How far is Andromeda

As I have written in my previous post, Edwin Powell Hubble expanded our understanding of the universe by calculating the distance of Andromeda using Cepheid Variable stars using 100 inch Hooker telescope at Mount Wilson. It was believed that it’s a nebula like many others inside our own galaxy but he calculated that its distance is 90,000 light years from earth.

 Later Walter Baade studied the RR Lyrae stars using the same telescope. RR Lyrae are also variable star similar to Cepheids but less luminous. It was shown before that like Cepheids, the variability of RR Lyrae stars can be used to measure distances. 

The movie shows RR Lyrae stars in a globular cluster. You can see their brightness changing; they look blue as they become brighter.

astro.princeton.edu

Walter Baade wanted to use these stars to measure the distance of Andromeda as it was done before using Cepheids. But the 100 inch telescope was not good enough to detect those stars. So he had to wait until the 200 inch (~5 meter) telescope was ready which was being built by George Hale but sadly he died two years after the project started. Later the telescope was named after him.

When the new Hale telescope became operational Baade used the telescope to search the faint variable stars in Andromeda but even after searching for a long time he was not able to find any sign of these stars.

He concluded that the only possible reason for this can be the distance of Andromeda previously measured is not correct!

At that time it was becoming evident that stars can be categorized into two broad types called populations. Older stars fall in Population2 and younger and brighter stars in Population1.

So Baade assumed that Cepheid variable stars will also have two different types. Thus he reasoned his argument that the previously measured distance of Andromeda was wrong using two points,

1.      Population1 Cepheids are brighter than Population2 Cepheids.

2.      Astronomers only saw the brighter Population1 stars in Andromeda and compared it to the dimmer population2 stars in Milky Way.

That’s what lacked in the calculations made by Hubble. And that’s why he measured the distance of Andromeda little less.

Baade calculated that Population1 stars are on average 4 times more luminous than Popluations2 stars of same period of variations. So if a star is moved twice as far away it will appear 4 times fainter. Thus Andromeda Galaxy should be twice as far away - approximately 2 million light years away!

More accurately,

How far is Andromeda? Its 2.537 million light years away.