Astrophysics: Experimental Astronomy + Theoretical Understanding of Universe – Cosmology

Astrophysics: experimental astronomy + theoretical understanding of universe – cosmology. Solar system is a collection of planets, moons, asteroids, comets, and other rocky objects travelling in elliptical orbits around the Sun under the influence of gravitational force. Sun: star formed from a giant cloud of molecular hydrogen gas that gravitated together, forming clumps of matter that collapsed and heated up. A gas disc around the young, spinning Sun evolved into the planets about 4.6 × 109 years ago. The planets move in elliptical orbits with only Mercury occupying a plane significantly different to that of the other planets. ~ 4.5 billion years ago, the Earth’s moon is believed to have been formed from Asteroid belt situated between material ejected when a collision occurred Mars and Jupiter, between a Mars-size object and the Earth. contains millions of asteroids. Jupiter is the biggest planet in terms of Kuiper belt, similar to asteroid mass and volume. Mercury is the smallest. belt but much larger; beyond Neptune. Asteroids and comets are both celestial In addition to asteroids it is the bodies orbiting our Sun, and they both can source of short-period comets have unusual orbits, sometimes straying and contains dwarf planets close to Earth or the other planets. They are both “leftovers” — made from Comets are irregular objects a few kilometres across comprising frozen gases (ice), rocky materials, and dust. Observable comets travel around the Sun in sharply elliptical Asteroids consist of metals and rocky material. Those of orbits with periods ranging from a few years to thousands of years. As they draw near size less than 300 km have irregular shape because their to the Sun the gases in the comet are vaporized, forming the distinctive comet tail that can be millions of kilometres long and always points away from the Sun. gravity is too weak to compress them into spheres. Star is a massive body of gas held together by gravity, with fusion going on at its center, giving off electromagnetic radiation. There is an (hydrostatic) equilibrium between radiation and gravitational pressure. This equilibrium is gained through nuclear fusion which provides the energy the star needs to keep it hot so that the star's radiation pressure is high enough to oppose gravitational contraction. This applies to all layers of a star. The fusing of hydrogen into helium takes up the majority of a star’s lifetime and is the reason Gravity pulls outer layers in, gas and why there are far more main sequence stars than those in other phases of their life-cycle. radiation pressure pushes them out. Stars initially form when gravity causes the gas in a nebula to condense. As the atoms move towards one another, they lose gravitational potential energy that is converted into kinetic energy. This raises the temperature of the atoms which then form a protostar. When the mass of the protostar is large enough, the temperature and pressure at the centre will be sufficient for hydrogen to fuse into helium, with the release of very large amounts of energy – the star has “ignited”. Ignition produces emission of radiation from the core, producing a radiation pressure that opposes the inward gravitational forces. Star will remain stable in hydrostatic equilibrium for up to billions of years. It is on the main sequence. As the hydrogen is used up the star will eventually undergo changes that will move it from the main sequence. During these changes the colour of the star alters as its surface temperature rises or falls and it will change size accordingly. The original mass of material in the star determines how the star will change during its lifetime. Groups of stars: believed ~ 50% of the stars are part of a star system comprising two or more stars. (Total estimates are higher, with NASA's reporting that up to 80% of all stars are in multiple star relationships. That is true for very massive stars). Binary stars consist of two stars that rotate about a common centre of mass. The ONLY way to find mass of the stars is when they are the part of binary stars. Knowing the period of the binary and the separation of the stars the total mass of the binary system can be calculated. Stellar cluster is a group of stars held together by gravitation in same region of space, formed roughly at the same time from the same nebula. Some clusters contain only a few dozen stars while others may contain millions. Open clusters consist of up to several hundred younger stars; contain some gas and dust; within our galaxy, and so lie within a single plane. Globular clusters contain many more older stars; contain very little gas and dust; spherically shaped just outside the Milky Way in its galactic halo galactic halo is a (nearly) spherical region surrounding the galaxy (like the diffuse light around the heads of