Galaxies

I was browsing through the Hubble Space Telescope site today and came across many beautiful images of astronomical objects taken with the space telescope.
Among them were the two galaxies displayed in this post.

The one at rightis M 101. It is a spiral galaxy that is about 25 Million Light Years away. It is nicknamed the "Pinwheel Galaxy" for obvious reasons. It was first discovered by Pierre Mechain in 1871 and was included by Charles Messier in his catalog. It is relatively large with a diameter of 170.000 light years. Our galaxy, the Milky Way has a diameter of about 100,000 light years. The Pinwheel is quite asymetrical (compare left and right sides in the picture). This asymetry is thought to have been caused by a close encounter with another galaxy in the relatively recent past. Galactic collisions are not particularly rare, photographs have been taken of galactic collisions. Our galaxy would look something like M 101 from the outside. We see it as a band across the sky as we are inside the Milky Way Galaxy.

The galaxy at left is also a spiral but with quite a different appearance. Its designation is NGC 1300. It type is called a "Barred Spiral" as the spiral arms are connected by a large bar rather than spiral in to the centre as in M 101. It is approximately 70 million light years away and about 100,00 light years across. It was discovered by John Herschel in 1835. His father was the more famous William Herschel.


A light year is the distance that light travels in one year, so the pictures show the galaxies as they were 25 and 70 million years ago respectively. The distance of a light year can be easily calculated by multiplying seconds in a minute X minutes in a hour X hours in a day X days in a year X distance light travels in one second. As light travels very close to 300,000 Kms in a second the calculation becomes 60 X 60 X 24 X 365 X 300,000. The answer is 9,460,000,000,000 Kms.

That means that M 101 is 236,000,000,000,000,000,000 Kms away and NGC 1300, 66,20,00,000,000,000,000,000 Kms away. These are large distances in anyone's estimation - except maybe for astronomers. Compared to the most distant galaxies known M 101 and NGC 1300 are just in our back yard. The most distant galaxies are of the order of 13 billion light years distant from us. I won't even bother with the kilometer distance which would be meaningless and silly. Galaxies much older than 13 billion years old are unlikely to be found as the universe is about 13.7 billion years old.

This post raises many questions, in particular, how are astronomical distances measured and how do we know that the universe is about 13.7 billion years old? They are obvious topics for later posts.