What Is Astrophysics?
Astrophysics is the branch of physics that applies the laws of physics and mathematics to astronomical objects and phenomena.
Astrophysicists investigate the physical nature of stars, planets, galaxies, black holes, neutron stars, interstellar gas, cosmic radiation and the universe itself.
The subject combines physics, mathematics, astronomy, computer science, chemistry and increasingly large-scale scientific data analysis.
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From Atoms to the Universe
Astrophysics applies fundamental physical laws across an enormous range of scales — from particles and atoms to stars, galaxies and the entire observable universe.
Physics and Astrophysics
Astrophysics draws upon almost every major branch of physics.
Classical Mechanics
Describes the motion of planets, stars and other astronomical objects.
Electromagnetism
Explains electromagnetic radiation, magnetic fields and many astronomical observations.
Thermodynamics
Describes energy, temperature and heat in stars and astrophysical systems.
Quantum Mechanics
Explains atomic, nuclear and microscopic processes occurring inside stars and compact objects.
Relativity
Einstein's theories describe gravity, spacetime, black holes and relativistic astrophysical phenomena.
Nuclear Physics
Explains nuclear reactions and energy generation inside stars.
Particle Physics
Provides theories of fundamental particles and interactions relevant to extreme astrophysical environments.
Plasma Physics
Describes ionised matter found throughout stars, the solar wind and many cosmic environments.
Astronomy and Astrophysics
Astronomy traditionally focuses on observing and describing objects in the universe, while astrophysics seeks to explain their physical properties and behaviour.
Modern astronomy and astrophysics are closely interconnected. Observational astronomy provides data, while astrophysics provides physical models that explain those observations.
Stars
Stars are enormous self-gravitating spheres of hot plasma. Their energy is primarily generated through nuclear fusion in their interiors.
Star Formation
Stars form when regions of cold molecular clouds collapse under gravity.
Main Sequence
During the main-sequence phase, hydrogen fusion provides the principal source of stellar energy.
Red Giants
Stars can expand dramatically after exhausting hydrogen in their cores.
White Dwarfs
Low- and intermediate-mass stars can end their lives as dense white-dwarf remnants.
Neutron Stars
Massive stars can leave behind extraordinarily dense neutron-star remnants after supernova explosions.
Black Holes
The most massive stellar remnants can collapse to form black holes.
The Sun
The Sun is the star at the centre of the Solar System and the nearest star to Earth.
Core
The region where nuclear fusion converts hydrogen into helium.
Radiative Zone
Energy is transported primarily through radiation.
Convection Zone
Hot plasma transports energy through convection.
Photosphere
The visible surface of the Sun.
Corona
The Sun's extremely hot outer atmosphere.
Solar Wind
A continuous flow of charged particles escaping from the Sun.
Planetary Astrophysics
Astrophysics is also used to understand planets and planetary systems.
Planetary Atmospheres
Physics explains atmospheric composition, temperature, circulation and escape.
Planetary Interiors
Models describe the internal structure of rocky and gaseous planets.
Magnetic Fields
Planetary magnetic fields influence radiation environments and interactions with stellar winds.
Planetary Climate
Radiative physics and atmospheric dynamics determine planetary climates.
Exoplanets
An exoplanet is a planet orbiting a star other than the Sun.
Transit Method
Detects the small decrease in stellar brightness when a planet crosses in front of its star.
Radial Velocity
Measures the motion of a star caused by the gravitational influence of an orbiting planet.
Gravitational Microlensing
Uses the gravitational field of a foreground planetary system to magnify light from a background star.
Direct Imaging
Attempts to detect light from a planet while suppressing the much brighter light of its parent star.
Galaxies
Galaxies are enormous systems containing stars, gas, dust, dark matter and other components.
Spiral Galaxies
Characterised by rotating disks and spiral arms. The Milky Way is a barred spiral galaxy.
Elliptical Galaxies
Generally contain older stellar populations and have ellipsoidal shapes.
Irregular Galaxies
Have less organised structures and can result from gravitational interactions.
Galaxy Clusters
Groups of galaxies bound together by gravity.
The Milky Way
The Milky Way is the galaxy containing the Solar System.
At the centre of the Milky Way lies Sagittarius A*, a supermassive black hole with a mass of approximately four million Suns.
Black Holes
A black hole is a region of spacetime in which gravity is sufficiently strong that, within the event horizon, nothing—not even light—can escape.
Black Hole
Mass → Curved Spacetime → Event Horizon
Stellar-Mass Black Holes
Can form from the collapse of massive stars.
Supermassive Black Holes
Found at the centres of most large galaxies.
Accretion Disks
Hot gas orbiting a black hole can produce intense radiation.
Jets
Some accreting black holes produce enormous relativistic jets of plasma.
Neutron Stars
A neutron star is the extremely dense remnant of a massive star following gravitational collapse.
Extreme Density
Matter is compressed to extraordinary densities, approaching nuclear scales.