saints, made up mostly of dark matter/ does not emit EM radiation - studied only through its gravitational interactions, in particular, with light passing by, an effect known as "gravitational lensing" the galactic spheroid (stars) the galactic corona (hot gas, i.e. a plasma) the dark matter halo. Constellation is a group of stars that form a pattern in the same general direction as seen from the Earth, but not bound by gravitation. Galaxy is a huge group of stars, dust, and gas held together by gravity, often containing billions of stars, measuring many light years across. Some in isolation, majority come in clusters which could have anything from a few dozen to a few thousand galaxies. Milky Way is part of a cluster of about 30 galaxies called the “Local Group” which includes Andromeda and Triangulum. Regular clusters consist of a concentrated core and are spherical in shape. Irregular clusters have no apparent shape and a lower concentration of galaxies within them. Hubble Space Telescope: superclusters In between the clusters there are voids that are apparently empty of galaxies. Superclusters – even larger structures observed since the launch of the Hubble Space Telescope Spiral galaxies the most common class of galaxies (both The Milky Way and Andromeda). a flat rotating disc-shape with spiral arms spreading out from a central galactic bulge that contains the greatest density of stars. Belief: at the centre of the galactic bulge, there is a black hole. The spiral arms contain many young blue stars and a great deal of dust and gas. Other galaxies are elliptical in shape, being ovoid or spherical – these contain much less gas and dust than spiral galaxies; they are thought to have been formed from collisions between spiral galaxies. Irregular galaxies are shapeless and may have been stretched by the presence of other massive galaxies – the Milky Way appears to be having this effect on some nearby dwarf galaxies. Nebulae (stellar nurseries ) are regions of intergalactic cloud of dust and gas. As all stars are “born” out of nebulae. two different origins of nebulae: ▪ nebulae formed in the “matter era” around 380 000 years after the Big Bang. At about this time, neutral atoms are formed as electrons link up with hydrogen and helium nuclei. Dust and gas clouds were formed when these atoms gravitated together. ▪ nebulae formed from the matter which has been ejected from a supernova explosion. nebulae can form in the final, red giant, stage of a low mass star such as the Sun. Astronomical distances Resulting from the huge distances involved in astronomical measurements, some unique, non-SI units have been developed. This avoids using large powers of ten and allows astrophysicists to gain a feel for relative sizes and distances. The astronomical unit (AU): the average distance between the Sun and the Earth. It is really only useful when dealing with the distances of planets from the Sun. 1 AU= 1.50 × 10 11 m ≈ 8 light minutes 1 parsec (pc): This is the most commonly used unit of distance in astrophysics. 1 pc= 3.26 ly = 3.09 × 10 16 m Distances between nearby stars are measured in pc, while distances between distant stars within a galaxy will be in kiloparsecs (kpc), and those between galaxies in megaparsecs (Mpc) or gigaparsecs (Gpc). Stars’ and planets’ radiation spectrum is approximately the same as black-body radiation/ Plank’s law. Intensity as a function of wavelength depends upon its temperature Wien’s law Wavelength at which the intensity : -3 of the radiation is a maximum λmax, is: 2.9×10 λ=max (m) T(K) Luminosity (of a star) is the total power (total energy per second) radiated by an object (star). If we regard stars as black body, then luminosity is: L = A σT4 = 4πR2σT4 (Watts) Stefan-Boltzmann’s law A is surface area of the star, R is the radius of the star, T surface temperature (K), σ is Stefan-Boltzmann constant. (Apparent) brightness (b) is the power from the star received per square meter of the Earth’s surface L 2 b = (W/m ) L is luminosity of the star; d its distance from the Earth 2 Can be measured, for example, by using a telescope4π and a charge-coupled device ? IB Internet says by Photometer!!! Explain how atomic spectra may be used to deduce chemical and physical data for stars. × •surface temperature of a star is determined by measuring the wavelength at which most of the radiation is emitted: ( ) = ( )−3 2 9 10 •Most stars essentially have the same chemical composition, yet show different absorption spectra as they have different temperatures. • Absorption spectra gives information about the temperature of the star and its chemical composition. • Doppler shift information of speed relative to earth (red shift → longer wavelength, blue shift → shorter wavelength) Techniques for determining stellar distances: stellar parallax, spectroscopic parallax and Cepheid variables.

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