Measuring the Universe

Publicado el 15/05/2012 por ciucinciu

Three useful interlinked methods for determining the distances across the universe

Measuring the Universe: from the transit of Venus to the edge of the cosmos

http://www.rmg.co.uk/visit/events/measuring-the-universe

Measuring the Universe: from the transit of Venus to the edge of the cosmos

http://www.rmg.co.uk/languages/espanol/

Measuring the Universe

http://vimeo.com/41434123

by Royal Observatory Greenwich Plus 3 weeks 5 hours ago

This is the film from our micro exhibition 'Measuring the Universe: from the transit of Venus to the edge of the cosmos'. If you can make it to Greenwich then come and see the exhibition - its on from 1 March–2 September 2012 and its absolutely FREE!

Design and direction: Richard Hogg
Animation: Robert Milne, Ross Philips, Kwok Fung Lam,
Music and sound effects: George Demure
Narration and Astro-smarts: Dr Olivia Johnson
Producer: Henry Holland
llllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll

Measuring the Universe
Fuente: http://www.iau.org/public/measuring/
The IAU and astronomical units
Scientists use units all the time. The concept of an internationally standardised system of units is one of the most fundamental in experimental science. Everyone uses familiar units such as kilograms, kilometres and seconds and they are indispensable in daily life. Scientists may need more exotic units such as measures of current, frequency and other scientific quantities, but the principle is the same, without an agreed scheme of measurement, scientists could not share results and there could be disastrous and costly mistakes.
The International Astronomical Union (IAU) is responsible for maintaining and approving a special set of units in astronomy, formally defined in 1976. One of the most important of these is the Astronomical Unit, abbreviated AU, which is defined by the IAU as equal to the distance from the centre of the Sun at which a particle of negligible mass, in an unperturbed circular orbit, would have a mean motion of 0.01720209895 radians/day. One AU is exactly 149,597,870.691 kilometres (roughly 150 million kilometres), slightly less than the mean Sun-Earth distance. The IAU also defines other astronomical units: the astronomical unit of time is 1 day (d) of 86,400 SI seconds (s) (SI is the International System of Units) and the astronomical unit of mass is equal to the mass of the Sun, 1.9891×10³⁰ kg.
Beyond the Solar System the distances in astronomy are so great that using the AU becomes too cumbersome. The IAU recognises several other distance units to be used on different scales. For studies of the structure of the Milky Way, our local galaxy, the parsec (pc) is the usual choice. This is equivalent to about 30.857×10¹² km, or about 206,000 AUs, and is itself defined in terms of the AU – as the distance at which one Astronomical Unit subtends an angle of one arcsecond. Alternatively the light-year (ly) is sometimes used in scientific papers as a distance unit, although its use is mostly confined to popular publications and similar media. The light-year is roughly equivalent to 0.3 parsecs, and is equal to the distance traveled by light in one Julian year in a vacuum, according to the IAU. To think of it in easily accessible terms, the light-year is 9,460,730,472,580.8 km or 63,241 AU. While smaller than the parsec, it is still an incredibly large distance.
Defining a unit is often more complex than first appears. For instance, to define a light-year it is necessary to understand exactly what a year is. When referring to a year in the precisely defined astronomical sense, it should be written with the indefinite article “a” as “a year”. Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. The IAU also recognises a Julian century of 36,525 days in the fundamental formulas for precession(more info). Other measurements of time such as sidereal, solar and universal time are not suitable for measuring precise intervals of time, since the rate of rotation of Earth, on which they ultimately depend, is variable with respect to the second.
Reference
Seidelmann, P. K. (Ed.), 1992, Explanatory supplement to the Astronomical Almanac, University Science Books