Pulsars
Rapidly rotating magnetised neutron stars can produce regular pulses of electromagnetic radiation.
Magnetars
Neutron stars possessing extraordinarily strong magnetic fields.
Binary Neutron Stars
Two neutron stars can orbit one another and eventually merge, producing gravitational waves.
Supernovae
A supernova is an extremely energetic stellar explosion.
Supernovae play a crucial role in astrophysics because they distribute heavy elements into space and influence the formation and evolution of galaxies.
Cosmic Nucleosynthesis
Astrophysics investigates how chemical elements are produced throughout the universe.
Big Bang Nucleosynthesis
The early universe produced primarily hydrogen, helium and small quantities of other light nuclei.
Stellar Nucleosynthesis
Nuclear fusion inside stars creates progressively heavier elements.
Supernovae
Stellar explosions create and distribute many elements.
Neutron-Star Mergers
Extreme neutron-rich environments can produce very heavy elements.
Gravity in Astrophysics
Gravity is the dominant force governing the large-scale structure and dynamics of astronomical systems.
Newtonian gravity provides an excellent approximation for many astronomical systems. Einstein's general relativity is required when gravitational fields become extremely strong or high precision is necessary.
General Relativity
Einstein's theory of general relativity describes gravity as the curvature of spacetime produced by matter and energy.
Spacetime
Matter and Energy → Curvature of Spacetime → Motion of Matter and Light
General relativity is essential for understanding black holes, gravitational waves, neutron stars, gravitational lensing and the large-scale dynamics of the universe.
Astrophysics and Cosmology
Cosmology studies the universe as a whole: its origin, evolution, structure and ultimate fate.
Big Bang
The observable universe evolved from an extremely hot, dense early state.
Cosmic Expansion
The large-scale universe has been expanding for billions of years.
Cosmic Microwave Background
Relic radiation from the early universe provides a detailed record of its early conditions.
Large-Scale Structure
Galaxies form an enormous cosmic web of filaments, clusters and voids.
Dark Matter
Dark matter is matter inferred from its gravitational effects but not directly observed through ordinary electromagnetic emission.
Evidence for dark matter comes from phenomena including galaxy rotation, gravitational lensing, galaxy clusters and the growth of cosmic structure.
Dark Energy
Dark energy is the name given to the unknown component associated with the accelerated expansion of the universe.
Understanding dark energy is one of the major unsolved problems in modern astrophysics and cosmology.
Gravitational Waves
Gravitational waves are propagating disturbances in spacetime produced by accelerating masses with changing quadrupole moments.
The detection of gravitational waves has created a new observational window on the universe.
Electromagnetic Radiation
Astronomers obtain much of their information about the universe by studying electromagnetic radiation.
| Region | Astrophysical applications |
|---|---|
| Radio | Cold gas, pulsars, galaxies and cosmic background radiation |
| Microwave | Cosmic microwave background and molecular emission |
| Infrared | Cool objects, dust and distant galaxies |
| Visible | Stars, galaxies and planetary systems |
| Ultraviolet | Hot stars and energetic astrophysical processes |
| X-ray | Black holes, neutron stars, hot gas and stellar coronae |
| Gamma Ray | The most energetic astrophysical phenomena |
Spectroscopy
Spectroscopy analyses how light is distributed across different wavelengths.
Spectra can reveal chemical composition, temperature, density, velocity, magnetic fields and other physical properties.
Astronomical Telescopes
Telescopes collect electromagnetic radiation and convert it into measurements that astronomers can analyse.
Optical Telescopes
Observe visible light from astronomical objects.
Radio Telescopes
Detect radio waves from astronomical sources.
Infrared Telescopes
Observe infrared radiation from cool or distant objects.
X-ray Telescopes
Detect high-energy radiation from extreme astrophysical environments.
Gamma-Ray Telescopes
Study the most energetic electromagnetic phenomena in the universe.
Space Telescopes
Space telescopes operate above Earth's atmosphere, avoiding atmospheric absorption and turbulence.
Hubble Space Telescope
A major optical, ultraviolet and near-infrared observatory that has transformed modern astronomy.
James Webb Space Telescope
A powerful infrared observatory designed to study galaxies, stars and planetary systems.
Chandra X-ray Observatory
Observes high-energy X-rays from black holes, neutron stars, galaxies and hot cosmic gas.
Nancy Grace Roman Space Telescope
A wide-field infrared observatory designed for large surveys of galaxies, dark matter, dark energy and exoplanets.
Computational Astrophysics
Modern astrophysics is heavily dependent on computer simulation and numerical modelling.
Galaxy Simulations
Model the formation and evolution of galaxies and cosmic structure.
Stellar Models
Calculate the internal structure and evolution of stars.
Black-Hole Simulations
Numerical relativity models the dynamics of black holes and compact-object systems.
Cosmological Simulations
Simulate the evolution of matter and structure throughout the universe.
Computational Astrophysics
Physics + Mathematics + Algorithms + Supercomputing
Astrophysical Data Science
Modern observatories generate enormous datasets. Astrophysicists use computational and statistical methods to extract physical information from them.
