Astronomy

Astronomy is a natural science that deals with the study of celestial objects (such as stars, planets, comets, nebulae, star clusters and galaxies) and phenomena that originate outside the Earth's atmosphere (such as the cosmic background radiation). It is concerned with the evolution, physics, chemistry, meteorology, and motion of celestial objects, as well as the formation and development of the universe.
Astronomy is one of the oldest sciences. Prehistoric cultures left behind astronomical artifacts such as the Egyptian monuments, Nubian monuments and Stonehenge, and early civilizations such as the Babylonians, Greeks, Chinese, Indians, and Maya performed methodical observations of the night sky. However, the invention of the telescope was required before astronomy was able to develop into a modern science. Historically, astronomy has included disciplines as diverse as astrometry, celestial navigation, observational astronomy, the making of calendars, and astrology, but professional astronomy is nowadays often considered to be synonymous with astrophysics.

Physical cosmology

Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution.[1] For most of human history, it was a branch of metaphysics and religion. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed us to understand those laws.

Metallicity

In astronomy and physical cosmology, the metallicity (also called Z[1]) of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium. Since stars, which comprise most of the visible matter in the universe, are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively.[2] Thus, a nebula rich in carbon, nitrogen, oxygen, and neon would be "metal-rich" in astrophysical terms even though those elements are non-metals in chemistry. This term should not be confused with the usual definition of "metal"; metallic bonds are impossible within stars, and the very strongest chemical bonds are only possible in the outer layers of cool K and M stars. Normal chemistry therefore has little or no relevance in stellar interiors.
The metallicity of an astronomical object may provide an indication of its age. When the universe first formed, according to the Big Bang theory, it consisted almost entirely of hydrogen which, through primordial nucleosynthesis, created a sizeable proportion of helium and only trace amounts of lithium and beryllium and no heavier elements. Therefore, older stars have lower metallicities than younger stars such as our Sun.

Star

A star is a massive, luminous ball of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible from Earth during the night when they are not outshone by the Sun or blocked by atmospheric phenomena. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

Interstellar space travel

Interstellar space travel is unmanned or manned travel between stars. The concept of interstellar travel in starships is a staple in science fiction. Interstellar travel is tremendously more difficult than interplanetary travel. Intergalactic travel, the travel between different galaxies, is even more difficult.
Many scientific papers have been published about related concepts. Given sufficient travel time and engineering work, both unmanned and generational interstellar travel seem possible, though representing a very considerable technological and economic challenge unlikely to be met for some time, particularly for crewed probes. NASA has been engaging in research into these topics for several years, and has accumulated a number of theoretical approaches.
The main difficulty of interstellar travel is the vast distances that have to be covered and therefore the time it takes with most realistic propulsion methods - from decades to millennia. Hence an interstellar ship would be much more severely exposed to the hazards found in interplanetary travel, including hard vacuum, radiation, weightlessness, and micrometeoroids. The long travel times make it difficult to design manned missions, and make economic justification of any interstellar mission nearly impossible, since benefits that do not become available for decades or longer have a present value close to zero.