Defining our Place in the Cosmos – the IAU and the Universal Frame of Reference
How do you know where you are now? How do we know where we are in space? How does the International Space Station or the latest space probe keep track of its location in the Universe? The best answer would be – with great difficulty! Ever since the earliest philosophers first considered our place in the Universe, it has always been a natural first step to define our position in the overall order and structure of the cosmos.
One of the earliest Greek philosophers, Heraclitus, is often credited with advancing the concept of “everything changes or panta rhei”; a philosophy that develops the notion that the Universe is continually in motion, like a river. If we consider the Earth, the Solar System and the Universe as a whole, from the ground beneath our feet to some of the largest objects in the Universe, nothing is, in fact, immobile. On Earth the tectonic plates under our feet are moving, albeit slowly! And when we look out beyond the Earth, there is still no absolute reference point. The Earth rotates at half a kilometre a second at the equator, and is moving around the Sun at 29 kilometres a second; our Sun is also moving through space at about 19 kilometres each second and is orbiting the centre of the Milky Way (our galaxy) at about 215 kilometres a second. Stepping up a scale, the Milky Way is moving towards the Virgo Cluster, which is also in motion. As an added complication, the continuing expansion of the Universe must be included in large-scale distance measurements. The light we see today arriving from distant objects has taken so long to reach us that the Universe has expanded in the travel time of the light. So the task of defining a single reference frame from which the location of any other object in space can be defined is particularly complex. Traditionally this has required very precise measurements of the positions of many reference stars, with a catalogue of their motion across the sky through the year, referenced to their position at a particular precise date and time.
The International Astronomical Union (IAU) is responsible for defining a Universal Frame of Reference. This work touches on many aspects of our daily lives, so much so, that without a standard reference frame many of our modern gadgets would, at best, be incompatible with each other and, at worst, inaccurate or not fit for purpose.
Many people nowadays use the Global Positioning System (GPS) in their everyday lives. GPS requires several aspects of the Universal Frame of Reference to be defined. For example, the systems that controlled the launches of the GPS satellites had to have an excellent understanding of the positions of the stars, orbital elements and the definitions of various units in order to position the satellite in the correct orbit needed to complete the “constellation” of satellites. The IAU Commission 8 (Astrometry) and 4 (Ephemerides) provide valuable information about “physical position” and “position in time” respectively, mainly to astronomers and space scientists. Astronomers also need to have accurate definitions for concepts such as the celestial equator — the imaginary line on the sky above the equator on Earth — and the ecliptic — the path of the Sun across the sky — as some earlier reference frames were based on these.
However, much more than basic positional input needs to be considered to establish a Universal Frame of Reference. Scientists have to agree on definitions for certain key reference units or parameters. These topics are covered by other IAU commissions, including Commission 31 (Time ). In the context of the Universal Frame of Reference the work of Commission 31 is also closely linked with Commission 19 on the Rotation of the Earth, as knowledge of the Earth's ever-changing orientation in space is necessary to link terrestrial and celestial frames. The Earth is not fixed, nor in moving in a way that is simply described, so much work goes into measuring and defining this complex movement. Phenomena such as precession, the slow, roughly 25 000 year cycle, of movement of the direction of the Earth’s axis, and nutation, the continual “nodding” of the Earth’s axis, due mainly to tidal forces from the Sun and Moon, all have to be taken into account when defining a Universal Reference Frame.
In 1997 and 1998 the IAU, in collaboration with the International Earth Rotation and Reference Systems Service (IERS) and the International Very Long Baseline Interferometry Service (IVS)International Celestial ReferenceFrame(ICRF). The ICRF uses the relative positions of 212 extragalactic radio sources to establish an origin for the system at the centre of mass of the Solar System, and coordinate axes that are aligned with the conventional axes of the celestial equator and equinox ( the point at which the Sun crosses the equatorial plane moving from south to north ) of the epoch J2000.0 (1200 hours Terrestrial Time on 1 January 2000), but are obtained in a way that is independent of the dynamics of the Earth’s rotation . On 20 August1997, a t the 23rd IAU General Assembly in Kyoto, Japan, the IAU adopted the ICRF, and the celestial equator and the ecliptic were no longer central in establishing a celestial or Universal Reference Frame.

