Printer Friendly

A scanner darkly.

Dr. Weaver,

In your March 2007 column, you state: "It is actually two clusters of galaxies, gasses and plasma found about four billion light years from Earth that collided around 100 million years ago."

Isn't there a typing error in this sentence? It seems to not make sense to see an event that happened 100 million years ago if the two clusters in question are four billion light years away from the Earth. I appreciate your kind response.



Hi Victorio,

I wrestled with this as well. The original paper, available at a, describes the bullet cluster: "... As the line-of-sight velocity difference between the components is only ~600 km/s, the merger must be occurring nearly in the plane of the sky and the cores passed through each other ~100 Myr ago." Various online sources (including describe the cluster as being" ... about 4 billion light years from Earth in the constellation of Carina. Observed from Earth, the subcluster passed through the cluster center 150 million years ago...."

I believe the important part is "observed from Earth," so that the temporal reference frame is our own reference frame, i.e., the reference frame of the observatories. So, from our reference frame, the collision happened approximately 100 million years ago, while from the reference frame of the Bullet cluster, it happened around 4 billion + 100 million years ago.

COPYRIGHT 2007 Advantage Business Media
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2007 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Victorio; Weaver, Bill
Publication:Scientific Computing
Article Type:Letter to the editor
Date:May 1, 2007
Previous Article:Seats helped ancient Greeks hear from back row.
Next Article:First deep-sea observatory.

Terms of use | Privacy policy | Copyright © 2018 Farlex, Inc. | Feedback | For webmasters