the top of the list of best-loved objects for any Northern Hemisphere amateur astronomer.  So, to see such a familiar object as these in breathtaking Hubble Space Telescope detail is thrilling indeed:

Messier 81 from the Hubble Space Telescope – click on the image for a larger view

Messier 82 from the Hubble Space Telescope – click on the image for a larger view

M81 and M82 lie little more than a moon-width apart in the constellation Ursa Major, 11.8 million and 11.5 million light years, respectively, from Earth.  Check out this pretty pair with either binoculars or a telescope any clear evening during the next few days.  Both galaxies transit the meridian on April 14 at the end of evening twilight, so this is the perfect time to observe them at their highest in the sky.  You can find Bode’s Galaxy (M81) and the “Silver Sliver” (M82) by drawing an imaginary diagonal across the bowl of the Big Dipper, opposite (rather than along) the handle, and extending the diagonal beyond the bowl almost as far as the two bowl stars are apart. Or, using the chart I created below, draw an imaginary line between Dubhe and 24 UMa, then go about four-fifths of the way to 24 UMa.  M81 & M82 lie about 0.4° (a little less than a moon-width) perpendicular to that line on the Polaris side.  Bingo, you’ve got ’em!

Skyline to M81 (and M82)