14 Pages

Astronomy & Astrophysics
Course Code
Michael Reid

This preview shows pages 1,2,3. Sign up to view the full 14 pages of the document.
ASTRO TEXTBOOK NOTES 2012 APRIL 18 2012 HUBBLE’S LAW/REDSHIFT/GALAXIES  Cosmological redshift – the expansion of the universe stretches out all the photons within it, shifting them to longer, redder wavelengths o Tells us how much space has expanded during the time since light from the galaxy left  An objects lookback time is directly related to its redshift  Lookback time – the difference between the current age of the universe and the age of the universe when the light left the object  Galaxies are moving away from one another; implies that galaxies must have been closer together in the past o The periods are very closely related to their luminosities: the longer the period, the more luminous the star  Obey a period-luminosity relationship o Once we measure its period, we know its luminosity and can use the inverse square law for light to determine its distance  Spiral galaxies (Milky Way): disks are filled with cool gas and dust, combined with hotter ionized gas, and usually display spiral arms  Elliptical galaxies: redder, rounded, often elongated (football shape), contain very little cool gas/dust but often contain very hot ionized gas  Irregular galaxies: appear neither disk-like nor rounded  Lenticular galaxies: have disk and spheroidal components like spiral galaxies but lack spiral arms; considered an intermediate class between spirals and ellipticals  Most large galaxies are spiral; some of the most massive are giant elliptical galaxies  Majority of ellipticals are small; they’re the most common type of galaxy o Have little/no ongoing star formation  Near hot stars we find ionization nebulae – ultraviolet photons from the hot stars can ionize the nebula’s atoms, causing them to glow  Spiral arms must be full of newly forming stars because they bear all the hallmarks of star formation  Disk population – contains both young and old stars  Spheroidal population – consists of stars in the bulge and halo; they are always old and low in mass  Sagittarius A contains a very massive black hole o By applying Newton’s version of Kepler’s Third Law to the orbits of these stars they concluded that this object must have a mass of about 4 million solar masses all packed into a region of space just a little larger than our solar system BUBBLES IN SPACE:  The high-speed gas ejected into space by supernovae or stellar winds sweeps up surrounding interstellar material, creating a bubble of hot, ionized gas (atoms are missing some of their electrons) around the exploding star Milky Way o Has more than 100 billion stars o Spiral galaxy because of its spiral arms o Fairly flat disk surrounding a bright central bulge o Entire disk is surrounded by a dimmer, rounder halo o Most bright stars reside in the disk o Most prominent stars in the halo are found in globular clusters o 100,000 light years in diameter, disk is only 1000 light years thick o Sun is located 28,000 light years from the center o Disk is filled with interstellar gas and dust – interstellar medium GALAXY FORMATION:  Most successful models for galaxy formation: o Hydrogen and helium gas filled all of space more or less uniformly when the universe was very young o The distribution of matter in the universe was not perfectly uniform  The regions of enhanced density originally expanded along with the rest of the universe; the slightly greater pull of gravity in these regions gradually slowed their expansion, so they began to contract into protogalactic clouds o Many galaxies may have formed from the merger of multiple protogalactic clouds  The disk population consists of stars born after the gas in the protogalactic cloud settled into a rotating disk which is why they all have similar orbits  All galaxies began their lives with gravity pulling matter into a patch of the universe that was slightly denser than its surroundings, then contracted into a protogalactic cloud and began to form stars  Explanations for the differences between spiral and elliptical galaxies trace a galaxy’s protogalactic cloud o Protogalactic spin – significant angular momentum would form a disk = spiral galaxy; little or no angular momentum would form no disk at all = elliptical o Protogalactic density – high gas density would have radiated energy more effectively and cooled more quickly allowing more rapid star formation; the gas could have turned into stars before any of it had time to settle into a disk = elliptical; lower-density clouds would have formed stars more slowly, leaving plenty of gas to form the disk of a spiral galaxy  Formation of a gas-rich disk depends on the angular momentum or density of the protogalactic cloud  A collision of 2 spiral galaxies can create an elliptical galaxy Tremendous tidal forces tear apart the 2 disks, randomizing the orbits of their stars, while a large fraction of their gas sinks to the center of the collision and rapidly  Starburst galaxies – currently forming stars at a rapid rate; represent a stage of evolution that many galaxies may have gone through o The rates of star formation in starbursts are unsustainable o After its starburst is over, the galaxy presumably returns to its spiral, elliptical or irregular state o Starbursts emit strong