In recent years more precise measurements have allowed the ICRF to be refined, allowing for a much more accurate system. At the IAU General Assembly in 2003 the IAU Working Group (WG) on the ICRF was dissolved and its work is now covered by the main Reference Frame Working Groups: Commission 8, Densification of the Optical Reference Frame and Division 1, Second Realization of International Celestial Reference Frame. On 24 August 2006, at the IAU General Assembly in Prague, new resolutions were adopted that aim to improve our definition of the Universal Frame of Reference.

llllllllllllllllllllllllllllllllll
http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/age.html

Imagine Home | Science | The Size and Age of the Universe - Introduction

How do we measure the size and the age of the Universe?

Astronomers estimate that the Big Bang occurred between 10 and 20 billion years ago. They estimate the age of the Universe in two ways: (a) by looking for the oldest stars; and (b) by measuring the rate of expansion of the Universe and extrapolating back to the Big Bang.

The Oldest Stars

Astronomers can figure out the ages of some of the oldest stars in the Universe by studying globular clusters. A globular cluster is a dense collection of close to a million stars, all of which formed at roughly the same time. The density of stars near the center of a globular cluster is enormous. If we lived near the center of a globular cluster, there would be several hundred thousand stars closer to us than Alpha Centauri, our current nearest stellar neighbor. The life cycle of a star depends upon its mass. High mass stars are much brighter than low mass stars; thus they rapidly burn through their supply of hydrogen fuel. A star like the Sun has enough fuel in its core to burn at its current brightness for approximately 9 billion years. A star that is twice as massive as the Sun will burn through its fuel supply in only 800 million years. A 10 solar mass star (a star that is 10 times more massive than the Sun) burns nearly a thousand times brighter and has only a 20 million year fuel supply. Conversely, a star that is half as massive as the Sun burns slowly enough for its fuel to last more than 20 billion years.
Since all of the stars in a globular cluster formed at roughly the same time, these clusters can serve as cosmic clocks. If a globular cluster is more than 10 million years old, then all of its hydrogen burning stars will be less massive than 10 solar masses. This implies that no individual hydrogen burning star will be more than 1000 times brighter than the Sun. If a globular cluster is more than 2 billion years old, then there will be no hydrogen-burning star more massive than 2 solar masses.
The oldest globular clusters contain only stars less massive than 0.7 solar masses. These low mass stars are much dimmer than the Sun. This suggests that the oldest globular clusters are between 11 and 18 billion years old. The uncertainty in this estimate is due to the difficulty in determining the exact distance to a globular cluster (hence, an uncertainty in the brightness (and mass) of the stars in the cluster). Another source of uncertainty in this estimate lies in our ignorance of some of the finer details of stellar evolution.

Extrapolating Back to the Big Bang

Another way to estimate the age of the Universe is to measure the "Hubble constant". The Hubble constant (H₀) is a measure of the current expansion rate of the Universe. Cosmologists use this measurement to extrapolate back to the Big Bang. This extrapolation depends upon the current density of the Universe and on the composition of the Universe. If the Universe is flat and composed mostly of matter, then the age of the Universe is 2/(3 H₀). If the Universe has a very low density of matter, then its extrapolated age is larger: 1/H₀. If the theory of general relativity is modified to include a cosmological constant, then the inferred age can be even larger.
Many astronomers are working hard to measure the Hubble constant using a variety of different techniques. The current best estimates of H₀ range from 50 kilometers/sec/Megaparsec to 100 km/s/Megaparsec. In more familiar units, astronomers believe that 1/H₀ is between 10 and 20 billion years.
If we compare the two age determinations, there is a potential crisis. If the astronomers who estimate that 1/H₀ is as small as 10 Billion years are correct, then the age of the Universe would be shorter than the age of its oldest stars. This contradiction implies that either the Big Bang theory is incorrect or that we need to modify general relativity by adding a cosmological constant.
Some astronomers believe that this crisis will pass as soon as our measurements improve. If the astronomers who have measured the larger values of 1/H₀ are correct and the smaller estimates of globular cluster ages are also correct, then all may be well for the Big Bang theory.

