Astrophysics

• The universe contains various objects such as stars, planets, and galaxies.
• Astronomical distances are a crucial concept in astrophysics.
• Stellar quantities include luminosity and apparent brightness.
• The solar system is a collection of eight major planets (Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus and Neptune) bound in elliptical orbits around a star we call the Sun.
• Pluto has been stripped of its status as a major planet and is now called a ‘dwarf planet’.
• The orbit of the Earth is almost circular; that of Mercury is the most elliptical.
• All planets revolve around the Sun in the same direction.
• Halley’s comet is an exception to the rule that all comets revolve around the Sun in the same direction.
• All the planets except Mercury and Venus have moons orbiting them.
• Interstellar space is the space between stars.
• Proxima Centauri, the nearest star to us after the Sun, is located at a distance of 4.2 light years.
• Stellar clusters are groupings of large numbers of stars that attract each other gravitationally and are relatively close to one another.
• The Milky Way is a huge assembly of stars that are kept together by gravity.
• The Local Group is a group of galaxies, including the Milky Way, that are relatively close to one another.
• Binary star: Two stars orbiting a common centre
• Neutron pressure now keeps the star from collapsing further, and the star becomes a neutron star.
• The light from a star is the best source of information about it, as it reveals its surface temperature and composition.
• Further observations are then needed to confirm or reject hypotheses.
• A main-sequence star is 15 times more massive than our Sun.
• The luminosity of a main-sequence star is 4500 times greater than the luminosity of our Sun.
• The mass of a star can be estimated from its luminosity.
• If a star's mass is higher than the Chandrasekhar limit of about 1.4 solar masses, it will become a stable white dwarf, in which electron pressure keeps the star from collapsing further.
• Chandrasekhar predicted a limit to the mass of a star that would become a white dwarf, while Oppenheimer and Volkoff predicted the mass above which it would become a black hole.
• The luminosity and temperature of a star are related, and together give us information about the evolution of stars of different masses.
• If the Oppenheimer–Volkoff limit is exceeded, the star will become a black hole.
• The development of theories of stellar evolution illustrates how, starting from simple observations of the natural world, science can build up a detailed picture of how the universe works.
• If a star's core is more massive than the Chandrasekhar limit but less than the Oppenheimer–Volkoff limit of about 2–3 solar masses, the core will collapse further until electrons are driven into protons, forming neutrons.
• Black dwarf: The remnant of a white dwarf after it has cooled down
• The spectrum of a star shows a peak wavelength, which can be used to determine its surface temperature using Wien’s law.
• Stars are divided into seven spectral classes according to their colour, which is related to their surface temperature.
• Spectral studies provide information on the star’s velocity and rotation through the Doppler shifting of spectral lines and the star’s magnetic field owing to the splitting of spectral lines in a magnetic field.
• The Sun has an approximate black-body spectrum with most of its energy radiated at a wavelength of 5
• Wien’s law states that the higher the temperature, the lower the wavelength at which most of the energy is radiated.
• Astronomers realised early on that there was a correlation between the luminosity of a star and its surface temperature.
• Most stars have essentially the same chemical composition, yet show different absorption spectra due to different temperatures.
• Different stars have different temperatures due to their different distances from the centre of the Milky Way.
• The surface temperature of a star is determined by measuring the wavelength at which most of its radiation is emitted.
• The surface temperature of the Sun can be calculated from Wien’s law, given that its approximate black-body spectrum is known.
• Hydrogen is the predominant element in normal main-sequence stars, making up upto 70% of their mass, followed by helium with at 28%; the rest is made up of heavier elements.
• Absorption spectra show dark lines representing the absorption of light of specific wavelengths by specific chemical elements in the star’s atmosphere.