NATIONAL GEOGRAPHIC'S BEST SPACE PICTURES THIS WEEK XXXIX

Week's Best Space Pictures: Galaxies Collide, Gullies Get Frosted, and Cities Light Up
By Jane J. Lee, National Geographic News, 19 December 2014.

Colliding galaxies put on a show, Martian gullies meet Jack Frost, and cities glitter for the holidays in this week's best space pictures.

1. A Colourful Collision

Two spiral galaxies in the Canis Major constellation collide in an encounter fraught with supernovae and incredibly bright x-rays.

X-ray binary systems - consisting of one star orbiting a neutron star or black hole - give off these luminous x-rays. The spiral galaxies NGC 2207 and IC 2163 contain one of the largest collections of such systems. (Learn why binary stars are so prevalent.)

Researchers believe these superbright x-rays could be evidence of an as-yet-undiscovered intermediate size of black hole. (Read about black holes in National Geographic magazine.)

The composite image above is a combination of views, with x-rays in pink; visible light in blue, white, orange, and brown; and infrared light in red.

2. Mapping a Retreat

Photographs of a Greenland glacier near the Sukkertoppen ice cap highlight how far the river of ice retreated between the summer of 1930 (bottom image) and summer 2013 (top).

Researchers estimate the glacier has shrunk by almost two miles (three kilometres).

The analysis was part of a study of the hidden movements of Greenland's glaciers, presented at the American Geophysical Union meeting this week in San Francisco. (See "Greenland Glacier Races to Ocean at Record Speed.")

3. Holiday Lights

It's official - all those holiday lights going up in cities across the U.S. can be seen from space.

A new NASA analysis finds that U.S. cities are 20 to 50 percent brighter during Christmas and New Year's, compared with other times of the year. In this satellite image of Texas and Louisiana, green spots indicate more light in December 2012 and 2013 than the average from 2012 to 2014.

In Texas, Dallas (left, centre) and Houston (left, bottom) are lit up like a green blobs. (Related: "Real Christmas Trees Save Water.")

Some Middle Eastern cities are more than 50 percent brighter during Ramadan than they are during the rest of the year.

4. Glittery Cradle

Dwarf galaxy Markarian 209 is quite the baby star factory. The image above captured a particularly productive period of star formation in the light-blue region at the top right portion of the galaxy.

The compact galaxy has continuously produced stars despite its advanced age. Markarian 209's current crop is relatively young, at just under three million years old. (See "Glowing, Green Space Blob Forming New Stars, Hubble Shows.")

5. A Frosty Dusting

Frost, which on Mars is normally made of dry ice (frozen carbon dioxide), coats gullies on the south-facing slopes of a small Martian crater in this image released December 17. North-facing slopes are frost-free this time of year on the red planet. (See "Alien Life on Mars? NASA Rover Spots Methane, a Possible Sign of Microbes.")

6. A Pulse of Life

This satellite image shows what happened when water in the Colorado River was allowed to flow to the Sea of Cortez earlier this year. Vegetation along the lower reaches of the river got a shot in the arm, appearing greener and healthier (green areas on image above) in August 2014 than they did in August 2013. (See "Saving the Colorado River Delta, One Habitat at a Time.")

7. Meteors Shower Earth

Photograph by Tunç Tezel, TWAN

A composite image shows the Geminid meteor shower in action over Turkey earlier this week. Taken over the course of an hour, the photographs feature one of the most reliable celestial events of the year. (See "Gem of a Meteor Shower on Tap This Weekend.")

Photo gallery by Mallory Benedict.

[Source: National Geographic News. Edited. Some links added.]