Most people would associate Castor with Pollux, which are the two brightest stars in the constellation Gemini, but Castor by itself is very interesting because it is actually a very complex system containing six stars.

The quasar QSO 1331+170 is best known for having a darker galaxy in front of it that is absorbing its light.

The cluster of galaxies MACS J1149.5+2223 is so massive that it has gravitationally bent (or lensed) the light from multiple things behind it, including one of the most distant galaxies in the universe and a supernova.

NGC 34 (also known as NGC 17) is a chaotic-looking galaxy that formed from two smaller galaxies merging together, and it is a place where astronomers have easily found lots of stars forming in a starburst but where they have had difficulty concluding whether the galaxy also contains a supermassive black hole.

The Monogem Ring, which is one of the largest sources of X-rays in the Earth's sky, was created by a supernova explosion about 86000 years ago, and the core of the star that exploded has been identified as the pulsar PSR B0656+14 at the center of the ring.

Even though Alpha Mensae is in one of the faintest and dumbest constellations in the sky, it's an intriguing star system because it is very close to the Earth, because one of the stars is very Sun-like, and because it may contain an exoplanet or a disk of dust in orbit around that Sun-like star.

The Type Ia supernova SN 2005cf was observed at multiple wavelengths for three months after its appearance, allowing astronomers to create templates of its spectrum that could be used to measure distances to other Type Ia supernovae.

Kappa Cassiopeiae is a large blue variaable star that is most potentially interesting because of the bow shock between its stellar winds and the interstellar medium.

47 Ursa Majoris is a nearby Sun-like star that astronomers have intensely studied in an effort to find an Earth-like exoplanet, and while three exoplanets have been found orbiting the star, none of them are remotely similar to the Earth.

While the planetary nebula NGC 1501 is a popular amateur astronomy target, the newly formed, hot, pulsating white dwarf at its center is much more interesting to professional astronomers.