An international team of astronomers, led by researchers at Japan’s Tohoku University, has just reported finding the faintest satellite galaxy ever seen orbiting our home galaxy, the Milky Way. All stars are born in great islands or groupings of stars called galaxies.
Big galaxies like the Milky Way are surrounded by smaller “baby galaxies” (or satellite galaxies), some of which collide with it over cosmic times. About 50 such galaxies are currently known to orbit our Milky Way – with the two “Magellanic Clouds” (discovered by Ferdinand Magellan’s crew) being the most famous of them.
Because many of the smallest galaxies are very faint, they are hard for us to make out. Remember, we are inside the Milky Way, and so (as we try to look outwards) we always have to observe through the stars and star clusters of our own galaxy. The faint baby galaxies can be hard to tell apart from clusters or groups of stars in the Milky Way itself. (This is why it’s hard to get a good photo of the Milky Way; we are inside it and so it’s like trying to take a selfie from inside your kidney. The view is not so clear.)
Still, using the giant Subaru telescope (whose mirror is more than 24 feet wide), the team was able to find the faintest baby galaxy ever found, which is being called Virgo I (since we see it in the constellation of Virgo.) At an estimated distance of 280,000 lightyears from us, Virgo I was much fainter than earlier surveys for our neighbor galaxies were able to reveal.
The whole Virgo I “dwarf galaxy” is only about 248 lightyears wide. Compare that to the 100,000 lightyear diameter of the Milky Way! The Magellanic Clouds are estimated to be 7,000 and 14,000 light across. So you can see that Virgo I really is just a baby. See the tiny smudge it makes on our accompanying image.
But if one such baby galaxy has escaped our notice until now, chances are many others like it may also be out there. Some of our theories predict that major galaxies like the Milky Way should be surrounded by many more dwarf galaxies that we have seen so far. Virgo I leads astronomers to think that more may be out there -- just waiting for bigger telescopes and more observations before they are discovered.