Our own Milky Way is a spiral galaxy. How do we know this? Sadly, we are not able to send a probe or spacecraft above the Milky Way to take a picture for us as this would make for too long a trip.
With present technology we would have to wait millions of years for a probe to fly far enough up above the plane of the Galaxy to take that precious panoramic photograph. All of this assumes, of course, that the camera equipment would still work after all that time.
The reason we know that we live in a spiral galaxy is because we can view the arms in a particular color seen only at radio wavelengths called ’21-cm.’ It’s not as stylish a name as ‘burgundy,’ ‘turquoise,’ or ‘chartreuse,’ but this color with the humble sounding name allows us to see where the spiral arms are, and even where they start and stop, without ever having to leave home.
This would be all well and good, except that as astronomers sit in an observatory merrily measuring the characteristics of the spiral arms, a collision is setting itself up with our nearest spiral galaxy that will change everything.
The nearest galaxy to us as seen from the northern hemisphere is called ‘Andromeda.’ It, and it’s 200 billion stars is hurtling towards the Milky Way and its 300 billion stars at a speed of about 250,000 miles per hour.
At this rate, given its present distance from us of 2.5 million light years, the two galaxies are fixing to collide with one another in about 4 billion years.
When this happens the orbits of the stars will be perturbed from their present, calm orbits into a more random arrangement about a new center. The spiral disk shape that we know and love from artist’s conceptions, because we cannot photograph it ourselves, will likely turn into more of a football shape called an elliptical galaxy. There is no need to worry just yet – 4 billion years is a long time away..