infrared light because of their interstellar dust ACTIVE GALACTIC NUCLEI/SUPRMASSIVE BLACK HOLES:  Active galactic nuclei – extreme amounts of radiation and sometimes powerful jets of material emanating from deep in the center of galaxies o The brightest of these are quasars  Quasars (quasi-stellar radio sources) are another temporary stage in the process if galaxy evolution  Find them at great distances, so we know that these were most common when galaxies were in their youth  Quasars in young galaxies must become dormant as their galaxies age  The energy output of a quasar comes from a gigantic accretion disk surrounding a supermassive black hole  They emit their energy across a wide swath of the electromagnetic spectrum, radiating approx. equal amounts of energy from infrared to gamma rays; they produce strong emission lines  Rapid changes in the luminosities of some active galactic nuclei point to an even smaller size  Occasionally, the luminosity of an active galactic nucleus doubles in a matter of hours. The fact that we see a clear signal indicates that the source must be less than a few light-hours across  In a radio galaxy, the active galactic nucleus is the power source and it drives 2 jets of particles that stream outward in opposite directions at nearly the speed of light  How radio galaxies, quasars, other active galactic nuclei release so much energy within such small central volumes = the energy comes from matter falling into a supermassive black hole  Explaining the extreme luminosities of quasars = matter falling into a black hole can generate awesome amounts of energy DARK MATTER AND DARK ENERGY  Dark matter – matter that appears to give off little or no light and influences the observed gravitational effects  Dark energy – whatever it is that may be causing the expansion of the universe to accelerate  Dark matter seems almost crucial to explaining the current structure of the universe  Dark matter and dark energy were each proposed to exist because they seemed the simplest ways to explain observed motions in the universe  Rotation curve: The flatness of the MW’s rotation curve implies that most of our galaxy’s mass lies well beyond the galactic center; most of the mass is located in the halo o Since we’ve detected very little radiation coming from this enormous amount of mass, it qualifies as dark matter o Composition of typical spiral galaxies = 90% or more dark matter, 10% or less matter in stars 1) x rays 2) gravitational lensing 3) Measuring the speeds of galaxies orbiting the center of the cluster 4) For individual galaxies, the case for dark matter rests primarily on applying Newton’s laws of motion and gravity to observations of the orbital speeds of stars and gas clouds 5) MACHOs (massive compact halo objects) – Trillions of faint red stars, brown dwarfs, and Jupiter-size objects left over from the MW’s formation could still roam our galaxy’s halo, providing much of its mass. They are too faint for us to see a. Gravitational lensing events demonstrate that dim, star-like objects (MACHOs) populate our galaxy’s halo, but not in large enough numbers to account for all the MW’s dark matter 6) The dark matter in galaxies can’t be made of neutrinos because these particles travel at enormous speeds and can easily escape a galaxy’s gravitational pull a. WIMPs (weakly interacting massive particles) – Other weakly interacting particles similar to neutrinos but considerably heavier b. It’s likely that WIMPs make up the vast majority of dark matter and hence the majority of all matter in the universe 7) Why is it distributed mostly in the halo? a. Works like oil and water in a cup: the water sinks to the bottom i. WIMPs are like the oil; they remain stuck on orbits far out in the galactic halo Cosmological Principle/FATE of UNIVERSE  Large-scale structures – much vaster than galaxy clusters o Galaxies are not scattered randomly through space; they are arranged in huge chains and sheets that span many millions of light years o Between these chains and sheets of galaxies lie giant empty regions called voids o On very large scales the universe looks much the same everywhere, in agreement with the Cosmological Principle  The greater the density, the greater the strength of gravity and the higher the likelihood that gravity will someday halt the expansion  Critical density – The precise density marking the dividing line between eternal expansion and eventual collapse  Recollapsing universe: if there is no dark energy and the matter density of the universe is larger than the critical density, then the collective gravity of all its matter will eventually halt the universe’s expansion and reverse it; the universe will end in a fiery “Big Crunch”  Critical universe: if there is no dark energy and the matter density of the universe equals the critical density, then the collective gravity of all its matter is exactly the amount needed to balance the expansion. The universe will never collapse but will expand more and more slowly as time progresses; called a critical universe because its density is the critical density o Its geometry is flat  Coasting universe: if there is no dark energy and the matter density of the universe is smaller than the critical density, then the collective gravity of all its matter can’t halt the expansion; the expansion