Supernova Remnant: The Cygnus Loop Nebula

Wispy tendrils of hot dust and gas glow brightly in this ultraviolet image of the Cygnus Loop nebula, taken by NASA's Galaxy Evolution Explorer. The nebula lies about 1,500 light-years away, and is a supernova remnant, left over from a massive stellar explosion that occurred between 5,000 to 8,000 years ago. The Cygnus Loop extends over three times the size of the full moon in the night sky, and is tucked next to one of the "swan's wings" in the constellation of Cygnus.

The filaments of gas and dust visible here in ultraviolet light were heated by the shockwave from the supernova, which is still spreading outward from the original explosion. The original supernova would have been bright enough to be seen clearly from Earth with the naked eye.

Image Credit: NASA/JPL-Caltech
Explanation from: https://www.nasa.gov/mission_pages/galex/pia15415.html

IC 4499: A globular cluster’s age revisited

This NASA/ESA Hubble Space Telescope image shows the globular cluster IC 4499.

Globular clusters are big balls of old stars that orbit around their host galaxy. It has long been believed that all the stars within a globular cluster form at the about same time, a property which can be used to determine the cluster's age. For more massive globulars however, detailed observations have shown that this is not entirely true — there is evidence that they instead consist of multiple populations of stars born at different times. One of the driving forces behind this behaviour is thought to be gravity: more massive globulars manage to grab more gas and dust, which can then be transformed into new stars.

IC 4499 is a somewhat special case. Its mass lies somewhere between low-mass globulars, which show a single generation build-up, and the more complex and massive globulars which can contain more than one generation of stars. By studying objects like IC 4499 astronomers can therefore explore how mass affects a cluster's contents. Astronomers found no sign of multiple generations of stars in IC 4499 — supporting the idea that less massive clusters in general only consist of a single stellar generation.

Hubble observations of IC 4499 have also helped to pinpoint the cluster's age: observations of this cluster from the 1990s suggested a puzzlingly young age when compared to other globular clusters within the Milky Way. However, since those first estimates new Hubble data been obtained, and it has been found to be much more likely that IC 4499 is actually roughly the same age as other Milky Way clusters at approximately 12 billion years old.

Image Credit: ESA/Hubble & NASA
Explanation from: https://www.spacetelescope.org/images/potw1431a/

Saturn's Rings

Specially designed Cassini orbits place Earth and Cassini on opposite sides of Saturn's rings, a geometry known as occultation. Cassini conducted the first radio occultation observation of Saturn's rings on May 3, 2005.

Three simultaneous radio signals of 0.94, 3.6, and 13 centimeter wavelength (Ka-, X-, and S-bands) were sent from Cassini through the rings to Earth. The observed change of each signal as Cassini moved behind the rings provided a profile of the distribution of ring material as a function of distance from Saturn, or an optical depth profile.

This simulated image was constructed from the measured optical depth profiles. It depicts the observed ring structure at about 10 kilometers (6 miles) in resolution. Color is used to represent information about ring particle sizes in different regions based on the measured effects of the three radio signals.

Purple color indicates regions where there is a lack of particles of size less than 5 centimeters (about 2 inches). Green and blue shades indicate regions where there are particles smaller than 5 centimeters (2 inches) and 1 centimeter (less than one third of one inch). The saturated broad white band near the middle of ring B is the densest region of ring B, over which two of the three radio signals were blocked at 10-kilometer (6-mile) resolution, preventing accurate color representation over this band. From other evidence in the radio observations, all ring regions appear to be populated by a broad range particle size distribution that extends to boulder sizes (several to many meters across).

Image Credit: NASA/JPL
Explanation from: http://photojournal.jpl.nasa.gov/catalog/PIA07873