Image Processing
Converts detector measurements into scientifically useful images.
Photometry
Measures the brightness of astronomical objects.
Astrometry
Measures positions and motions of astronomical objects.
Machine Learning
Can classify astronomical objects and identify patterns in large datasets.
Measuring Cosmic Distances
Determining distances is one of the fundamental challenges of observational astrophysics.
Different methods apply at different distances, creating a cosmic distance ladder extending from nearby stars to the distant universe.
Cosmological Redshift
Light from distant galaxies is shifted towards longer wavelengths as the universe expands.
Redshift provides information about the motion and distance of astronomical objects and is a fundamental observational tool in cosmology.
History of Astrophysics
People developed systematic observations of the Sun, Moon, planets and stars.
The telescope transformed observational astronomy. Galileo's observations provided important evidence for a new understanding of the Solar System.
Spectroscopy established that stars could be studied through their light and that their chemical compositions could be determined.
Quantum theory and relativity revolutionised the physical understanding of stars and the universe.
Observational evidence established that the universe contains galaxies beyond the Milky Way and that the universe is expanding.
Radio astronomy, nuclear physics and space technology opened new observational windows on the universe.
Space telescopes transformed ultraviolet, X-ray, gamma-ray and infrared astronomy.
Gravitational-wave astronomy, exoplanet research, large digital surveys and advanced space telescopes have expanded the field dramatically.
Studying Astrophysics
Astrophysics is an interdisciplinary subject requiring a strong foundation in physics and mathematics.
| Subject | Astrophysical application |
|---|---|
| Mechanics | Orbital motion and gravitational dynamics |
| Electromagnetism | Radiation, magnetic fields and plasma |
| Quantum Mechanics | Atoms, radiation and microscopic processes |
| Thermodynamics | Stars, gases and thermal processes |
| Nuclear Physics | Fusion, stellar nucleosynthesis and compact objects |
| General Relativity | Black holes, gravitational waves and cosmology |
| Mathematics | Modelling and solving astrophysical equations |
| Computational Physics | Numerical simulation and scientific computing |
| Statistics | Observational data analysis and uncertainty |
| Programming | Simulation, data analysis and astronomical software |
Careers in Astrophysics
Astrophysicist
Investigates the physical processes governing astronomical objects and systems.
Cosmologist
Studies the origin, evolution and large-scale structure of the universe.
Observational Astronomer
Uses telescopes and scientific instruments to collect astronomical data.
Theoretical Physicist
Develops mathematical theories and models of astrophysical phenomena.
Computational Astrophysicist
Uses numerical simulations and high-performance computing to investigate astronomical systems.
Data Scientist
Analyses large astronomical datasets using statistics, programming and machine learning.
Space Scientist
Studies the Sun, Solar System and wider space environment.
Instrumentation Scientist
Develops detectors, cameras, spectrographs and other astronomical instruments.
Major Research Questions
How Do Stars Form?
How do clouds of gas and dust collapse to produce stars and planetary systems?
How Do Galaxies Evolve?
What processes control the growth and transformation of galaxies?
What Is Dark Matter?
What particle or physical phenomenon produces the gravitational effects attributed to dark matter?
What Is Dark Energy?
What causes the accelerated expansion of the universe?
How Do Black Holes Grow?
How do supermassive black holes form and evolve with their host galaxies?
Are We Alone?
How common are planetary systems and environments capable of supporting life?
The Future of Astrophysics
Large Space Telescopes
Future observatories will investigate the earliest galaxies, stars and planetary systems.
Exoplanet Characterisation
Astronomers will increasingly study the atmospheres and physical properties of planets beyond the Solar System.
Gravitational-Wave Astronomy
New detectors will expand observations of black holes, neutron stars and other compact objects.
Dark Universe
Large cosmological surveys will investigate dark matter and dark energy.
Artificial Intelligence
Machine-learning methods will assist with astronomical classification, simulation and data analysis.
Computational Astrophysics
Increasing computing power will allow more detailed simulations of complex astrophysical systems.
Summary
Astrophysics uses the laws of physics and mathematics to understand the universe.
Its major subjects include:
- stars and stellar evolution;
- the Sun and solar physics;
- planets and exoplanets;
- galaxies and galaxy evolution;
- black holes;
- neutron stars;
- supernovae;
- nuclear astrophysics;
- plasma astrophysics;
- gravitational physics;
- general relativity;
- gravitational waves;
- dark matter;
- dark energy;
- cosmology;
- astronomical spectroscopy;
- computational astrophysics; and
- astronomical data science.
Astrophysics
The Physics of Stars, Galaxies and the Universe
From the smallest particles to the largest structures in the cosmos.
Understanding the Universe Through Physics
Astrophysics connects fundamental physics with observations of the cosmos, allowing humanity to investigate the origin, evolution and physical nature of everything from planets and stars to black holes, galaxies and the universe itself.