Calamity looms for Milky Way 27 million years out
By Amanda Jermyn

Astronomers have revealed that a massive cloud of gas will crash into the Milky Way, but not to worry, it won’t be for another 27 million years or so, and even then, it’s unlikely to do much damage to our home galaxy.

Doctoral astronomy student Gail Smith first discovered the Smith Cloud back in 1963 when she detected the radio waves emitted by its hydrogen. New observations by NASA’s Hubble Space Telescope indicate the high velocity gas cloud was ejected from the outer regions of our galaxy’s disk about 70 million years ago and is now hurtling back towards the Milky Way at a speed of 700,000 miles per hour. At this rate, it is expected to collide with the Perseus arm, one of two major spiral arms of the galaxy, in about 27 million years.

The cloud, whose trajectory is well known, has been stretched into the shape of a comet by gravity and gas pressure. It is 11,000 light-years long and 2,500 light-years across. To give you an idea of how large this is, a light-year is the distance light travels in one year, which is almost 6 trillion miles. While the cloud is not visible because the radiation it emits is not in the visible light spectrum, if it were visible in the sky it would have an apparent diameter 30 times greater than the diameter of the full Moon.

One theory held that the Smith Cloud had its origins outside the Milky Way, as a failed galaxy or gas from intergalactic space that was captured by our galaxy’s gravity. However, new evidence suggests it originated within the Milky Way but was later ejected. A team of researchers, led by Andrew Fox, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, used the Hubble Space Telescope’s Cosmic Origins Spectrograph to examine the cloud’s chemical composition. They discovered that it contained about the same amount of sulfur as that found in the outer disk of the Milky Way, suggesting that the cloud formed in this region and was enriched by material from the region’s stars.

While there are many other high velocity gas clouds in the vicinity of the Milky Way this is the only one whose trajectory is well known, so there may be others interacting with our galaxy in similar ways. Fox notes: “Our galaxy is recycling its gas through clouds, the Smith Cloud being one example, and will form stars in different places than before.”

Why the cloud was ejected is still unknown, though a cluster of supernova explosions has been suggested as a possible trigger. Another mystery is how it has remained relatively intact throughout its long journey. As its trajectory brings it closer to the Milky Way that is starting to change. According to Fox, “The cloud is fragmenting and evaporating as it plows through a halo of diffuse gas surrounding our galaxy. It is basically falling apart. This means that not all of the material in Smith’s Cloud will survive to form new stars.” However, when the material that does survive collides with gas in the galaxy there should be a dramatic surge in star formation.

While there is no public Stars Club meeting in December, on Friday, January 6th at 7:30pm, the Stars Club and Springfield Science Museum will host “Stars over Springfield,” an astronomy adventure for the whole family. A fee of $3 for adults and $2 for children under 18 will be charged.

Also, join the Springfield Stars Club on Tuesday, January 24th at 7:00pm at the Springfield Science Museum for a talk by Alan Rifkin and Richard Sanderson on Solar Eclipses. The talk will include information on the August 2017 solar eclipse. Refreshments will be served, and the public is welcome. The meeting is free of charge for members, with a suggested donation of $2 per nonmember.