will continue to coast forever o Its geometry is like the open surface of a saddle  Accelerating universe: if dark energy exerts a repulsive force that causes the expansion of the universe to accelerate with time, then the expansion rate will grow with time. Galaxies will recede from one another with increasing speed, and the universe will become cold and dark more quickly than it would in a coasting universe o Depending on the strength of gravity relative to the repulsive force, its geometry could be flat, open or closed; current evidence suggests a flat geometry KEPLER’S LAWS/NEWTONS LAWS  Kepler’s Laws 1. The orbit of each planet around the Sun is an ellipse with the Sun at one focus; nothing is at the other focus. A planet’s distance from the Sun varies based on its location in its orbit; the point closest to the Sun is called perihelion and the point farthest from the Sun is aphelion. 2. As a planet moves around its orbit, it sweeps out equal areas in equal times. The planet moves faster when at perihelion and slower at its aphelion 3. More distant planets orbit the Sun at slower average speeds. To calculate average 2 3 orbital speed, use equation: p = a -> p=planet’s orbital period in years, a=average distance from Sun in AU  Newton explained planetary motion as a consequence of gravity  Newton’s 3 laws of motion: 1. An object moves at constant velocity if there is no net force acting upon it  Explains why space ships don’t need fuel while they are in space – there is no force to interrupt their momentum 2. Force = mass x acceleration  Explains why Jupiter has a greater effect on asteroids than Earth  The amount of acceleration depends on the object’s mass and the strength of the net force  Gravity keeps orbiting planets always accelerating towards the Sun 3. For any force, there is always an equal and opposite reaction force o Objects always attract each other through gravity  When you jump, the force pulling you back to the ground is the same force that’s pulling the Earth up to you LAWS OF GRAVITY  Universal law of gravitation o Every mass attracts every other mass through gravity o The strength of the gravitational force is directly proportional to the product of their masses o The strength of gravity between 2 objects decreases with the square of the distance between their centers; gravitational force follows an inverse square law -> doubling the distance between 2 objects weakens the force of gravity  An object is in free-fall when the main force acting on it is gravity SPECTRA:  Continuous spectrum – rainbow spectrum without interruption  Emission line spectrum – bright emission lines against a black background; a low- density gas emits specific wavelengths (colours) based on its chemical makeup  Absorption line spectrum – a cloud of gas absorbs specific elements from a hot light source; we see a rainbow spectrum with specific colours missing TELESCOPES have 3 powers: o Light gathering power – more light = able to see fainter features o Resolution – more resolution = more detail o Magnification – but not important  Light-collecting area – this area is proportional to the square of a telescope’s diameter  Radio telescope – most common (satellite dishes); the long wavelengths means that telescopes with a large diameter are required to achieve good angular resolution; multiple telescopes can work together to achieve better angular resolution (interferometry)  Infrared – infrared light from space doesn’t reach the ground, but we can collect it from higher up in the atmosphere (airplanes); cosmic infrared energy is disrupted by other sources of infrared light on Earth, so the best solution is to put a telescope in space  Ultraviolet – atmosphere almost completely absorbs UV rays, so these telescopes are in space  X-Ray – need to be in space; need special mirrors because X-Rays can penetrate through normal mirrors; these telescopes have angled mirrors so the X ray slightly grazes it, until it reaches the focus (by this point, it’s not as strong and can be measured)  Gamma ray – a huge detector is required; the most mysterious objects in space produce very short bursts of gamma rays that quickly fade  Neutrino – produced by nuclear fusion o Neutrino telescopes – located deep underground because all other light can’t reach it THE SUN  Nuclear fission – splitting a nucleus into 2 smaller nuclei  Nuclear fusion – combining nuclei to make one bigger nucleui PHYSICS  Special theory of relativity shows that space and time are intertwined o Special because it deals with only with the special case in which we ignore the role of gravity  General theory of relativity is a new view of gravity; helps us understand the expansion and fate of the universe and black holes o General because it applies with or without gravity HR DIAGRAM/STAR FORMATION  Main-sequence turnoff point determines the ages of star clusters  Youngest star clusters are always associated with dark clouds of gas and dust o Stars form only in clouds that are unusually cold and dense o Infrared allows us to see directly through a molecular cloud  Much of the radiation produced by young stars within the molecular cloud can’t escape because dust grains absorb the visible light o The absorbed ra
More Less
Unlock Document

Only pages 1,2,3 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.