Fuente: http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971124x.html

Imagine Home | Ask an Astrophysicist | Measuring the Size of the Universe

The Question

(Submitted November 24, 1997) I'm a 13 year old student from Denmark, who wants to know how big the Universe is and how the size of it is measured.

The Answer

The simple answer is that the observable Universe is about 10 billion light years in radius. That number is obtained by multiplying how old we think the Universe is by the speed of light. The reasoning there is quite straightforward: we can only see out to that distance from which light can have reached us since the Universe began. (But see my note marked * below). We determine the age of the Universe in a number of ways. One is to estimate the age of the oldest stars we see. Our knowledge of how stars of a given size evolve with time is very good (based on what we know about atomic and nuclear physics) so the major uncertainty here is usually measuring how far away (and so how big) such stars are. The standard method is to look for very small changes in the apparent positions of the stars as the Earth moves around the Sun. (This effect is called parallax). A second way to get an age for the Universe is to try to figure out the time of the big bang itself. Here the method is to use a series of techniques (based on how bright things appear to be - like Cepheid variable stars - that we think we know the true brightness of) to determine first the distance of the nearby galaxies, then increasingly distant galaxies, until we have estimated distances for many galaxies for which relative velocity measurements have been made (using the Doppler red shift of features in their spectra). The relative velocities we observe for distant galaxies have been largely determined by the expansion of the Universe begun with the 'big bang'. So, once we've determined how expansion velocity correlates with distance for some range of distances, it's possible to extrapolate back (with some assumptions) to calculate the instant of the big bang, when all the matter in the Universe was at a single point.
(If any of these terms like 'parallax', 'Cepheid' and 'red shift' are unfamiliar, try entering them in the search window on our home page).
The determination of greater and greater distances is one of the great themes of astronomy. Most introductory books will give you an outline of the story, which you can then fill in to any level of detail with further reading.
Our website has a lot of material on recent developments. For instance, there are already several answers in the 'Ask an Astrophysicist' archive which deal with the size and age of the Universe. If you enter things like 'size of the Universe', 'age of the Universe', or 'distance scale' in our search window you will get lists of links to many of the most relevant discussions.
Paul Butterworth
for the Ask an Astrophysicist team
* Note: The observable Universe may be only a small part of the physical Universe. In some theories, the Universe may have expanded very fast just after the 'big bang', and only a little bit may have remained within range of detection.

