Astrophysics

Stars, galaxies, black holes, and the expanding universe

What is astrophysics?

Astrophysics is the branch of physics that studies stars, planets, galaxies, and the universe as a whole, using the same physical laws covered elsewhere in this encyclopedia — mechanics, thermodynamics, electromagnetism, nuclear physics, and relativity — applied to objects on an enormous scale.

Stars: giant nuclear furnaces

A star is an enormous ball of gas, mostly hydrogen, held together by its own gravity. Deep in its core, the pressure and temperature are high enough for nuclear fusion (covered in the Nuclear Physics article) to occur continuously, fusing hydrogen into helium and releasing enormous amounts of energy — this is what makes stars shine, including our own Sun. A star spends most of its life in a stable balance: gravity constantly pulling inward, and the outward pressure from the energy released by fusion constantly pushing back.

How we know what's out there

Almost everything we know about distant stars and galaxies comes from analyzing the light they send us. By spreading starlight into its spectrum (using the same dispersion principle covered in the Optics article) and studying the specific patterns of light each chemical element absorbs or emits, astronomers can determine a star's composition, temperature, and even its motion toward or away from us — all without ever traveling there.

Galaxies

A galaxy is an enormous, gravitationally bound collection of stars, gas, and dust — our own galaxy, the Milky Way, contains several hundred billion stars. Galaxies themselves are often grouped together by gravity into even larger clusters, and the observable universe contains an estimated hundreds of billions of galaxies.

Why astrophysics matters

Astrophysics answers fundamental questions about where we came from — the elements that make up the Earth and our own bodies were themselves forged inside earlier generations of stars — and has driven the development of major technologies, from the precision timekeeping behind GPS (covered in the Relativity article) to advances in imaging and detector technology used far beyond astronomy itself.