20 years of Hubble

[sheeny]

Hubble has been going for 20 years. Katherine Sanderson looks back at 20 years of Hubble:

http://www.nature.com/news/2010/100422/full/news.2010.195.html

Al.

[Robh]

Al, very interesting!

The early images were really quite fuzzy (slide 2).

Slide 12. It says that hot gas is slowed by drag but dark matter, which is only affected by gravity, is not slowed. If it is not gravitational drag then what causes it? Physical obstruction by galaxies like pebbles in a river? Anyone.

Regards, Rob.

[sheeny]

The term drag means friction due to collisions of the actual molecules in the gas. While the distances between the molecules will be relatively large as it is hot gas they are moving quickly. Collisions between the gas molecules are therefore much more likely than the improbable collisions of stars as galaxies merge.

Gravitational attraction is not usually (or correctly) referred to as drag.

:)

Al.

[Robh]

Hi Al,

Thanks for that. It was the friction of molecules that produced the drag.
I tracked down an article on it. The two pink clumps are the hot gases from two large clusters of galaxies that collided. The hot gases passed through each other, hence the drag.
http://chandra.harvard.edu/photo/2006/1e0657/

A bit on gravitational drag or dynamical friction...
http://en.wikipedia.org/wiki/Dynamical_friction

Regards, Rob