The Shadow Knows

twhydraeThese images were taken a year apart by the Hubble Space Telescope. They reveal a shadow moving counterclockwise around a gas-and-dust disk encircling the young star TW Hydrae. The two images at the top show an uneven brightness across the disk. In the bottom pair of images, which have have been subjected to enhanced image processing, the darkening becomes even more apparent. The dimmer areas of the disk are caused by a shadow spreading across the outer disk. The dotted lines approximate the shadow’s coverage, and the long arrows show how far the shadow has moved in a year (from 2015 to 2016), roughly 20 degrees. Using Hubble archival data, astronomers have determined that the shadow completes a rotation around the central star every 16 years. The feature is certainly a shadow because dust and gas in the disk do not orbit the star nearly that quickly. The, the feature must not be caused by a part of the physical disk. The shadow might be caused by the gravitational effect of an unseen planet orbiting close to the star. Such a planet could  disturb material from the main disk, creating a warped inner disk, and the twisted disk might block light from the star, casting a shadow onto the disk’s outer region.

Image Credits: NASA / ESA

A Bipolar Nebula

pn-hb-12This is the bipolar planetary nebula PN Hb 12 in the constellation of Cassiopeia. The striking shape of this nebula, reminiscent of a butterfly or an hourglass, was formed as a Sun-like star blew its its outer layers into the surrounding space near the end of its life. The bipolar nebula resulted from that material being spewed toward the poles of the ageing star, creating the distinctive double-lobed structure.

Image Credit: ESA / NASA

A Megamaser

A cosmic megamaser“OK,” I hear the Gentle Reader ask, “what’s a maser?”

The acronym laser stands for Light Amplification by Stimulated Emission of Radiation. Stimulated emission is a process by which an incoming photon interacts with a exited electron in an atom or molecule in such a way as to cause the electron to move to a lower energy state which in turn causes the emission of a new photon with a specific phase, frequency, polarization, and direction of travel. A maser operates on the same principle as a laser, but at a lower frequency. It emits a microwave signal rather than light.

This megamaser galaxy is named IRAS 16399-0937. It’s 370 million light-years away. Some components within the galaxy (like gas clouds) are in the right stimulated physical condition to radiate intense microwave energy, causing the whole galaxy to act as a giant maser that is on the order of 100 million times more powerful than any of the masers found in the Milky Way.

IRAS 16399-0937 has another distinction. It has two cores which appear to be in the process of merging. The southern core is a starburst region where new stars are being rapidly formed. The northern core contains a supermassive black hole.

Image Credit: ESA / NASA

The Cartwheel Galaxy

cartwheel_galaxyThe Cartwheel Galaxy is a ring galaxy about 500 million light-years away in the constellation Sculptor. It is an estimated 150,000 light-years diameter. The galaxy was once a normal spiral galaxy before it apparently underwent a head-on collision with a smaller companion approximately 200 million years ago. When the other galaxy passed through the Cartwheel Galaxy, the collision caused a powerful shock wave. Moving at high speed, the shock wave swept up gas and dust, creating a starburst around the galaxy’s center portion forming the bluish ring around the central brighter portion. The galaxy appears to be retaking the form a spiral galaxy with thin arms beginning to spread from its central core.

Image Credit: ESA / NASA