10 most difficult space riddles

A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances.

The 'avenue of heroes' at Baikonur in Kazakhstan, where the shuttle will launch, provides a poignant memorial to each mission conducted over the past 50 years. The tradition started with Yuri Gagarin, the first human to enter space, when he planted a single tree before blasting off further from Earth than anyone had ever been. Every Russian cosmonaut, and then later astronauts from other countries, that has left the bounds of Earth from the Cosmodrome has first planted a tree on the path to the launchpad.

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light

The diameters of Uranus and Neptune are respectively 25 362 km and 24 622 km - the difference is only 2.92%.

Canopus is the brightest star in the southern constellation of Carina, and the second brightest star in the night time sky, after Sirius. Its name is generally considered to originate from the mythological Canopus, who was a navigator for Menelaus, king of Sparta.

Sojourner was the first Mars rover. It was much smaller than Curiosity, whose wheel is about the size of the whole Sojourner. It landed on July 4, 1997 as part of the Mars Pathfinder mission on Mars's Ares Vallis and remained in contact with Earth till September 27, 1997.

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae. A typical diffuse nebula is a few hundred light-years across. (NGC 2264 shown; see also the Horsehead Nebula)

Galaxy filaments are the largest known structures in the universe. They group galaxy superclusters, which in turn are formed by cluster composed of galaxies. Hercules–Corona Borealis Great Wall is considered the largest known structure of Universe. It is estimated to span 10 billions light years.

A fast radio burst (FRB) is a high-energy astrophysical phenomenon manifested as a transient radio pulse lasting a few milliseconds on average. The origin of the FRBs has yet to be determined. Proposals range from a rapidly rotating neutron star and a black hole to an extraterrestrial intelligence.

The Armstrong limit or Armstrong's line is a measure of altitude above which atmospheric pressure is sufficiently low that water boils at the normal temperature of the human body. Humans absolutely cannot survive above this limit in an unpressurized environment. Above Earth, this begins at 18–19 km (11–12 mi; 59,000–62,000 ft) above sea level. The term is named after United States Air Force General Harry George Armstrong, who was the first to recognize this phenomenon