These images, taken a year apart by the NASA/ESA Hubble Space Telescope, reveal a shadow moving counterclockwise around a gas and dust disc encircling the young star TW Hydrae.

The two images at the top, taken by the Space Telescope Imaging Spectrograph, show an uneven brightness across the disc. Through enhanced image processing (images at bottom), the darkening becomes even more apparent. These enhanced images allowed astronomers to determine the reason for the changes in brightness.

The dimmer areas of the disc (top left) are caused by a shadow spreading across the outer disc. The dotted lines approximate the shadow's coverage. The long arrows show how far the shadow has moved in a year (from 2015-2016), which is roughly 20 degrees.

Based on Hubble archival data, astronomers determined that the shadow completes a rotation around the central star every 16 years. They know the feature is a shadow because dust and gas in the disc do not orbit the star nearly that quickly. So, the feature must not be part of the physical disc.

The shadow may be caused by the gravitational effect of an unseen planet orbiting close to the star. The planet pulls up material from the main disc, creating a warped inner disc. The twisted disc blocks light from the star and casts a shadow onto the disc's outer region.

Credit:
NASA/ESA and The Hubble Heritage Team (STScI/AURA)

This image from Hubble's Wide Field and Planetary Camera 2 (WFPC2) is likely the best of ancient and brilliant quasar 3C 273, which resides in a giant elliptical galaxy in the constellation of Virgo (The Virgin). Its light has taken some 2.5 billion years to reach us. Despite this great distance, it is still one of the closest quasars to our home. It was the first quasar ever to be identified, and was discovered in the early 1960s by astronomer Allan Sandage.

The term quasar is an abbreviation of the phrase "quasi-stellar radio source", as they appear to be star-like on the sky. In fact, quasars are the intensely powerful centres of distant, active galaxies, powered by a huge disc of particles surrounding a supermassive black hole. As material from this disc falls inwards, some quasars - including 3C 273 - have been observed to fire off super-fast jets into the surrounding space. In this picture, one of these jets appears as a cloudy streak, measuring some 200 000 light-years in length.

Quasars are capable of emitting hundreds or even thousands of times the entire energy output of our galaxy, making them some of the most luminous and energetic objects in the entire Universe. Of these very bright objects, 3C 273 is the brightest in our skies. If it was located 30 light-years from our own planet - roughly seven times the distance between Earth and Proxima Centauri, the nearest star to us after the Sun - it would still appear as bright as the Sun in the sky.

WFPC2 was installed on Hubble during shuttle mission STS-61. It is the size of a small piano and was capable of seeing images in the visible, near-ultraviolet, and near-infrared parts of the spectrum.

Credit:
NASA & ESA