lllllllllllllllllllllllllllllllllllllllllllllllllllll

The Woman Who Discovered the Key to Measuring the Universe

By Sarah Zielinski | February 17, 2012

By the late 1800s, astronomy had moved on from simple human observation to the collection of images of the sky on photographic plates — pieces of glass coated with light-sensitive silver salts. At the time they were made, these plates could be analyzed only through tedious, labor-intensive work. A person had to scan and measure and compare stars in the images before their position and brightness could be calculated and discoveries made.
In 1879, Edward Pickering, head of the Harvard College Observatory, began hiring women to do this work. Paid just 25 to 30 cents an hour for their labors, women were cheaper than men, but Pickering found that they were also better than the male scientists who had done the work previously. The women were more detail-oriented and worked harder. (That didn’t mean they were more respected, however. Today this group of women is often called the “Harvard Computers,” but when they were working they were called “Pickering’s Harem.”) One of the computers was Henrietta Snow Leavitt.
Leavitt joined the team as a volunteer in 1895, after studying astronomy at (what would become) Radcliffe College. In 1902, Leavitt became a permanent—and paid—member of the staff and eventually headed up the photographic photometry department.
Leavitt’s job was to identify variable stars, which can change in brightness over hours to weeks. She used a blink comparator to look back and forth between two plates that showed the same spot in space days or weeks apart. A star that had changed in brightness over that time would appear as a blinking spot, and Leavitt identified more than 2,400 variable stars using this method.
Anyone studying the brightness of stars quickly runs up against a problem—the brightness alone doesn’t give any information about the star. A very bright star from far away looks the same as a dimmer one closer to Earth. But Leavitt eliminated that problem by studying Cepheid variables in the Magellanic Clouds, which are really two tiny galaxies orbiting the Milky Way. She began studying these stars, which are approximately the same distance from Earth whatever their appearance, to determine whether there was a relationship between a variable star’s brightness and the period of its dimming-brightening cycle.
Leavitt identified 1,777 variable stars in the Magellanic Clouds but, due to the difficulty involved in determining the period and maximum and minimum brightnesses for a single star, was able to gather this data for only 25 stars by 1912. But that was enough data for her to find a pattern. When Leavitt plotted these stars’ brightnesses versus their periods on a graph, she found that they were related logarithmically—the brighter the star, the longer its period. (Her study was published in the Harvard College Observatory Circular, dated March 3, 1912, with Pickering as the official author on the paper; he did, however, credit Leavitt for the discovery and the write-up.)
Other astronomers soon realized the value of Leavitt’s discovery. A year later, Danish astronomer Ejnar Hertzsprung determined the distance to several Cepheid variables in the Milky Way, and once this was combined with Leavitt’s data, astronomers could calculate the distance to any Cepheid variable in the sky.
In 1922 and 1923, Edwin Hubble found Cepheid variables in several spiral nebulae and, when he calculated their distances, found they were too far away to be part of the Milky Way and concluded that our galaxy wasn’t the only one in the universe. Leavitt’s finding would also prove to play a key part in Hubble’s later discovery that the universe is expanding.
Hubble recommended that Leavitt be awarded the Nobel Prize in Physics and the head of the Swedish Academy of Sciences began the paperwork for her nomination. That came to a halt, however, when they realized that she had died of cancer in 1921.
After her death, Harlow Shapley, then director of the Harvard College Observatory, wrote:

Much of the time [Leavitt] was engaged at the Harvard Observatory, her efforts had to be devoted to the heavy routine of establishing standard magnitudes upon which later we can base our studies of the galactic system. If she had been free from those necessary chores, I feel sure that Miss Leavitt’s scientific contributions would have been even more brilliant than they were.

We can only imagine what Leavitt and the other computers during that time might have accomplished had they had the time and freedom to devote themselves to efforts outside that of routine astronomical data gathering.
__________
Sarah Zielinski is an award-winning science writer whose work has spanned the range of science, from astronomy to zoology, with a healthy dose of kitty science along the way. Find out more at sarahzielinski.com.

lllllllllllllllllllllllllllllllllllllllllllllllllllll

Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley

Albert Van Helden

University of Chicago Press, 15/09/1986 - 203 páginas

Measuring the Universeis the first history of the evolution of cosmic dimensions, from the work of Eratosthenes and Aristarchus in the third century B.C. to the efforts of Edmond Halley (1656--1742). "Van Helden's authoritative treatment is concise and informative; he refers to numerous sources of information, draws on the discoveries of modern scholarship, and presents the first book-length treatment of this exceedingly important branch of science."--Edward Harrison, American Journal of Physics"Van Helden writes well, with a flair for clear explanation. I warmly recommend this book."--Colin A. Ronan, Journal of the British Astronomical Association

<iframe frameborder="0" scrolling="no" style="border:0px" src="http://books.google.com.au/books?id=L-yb7GX9mQIC&lpg=PP1&hl=es&pg=PP1&output=embed" width=500 height=500></iframe>

---

------

ENCUENTROS UNIVERSITARIOS 3 CANTOS

jueves, 1 de marzo de 2012

Measuring the Universe (Midiendo el universo)

Measuring the Universe

Measuring the Universe: from the transit of Venus to the edge of the cosmos

Measuring the Universe: from the transit of Venus to the edge of the cosmos

Measuring the Universe

How do we measure the size and the age of the Universe?

The Oldest Stars

Extrapolating Back to the Big Bang

Imagine Home | Ask an Astrophysicist | Measuring the Size of the Universe

The Question

The Answer

The Woman Who Discovered the Key to Measuring the Universe

Measuring the Universe: Cosmic Dimensions from Aristarchus to Halley

No hay comentarios:

Publicar un comentario