Space Friday: Death Star, Enceladus and the Milky Way

Real-Life Death Star

Just in time for Halloween… Imagine our sun destroying planet Earth. Scary, right? Well, it may not be far from the truth. “Dead Star Vaporizing a Planet,” read the headlines last week. Using the Kepler telescope (now renamed the K2 Mission), scientists observed a white dwarf, WD 1145+017, with a disk of rocky dust surrounding it. (The system lies about 570 light years from Earth.) The astronomers reported in Nature that the dust was likely a disintegrating planet (or planets), “killed” (according to Discovery News) by the white dwarf.

When a star similar to our sun dies, it swells into a red giant and sloughs off its outer layers. The hot, Earth-sized core that remains is a white dwarf, generally consisting of carbon and oxygen with a thin hydrogen or helium shell. (This process is different with larger stars, which become supernovae.) However, for years, researchers have found evidence that some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium, and iron—elements a smaller star could not have created on its own. They suspected that the source of this pollution was an asteroid or a small planet being torn apart by the white dwarf's intense gravity.

“We now have a ‘smoking gun’ linking white dwarf pollution to the destruction of rocky planets,” says lead author Andrew Vanderburg of the Harvard-Smithsonian Center for Astrophysics (CfA).

Now that scientists have validated the long-held theory that white dwarfs can cannibalize remnant planets, we can look forward to the same destiny here on Earth. In about five billion years, as the Sun expands, it will gobble up Mercury, Venus, and likely our planet, too. –Molly Michelson

Cassini’s Close Enceladus Flyby

Orbiting Saturn since 2004, NASA’s Cassini mission has taken an up-close look at the planet, its rings, and even some of the more intriguing of its 62 moons. As the mission rapidly approaches its Grand Finale in 2017, this past Wednesday marked the second of a trio of final flybys of the icy moon Enceladus and Cassini’s deepest ever dive through the moon’s southern plumes of icy spray.

Water-rich plumes of material spewing from the “tiger stripes” cracks near the moon’s south pole were first discovered by Cassini in 2005—evidence of a sub-surface ocean of water kept warm and liquid by tidal flexing. And since the search for life begins with liquid water, Enceladus is a top candidate in place to search for signs of life. The flyby is not intended to detect life; however, it will provide powerful new insights about the potential habitability of Enceladus’ sub-ice ocean environment.

At 30 miles (49 kilometers) above the moon’s southern polar region, it is not the closest approach to Enceladus’ surface ever completed, but it is the deepest dive through an active plume. Speeding past at 19,000 miles per hour, Cassini’s encounter with the plume itself only lasted tens of seconds, but the probe’s dust analyzer sampled and identified about 10,000 particles per second in that short time. Scientist hope that analysis of the material in the coming weeks will provide insight to how much hydrothermal activity is occurring in the moon, what kind of heavier molecules (such as organics) the plumes may contain, and whether the plumes are individual jets, sinuous curtain eruptions, or a combination. The answer to the last question will inform how material arrives at the surface from the sub-surface ocean.

Cassini’s final Enceladus flyby will take place on December 19, but it won’t pass nearly as close, only 3,100 miles (5,000 kilometers) above the surface. Cassini is scheduled to begin maneuvering between Saturn and its innermost ring in late 2016, complete 22 tight orbits, and plunge into Saturn’s atmosphere in September 2017. –Elise Ricard

A New Discovery About the Milky Way Galaxy

Conventional wisdom has long held that the cores of spiral galaxies contain older, redder stars, while the spiral arms contain gas, dust, and younger, bluer stars, indicative of star formation taking place in the arms. This is apparent in color images of spiral galaxies. However, astronomers using the Visible and Infrared Survey Telescope for Astronomy (VISTA) at the Paranal Observatory in Chile have discovered that not all of the stars in the central bulge are older. Analysis of observations made between 2010 and 2014 reveal an unexpected disk of young stars around the center of the Milky Way Galaxy, hidden behind thick clouds of galactic dust. These stars include a type known as Cepheid variables, which were discovered in the early 1900s by Henrietta Swan Leavitt.

Cepheid variables—named after the first such object discovered in the constellation Cepheus the King—pulsate, varying their size and brightness regularly and at a rate that corresponds to their intrinsic brightness, or luminosity. Knowing a Cepheid variable’s intrinsic brightness makes it possible to determine how bright it should appear at a given distance. This, in turn, allows astronomers to calculate the distance to any Cepheid variable by measuring the rate of its variations. In the 1920s, this relationship was the key to Edwin Hubble’s determination that other galaxies exist far beyond the Milky Way, when he discovered Cepheid variable stars in M-31 and other objects then known as “spiral nebulae” (and today as spiral galaxies).

The VISTA team further analyzed the light of thirty-five of the Cepheids and found that they are very young for stars—less than 100 million years old. The disk they form across the galaxy’s central bulge was completely unknown until now.

“This study is a powerful demonstration of the unmatched capabilities of the VISTA telescope for probing extremely obscured galactic regions that cannot be reached by any other current or planned surveys,” says the study’s lead author Istvan Dékány of Instituto Milenio de Astrofísica and Pontificia Universidad Católica de Chile, in Santiago.

Follow-up observations will determine where the Cepheid variables formed—whether at their present locations or farther away from the center of the galaxy (before migrating inward to their current locations). –Bing Quock